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Robert Stevens Robert Stevens is offline
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Default Revox A77 biasing question


That also really tells you nothing. Get the heads checked.


Fair enough, took me an hour to figure out how to get the rec/play heads off, I don't think I see how to get the erase head off. But I will figure it out.

Thanks.

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Default Revox A77 biasing question



or more basic, check the DC supply voltage to the bias osc.

m


21V.

Thx.
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Default Revox A77 biasing question

Robert Stevens wrote:

That also really tells you nothing. Get the heads checked.


Fair enough, took me an hour to figure out how to get the rec/play heads off, I don't think I see how to get the erase head off. But I will figure it out.


The manual should tell you how to do the R/P heads.

The erase head is comparatively easy since the alignment on it isn't very
precise.
--scott
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"C'est un Nagra. C'est suisse, et tres, tres precis."
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Default Revox A77 biasing question

Thx, I got it off. All 3 are off to JRF.
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On Wednesday, May 10, 2017 at 4:23:05 PM UTC-4, Robert Stevens wrote:
Thx, I got it off. All 3 are off to JRF.


Heads are back, installed, aligned. Still cannot overbias .


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Robert Stevens wrote:
On Wednesday, May 10, 2017 at 4:23:05 PM UTC-4, Robert Stevens wrote:
Thx, I got it off. All 3 are off to JRF.


Heads are back, installed, aligned. Still cannot overbias .


And what did the head report say? Do the record heads now have the same
inductance top to bottom? Was the tape contact pattern on the heads even?
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Default Revox A77 biasing question

Erase: 0.144, 0.141
Rec: 8.15, 7,80
PB: 221.55, 241.04

Tape contact looks even. PB aligned nicely with MRL. Azimuth is very good at 8K and 16K. Rec Azimuth is also very good at both 8K and 16K.
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Default Revox A77 biasing question

In article ,
Robert Stevens wrote:

Erase: 0.144, 0.141
Rec: 8.15, 7,80
PB: 221.55, 241.04

Tape contact looks even. PB aligned nicely with MRL. Azimuth is very good at
8K and 16K. Rec Azimuth is also very good at both 8K and 16K.


Were these an upgraded set of heads from Revox? I seem to recall that
Revox had a half-track upgrade kit that included an additional
capacitor meant to be placed in parallel with the bias adjust trimmer.
This would supply the additional bias current that you'd need for a
wider track width head. It would probably be the 100 pF range. I'm a
little fuzzy on the details after all these years, but maybe John
French would know, or a quick check of the schematics might be helpful.
Does a Studer/Revox service dept still exist? I doubt that there's
anyone still on the payroll who'd remember, but that might be worth an
email or phone call.
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Default Revox A77 biasing question

Robert Stevens wrote:
Erase: 0.144, 0.141
Rec: 8.15, 7,80
PB: 221.55, 241.04


Yeah, that record head has a little more metal on top than on bottom, so if
you are really trying to get the absolute most bias flux, you'll need a little
more voltage on one track than the other. So this explains why you can get
the peak on one channel but can't quite get to it on the other. Still, you
need to be able to get a little past that peak.

I'm still in Berlin and don't have the A77 manual in front of me. But if
you get the manual and go to the page with the oscillator board, it will
show you test points A and B and you have verified that the levels on those
are a bit low. Check the +20V input on pin 8 of the board and make sure that
is correct. If that is low, everything is going to be low.

You can try measuring at points A and B with the record board removed from the
machine, to see if the record board is pulling the bias down. I have never
seen that kind of failure before but there is a first time for everything and
it's a thing you can check without too much difficulty.

Now, take a look at the inductor on the oscillator board and compare it with
the one that you have. It's different for the 2-track and 4-track machines
and if you have a quarter track to 2-track conversion, they may not have done
that correctly.

Oh, and...
Replace the 250uF cap on the oscillator board. It has nothing to do with
your problem but it frequently fails and while you have the board out,
you might as well do the job. Putting a 330uF long-life axial in there is
a good plan.

Tape contact looks even. PB aligned nicely with MRL. Azimuth is very good at 8K and 16K. Rec Azimuth is also very good at both 8K and 16K.


