Behringer VT1951 Tube Ultra-Q repair

[bitrex]

I have one of these on my bench that I'm repairing:

<https://www.globalmediapro.com/att/a/0/1/a/a01aq8/lbx.jpg>

Currently the only problem is a weak left channel when the "Warmth" knob
is increased. When the "Warmth" control (VR200, bottom left of the
schematic) for that channel is fully counterclockwise signal passes
fine, however when it's brought up the channel goes almost completely
dead, and the meter barely deflects.

<http://imgur.com/a/JMosx>

I've traced the problem down to the circuitry associated with R4 and
that tube section; with no signal applied the DC voltage across the
anode resistors of all tubes sit at approximately half the 48V supply;
however with the bad channel's "Warmth" control is brought up the
average DC voltage across R4 collapses to around 200mV, while the others
remain about the same.

In hopes of a quick fix I replaced the cathode bypass capacitor C38 and
coupling capacitor C77, but no change in the symptoms. Tried several
different sets of tubes and also no change. Suggestions?

 

[megamusic]

"bitrex" <bitrex@de.lete.earthlink.net> wrote in message
news:9ZlYA.60527$Lf4.14846@fx02.iad...

I have one of these on my bench that I'm repairing:

https://www.globalmediapro.com/att/a/0/1/a/a01aq8/lbx.jpg

Currently the only problem is a weak left channel when the "Warmth" knob
is increased. When the "Warmth" control (VR200, bottom left of the
schematic) for that channel is fully counterclockwise signal passes fine,
however when it's brought up the channel goes almost completely dead, and
the meter barely deflects.

http://imgur.com/a/JMosx

I've traced the problem down to the circuitry associated with R4 and that
tube section; with no signal applied the DC voltage across the anode
resistors of all tubes sit at approximately half the 48V supply; however
with the bad channel's "Warmth" control is brought up the average DC
voltage across R4 collapses to around 200mV, while the others remain about
the same.

In hopes of a quick fix I replaced the cathode bypass capacitor C38 and
coupling capacitor C77, but no change in the symptoms. Tried several
different sets of tubes and also no change. Suggestions?

Check R8, R224, C142 and ic2.

 

[Phil Allison]

bitrex wrote:

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

I have one of these on my bench that I'm repairing:

https://www.globalmediapro.com/att/a/0/1/a/a01aq8/lbx.jpg

Currently the only problem is a weak left channel when the "Warmth" knob
is increased. When the "Warmth" control (VR200, bottom left of the
schematic) for that channel is fully counterclockwise signal passes
fine, however when it's brought up the channel goes almost completely
dead, and the meter barely deflects.

http://imgur.com/a/JMosx

I've traced the problem down to the circuitry associated with R4 and
that tube section; with no signal applied the DC voltage across the
anode resistors of all tubes sit at approximately half the 48V supply;
however with the bad channel's "Warmth" control is brought up the
average DC voltage across R4 collapses to around 200mV, while the others
remain about the same.

In hopes of a quick fix I replaced the cathode bypass capacitor C38 and
coupling capacitor C77, but no change in the symptoms. Tried several
different sets of tubes and also no change. Suggestions?

** Use your scope FFS.

The halves of each 12AX7 are driven in reverse phase and the plate outputs fed to a differential stage. With sine wave input, the waveforms on each plate should be similar amplitude with one inverted.

If your post is correct this is not the case with your unit.

Suspect a dud section in one of the op-amps.


...... Phil

 

[bitrex]

On 06/02/2017 09:36 PM, Phil Allison wrote:

bitrex wrote:

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


I have one of these on my bench that I'm repairing:

https://www.globalmediapro.com/att/a/0/1/a/a01aq8/lbx.jpg

Currently the only problem is a weak left channel when the "Warmth" knob
is increased. When the "Warmth" control (VR200, bottom left of the
schematic) for that channel is fully counterclockwise signal passes
fine, however when it's brought up the channel goes almost completely
dead, and the meter barely deflects.

http://imgur.com/a/JMosx

I've traced the problem down to the circuitry associated with R4 and
that tube section; with no signal applied the DC voltage across the
anode resistors of all tubes sit at approximately half the 48V supply;
however with the bad channel's "Warmth" control is brought up the
average DC voltage across R4 collapses to around 200mV, while the others
remain about the same.

