[EADGBE]

I have a Nakamichi CR-7A that works and sounds great, but it has
developed a problem, and I am wondering how difficult it will be to
repair.

The flourescent meter window has stopped working.

I know a little about electronics, so I opened up the deck and checked
the voltages coming off of the transformer.

As expected, there were various different outputs of AC voltage,
including an output of 3.5v AC.

This 3.5v AC output feeds the flourescent meter window AND NOTHING
ELSE.

But this output has stopped working. No smoke, no fire, everything
else on the deck works perfectly fine. There is just no more 3.5v AC
output from the transformer.

QUESTION 1: What happened? Did the transformer have a meltdown or is
it as simple as some kind of internal fuse? (I should point out that
this deck's power supply circuit had been damaged at some point,
possibly by a power surge, but it was professionally repaired by an
authorized Nak repair center).

QUESTION 2: Can this be fixed - and if so, how difficult is it?

QUESTION 3: Will it be possible to find a replacement transformer?
For the record, this transformer has the following PAIRS of outputs:

3.5v AC (two of them)
5v AC (two of them)
15v AC (two of them)
18v AC (two of them)

Any help would be much appreciated.

[EADGBE]

Thanks for the link - I have already downloaded the service manual.
That's how I know the voltages for the secondary outputs.

I should have made that more clear in my original post (sorry).

[Eeyore]

EADGBE wrote:

Thanks for the link - I have already downloaded the service manual.
That's how I know the voltages for the secondary outputs.

I should have made that more clear in my original post (sorry).

The 3.5V you've lost is almost certainly the filament supply for the vacuum
fluourescent display. That explains why it's gone out.

Where does the other 3.5V go to ?

Graham

[Chris Hornbeck]

On Thu, 27 Sep 2007 18:24:01 -0700, EADGBE
wrote:

The flourescent meter window has stopped working.

As expected, there were various different outputs of AC voltage,
including an output of 3.5v AC.

This 3.5v AC output feeds the flourescent meter window AND NOTHING
ELSE.

But this output has stopped working. No smoke, no fire, everything
else on the deck works perfectly fine. There is just no more 3.5v AC
output from the transformer.

You are possibly the luckiest critter on the planet. If your
measurements are correct, and I'm not in the least convinced that
they are, the display itself (unobtainium) is still alive. A
small transformer to provide lamp voltage is all you'll need.

But, this never happens except in the movies. Sadly, the vast bulk
of these issues are failure of the *incandescent* internal lamp.
Yeah, who knew? This was before LED's and relative immortality.

Before you panic, disconnect the supply lines and measure across
the lamp filament. Got continuity? If so, cuss me loudly for being
such a panic-monger. If not, you have my sympathies.

All good fortune,

Chris Hornbeck

[Paul Stamler]

"EADGBE" wrote in message
oups.com...
Thanks for the link - I have already downloaded the service manual.
That's how I know the voltages for the secondary outputs.

I should have made that more clear in my original post (sorry).

If you're not getting 3.5VAC from the winding there are a couple of
possibilities. First is a fuse -- is there one between the transformer and
the diode bridge? If so, look for a blown one. If you find one, figure out
why it blew before replacing it -- possibilities include a shorted
capacitor.

If no fuse, then your transformer's winding may have burnt out -- but
usually you can smell that.

Try detaching the transformer winding from the board, if that's doable, and
see if the voltage is still missing.

Peace,
Paul

[Arfa Daily]

"Paul Stamler" wrote in message
...
"EADGBE" wrote in message
oups.com...
Thanks for the link - I have already downloaded the service manual.
That's how I know the voltages for the secondary outputs.

I should have made that more clear in my original post (sorry).

If you're not getting 3.5VAC from the winding there are a couple of
possibilities. First is a fuse -- is there one between the transformer and
the diode bridge? If so, look for a blown one. If you find one, figure out
why it blew before replacing it -- possibilities include a shorted
capacitor.

If no fuse, then your transformer's winding may have burnt out -- but
usually you can smell that.

Try detaching the transformer winding from the board, if that's doable,
and
see if the voltage is still missing.

Peace,
Paul



Where are you measuring the voltage ? As others have said, the 3.5v AC going
to the display, is its filament supply. Assuming that it does go direct to
the front panel as an AC supply (90% of manufacturers do it like this, but a
few don't), then it will eventually arrive at either end of the VFD pin row,
where you will find, a pair of pins separated from the rest of the row, by
one pin gap. The trick is that sometimes, there is a safety (fuse) resistor
or small wire-ended fuse in series with the supply to one end. Several of
the Aiwa models have such items, and you can't see them unless the front
panel PCB is removed. They go open for no good reason. I would try measuring
across either end of the display, and where the winding arrives at the
board. Don't trust that any such fuse will be shown on the schematics
either. A further check for no display, just in case you are fooling
yourself with the missing filament supply, is to measure from either one of
the filament pin pairs, to ground. You shoud get a reading of between about
minus 27v and minus 35 volts. You can measure it from filament to ground, as
the filament is biased to this figure to reduce the inter-electrode stress
in the VFD. It is very common in all manufacturer's kit for this supply to
fail, as it is often derived from a voltage multiplier using ordinary
electrolytic caps, which dry out and go open over the years, due to the
continuous pounding they are taking from having high level AC across them.

Arfa

[EADGBE]

On Sep 27, 11:55 pm, Eeyore
wrote:

Where does the other 3.5V go to ?

