[bk]

I recently decideded to upgrade my Dynaco st-70 to a VST driver board.
The install semmed to go smooth, all the bias checks were perfect
however when i finally turned everything on i got some sound and than
id faded into nothing. if i turn the volume all the up i can faintly
hear it and it sounds extremely static. I decied to replace the cap
and still had the same problem, i also replaced the rectifier tube and
all the tube sockets. still the exact same problem. the amp was
working fine before th e"upgrade" anyone have any hints as to where to
check next?

[Jon Yaeger]

in article , bk at
wrote on 1/9/07 9:58 PM:

I recently decideded to upgrade my Dynaco st-70 to a VST driver board.
The install semmed to go smooth, all the bias checks were perfect
however when i finally turned everything on i got some sound and than
id faded into nothing. if i turn the volume all the up i can faintly
hear it and it sounds extremely static. I decied to replace the cap
and still had the same problem, i also replaced the rectifier tube and
all the tube sockets. still the exact same problem. the amp was
working fine before th e"upgrade" anyone have any hints as to where to
check next?


Fundamental troubleshooting rule: Look at the last thing that was done
before the amp went "south" and work backward from that. Check solder
joints and wires for breaks.

[Peter Wieck]

bk wrote:
I recently decideded to upgrade my Dynaco st-70 to a VST driver board.
The install semmed to go smooth, all the bias checks were perfect
however when i finally turned everything on i got some sound and than
id faded into nothing. if i turn the volume all the up i can faintly
hear it and it sounds extremely static. I decied to replace the cap
and still had the same problem, i also replaced the rectifier tube and
all the tube sockets. still the exact same problem. the amp was
working fine before th e"upgrade" anyone have any hints as to where to
check next?

A couple of things following up on Jon's advice. First some basic
stuff:

Dynaco used solid wire for their point-to-point work. They were not at
all particular to get the annealed stuff either. So it is quite common
for the *other end* of a wire to crack when de-soldered from one or
another point. It is also quite common to see what look like good
connections actually be poor or open, but held in place by a bit of
melted insulation. Sometimes the transformer wires rub against the
chassis holes as there is no rub-guard or strain-relief. Sometimes
careless soldering technique will melt the insulation on a twisted
pair. So:

1. Check _ALL_ wiring throughout the chassis. Do a real "pull" test.
Most especially, examine the tube socket wiring.
2. Check the transformer wiring as it passes through the chassis. Also
check the connections.
3. Look for shorts in twisted pairs, or where two wires cross.
4. Check all your connections. Then check the other end of each
connection specifically and all over again. Yes, you did the overall
check in #1 above. But do it again.

Did you change the bias pots? And have you set the bias voltage. Dynaco
used the cheapest-available pots in this application. It may be
coincidence but worth checking. Similarly the bias diode. One hopes
that the selenium diode has been replaced?

Are the 'new' tubes in the 'new' board good? How did you verify this?

Last: What happens if you put the old board back in? Does the amp come
back?

Let us know.

Peter Wieck
Wyncote, PA

[Leigh Orf]

bk wrote:
I recently decideded to upgrade my Dynaco st-70 to a VST driver board.
The install semmed to go smooth, all the bias checks were perfect
however when i finally turned everything on i got some sound and than
id faded into nothing. if i turn the volume all the up i can faintly
hear it and it sounds extremely static. I decied to replace the cap
and still had the same problem, i also replaced the rectifier tube and
all the tube sockets. still the exact same problem. the amp was
working fine before th e"upgrade" anyone have any hints as to where to
check next?

Check the board to make sure you stuffed it right. I had almost exactly
the same symptoms and the problem was (partly due to being coloblind)
swapping 10 kohm and 10 ohm resistors in the Triode Electronics board.



Leigh




--
Leigh Orf http://orf.cx
SDF Public Access UNIX System - http://sdf.lonestar.org

[Ian Iveson]

Peter Wieck wrote

I recently decideded to upgrade my Dynaco st-70 to a VST driver
board.
The install semmed to go smooth, all the bias checks were perfect
however when i finally turned everything on i got some sound and
than
id faded into nothing. if i turn the volume all the up i can
faintly
hear it and it sounds extremely static. I decied to replace the
cap
and still had the same problem, i also replaced the rectifier tube
and
all the tube sockets. still the exact same problem. the amp was
working fine before th e"upgrade" anyone have any hints as to
where to
check next?