In hopes of a quick fix I replaced the cathode bypass capacitor C38 and
coupling capacitor C77, but no change in the symptoms. Tried several
different sets of tubes and also no change. Suggestions?



** Use your scope FFS.

The halves of each 12AX7 are driven in reverse phase and the plate outputs fed to a differential stage. With sine wave input, the waveforms on each plate should be similar amplitude with one inverted.

If your post is correct this is not the case with your unit.

Suspect a dud section in one of the op-amps.


..... Phil

Better schematic image:

<http://imgur.com/a/5RtGH>

Here's what the scope shows: VR28A and VR28B are ganged. With VR28 all
the way counterclockwise, there is similar but inverted waveforms at the
anode connections of R4 and R6. As VR28 is rotate clockwise, the signal
at R4 anode decreases while the signal at R6 anode connection increases.

That's the channel that appears to be working OK. There's almost no
signal at all on the anode connections of the other channel, for any
position of VR27A/B, and the channel goes quiet as the pot is rotated
clockwise.

There's normal-looking signal at pin 8 of both IC1C and IC2C for each
channel, the op-amp phase inverter, though.

 

[Phil Allison]

On Saturday, June 3, 2017 at 1:39:48 PM UTC+10, bitrex wrote:

On 06/02/2017 09:36 PM, Phil Allison wrote:
bitrex wrote:

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


I have one of these on my bench that I'm repairing:

https://www.globalmediapro.com/att/a/0/1/a/a01aq8/lbx.jpg

Currently the only problem is a weak left channel when the "Warmth" knob
is increased. When the "Warmth" control (VR200, bottom left of the
schematic) for that channel is fully counterclockwise signal passes
fine, however when it's brought up the channel goes almost completely
dead, and the meter barely deflects.

http://imgur.com/a/JMosx

I've traced the problem down to the circuitry associated with R4 and
that tube section; with no signal applied the DC voltage across the
anode resistors of all tubes sit at approximately half the 48V supply;
however with the bad channel's "Warmth" control is brought up the
average DC voltage across R4 collapses to around 200mV, while the others
remain about the same.

In hopes of a quick fix I replaced the cathode bypass capacitor C38 and
coupling capacitor C77, but no change in the symptoms. Tried several
different sets of tubes and also no change. Suggestions?



** Use your scope FFS.

The halves of each 12AX7 are driven in reverse phase and the plate outputs fed to a differential stage. With sine wave input, the waveforms on each plate should be similar amplitude with one inverted.

If your post is correct this is not the case with your unit.

Suspect a dud section in one of the op-amps.


..... Phil


Better schematic image:

http://imgur.com/a/5RtGH

Here's what the scope shows: VR28A and VR28B are ganged. With VR28 all
the way counterclockwise, there is similar but inverted waveforms at the
anode connections of R4 and R6. As VR28 is rotate clockwise, the signal
at R4 anode decreases while the signal at R6 anode connection increases.

That's the channel that appears to be working OK. There's almost no
signal at all on the anode connections of the other channel, for any
position of VR27A/B, and the channel goes quiet as the pot is rotated
clockwise.

There's normal-looking signal at pin 8 of both IC1C and IC2C for each
channel, the op-amp phase inverter, though.

** You have made a start and you story has changed.

You need to scope the X7 tube grids in the bad channel.

Plus recheck all DC voltages too.