Graham

GRAHAM:

Both of the 3.5v AC supplies go directly to the flourescent display.
They don't go anywhere else.

[EADGBE]

On Sep 28, 12:58 am, "Paul Stamler" wrote:

If you're not getting 3.5VAC from the winding there are a couple of
possibilities. First is a fuse -- is there one between the transformer and
the diode bridge? If so, look for a blown one. If you find one, figure out
why it blew before replacing it -- possibilities include a shorted
capacitor.

If no fuse, then your transformer's winding may have burnt out -- but
usually you can smell that.

Try detaching the transformer winding from the board, if that's doable, and
see if the voltage is still missing.

PAUL:

It's pretty much a direct path from the transformer to the flourescent
display. They don't go to a diode bridge because they are never
converted to DC voltage. The two 3.5v leads are soldered to the power
supply board, where each of them runs into a 1-watt "r-fuse". (Both
of the 3.5v leads have their own "r-fuse".) Then these lines go to a
connector near the back of the deck that's mounted on the deck's logic
board. That connector then has straight circuit board lines to a
connector at the front of the deck which attaches to the flourescent
display.

I am measuring voltage directly at the ends of the two 3.5vAC leads
coming out of the transformer. No output at all.

[Eeyore]

EADGBE wrote:

Eeyore wrote:

Where does the other 3.5V go to ?

Graham

GRAHAM:

Both of the 3.5v AC supplies go directly to the flourescent display.
They don't go anywhere else.

How do TWO supplies connect to the display ?

Do you actually mean two WIRES (you said pairs of wires originally) ?

Graham

[Arfa Daily]

"EADGBE" wrote in message
oups.com...
On Sep 28, 12:58 am, "Paul Stamler" wrote:

If you're not getting 3.5VAC from the winding there are a couple of
possibilities. First is a fuse -- is there one between the transformer
and
the diode bridge? If so, look for a blown one. If you find one, figure
out
why it blew before replacing it -- possibilities include a shorted
capacitor.

If no fuse, then your transformer's winding may have burnt out -- but
usually you can smell that.

Try detaching the transformer winding from the board, if that's doable,
and
see if the voltage is still missing.

PAUL:

It's pretty much a direct path from the transformer to the flourescent
display. They don't go to a diode bridge because they are never
converted to DC voltage. The two 3.5v leads are soldered to the power
supply board, where each of them runs into a 1-watt "r-fuse". (Both
of the 3.5v leads have their own "r-fuse".) Then these lines go to a
connector near the back of the deck that's mounted on the deck's logic
board. That connector then has straight circuit board lines to a
connector at the front of the deck which attaches to the flourescent
display.

I am measuring voltage directly at the ends of the two 3.5vAC leads
coming out of the transformer. No output at all.


Based on that, I would have to say that the 3.5v winding is open, but by the
same token, I would also have to say that it is extremely unusual for a
single secondary winding - particularly one that has to do little 'work'
like this one - to fail without there being some kind of fault like a dead
short on the end of it. If the windings come out to tags on the tranny, it
would be worth checking that the soldering between the winding wire and the
tag itself is good. I have known the laquer coating of the wire to never
have been stripped properly, which has led to a bad joint at the tag which
has managed to 'make' ok for years, before finally giving up. Also check
that the winding wire has not fractured right at the tag. I have seen that
as well, presumably caused by heat expansion and contraction over the years,
working to break the wire.

As a final check, you could sub the supply. It doesn't have to be AC. You
could just hook up a couple of 1.5v dry cells to give you 3v, and hang that
across the two wires going back to the tranny, having first disconnected
them, of course. The reason I say connect it there, is so that any bias
supply that the filament supply is floated on, still gets applied.

Arfa

[Eeyore]

Arfa Daily wrote:

As a final check, you could sub the supply. It doesn't have to be AC. You
could just hook up a couple of 1.5v dry cells to give you 3v, and hang that
across the two wires going back to the tranny, having first disconnected
them, of course.

DC on the filament will actually cause a brightness variation across the
display bit it's fine for fault finding.

Graham

[EADGBE]

On Sep 28, 7:14 am, "Arfa Daily" wrote:

Based on that, I would have to say that the 3.5v winding is open, but by the
same token, I would also have to say that it is extremely unusual for a
single secondary winding - particularly one that has to do little 'work'
like this one - to fail without there being some kind of fault like a dead
short on the end of it.

ARFA:

There is one important factor that I have not mentioned.

I bought this deck on eBay, from an honest seller who clearly stated
that the deck was NOT in working order. I bought it, figuring that I
could fix it up.

When I looked it over, I found problems that clearly indicate that
this deck had encountered some sort of power surge or maybe even a
lightning strike. All three fuses on the power supply board were
blown (two for 15v AC; the other one is for 5v AC), PLUS two voltage-
regulating transistors on the power supply board were blown (one of
them burnt to a crisp), PLUS a couple of diodes had shorted out. Oh,
and a buffering capacitor in another part of the deck had shorted
completely.

After fixing those problems, the power supply is stable and gives good
voltage levels throughout the deck, and the deck runs perfectly.

Perhaps the failure of the 3.5v tap on the power transformer is the
result of some stress on the transformer from the power
surge?????????????????

QUESTION: Could it be possible that the 3.5v tap had been damaged by
the power surge, and was "tired"?

ANOTHER QUESTION: Is it possible that such a low-voltage tap could
have its own thermal fuse inside the transformer?