A couple of things following up on Jon's advice. First some basic
stuff:

Dynaco used solid wire for their point-to-point work. They were not
at
all particular to get the annealed stuff either. So it is quite
common
for the *other end* of a wire to crack when de-soldered from one or
another point. It is also quite common to see what look like good
connections actually be poor or open, but held in place by a bit of
melted insulation. Sometimes the transformer wires rub against the
chassis holes as there is no rub-guard or strain-relief. Sometimes
careless soldering technique will melt the insulation on a twisted
pair. So:

1. Check _ALL_ wiring throughout the chassis. Do a real "pull" test.
Most especially, examine the tube socket wiring.
2. Check the transformer wiring as it passes through the chassis.
Also
check the connections.
3. Look for shorts in twisted pairs, or where two wires cross.
4. Check all your connections. Then check the other end of each
connection specifically and all over again. Yes, you did the overall
check in #1 above. But do it again.

Did you change the bias pots? And have you set the bias voltage.
Dynaco
used the cheapest-available pots in this application. It may be
coincidence but worth checking. Similarly the bias diode. One hopes
that the selenium diode has been replaced?

I wonder if this is a wise strategy. Seems good for someone in the
business of overhaul, but not necessarily best for an end user.

Considering the amp did work before something went wrong, I would want
to find the cause of the failure first. Pulling at all the wires and
replacing the bias rectifier may obscure the original problem.

I note that the OP said that it worked briefly after the mod, but then
"faded". I wonder how soon, and how quickly, this fading happened.
Could be a clue. Perhaps: a failed power transformer; open or poorly
connected small-valve anode or HT dropping resistor; ditto cathode
resistor; failed small valve or heater supply to same; failed bias
rectifier diode.

Also, the bias measurements were good before the failure. I wonder if
we can assume that they remained good afterwards? And do they apply
only to the power valves?

So, OP, I advise if you haven't already done so to recheck voltage
measurements, including for the small valves. Start with HT and power
valve grid voltages, then output stage bias current, just so you know
you aren't risking burning out a transformer. There is no indication
that the problem is intermittent, so this process is likely to reveal
the cause.

Once you have it working, then swap the rectifier if it's selenium,
and do whatever other overhauling seems necessary.

Are the 'new' tubes in the 'new' board good? How did you verify
this?

Last: What happens if you put the old board back in? Does the amp
come
back?

I would measure the voltages on the new board first. If he takes it
out, and finds that the amp works, what does he do next?

cheers, Ian

[Ian Iveson]

Leigh Orf wrote

Check the board to make sure you stuffed it right. I had almost
exactly
the same symptoms and the problem was (partly due to being
coloblind)
swapping 10 kohm and 10 ohm resistors in the Triode Electronics
board.

My sympathy. I'm bad with red/green and grey/violet. Only safe with
black, white, silver, gold and yellow.

I have to measure all resistors as I fit them.

Installing medical piping in a hospital was a nightmare. Fortunately
the gases taste different.

cheers, Ian

[bk]

Thanks for all the assistance so far. Ill give an update as to what
has been done so far.

I rechecked all the bias on the power tubes and all check out fine.
..40 volts which is what they should be(this is a VTA board, its fair to
say its a rebuild of the stock circuit). I checked the voltage on pin
1 of the first PreAmp tube and it was around 450 volts which is about
300 volts HIGHER than its supposed to be. I got a bit scared and
turned the amp off. heres a few other things ive done:

did the chopstick test on anything i could find but nothing

found that sound is actually comming out of both sides. when i fire
the amp up it stats up as normal and the music starts to come up for
roughly 2 seconds (as the tubes warm) it than reaches a peak and
instantly dies. the whole process lasts about 2 seconds.

I had a replacement choke i decided to try to replace since the old one
was leaking wax however that did nothing. havent swapped out the tubes
however they were working fine before i started. Another thing to
note is that if i do turn the amp up full volume I can faintly hear
music however it sounds crunchy.

Ive checked values of alomost all the resistors and everything checks
ok. - not fully completed but will be shortly. all wire connections
seem fine and ive gone over the solder connections and removed any
excess blobs which may have occurred and resoldered most connections.