..... Phil

 

[N_Cook]

On 02/06/2017 23:50, bitrex wrote:

I have one of these on my bench that I'm repairing:

https://www.globalmediapro.com/att/a/0/1/a/a01aq8/lbx.jpg

Currently the only problem is a weak left channel when the "Warmth" knob
is increased. When the "Warmth" control (VR200, bottom left of the
schematic) for that channel is fully counterclockwise signal passes
fine, however when it's brought up the channel goes almost completely
dead, and the meter barely deflects.

http://imgur.com/a/JMosx

I've traced the problem down to the circuitry associated with R4 and
that tube section; with no signal applied the DC voltage across the
anode resistors of all tubes sit at approximately half the 48V supply;
however with the bad channel's "Warmth" control is brought up the
average DC voltage across R4 collapses to around 200mV, while the others
remain about the same.

In hopes of a quick fix I replaced the cathode bypass capacitor C38 and
coupling capacitor C77, but no change in the symptoms. Tried several
different sets of tubes and also no change. Suggestions?

I recently came across a pot problem , not seen before, Alps or Alpha ,
one of the main makers. Pot track DVM_R checked out fine. Not been
abused or distorted but manufacturing tolerances problem presumably, the
rear grounded screen/casing was touching outer part of the active track
at one point. Radial tolerance of the track meeting radial tolerance of
the casing? +/- tolerances.

 

[bitrex]

On 06/03/2017 03:09 AM, N_Cook wrote:

On 02/06/2017 23:50, bitrex wrote:
I have one of these on my bench that I'm repairing:

https://www.globalmediapro.com/att/a/0/1/a/a01aq8/lbx.jpg

Currently the only problem is a weak left channel when the "Warmth" knob
is increased. When the "Warmth" control (VR200, bottom left of the
schematic) for that channel is fully counterclockwise signal passes
fine, however when it's brought up the channel goes almost completely
dead, and the meter barely deflects.

http://imgur.com/a/JMosx

I've traced the problem down to the circuitry associated with R4 and
that tube section; with no signal applied the DC voltage across the
anode resistors of all tubes sit at approximately half the 48V supply;
however with the bad channel's "Warmth" control is brought up the
average DC voltage across R4 collapses to around 200mV, while the others
remain about the same.

In hopes of a quick fix I replaced the cathode bypass capacitor C38 and
coupling capacitor C77, but no change in the symptoms. Tried several
different sets of tubes and also no change. Suggestions?

I recently came across a pot problem , not seen before, Alps or Alpha ,
one of the main makers. Pot track DVM_R checked out fine. Not been
abused or distorted but manufacturing tolerances problem presumably, the
rear grounded screen/casing was touching outer part of the active track
at one point. Radial tolerance of the track meeting radial tolerance of
the casing? +/- tolerances.

Thanks for the reply. Yep, a shorted ganged pot in the grid circuit
could definitely explain the symptoms - as Phil mentioned I need to
check all the grid voltages on the bad channel at quiescence and with
signal applied.

Signal coming into the junction of R206 and C141 looks good, signal
coming out of IC1C pin 8 also looks good, so it seems really the only
options for where the problem lies are with the 100n coupling
capacitors, the pot, or shorted pins on the tube socket.

 

[bitrex]

On 06/02/2017 11:52 PM, Phil Allison wrote:

On Saturday, June 3, 2017 at 1:39:48 PM UTC+10, bitrex wrote:
On 06/02/2017 09:36 PM, Phil Allison wrote:
bitrex wrote:

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


I have one of these on my bench that I'm repairing:

https://www.globalmediapro.com/att/a/0/1/a/a01aq8/lbx.jpg

Currently the only problem is a weak left channel when the "Warmth" knob
is increased. When the "Warmth" control (VR200, bottom left of the
schematic) for that channel is fully counterclockwise signal passes
fine, however when it's brought up the channel goes almost completely
dead, and the meter barely deflects.

http://imgur.com/a/JMosx

I've traced the problem down to the circuitry associated with R4 and
that tube section; with no signal applied the DC voltage across the
anode resistors of all tubes sit at approximately half the 48V supply;
however with the bad channel's "Warmth" control is brought up the
average DC voltage across R4 collapses to around 200mV, while the others
remain about the same.