NOTE: The 3.5v AC is the ONLY voltage line that comes directly from
the transformer and is not "handled" or "modified" in any way by the
power supply circuitry.

Thanks for the information on what to look for on the transformer
itself. I'll be sure to check it out.

[EADGBE]

On Sep 28, 7:10 am, Eeyore
wrote:

How do TWO supplies connect to the display ?

Do you actually mean two WIRES (you said pairs of wires originally) ?

GRAHAM:

I must admit that I was assuming that the two 3.5v wires were two
supplies. They could easily be two parts of ONE 3.5v supply.

I was assuming (I know--a dangerous practice) that they were two
supplies because the transformer seems to have a common ground wire
(black wire) soldered to the ground circuit on the power supply
board. Or maybe the black wire isn't a common ground for the entire
transformer....read below:

Besides the outputs I have already mentioned, there is a black wire
soldered to the ground circuit of the power supply board, and this
wire is labeled "CT GND". There is also a grey wire soldered to the
power supply board in another spot, and it is labeled "CT". What does
"CT" stand for?

[Eeyore]

EADGBE wrote:

On Sep 28, 7:10 am, Eeyore
wrote:

How do TWO supplies connect to the display ?

Do you actually mean two WIRES (you said pairs of wires originally) ?

GRAHAM:

I must admit that I was assuming that the two 3.5v wires were two
supplies. They could easily be two parts of ONE 3.5v supply.

I was assuming (I know--a dangerous practice) that they were two
supplies because the transformer seems to have a common ground wire
(black wire) soldered to the ground circuit on the power supply
board. Or maybe the black wire isn't a common ground for the entire
transformer....read below:

The filament supply will most certainly NOT share a comon ground. It has to
'float' at a negative voltage provided by the rest of the psu, so it seems you
have one winding only. Have you measured it with a DVM ? I would expect it to
show 'infinite' ohms indicating it's 'burnt out'.


Besides the outputs I have already mentioned, there is a black wire
soldered to the ground circuit of the power supply board, and this
wire is labeled "CT GND". There is also a grey wire soldered to the
power supply board in another spot, and it is labeled "CT". What does
"CT" stand for?

Centre tap. Probably just for the main audio +/- supply.

So, I'd suggest you follow Arfa's advice and connect 2 1.5 batteries in series
to the display filament. That should bring it back to life.

What one does about it in the long term is another matter. Is there room to fit
another small transformer inside ?

Graham

[EADGBE]

On Sep 28, 8:27 am, Eeyore
wrote:

The filament supply will most certainly NOT share a comon ground. It has to
'float' at a negative voltage provided by the rest of the psu, so it seems you
have one winding only. Have you measured it with a DVM ? I would expect it to
show 'infinite' ohms indicating it's 'burnt out'.

GRAHAM:

I'm a happily married man, but nevertheless, I could kiss you! With
your latest suggestion, you have given me some hope.

Up to this point, I had always assumed that the two blue 3.5v wires
coming out of the transformer were TWO 3.5v supplies. It never
occured to me that it was a single winding.

I disconnected the jumper wire carrying the 3.5v charge to the deck's
logic board and connected my DVM up to the two wires, where they are
soldered to the power supply board.

It measured resistance of 1.6 ohms! It is NOT infinite, and therefore
NOT open circuit!

BUT THE NAGGING QUESTION REMAINS: WHY AM I NOT GETTING *ANY* VOLTAGE
READINGS ON EITHER OF THE TWO BLUE WIRES WHEN THE DECK IS SWITCHED
ON???

Thank you VERY much for your help so far, Graham! I at least have a
glimmer of light at the end of the tunnel.

[Gareth Magennis]

"EADGBE" wrote in message
ups.com...
On Sep 28, 7:14 am, "Arfa Daily" wrote:

Based on that, I would have to say that the 3.5v winding is open, but by
the
same token, I would also have to say that it is extremely unusual for a
single secondary winding - particularly one that has to do little 'work'
like this one - to fail without there being some kind of fault like a
dead
short on the end of it.

ARFA:

There is one important factor that I have not mentioned.

I bought this deck on eBay, from an honest seller who clearly stated
that the deck was NOT in working order. I bought it, figuring that I
could fix it up.

When I looked it over, I found problems that clearly indicate that
this deck had encountered some sort of power surge or maybe even a
lightning strike. All three fuses on the power supply board were
blown (two for 15v AC; the other one is for 5v AC), PLUS two voltage-
regulating transistors on the power supply board were blown (one of
them burnt to a crisp), PLUS a couple of diodes had shorted out. Oh,
and a buffering capacitor in another part of the deck had shorted
completely.

After fixing those problems, the power supply is stable and gives good
voltage levels throughout the deck, and the deck runs perfectly.

Perhaps the failure of the 3.5v tap on the power transformer is the
result of some stress on the transformer from the power
surge?????????????????

QUESTION: Could it be possible that the 3.5v tap had been damaged by
the power surge, and was "tired"?



Maybe yes, maybe no, definately irrelevent.



ANOTHER QUESTION: Is it possible that such a low-voltage tap could
have its own thermal fuse inside the transformer?


Almost certainly not, almost certainly irrelevent since you won't be able to
fix it.


NOTE: The 3.5v AC is the ONLY voltage line that comes directly from
the transformer and is not "handled" or "modified" in any way by the
power supply circuitry.

Thanks for the information on what to look for on the transformer
itself. I'll be sure to check it out.