I also put in a new rectifier tube and ive removed and installed all
brand new ceramic tube sockets. exact same problem as initally. seems
anything i do produces origional results. once again this was a fully
functional amp, everything worked until i put this new board in. dont
have the original board anymore so cant test.

thanks fora the assistance.
Ian Iveson wrote:
Peter Wieck wrote

I recently decideded to upgrade my Dynaco st-70 to a VST driver
board.
The install semmed to go smooth, all the bias checks were perfect
however when i finally turned everything on i got some sound and
than
id faded into nothing. if i turn the volume all the up i can
faintly
hear it and it sounds extremely static. I decied to replace the
cap
and still had the same problem, i also replaced the rectifier tube
and
all the tube sockets. still the exact same problem. the amp was
working fine before th e"upgrade" anyone have any hints as to
where to
check next?

A couple of things following up on Jon's advice. First some basic
stuff:

Dynaco used solid wire for their point-to-point work. They were not
at
all particular to get the annealed stuff either. So it is quite
common
for the *other end* of a wire to crack when de-soldered from one or
another point. It is also quite common to see what look like good
connections actually be poor or open, but held in place by a bit of
melted insulation. Sometimes the transformer wires rub against the
chassis holes as there is no rub-guard or strain-relief. Sometimes
careless soldering technique will melt the insulation on a twisted
pair. So:

1. Check _ALL_ wiring throughout the chassis. Do a real "pull" test.
Most especially, examine the tube socket wiring.
2. Check the transformer wiring as it passes through the chassis.
Also
check the connections.
3. Look for shorts in twisted pairs, or where two wires cross.
4. Check all your connections. Then check the other end of each
connection specifically and all over again. Yes, you did the overall
check in #1 above. But do it again.

Did you change the bias pots? And have you set the bias voltage.
Dynaco
used the cheapest-available pots in this application. It may be
coincidence but worth checking. Similarly the bias diode. One hopes
that the selenium diode has been replaced?

I wonder if this is a wise strategy. Seems good for someone in the
business of overhaul, but not necessarily best for an end user.

Considering the amp did work before something went wrong, I would want
to find the cause of the failure first. Pulling at all the wires and
replacing the bias rectifier may obscure the original problem.

I note that the OP said that it worked briefly after the mod, but then
"faded". I wonder how soon, and how quickly, this fading happened.
Could be a clue. Perhaps: a failed power transformer; open or poorly
connected small-valve anode or HT dropping resistor; ditto cathode
resistor; failed small valve or heater supply to same; failed bias
rectifier diode.

Also, the bias measurements were good before the failure. I wonder if
we can assume that they remained good afterwards? And do they apply
only to the power valves?

So, OP, I advise if you haven't already done so to recheck voltage
measurements, including for the small valves. Start with HT and power
valve grid voltages, then output stage bias current, just so you know
you aren't risking burning out a transformer. There is no indication
that the problem is intermittent, so this process is likely to reveal
the cause.

Once you have it working, then swap the rectifier if it's selenium,
and do whatever other overhauling seems necessary.

Are the 'new' tubes in the 'new' board good? How did you verify
this?

Last: What happens if you put the old board back in? Does the amp
come
back?

I would measure the voltages on the new board first. If he takes it
out, and finds that the amp works, what does he do next?

cheers, Ian

[Patrick Turner]

bk wrote:

Thanks for all the assistance so far. Ill give an update as to what
has been done so far.

I rechecked all the bias on the power tubes and all check out fine.
.40 volts which is what they should be(this is a VTA board, its fair to
say its a rebuild of the stock circuit). I checked the voltage on pin
1 of the first PreAmp tube and it was around 450 volts which is about
300 volts HIGHER than its supposed to be. I got a bit scared and
turned the amp off. heres a few other things ive done:

did the chopstick test on anything i could find but nothing

found that sound is actually comming out of both sides. when i fire
the amp up it stats up as normal and the music starts to come up for
roughly 2 seconds (as the tubes warm) it than reaches a peak and
instantly dies. the whole process lasts about 2 seconds.

I had a replacement choke i decided to try to replace since the old one
was leaking wax however that did nothing. havent swapped out the tubes
however they were working fine before i started. Another thing to
note is that if i do turn the amp up full volume I can faintly hear
music however it sounds crunchy.

Ive checked values of alomost all the resistors and everything checks
ok. - not fully completed but will be shortly. all wire connections
seem fine and ive gone over the solder connections and removed any
excess blobs which may have occurred and resoldered most connections.

I also put in a new rectifier tube and ive removed and installed all
brand new ceramic tube sockets. exact same problem as initally. seems
anything i do produces origional results. once again this was a fully
functional amp, everything worked until i put this new board in. dont
have the original board anymore so cant test.

thanks fora the assistance.



What is the URL for the schematic you have used for this amp?

Have you followed it carefully?