These machines have remarkably stable and clean transports considering how
flimsy they are and how much static stuff they have in contact with the
tape. They are not professional machines, but they were likely the best
consumer-grade machines ever made. You should have a rock solid azimuth
pattern, better than on some professional decks.

Part of what makes them not professional machines you are already figuring
out... it is nearly impossible to get to the control pots while looking at
the meters... daily alignment is not a 10-minute touchup like it is on the 440.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Default Revox A77 biasing question

Bruce Seifried wrote:
In article ,
Robert Stevens wrote:

Erase: 0.144, 0.141
Rec: 8.15, 7,80
PB: 221.55, 241.04

Tape contact looks even. PB aligned nicely with MRL. Azimuth is very good at
8K and 16K. Rec Azimuth is also very good at both 8K and 16K.


Were these an upgraded set of heads from Revox? I seem to recall that
Revox had a half-track upgrade kit that included an additional
capacitor meant to be placed in parallel with the bias adjust trimmer.


The idea being that instead of changing out the inductor you'd just change
the effective value of the trimmer?
--scott


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


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Default Revox A77 biasing question


Were these an upgraded set of heads from Revox


The Oscillator board is the red dot version for half tracks. I have a spare which i get same results with. I also have a yellow dot version spare (meant for 1/4 tracks) for another machine. As you would expect, results with this card are even worse.

The supply voltage to the oscillator card is 21V with the card out. I'll try to get a reading tonight with the card installed.

I strongly suspect that this has always been a half track machine, since the markings on the pinch roller assembly also indicate it is a 2 track version. But of course that could be a swap out as well.
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Default Revox A77 biasing question

In article ,
Robert Stevens wrote:

Were these an upgraded set of heads from Revox


The Oscillator board is the red dot version for half tracks. I have a spare
which i get same results with. I also have a yellow dot version spare (meant
for 1/4 tracks) for another machine. As you would expect, results with this
card are even worse.

The supply voltage to the oscillator card is 21V with the card out. I'll try
to get a reading tonight with the card installed.

I strongly suspect that this has always been a half track machine, since the
markings on the pinch roller assembly also indicate it is a 2 track version.
But of course that could be a swap out as well.


Alright, scrap that theory then...

As Scott points out, the difference between the half- and quarter-
track oscillator boards is in the inductor, which for the half-track
heads is labeled L703. Also, the bias adjust trimmer is a variable
resistor, not a variable capacitor as I wrongly remembered.

Sounds like the problem exists external to your oscillator card anyway,
since you say both of your cards act the same. You might try measuring
the resistance of the 100 ohm resistors located in the ground leads of
each record head winding. Seems unlikely that they would drift in
value, but it's simple enough to check.

If all else fails, and you can't find the actual cause of your problem,
you could try bridging a small value cap, say 30-50 pF or so, across
C710 and C711 on the osc pcb. That -might- give you a little bit more
bias signal to work with.
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Default Revox A77 biasing question



If all else fails, and you can't find the actual cause of your problem,
you could try bridging a small value cap, say 30-50 pF or so, across
C710 and C711 on the osc pcb. That -might- give you a little bit more
bias signal to work with.


again,
don't forget the erase head is part of the circuit. The problem could lie there.
m
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In article ,
wrote:


If all else fails, and you can't find the actual cause of your problem,
you could try bridging a small value cap, say 30-50 pF or so, across
C710 and C711 on the osc pcb. That -might- give you a little bit more
bias signal to work with.


again,
don't forget the erase head is part of the circuit. The problem could lie there.
m


No, that was actually my first guess, but the erase head checks out okay
with JRF.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Default Revox A77 biasing question


Sounds like the problem exists external to your oscillator card anyway,
since you say both of your cards act the same. You might try measuring
the resistance of the 100 ohm resistors located in the ground leads of
each record head winding. Seems unlikely that they would drift in
value, but it's simple enough to check.

If all else fails, and you can't find the actual cause of your problem,
you could try bridging a small value cap, say 30-50 pF or so, across
C710 and C711 on the osc pcb. That -might- give you a little bit more
bias signal to work with.