I would go back and double check with an ohm meter, not a voltmeter, that
the 3.5 volt winding is definately open circuit - likewise check that the
display filament isn't open circuit, again with an ohm meter. There is no
point wasting time with the transformer until you know the display is OK.


Gareth.

[Gareth Magennis]

"EADGBE" wrote in message
oups.com...
On Sep 28, 8:27 am, Eeyore
wrote:

The filament supply will most certainly NOT share a comon ground. It has
to
'float' at a negative voltage provided by the rest of the psu, so it
seems you
have one winding only. Have you measured it with a DVM ? I would expect
it to
show 'infinite' ohms indicating it's 'burnt out'.

GRAHAM:

I'm a happily married man, but nevertheless, I could kiss you! With
your latest suggestion, you have given me some hope.

Up to this point, I had always assumed that the two blue 3.5v wires
coming out of the transformer were TWO 3.5v supplies. It never
occured to me that it was a single winding.

I disconnected the jumper wire carrying the 3.5v charge to the deck's
logic board and connected my DVM up to the two wires, where they are
soldered to the power supply board.

It measured resistance of 1.6 ohms! It is NOT infinite, and therefore
NOT open circuit!

BUT THE NAGGING QUESTION REMAINS: WHY AM I NOT GETTING *ANY* VOLTAGE
READINGS ON EITHER OF THE TWO BLUE WIRES WHEN THE DECK IS SWITCHED
ON???

Thank you VERY much for your help so far, Graham! I at least have a
glimmer of light at the end of the tunnel.



Are you measuring between the two wires and is your meter on AC volts or DC
volts?


Gareth.

[John Williamson]

EADGBE wrote:


BUT THE NAGGING QUESTION REMAINS: WHY AM I NOT GETTING *ANY* VOLTAGE
READINGS ON EITHER OF THE TWO BLUE WIRES WHEN THE DECK IS SWITCHED
ON???

Have you checked for voltage *across* the blue wires?
If it's an isolated heater supply, either wire on its own will show zero
with respect to chassis ground, but will show 3.5 volts with respect to
the other blue wire.

--
Tciao for Now!

John.

[[email protected]]

On Sep 28, 8:50 am, EADGBE wrote:
It measured resistance of 1.6 ohms! It is NOT infinite, and therefore
NOT open circuit!

Which is not at all out of the realm of what such a winding
might be.

BUT THE NAGGING QUESTION REMAINS: WHY AM I NOT
GETTING *ANY* VOLTAGE READINGS ON EITHER OF THE
TWO BLUE WIRES WHEN THE DECK IS SWITCHED
ON???

Because if that winding is NOT connected with the common
ground, and you are trying to measure the voltage of either
one in reference to that common ground, you won't get
any meaningful reading.

Instead, you need to measure the AC voltage between these
two wires, NOT between each of them and ground.

I'd bet when you do so, you get the voltage you're looking
for, which probably indicated the display is shot, among other
things.

[Scott Dorsey]

EADGBE wrote:
I know a little about electronics, so I opened up the deck and checked
the voltages coming off of the transformer.

As expected, there were various different outputs of AC voltage,
including an output of 3.5v AC.

This 3.5v AC output feeds the flourescent meter window AND NOTHING
ELSE.

But this output has stopped working. No smoke, no fire, everything
else on the deck works perfectly fine. There is just no more 3.5v AC
output from the transformer.

Disconnect the transformer from the board. Is there still no output
across it?

QUESTION 1: What happened? Did the transformer have a meltdown or is
it as simple as some kind of internal fuse? (I should point out that
this deck's power supply circuit had been damaged at some point,
possibly by a power surge, but it was professionally repaired by an
authorized Nak repair center).

Either the transformer is bad, or something on the board has failed
into a short.

QUESTION 2: Can this be fixed - and if so, how difficult is it?

You can either spend big money to have the old one rewound, or you could
retrofit a second transformer in there to replace the 3.5V winding that
is bad, or you could call Steven Sank and have him send you a new transformer
from his large stock of Nak parts.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Scott Dorsey]

EADGBE wrote:

Besides the outputs I have already mentioned, there is a black wire
soldered to the ground circuit of the power supply board, and this
wire is labeled "CT GND". There is also a grey wire soldered to the
power supply board in another spot, and it is labeled "CT". What does
"CT" stand for?

Center tap. One of the windings has a grounded center tap. Which one?
the schematic will tell you.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[EADGBE]

I have discovered a fault on the power supply board - in fact, it's
the only possible fault there could be, other than the transformer.

And it looks like my initial panic about a defective transformer MIGHT
have been an overreaction.

First of all, I must apologize for my hasty (and incorrect) assumption
that there were TWO 3.5v taps, connected to common ground.

This was NOT true. The two blue wires coming from the transformer are
ONE complete AC supply.

The GOOD news: This 3.5v tap is NOT dead. It DOES produce voltage.

The PUZZLING news: Instead of the 3.5v AC specified in the service
manual, I am measuring a steady 7.97v AC.

HERE IS THE FAULT I DISCOVERED: As I mentioned before, the two wires
of the 3.5v supply (which is actually measuring at 7.97v) are each
connected to their own 1-ohm 1/4-watt "R-FUSE" before proceeding to
the jumper wire that goes to the flourescent display.

I don't really know what an "R-FUSE" is, actually. They just seem to
be regular dull blue 1-ohm metal film resistors. But the schematic
symbol shows a small rectangle with symbols for a fuse and a resistor
inside the rectangle, connected in series.