One must be able to service anything one does diy, and asking
why you have no music at this NG here is not always going to be helpful
although you will get at least several overlapping sources of advice.

But without the EXACT schematic you have in front of us
how do we know what could be wrong?
We need to be able to talk about the circuit in front of you.

The 450V at the anode of V1 won't cause any damage BTW, and there was
nothing to fear.
What was the 0V rail voltage with respect to the chassis?
But you didn't then measure the dc current in the anode supply
resistors,
or cathode resistors and probably you have not grounded something
as you should have so no Ia flows in the RL or the tube, so the anode
voltage is
up at the supply voltage.
It would seem you are fairly untrained in the basics; I was there once,
but
you should be asking 101 questions about voltages and currents in ALL
the R components around the circuit
as you prod places with the voltmeter knowing exactly where you are on
the schematic.
be careful measuring around the circuit because a shock from 450V can be
a real bother.
As you prod and calculate current using ohm's law with your calculator
you should record the
voltages on a copy of the schematic, printed up nice and big, or
carefully
written out in front of you, which BTW teaches you to concentrate where
every part is
and familiarize yourself with the parts.

Never assume anything you've done is correct until you've proven it to
be correct, so assume
until the proof is in that you have probably made at least a few
mistakes.
The man who makes no mistakes makes nothing, usually; we all make them,
so don't be afraid.

Patrick Turner.

[Sander deWaal]

"bk" said:

I recently decideded to upgrade my Dynaco st-70 to a VST driver board.
The install semmed to go smooth, all the bias checks were perfect
however when i finally turned everything on i got some sound and than
id faded into nothing. if i turn the volume all the up i can faintly
hear it and it sounds extremely static. I decied to replace the cap
and still had the same problem, i also replaced the rectifier tube and
all the tube sockets. still the exact same problem. the amp was
working fine before th e"upgrade" anyone have any hints as to where to
check next?


Dumb question: is there a fuse between CT of the power transformer and
ground?
If so, check it with an ohm meter.

If there's no fuse, check whether the CT is actually connected to
ground (chassis).

An open CT connection would also explain your rather high voltage
reading.

--

- Maggies are an addiction for life. -

[Ian Iveson]

bk wrote

found that sound is actually comming out of both sides. when i fire
the amp up it stats up as normal and the music starts to come up for
roughly 2 seconds (as the tubes warm) it than reaches a peak and
instantly dies. the whole process lasts about 2 seconds.

I had a replacement choke i decided to try to replace since the old
one
was leaking wax however that did nothing. havent swapped out the
tubes
however they were working fine before i started. Another thing to
note is that if i do turn the amp up full volume I can faintly hear
music however it sounds crunchy.

Ive checked values of alomost all the resistors and everything
checks
ok. - not fully completed but will be shortly. all wire
connections
seem fine and ive gone over the solder connections and removed any
excess blobs which may have occurred and resoldered most
connections.

I also put in a new rectifier tube and ive removed and installed all
brand new ceramic tube sockets. exact same problem as initally.
seems
anything i do produces origional results. once again this was a
fully
functional amp, everything worked until i put this new board in.
dont
have the original board anymore so cant test.

The high anode voltage means there is no or hardly any current to that
anode. Possibilities: the valve is at fault; the cathode resistor has
failed open or otherwise doesn't connect the cathode to ground; the
grid leak resistor likewise; the grid stopper resistor likewise.

A circuit for your amp (schematics index), and information on valve
pinouts (data sheet locator), can be found via

http://www.duncanamps.com/

The grid stopper is the one connected to the grid. The grid leak is
between the other end of the stopper and ground. No stoppers for the
input valves in the original, looking at the schemo, and the leaks
aren't obvious...the 470k Rs just after the input jack. A link to the
circuit for your driver board would be useful. The diagram I'm looking
at ( http://www.curcioaudio.com/st7_mnl.pdf) doesn't show a ground
connection at all, ahem. Should point "D 9" be grounded? Anybody?

Possibly your ground isn't, either. If by "both sides" you mean both
channels, rather than both PP phases, then prime suspect is the
connection between both cathode resistors and ground, for the valve
with the high anode voltage, and presumably its mate in the other
channel.

If it wasn't going to be OK, it would have got worse by now.

cheers, Ian

[Peter Wieck]

Ian:

Note the interpolations:

Ian Iveson wrote:

I wonder if this is a wise strategy. Seems good for someone in the
business of overhaul, but not necessarily best for an end user.