Okay, ground pins of record head to sheild of head are about 100ohms. But this is odd, the hot pins to shield are not infinite. Come in around 60 ohms. I may need to unsolder the hot wires and see if it stays that way.


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In article ,
Robert Stevens wrote:

Sounds like the problem exists external to your oscillator card anyway,
since you say both of your cards act the same. You might try measuring
the resistance of the 100 ohm resistors located in the ground leads of
each record head winding. Seems unlikely that they would drift in
value, but it's simple enough to check.

If all else fails, and you can't find the actual cause of your problem,
you could try bridging a small value cap, say 30-50 pF or so, across
C710 and C711 on the osc pcb. That -might- give you a little bit more
bias signal to work with.


Okay, ground pins of record head to sheild of head are about 100ohms. But this is odd, the hot pins to shield are not infinite. Come in around 60 ohms. I may need to unsolder the hot wires and see if it stays that way.


That's probably fine. Pull the record and erase boards out and that resistance
will disappear most likely. Check the schematics on the record and erase
diagrams on the schematic.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Thanks for all the help everybody, I've dug into the machine as deep as I dare. I'll get a tech to take it from here.
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Robert Stevens wrote:
Thanks for all the help everybody, I've dug into the machine as deep as I dare. I'll get a tech to take it from here.


I think you're doing better than any of the techs I know these days. If
you actually find someone who knows the A77 well, PLEASE let me know because
there is a real demand for work on those machines. Nobody likes working on
them because they really are kind of cantankerous.

You've got the hard part down.

I _may_ have a test jig somewhere... if you send me an oscillator board I
might be able to see that it meets spec, but I have to admit that I last
used the thing in the mid-eighties....
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Default Revox A77 biasing question

All the mods I made were on the removable cards plus 2 resistors on the main connector board which I made without physically removing the board. I'm afraid experience tells me that once I start removing major components, I break more than I fix.

I may however, have to replace the connector board as some of the card headers are starting to crumble. The Relay and Bias card headers are not among them though. 1 Rec card is now hot glued in place at this time. A gift from 2 nights ago.



On Friday, May 26, 2017 at 10:30:48 AM UTC-4, Scott Dorsey wrote:
Robert Stevens wrote:
Thanks for all the help everybody, I've dug into the machine as deep as I dare. I'll get a tech to take it from here.


I think you're doing better than any of the techs I know these days. If
you actually find someone who knows the A77 well, PLEASE let me know because
there is a real demand for work on those machines. Nobody likes working on
them because they really are kind of cantankerous.

You've got the hard part down.

I _may_ have a test jig somewhere... if you send me an oscillator board I
might be able to see that it meets spec, but I have to admit that I last
used the thing in the mid-eighties....
--scott

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


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On Friday, May 26, 2017 at 11:23:42 AM UTC-4, Robert Stevens wrote:
All the mods I made were on the removable cards plus 2 resistors on the main connector board which I made without physically removing the board. I'm afraid experience tells me that once I start removing major components, I break more than I fix.

I may however, have to replace the connector board as some of the card headers are starting to crumble. The Relay and Bias card headers are not among them though. 1 Rec card is now hot glued in place at this time. A gift from 2 nights ago.



On Friday, May 26, 2017 at 10:30:48 AM UTC-4, Scott Dorsey wrote:
Robert Stevens wrote:
Thanks for all the help everybody, I've dug into the machine as deep as I dare. I'll get a tech to take it from here.


I think you're doing better than any of the techs I know these days. If
you actually find someone who knows the A77 well, PLEASE let me know because
there is a real demand for work on those machines. Nobody likes working on
them because they really are kind of cantankerous.

You've got the hard part down.

I _may_ have a test jig somewhere... if you send me an oscillator board I
might be able to see that it meets spec, but I have to admit that I last
used the thing in the mid-eighties....
--scott

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


Looks like someone modified this one, too....

http://www.ebay.com/itm/REVOX-A77-1-...-/222519962937

Jack


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Default Revox A77 biasing question

OK here is another thing to check. The record amp has a bias trap at its output
to keep the bias out of the amp and to keep the amp from loading down the
bias. This could be out of tune or defective. Try swapping the record amps
and see if the problem follows one amp.
Mark
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So one final "Hail Mary". I replaced the 100 ohm resistors in the bias card with 10 ohm resistors. This seems to have done the trick for 456 tape. Still cannot get enough bias for 499. But I'll take it!
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On Friday, May 26, 2017 at 10:26:56 PM UTC-5, Robert Stevens wrote:
So one final "Hail Mary". I replaced the 100 ohm resistors in the bias card with 10 ohm resistors. This seems to have done the trick for 456 tape. Still cannot get enough bias for 499. But I'll take it!