Is it possible that these are indeed metal film resistors BEING USED
AS FUSES as well as being a voltage drop for the "3.5v" supply which
is actually 7.97v?

[EADGBE]

Whoops, I forgot to specifically mention what the fault was.

One of the "R-FUSES" has become open circuit.

So the transformer voltage cannot reach the flourescent display.

[Gareth Magennis]

"EADGBE" wrote in message
ups.com...
I have discovered a fault on the power supply board - in fact, it's
the only possible fault there could be, other than the transformer.

And it looks like my initial panic about a defective transformer MIGHT
have been an overreaction.

First of all, I must apologize for my hasty (and incorrect) assumption
that there were TWO 3.5v taps, connected to common ground.

This was NOT true. The two blue wires coming from the transformer are
ONE complete AC supply.

The GOOD news: This 3.5v tap is NOT dead. It DOES produce voltage.

The PUZZLING news: Instead of the 3.5v AC specified in the service
manual, I am measuring a steady 7.97v AC.

HERE IS THE FAULT I DISCOVERED: As I mentioned before, the two wires
of the 3.5v supply (which is actually measuring at 7.97v) are each
connected to their own 1-ohm 1/4-watt "R-FUSE" before proceeding to
the jumper wire that goes to the flourescent display.

I don't really know what an "R-FUSE" is, actually. They just seem to
be regular dull blue 1-ohm metal film resistors. But the schematic
symbol shows a small rectangle with symbols for a fuse and a resistor
inside the rectangle, connected in series.

Is it possible that these are indeed metal film resistors BEING USED
AS FUSES as well as being a voltage drop for the "3.5v" supply which
is actually 7.97v?




Fuse resistors are pretty common. They are not meant to be replaced in the
same way as a normal fuse as they do not normally burn unless there has been
a serious problem. (Fuses often just get tired and blow when they haven't
been told to). You have to correct the problem, then replace the fuse
resistor. If yours are in series with the filament supply and one has
blown, it is because FAR TOO MUCH CURRENT HAS GONE THROUGH THE FILAMENTS AT
SOME POINT.

AND EXCUSE ME FOR SHOUTING AS LOUD AS I CAN.

So it does look as though the unit has suffered some kind of over voltage
thingy, and the filamant may now be a piece of curly toast. Unless of
course the problem is just that the cabling and circuit board from
transformer to display has a fault instead. Go and check the resistance of
the filament right now, as I suggested earlier on. This may all be academic
if you need a new display.

Incidentally, fuse resistors are not just resistors. They are flame proof
so they wont make a lot of alarming smoke and a cringeworthy smell, and have
been designed as a fuse, not a resistor.


Gareth.

[Scott Dorsey]

EADGBE wrote:
First of all, I must apologize for my hasty (and incorrect) assumption
that there were TWO 3.5v taps, connected to common ground.

This was NOT true. The two blue wires coming from the transformer are
ONE complete AC supply.

You need to get the service manual, which will detail how the transformer
is wired.

The GOOD news: This 3.5v tap is NOT dead. It DOES produce voltage.

The PUZZLING news: Instead of the 3.5v AC specified in the service
manual, I am measuring a steady 7.97v AC.

This is because it is completely unloaded. Inexpensive transformers will
often have poor regulation, and will shoot up to fairly high voltages when
unloaded.

HERE IS THE FAULT I DISCOVERED: As I mentioned before, the two wires
of the 3.5v supply (which is actually measuring at 7.97v) are each
connected to their own 1-ohm 1/4-watt "R-FUSE" before proceeding to
the jumper wire that goes to the flourescent display.

I don't really know what an "R-FUSE" is, actually. They just seem to
be regular dull blue 1-ohm metal film resistors. But the schematic
symbol shows a small rectangle with symbols for a fuse and a resistor
inside the rectangle, connected in series.

Is it possible that these are indeed metal film resistors BEING USED
AS FUSES as well as being a voltage drop for the "3.5v" supply which
is actually 7.97v?

They are fusible resistors; 1 ohm resistors that have controlled burnout
characteristics for use as fuses. MCM Electronics carries replacements.
You can substitute a stock 1 ohm resistor if you are adventurous, or
replace it with a picofuse.

Does an ohmmeter show them as open or not?
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[EADGBE]

On Sep 28, 10:42 am, "Gareth Magennis"
wrote:
Go and check the resistance of
the filament right now, as I suggested earlier on. This may all be academic
if you need a new display.

Incidentally, fuse resistors are not just resistors. They are flame proof
so they wont make a lot of alarming smoke and a cringeworthy smell, and have
been designed as a fuse, not a resistor.

GARETH:

I have just checked the resistance of the filament, as you asked
(sorry I didn't do it sooner).

It seems to be fine. There is a solid reading of 7.2 ohms.

This is an accurate reading because I disconnected the filament from
the rest of the display window circuitry before measuring it.

It's interesting to note that the filament circuit consists entirely
of just these items:

1) AC voltage coming from the transformer - dedicated to just the
filament and nothing else.
2) Two (2) inline fuse resistors - 1 ohm, 1/4 watt each
3) The filament itself
4) A jumper wire connecting the whole thing together.

[Gareth Magennis]

"EADGBE" wrote in message
oups.com...
On Sep 28, 10:42 am, "Gareth Magennis"
wrote:
Go and check the resistance of
the filament right now, as I suggested earlier on. This may all be
academic
if you need a new display.