Anyone replacing a driver board (functional equivalent of replacing a
car engine or transmission) is in the business of overhaul. And as much
as one would not want to put a new engine in a rotten chassis or
coupled to a bad transmission (or vice-versa), one should _never_
undertake an overhaul until one has done 100% of the "other"
maintenance first. It is part of the learning process if nothing else.

Considering the amp did work before something went wrong, I would want
to find the cause of the failure first. Pulling at all the wires and
replacing the bias rectifier may obscure the original problem.

That failure could be anything from the board itself, to a broken wire
to a coincidental 'elsewhere' failure. So, the point is to eliminate
the potential for those 'elsewheres'. See my first paragraph above.

I note that the OP said that it worked briefly after the mod, but then
"faded". I wonder how soon, and how quickly, this fading happened.
Could be a clue. Perhaps: a failed power transformer; open or poorly
connected small-valve anode or HT dropping resistor; ditto cathode
resistor; failed small valve or heater supply to same; failed bias
rectifier diode.

Again, new board, let's isolate the difficulty to the board. Or not.
Other than the bias supply and the filter caps (and associated
resistors), there are no active components except on the board.

Also, the bias measurements were good before the failure. I wonder if
we can assume that they remained good afterwards? And do they apply
only to the power valves?

Were they good? Or were they assumed to be good? And, no, you may not
assume that they would be good after the driver board is replaced. Bias
voltage will change with a change in transformer load (any winding),
line-voltage, almost with the phases of the moon, it is that sensitive
on that beast.

So, OP, I advise if you haven't already done so to recheck voltage
measurements, including for the small valves. Start with HT and power
valve grid voltages, then output stage bias current, just so you know
you aren't risking burning out a transformer. There is no indication
that the problem is intermittent, so this process is likely to reveal
the cause.

Once you have it working, then swap the rectifier if it's selenium,
and do whatever other overhauling seems necessary.

Are the 'new' tubes in the 'new' board good? How did you verify
this?

Last: What happens if you put the old board back in? Does the amp
come
back?

I would measure the voltages on the new board first. If he takes it
out, and finds that the amp works, what does he do next?

He has three choices of what to do next. They are in order of what I
would suggest first:

a) have another person check the new board top-to-toe for any errors.
Check the traces as well. Often the original assembler will overlook a
mistake over and over and.... Correct anything found and try again.
b) check the new board himself. Painstakingly. Check _EVERYTHING_. It
could be something as simple as a mechanical connection or cracked
trace that fails upon heating. Thus accounting for the fading as
described.
c) stick with the OEM board.

But in any case, he has isolated the problem to the new board. As it is
now, it could be anything up to and including multiple coincidental
failure... which can be terribly frustrating. Another bad analogy: This
would be like moving a door in your house by say half-a-meter (~20" for
in the old currency). Seems simple enough. Would you do it without a
thorough investigation and understanding of both the wall and what may
be in it? I wouldn't.

Peter Wieck
Wyncote, PA

[Ian Iveson]

Peter Wieck wrote

Note the interpolations:

Every engineer should know that interpolation can be dangerous.


I wonder if this is a wise strategy. Seems good for someone in the
business of overhaul, but not necessarily best for an end user.

Anyone replacing a driver board (functional equivalent of replacing
a
car engine or transmission) is in the business of overhaul. And as
much
as one would not want to put a new engine in a rotten chassis or
coupled to a bad transmission (or vice-versa), one should _never_
undertake an overhaul until one has done 100% of the "other"
maintenance first. It is part of the learning process if nothing
else.

I disagree. Fitting a new assembly is not an overhaul. An overhaul is
generally a bad first response to a fault condition. Specific
fault-finding is also more immediately instructive. Overhaul can be a
low-skill, production-line activity. Not so with fault-finding.

Considering the amp did work before something went wrong, I would
want
to find the cause of the failure first. Pulling at all the wires
and
replacing the bias rectifier may obscure the original problem.

That failure could be anything from the board itself, to a broken
wire
to a coincidental 'elsewhere' failure. So, the point is to eliminate
the potential for those 'elsewheres'. See my first paragraph above.

Er, eh? Some tricky logic would be required to make that one stick. To
me, as it is, progress by elimination of elsewheres is likely to be
slow, dull, and desperately inefficient.

I note that the OP said that it worked briefly after the mod, but
then
"faded". I wonder how soon, and how quickly, this fading happened.
Could be a clue. Perhaps: a failed power transformer; open or
poorly
connected small-valve anode or HT dropping resistor; ditto cathode
resistor; failed small valve or heater supply to same; failed bias
rectifier diode.