Congratulations. If it'll bias 456, it'll do 468 and probably 911. My recollection is that the A77 was never up to 499, so I wouldn't worry about that too much.

Peace,
Paul
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In article ,
wrote:
OK here is another thing to check. The record amp has a bias trap at its output
to keep the bias out of the amp and to keep the amp from loading down the
bias. This could be out of tune or defective. Try swapping the record amps
and see if the problem follows one amp.


Yes. This is why I asked earlier about whether the voltage measurements
change when you pull the record amp boards.

I am not sure how much a bad bias trap would affect the bias point, I would
think it would cause record distortion that is pretty severe, but it's a
thing you ought to check.

The problem is clearly on both channels but it seems to be worse on one than
the other... but since it's so close to the edge that may not indicate more
than just a normal channel difference.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Default Revox A77 biasing question

In article ,
Robert Stevens wrote:
So one final "Hail Mary". I replaced the 100 ohm resistors in the bias card with 10 ohm resistors. This seems to have done the trick for 456 tape. Still cannot get enough bias for 499. But I'll take it!


This isn't fixing the problem, though, this is working around it. You still
need to find it.

And no, a brand new and perfectly functioning A77 won't bias up 499 so do
not expect that.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."


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Default Revox A77 biasing question

I found what appear to be a pair of 2nF caps across the wires that send signal to the record heads and ground. I removed them and now biasing works great. No idea what they were there for, but they are gone now.

Thanks everybody.
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On Wednesday, June 14, 2017 at 4:54:35 PM UTC-4, Robert Stevens wrote:
I found what appear to be a pair of 2nF caps across the wires that send signal to the record heads and ground. I removed them and now biasing works great. No idea what they were there for, but they are gone now.

Thanks everybody.


I should also point out they are not on the schematic, so probably a mod someone did in the past for some unknowable reason.
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On Wednesday, June 14, 2017 at 4:59:25 PM UTC-4, Robert Stevens wrote:
On Wednesday, June 14, 2017 at 4:54:35 PM UTC-4, Robert Stevens wrote:
I found what appear to be a pair of 2nF caps across the wires that send signal to the record heads and ground. I removed them and now biasing works great. No idea what they were there for, but they are gone now.

Thanks everybody.


I should also point out they are not on the schematic, so probably a mod someone did in the past for some unknowable reason.


And I put the 100ohm resistors on the oscillator card back in where they go and still plenty of overbias!
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Default Revox A77 biasing question

Robert Stevens wrote:

-------------------------

I found what appear to be a pair of 2nF caps across the wires that
send signal to the record heads and ground. I removed them and now
biasing works great. No idea what they were there for,
but they are gone now.

Thanks everybody.



** Were they fitted on the PCB ??

How come you never spotted them before ?

This sort of thing demonstrates the UTTER FOLLY of attempting remote diagnosis.



..... Phil
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In article ,
Robert Stevens wrote:
I found what appear to be a pair of 2nF caps across the wires that send signal to the record heads and ground. I removed them and now biasing works great. No idea what they were there for, but they are gone now.


That is very, very weird.

I can't think of any reason why anyone would do that.... if the machine had
sel-sync maybe I could think of a misguided attempt at dealing with radio
interference maybe but even that doesn't make sense.

Everything that you have done to this machine and figured out about it... write
it down and tape it to the inside back cover. It will save you, or someone
else, a lot of time someday.
--scott

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


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O
** Were they fitted on the PCB ??


Sort of. They were soldered to the header pins where the signal goes from the VU meter board to the heads. As for why I never noticed before, probably because they did not look out of place. I only noticed because I was going through the BOM to buy parts for a proper recap and could not find them listed. Then started looking at the schematic closer and could not find them there either.

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