Incidentally, fuse resistors are not just resistors. They are flame
proof
so they wont make a lot of alarming smoke and a cringeworthy smell, and
have
been designed as a fuse, not a resistor.

GARETH:

I have just checked the resistance of the filament, as you asked
(sorry I didn't do it sooner).

It seems to be fine. There is a solid reading of 7.2 ohms.

This is an accurate reading because I disconnected the filament from
the rest of the display window circuitry before measuring it.

It's interesting to note that the filament circuit consists entirely
of just these items:

1) AC voltage coming from the transformer - dedicated to just the
filament and nothing else.
2) Two (2) inline fuse resistors - 1 ohm, 1/4 watt each
3) The filament itself
4) A jumper wire connecting the whole thing together.



Well you may be lucky and find the fuse resistor blew before the filament
then.

I have no idea what the filament resistance should read, but calculating 7.2
ohms + 2 x 1 ohm resistors at 3.5v gives about 380 mA max, thats 0.14 watts
per fusible resistor. I shuld imagine that as the filament heats up its
resistance will increase dramatically, reducing the current, so it looks
like you may be in the right ball park.



Gareth.

[Arfa Daily]

"EADGBE" wrote in message
oups.com...
On Sep 28, 8:27 am, Eeyore
wrote:

The filament supply will most certainly NOT share a comon ground. It has
to
'float' at a negative voltage provided by the rest of the psu, so it
seems you
have one winding only. Have you measured it with a DVM ? I would expect
it to
show 'infinite' ohms indicating it's 'burnt out'.

GRAHAM:

I'm a happily married man, but nevertheless, I could kiss you! With
your latest suggestion, you have given me some hope.

Up to this point, I had always assumed that the two blue 3.5v wires
coming out of the transformer were TWO 3.5v supplies. It never
occured to me that it was a single winding.

I disconnected the jumper wire carrying the 3.5v charge to the deck's
logic board and connected my DVM up to the two wires, where they are
soldered to the power supply board.

It measured resistance of 1.6 ohms! It is NOT infinite, and therefore
NOT open circuit!

BUT THE NAGGING QUESTION REMAINS: WHY AM I NOT GETTING *ANY* VOLTAGE
READINGS ON EITHER OF THE TWO BLUE WIRES WHEN THE DECK IS SWITCHED
ON???

Thank you VERY much for your help so far, Graham! I at least have a
glimmer of light at the end of the tunnel.


Again, where are you measuring exactly ? If you are measuring between ground
and either of the two wires, you will not get a reading, because as Graham
says, the filament winding is completely floating. You must measure *across*
the two wires, with your meter set to its AC 10v range. If you have a
reading of 1.6 ohms across the winding, and the rest of the transformer is
working, it is highly unlikely that there is anything wrong to be causing
the 3.5v winding to have no output. Also, when the winding is connected
where it's supposed to be, you need to measure across the two sets of double
and slightly spaced pins at either end of the VFD pin row to read the AC
filament voltage. If you read between ground and those pins, you will not
read filament AC voltage, but with the meter set to DC you should read a
negative voltage of 25 to 35v. If you do not, then it is the negative supply
which is missing, not the filament supply. This is the much more common
fault condition in my experience.

Arfa

[Arfa Daily]

"Gareth Magennis" wrote in message
...

"EADGBE" wrote in message
ups.com...
I have discovered a fault on the power supply board - in fact, it's
the only possible fault there could be, other than the transformer.

And it looks like my initial panic about a defective transformer MIGHT
have been an overreaction.

First of all, I must apologize for my hasty (and incorrect) assumption
that there were TWO 3.5v taps, connected to common ground.

This was NOT true. The two blue wires coming from the transformer are
ONE complete AC supply.

The GOOD news: This 3.5v tap is NOT dead. It DOES produce voltage.

The PUZZLING news: Instead of the 3.5v AC specified in the service
manual, I am measuring a steady 7.97v AC.

HERE IS THE FAULT I DISCOVERED: As I mentioned before, the two wires
of the 3.5v supply (which is actually measuring at 7.97v) are each
connected to their own 1-ohm 1/4-watt "R-FUSE" before proceeding to
the jumper wire that goes to the flourescent display.

I don't really know what an "R-FUSE" is, actually. They just seem to
be regular dull blue 1-ohm metal film resistors. But the schematic
symbol shows a small rectangle with symbols for a fuse and a resistor
inside the rectangle, connected in series.

Is it possible that these are indeed metal film resistors BEING USED
AS FUSES as well as being a voltage drop for the "3.5v" supply which
is actually 7.97v?




Fuse resistors are pretty common. They are not meant to be replaced in
the same way as a normal fuse as they do not normally burn unless there
has been a serious problem. (Fuses often just get tired and blow when they
haven't been told to). You have to correct the problem, then replace the
fuse resistor. If yours are in series with the filament supply and one
has blown, it is because FAR TOO MUCH CURRENT HAS GONE THROUGH THE
FILAMENTS AT SOME POINT.

AND EXCUSE ME FOR SHOUTING AS LOUD AS I CAN.

So it does look as though the unit has suffered some kind of over voltage
thingy, and the filamant may now be a piece of curly toast. Unless of
course the problem is just that the cabling and circuit board from
transformer to display has a fault instead. Go and check the resistance of
the filament right now, as I suggested earlier on. This may all be
academic if you need a new display.