Again, new board, let's isolate the difficulty to the board. Or not.
Other than the bias supply and the filter caps (and associated
resistors), there are no active components except on the board.

This makes more sense as a strategy. Connections to the board are the
most likely, then a fault with the board itself or its power supply or
ground connection. As Jon pointed out, events leading up to failure
can be a useful indication of where a problem might be.

Also, the bias measurements were good before the failure. I wonder
if
we can assume that they remained good afterwards? And do they apply
only to the power valves?

Were they good? Or were they assumed to be good?

Read the post and make up your own mind.

And, no, you may not
assume that they would be good after the driver board is replaced.
Bias
voltage will change with a change in transformer load (any winding),
line-voltage, almost with the phases of the moon, it is that
sensitive
on that beast.

That's as may be. I asked because it seemed likely that the OP, having
measured once, would measure again, and would have reported a change
had he found one. I was making a guess about the person, not the
circuit.

So, OP, I advise if you haven't already done so to recheck voltage
measurements, including for the small valves. Start with HT and
power
valve grid voltages, then output stage bias current, just so you
know
you aren't risking burning out a transformer. There is no
indication
that the problem is intermittent, so this process is likely to
reveal
the cause.

Once you have it working, then swap the rectifier if it's selenium,
and do whatever other overhauling seems necessary.

Are the 'new' tubes in the 'new' board good? How did you verify
this?

Last: What happens if you put the old board back in? Does the amp
come
back?

I would measure the voltages on the new board first. If he takes it
out, and finds that the amp works, what does he do next?

He has three choices of what to do next. They are in order of what I
would suggest first:

a) have another person check the new board top-to-toe for any
errors.
Check the traces as well. Often the original assembler will overlook
a
mistake over and over and.... Correct anything found and try again.
b) check the new board himself. Painstakingly. Check _EVERYTHING_.
It
could be something as simple as a mechanical connection or cracked
trace that fails upon heating. Thus accounting for the fading as
described.
c) stick with the OEM board.

But in any case, he has isolated the problem to the new board. As it
is
now, it could be anything up to and including multiple coincidental
failure... which can be terribly frustrating. Another bad analogy:
This
would be like moving a door in your house by say half-a-meter (~20"
for
in the old currency). Seems simple enough. Would you do it without a
thorough investigation and understanding of both the wall and what
may
be in it? I wouldn't.

I have an AVO valve tester full of brittle wires. It had a faulty
meter because one of the wires snapped. I first found the fault by
non-invasive measuring and logic, then fixed the wire and it has
worked ever since. If I had pulled and poked and messed with this and
that, I would have multiplied the faults and ended up rewiring it
completely for no good reason. Much quicker, and with my colour vision
perhaps safer, to find the fault and fix it, and leave the seperate
issue of overhaul for some other time. Yes it will fail eventually.
Then maybe I will fix it again, or maybe I'll overhaul it but probably
not because it is such a dull activity.

Were I in the business of fixing things, my attitude to working for
clients would necessarily be very different. In some ways that would
be a shame.

It's not the first time the basic issue here has been explored. Every
manager argues about it every day. Determining the appropriate level,
or breadth, of response to each problem is a constant headache. Top
down or bottom up, root and branch or surgical intervention, fix or
replace, etc. etc.

Just as I mull it over, I wonder if this dialectic determines to some
extent the difference between the professional practioner and the
amateur. Although we should approach every problem from both
directions, we have to start somewhe what in dialectics is
considered to determine the *primary* direction in each case. Perhaps,
for the pros, the primary direction is top down, root and branch,
whereas for the amateur it is bottom up, surgical intervention? OTOH,
perhaps some amateurs actually enjoy the process of being thorough?

Perhaps some ppl wouldn't know what to do with freedom if they found
it.

Talking of building disasters, there have been several TV series here
where teams of pros have gone to fix amateur home improvement
disasters. The biggest and most dangerous domestic catastrophes were
created by incompetents setting out to do a "thorough job"...basically
ripping rooms apart with precious little idea of how to put them back
together.

I don't have to be thorough, and it is a welcome relief not to be.
Hardly anything I have personally made has ever gone wrong, and if it
does I know what the problem is almost immediately, because it is my
design and I made it and it runs in my head and it is in front of me
now.

cheers, Ian

[Peter Wieck]

More interpolations... Damned the torpedoes, and so forth.