Incidentally, fuse resistors are not just resistors. They are flame
proof so they wont make a lot of alarming smoke and a cringeworthy smell,
and have been designed as a fuse, not a resistor.


Gareth.


Whilst in theory I would agree that fuse resistors often fail because of a
following problem, I would have to disagree with it as an electronics
'generalisation'. I have been repairing kit for as long as fuse resistors
have been in common usage ( and a good many years before that ), and I find
that as many go open circuit for no detectable reason, with no physical
signs of distress, or having been stressed, as fail for a valid reason. This
seems to be particularly true of ones that are positioned in AC feeds, so
maybe they don't like AC across them too much. As I said in an earlier post
in this thread, several models of Aiwa hifi employ a 1 ohm fuse resistor in
series with the VFD filament supply, and it is very common for them to fail.
A replacement always restores completely normal operation of the display,
and you don't get a 2 week bounce from them either.

Arfa

[EADGBE]

On Sep 28, 12:04 pm, "Arfa Daily" wrote:

Again, where are you measuring exactly ? If you are measuring between ground
and either of the two wires, you will not get a reading, because as Graham
says, the filament winding is completely floating.

ARFA:

I measured the resistance between the two blue 3.5v wires of the
transformer, with the jumper that leads to the flourescent display
DISCONNECTED.

I got the resistance reading of 1.6 ohms.

I measured the voltage the same way.

I took NO readings involving the ground...only between the two 3.5v
wires coming out of the transformer.

[EADGBE]

On Sep 28, 10:46 am, (Scott Dorsey) wrote:

Does an ohmmeter show them as open or not?
--scott

SCOTT:

The fuse resistor that didn't blow reads exactly 1.04 ohms (that's an
in-circuit reading).

The fuse resistor that blew reads open circuit....and that is with the
resistor TAKEN OUT of circuit completely.

I figured I could replace the fuse resistor with a regular 1-ohm
resistor and get things working again, but I also realize that that is
NOT safe, and so the deck will not be powered up again until it has a
new 1-ohm fuse resistor in place...and I might even replace the other
1-ohm fuse resistor whilst I'm at it.

[Paul Stamler]

"EADGBE" wrote in message
ups.com...

The GOOD news: This 3.5v tap is NOT dead. It DOES produce voltage.

The PUZZLING news: Instead of the 3.5v AC specified in the service
manual, I am measuring a steady 7.97v AC.

Are you measuring with the display not connected? The transformer's output
without a load will be higher than with a load.

HERE IS THE FAULT I DISCOVERED: As I mentioned before, the two wires
of the 3.5v supply (which is actually measuring at 7.97v) are each
connected to their own 1-ohm 1/4-watt "R-FUSE" before proceeding to
the jumper wire that goes to the flourescent display.

I don't really know what an "R-FUSE" is, actually. They just seem to
be regular dull blue 1-ohm metal film resistors. But the schematic
symbol shows a small rectangle with symbols for a fuse and a resistor
inside the rectangle, connected in series.

Is it possible that these are indeed metal film resistors BEING USED
AS FUSES as well as being a voltage drop for the "3.5v" supply which
is actually 7.97v?

Quite possible. These are called "fusible resistors", and they're designed
to protect the circuit by blowing up if too much current flows through them.
If you combine loading on the transformer and two 1-ohm resistors in series
with the filament, that might give you the 3.5V you want.

Next step: put the ohmmeter across each of the 1-ohm resistors and see if
they're blown.

Peace,
Paul

[Dave Platt]

In article .com,
EADGBE wrote:

How do TWO supplies connect to the display ?

Do you actually mean two WIRES (you said pairs of wires originally) ?

GRAHAM:

I must admit that I was assuming that the two 3.5v wires were two
supplies. They could easily be two parts of ONE 3.5v supply.

It's not uncommon for the filament on a VFD to be fed from a
centertapped transformer secondary winding. The center tap is
grounded, and the two ends of the winding feed the two ends of the
filament. The same effect can be created using a dual secondary
winding, of course.

This approach has the effect of balancing the filament voltage around
the system-ground voltage, and helps ensure a consistent
anode-to-cathode voltage (and consistent brightness) across the width
of the VFD.

If you feed the filament from a single secondary winding, and ground
one end or the other, the display brightness will vary somewhat from
one side to the other. DC drive is even worse in this respect.

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

[Eeyore]

EADGBE wrote:

On Sep 28, 8:27 am, Eeyore
wrote:

The filament supply will most certainly NOT share a comon ground. It has to
'float' at a negative voltage provided by the rest of the psu, so it seems you
have one winding only. Have you measured it with a DVM ? I would expect it to
show 'infinite' ohms indicating it's 'burnt out'.

GRAHAM:

I'm a happily married man, but nevertheless, I could kiss you! With
your latest suggestion, you have given me some hope.

Up to this point, I had always assumed that the two blue 3.5v wires
coming out of the transformer were TWO 3.5v supplies. It never
occured to me that it was a single winding.

I disconnected the jumper wire carrying the 3.5v charge to the deck's
logic board and connected my DVM up to the two wires, where they are
soldered to the power supply board.

It measured resistance of 1.6 ohms! It is NOT infinite, and therefore
NOT open circuit!

BUT THE NAGGING QUESTION REMAINS: WHY AM I NOT GETTING *ANY* VOLTAGE
READINGS ON EITHER OF THE TWO BLUE WIRES WHEN THE DECK IS SWITCHED
ON???