Ian Iveson wrote:

I have an AVO valve tester full of brittle wires. It had a faulty
meter because one of the wires snapped. I first found the fault by
non-invasive measuring and logic, then fixed the wire and it has
worked ever since. If I had pulled and poked and messed with this and
that, I would have multiplied the faults and ended up rewiring it
completely for no good reason. Much quicker, and with my colour vision
perhaps safer, to find the fault and fix it, and leave the seperate
issue of overhaul for some other time. Yes it will fail eventually.
Then maybe I will fix it again, or maybe I'll overhaul it but probably
not because it is such a dull activity.

The essential difference between you and the OP is that you know the
limitations of the instrument you are working on or with and he does
not. You are aware of the faults, the unit is for your use only, you
are not representing it as anything other than what it is...something
for your own use. Whether you are professional or amateur or something
in between, this is an important distinction.

Were I in the business of fixing things, my attitude to working for
clients would necessarily be very different. In some ways that would
be a shame.

Define "fix". Lots of stuff relative to my available time is brought to
me to "fix". That I specialize in vintage stuff and that I do not
charge for it beyond parts may be the reason that the demand always
exceeds the supply of time, but it does not change the single question
I ask every owner: Do you want me to fix the fault and its immediate
cause (of course) (and tell you what it is and how it may have
happened), or do you want me to make the item fit for polite society
into the foreseeable future? If the former (about 80%), I will very
carefully and as passively as possible diagnose the fault, back-track
causes until they are cleared and stop right there. Sometimes this is a
matter of minutes... a bad capacitor, a burnt resistor, a bad solder
joint, failed insulation, whatever.

For that 20% that want a reliable and reasonably predictable piece, it
will get the full restoration including marginal caps, drifted
resistors, matching tubes (if and where applicable) and so forth. This
may take days in real-time and several hours of my actual work time.
Weeks if I need to source parts I do not have.

Just as I mull it over, I wonder if this dialectic determines to some
extent the difference between the professional practioner and the
amateur. OTOH,
perhaps some amateurs actually enjoy the process of being thorough?

Now, let's consider the heart-transplant as replacing the driver board
is essentially for the ST-70. About any careful surgeon who understands
the principles of good surgery could do it in a perfect world with no
complications whatsoever. But that is seldom the case in the real
world. Any nearly-50-YO piece of equipment will have at least some
age-related flaws. One would think that clearing up those flaws would
be the first order of business before a new heart is introduced. That
would be my strategy whether I intended to keep the piece forever or
sell it down the river the moment I am done with it. Gee Whiz... I can
get it 'working', and put it into the line-up... until the
next-little-thing fails. Then out it comes, onto the bench, take it
apart, diagnose and replace that next little thing... repeat as
necessary. Not my style.

Perhaps some ppl wouldn't know what to do with freedom if they found
it.

It's a hobby. At least in this house. I am privileged to be able to
spend as much time as I choose at it (working around maintaining a good
marriage, making a living, traveling and all the other aspects of an
active life, of course). Or as little time. But any time I spend at it
is usually enjoyable. It can be frustrating, annoying, many other
things... but at the end of the process, when whatever it is is now
doing as it should... there is tremendous satisfaction as well.

Talking of building disasters, there have been several TV series here
where teams of pros have gone to fix amateur home improvement
disasters. The biggest and most dangerous domestic catastrophes were
created by incompetents setting out to do a "thorough job"...basically
ripping rooms apart with precious little idea of how to put them back
together.

All the time. Removing structural walls is my favorite. Or cutting
through a soil-stack with a SawzAll....

I don't have to be thorough, and it is a welcome relief not to be.
Hardly anything I have personally made has ever gone wrong, and if it
does I know what the problem is almost immediately, because it is my
design and I made it and it runs in my head and it is in front of me
now.

Sure. What you make you control and you understand. And you take it
exactly as far as you choose to do. However, what you do for others
musts-needs at a higher standard. Or, as they say in the business,
call-backs are a %^&*#!!

Peter Wieck
Wyncote, PA

cheers, Ian

[Jon Yaeger]

in article , Leigh Orf at
wrote on 1/10/07 9:05 AM:


bk wrote:
I recently decideded to upgrade my Dynaco st-70 to a VST driver board.
The install semmed to go smooth, all the bias checks were perfect
however when i finally turned everything on i got some sound and than
id faded into nothing. if i turn the volume all the up i can faintly
hear it and it sounds extremely static. I decied to replace the cap
and still had the same problem, i also replaced the rectifier tube and
all the tube sockets. still the exact same problem. the amp was
working fine before th e"upgrade" anyone have any hints as to where to
check next?