That suggests the 3.5V winding is 'floating' at zero volts and somehow the negative
supply to the VFD has failed. This would give the same symptoms but is probably much
easier to fix.

One now needs to check out the circuitry driving the VFD. It'll be a case of
examining the schematics.

Graham

[Eeyore]

John Williamson wrote:

EADGBE wrote:

BUT THE NAGGING QUESTION REMAINS: WHY AM I NOT GETTING *ANY* VOLTAGE
READINGS ON EITHER OF THE TWO BLUE WIRES WHEN THE DECK IS SWITCHED
ON???

Have you checked for voltage *across* the blue wires?
If it's an isolated heater supply, either wire on its own will show zero
with respect to chassis ground, but will show 3.5 volts with respect to
the other blue wire.

And when the equipment is working correctly, both blue wires will show a
negative DC voltage too (from the VFD supply).

The filament btw acts as a directly heated cathode in a VFD. The segments that
illuminate are the anodes and the remaining connections are control grids.

It's a big multipin triode.

Graham

[Eeyore]

wrote:

On Sep 28, 8:50 am, EADGBE wrote:
It measured resistance of 1.6 ohms! It is NOT infinite, and therefore
NOT open circuit!

Which is not at all out of the realm of what such a winding
might be.

BUT THE NAGGING QUESTION REMAINS: WHY AM I NOT
GETTING *ANY* VOLTAGE READINGS ON EITHER OF THE
TWO BLUE WIRES WHEN THE DECK IS SWITCHED
ON???

Because if that winding is NOT connected with the common
ground, and you are trying to measure the voltage of either
one in reference to that common ground, you won't get
any meaningful reading.

Instead, you need to measure the AC voltage between these
two wires, NOT between each of them and ground.

Plus, if it's working correctly, they'll be at some negative DC potential too.


I'd bet when you do so, you get the voltage you're looking
for, which probably indicated the display is shot, among other
things.

It would make sense to measure the resistance of the filament. If there's no
continuity, then the display would need replacement.

Graham

[Eeyore]

EADGBE wrote:

On Sep 28, 10:42 am, "Gareth Magennis"
wrote:
Go and check the resistance of
the filament right now, as I suggested earlier on. This may all be academic
if you need a new display.

Incidentally, fuse resistors are not just resistors. They are flame proof
so they wont make a lot of alarming smoke and a cringeworthy smell, and have
been designed as a fuse, not a resistor.

GARETH:

I have just checked the resistance of the filament, as you asked
(sorry I didn't do it sooner).

It seems to be fine. There is a solid reading of 7.2 ohms.

This is an accurate reading because I disconnected the filament from
the rest of the display window circuitry before measuring it.

It's interesting to note that the filament circuit consists entirely
of just these items:

1) AC voltage coming from the transformer - dedicated to just the
filament and nothing else.
2) Two (2) inline fuse resistors - 1 ohm, 1/4 watt each
3) The filament itself
4) A jumper wire connecting the whole thing together.

The filament needs to be 'tied' to some negative DC voltage for the display to
work.

Check for this DC voltage. If not present you won't get a display either.

Graham

[Eeyore]

Gareth Magennis wrote:

"EADGBE" wrote

It's interesting to note that the filament circuit consists entirely
of just these items:

1) AC voltage coming from the transformer - dedicated to just the
filament and nothing else.
2) Two (2) inline fuse resistors - 1 ohm, 1/4 watt each
3) The filament itself
4) A jumper wire connecting the whole thing together.


Well you may be lucky and find the fuse resistor blew before the filament
then.

I have no idea what the filament resistance should read, but calculating 7.2
ohms + 2 x 1 ohm resistors at 3.5v gives about 380 mA max, thats 0.14 watts
per fusible resistor. I shuld imagine that as the filament heats up its
resistance will increase dramatically, reducing the current, so it looks
like you may be in the right ball park.

I agree. That's certainly the right kind of value.

Graham

[Eeyore]

EADGBE wrote:

(Scott Dorsey) wrote:

Does an ohmmeter show them as open or not?

SCOTT:

The fuse resistor that didn't blow reads exactly 1.04 ohms (that's an
in-circuit reading).

The fuse resistor that blew reads open circuit....and that is with the
resistor TAKEN OUT of circuit completely.

I figured I could replace the fuse resistor with a regular 1-ohm
resistor and get things working again, but I also realize that that is
NOT safe, and so the deck will not be powered up again until it has a
new 1-ohm fuse resistor in place...and I might even replace the other
1-ohm fuse resistor whilst I'm at it.

I wouldn't excessively fret over not fitting a genuine fusible resistor. The
only difference between them and normal resistors is that they are (a) designed
to fail open and (b) are flameproof. There's still another one in circuit !

If one of those fusibles had failed (as it seems) that's why you have no
display.

Graham

[Eeyore]

Dave Platt wrote:

EADGBE wrote:

How do TWO supplies connect to the display ?

Do you actually mean two WIRES (you said pairs of wires originally) ?

GRAHAM:

I must admit that I was assuming that the two 3.5v wires were two
supplies. They could easily be two parts of ONE 3.5v supply.

It's not uncommon for the filament on a VFD to be fed from a
centertapped transformer secondary winding. The center tap is
grounded, and the two ends of the winding feed the two ends of the
filament. The same effect can be created using a dual secondary
winding, of course.

Actually, the centre tap of such a winding for a VFD would usually be at some
negative potential, NOT grounded.

However he doesn't have a centre tap so please don't confuse the issue.

Graham