Check the board to make sure you stuffed it right. I had almost exactly
the same symptoms and the problem was (partly due to being coloblind)
swapping 10 kohm and 10 ohm resistors in the Triode Electronics board.



Leigh




If you've changed tube sockets, caps, etc. since you first noticed the
problem it is possible that you could have compounded the problem.

I don't know anything about the VST mod so I'm really shooting from the hip
into the dark . . .

Two things that I'd be sure to check, in addition to reviewing the PCB:

1. Make sure that you have correctly identified the pin numbers on the
6CA7/EL34 tubes. Sometimes it is hard to see the numbers. Make sure all of
the wiring to these tubes is correct.

2. Make sure you've correctly identified & wired points A, B, & C on the
multisection capacitor.

Good luck.

Jon

[Bob H.]

bk wrote:

found that sound is actually comming out of both sides. when i fire
the amp up it stats up as normal and the music starts to come up for
roughly 2 seconds (as the tubes warm) it than reaches a peak and
instantly dies. the whole process lasts about 2 seconds.


If sound is common to both sides, the culprit should be common to both
sides. If the driver uses a single tube for both channels, maybe the
filament supply should be checked more closely, both for that tube and
all others. Wrap thin wire around individual tube pins to check what
is actually present. This is good for all voltage checks.
Especially check for wires which may be partially grounding or
otherwise touching.

Nothing like a difficult problem to liven up the day.

good luck
Bob H.

[bk]

thanks again for all the help. To answer a few of the questions youve
all had:

a. yes I was an electronics engineering studnt 10 years ago before i
moved over to computers so im very very rusty and with the exception of
working and building a few tube guitar amps im pretty green when
troubleshooting these. Im also used to turrett boards so they are much
easier to work on. im more of the jump in and get the hands dirty and
even though its frustrating i have to admit im learnign alot and just
bought a thorens turntable which has some motor issues. just a preview
of whats around the corner if anyones intrested!

b. Its a VTA board that i installed, sorry if i said VST. I have a
scan of the schematic but do not know how to post it. im new to this
group so if anyone has any suggestions as to a hosting site frequesntly
used by this group ill post it there.

c. The cathode resistors are pretty much brand new and i replaced them
a second time when i installed the new ceramic tube sockets with the
same results so i find it hard to believe that would be the same issue.
ive also measured the resistance on the resistors and they appeared
within specs. I also have done the filament wires 3 times already and
the last resolder i decided to bypass the old bias sockets in the front
and just solder them directly from the output tubes to the board.

d. Even though i live in nyc I dont know anyone who even knows what a
dynaco st 70 is no less someone who would check my work short of
bringing it to a repair shop(I figured this would be my last stab at
fixing it before i brough it there though)

E. By both sides i did mean both channels.

F. double checked for wires touching and pulled on all connections and
everything seems fine. per the instructions the old resistors
connection to the inputs were removed and the connections are connected
directly to the board. I also totally removed the mono/stereo switch.


G. my next course of action was going ot be removing the board and
reching every single component to ensure all values are correct and to
remove any excess solder that may exsists. its difficult to see all
the components in its mounted states many of the parts needed to be
mounted on the bottom of the board. Ive double and triple checked cap
polarity as well so that should be fine. I do have a feeling it may be
a ground issue or a cap issue since it warms up palys briefly for 2
seconds form each channel and dissipates quickly. also i can hear both
channels if i turn it up but the sound is distorted and faint.

i think i answered most questions, thanks again for all the help.
boards like this are invaluable and truley what the internet was for in
the first place. an exchange of thoughts and ideas.


On Jan 10, 11:53 pm, "Bob H." wrote:
bk wrote:

found that sound is actually comming out of both sides. when i fire
the amp up it stats up as normal and the music starts to come up for
roughly 2 seconds (as the tubes warm) it than reaches a peak and
instantly dies. the whole process lasts about 2 seconds.If sound is common to both sides, the culprit should be common to both
sides. If the driver uses a single tube for both channels, maybe the
filament supply should be checked more closely, both for that tube and
all others. Wrap thin wire around individual tube pins to check what
is actually present. This is good for all voltage checks.
Especially check for wires which may be partially grounding or
otherwise touching.

Nothing like a difficult problem to liven up the day.

good luck
Bob H.