In hopes of a quick fix I replaced the cathode bypass capacitor C38 and
coupling capacitor C77, but no change in the symptoms. Tried several
different sets of tubes and also no change. Suggestions?



** Use your scope FFS.

The halves of each 12AX7 are driven in reverse phase and the plate outputs fed to a differential stage. With sine wave input, the waveforms on each plate should be similar amplitude with one inverted.

If your post is correct this is not the case with your unit.

Suspect a dud section in one of the op-amps.


..... Phil


Better schematic image:

http://imgur.com/a/5RtGH

Here's what the scope shows: VR28A and VR28B are ganged. With VR28 all
the way counterclockwise, there is similar but inverted waveforms at the
anode connections of R4 and R6. As VR28 is rotate clockwise, the signal
at R4 anode decreases while the signal at R6 anode connection increases.

That's the channel that appears to be working OK. There's almost no
signal at all on the anode connections of the other channel, for any
position of VR27A/B, and the channel goes quiet as the pot is rotated
clockwise.

There's normal-looking signal at pin 8 of both IC1C and IC2C for each
channel, the op-amp phase inverter, though.


** You have made a start and you story has changed.

You need to scope the X7 tube grids in the bad channel.

Plus recheck all DC voltages too.


.... Phil

First thing I need to make sure of is that I understand how the circuit
is _supposed_ to function so I understand what should be considered
"normal" operation. It seems that the channel associated with R4 _is_
the one that's functioning properly; as the "Warmth" control is
increased from its fully counterclockwise position the AC signal across
R4 decreases (this is what I was seeing with my meter) while the signal
across R6 increases. The panel meter begins deflecting to the right in
response to signal.

The overall level of the signal doesn't change, however, it just becomes
well..."warmer." With a 12AX7 tube the additional harmonic content is
quite noticeable at full clockwise rotation and IMO doesn't sound that
good. With a NOS 12AT7 in the socket it's more subtle and (also IMO)
more appropriate for an EQ/exciter.

It seems the control is converting the operation from differential to
single-ended, but I'm not sure exactly how the circuitry associated with
IC2A and IC19B is accomplishing this.

There are also two trimmer pots on the board associated with the tube
section VR29 and VR30.

 

[megamusic]

"bitrex" <bitrex@de.lete.earthlink.net> wrote in message
news:4cqYA.28135$5D5.24639@fx37.iad...

On 06/02/2017 09:36 PM, Phil Allison wrote:
bitrex wrote:

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


I have one of these on my bench that I'm repairing:

https://www.globalmediapro.com/att/a/0/1/a/a01aq8/lbx.jpg

Currently the only problem is a weak left channel when the "Warmth" knob
is increased. When the "Warmth" control (VR200, bottom left of the
schematic) for that channel is fully counterclockwise signal passes
fine, however when it's brought up the channel goes almost completely
dead, and the meter barely deflects.

http://imgur.com/a/JMosx

I've traced the problem down to the circuitry associated with R4 and
that tube section; with no signal applied the DC voltage across the
anode resistors of all tubes sit at approximately half the 48V supply;
however with the bad channel's "Warmth" control is brought up the
average DC voltage across R4 collapses to around 200mV, while the others
remain about the same.

In hopes of a quick fix I replaced the cathode bypass capacitor C38 and
coupling capacitor C77, but no change in the symptoms. Tried several
different sets of tubes and also no change. Suggestions?



** Use your scope FFS.

The halves of each 12AX7 are driven in reverse phase and the plate
outputs fed to a differential stage. With sine wave input, the waveforms
on each plate should be similar amplitude with one inverted.

If your post is correct this is not the case with your unit.

Suspect a dud section in one of the op-amps.


..... Phil


Better schematic image:

http://imgur.com/a/5RtGH

Here's what the scope shows: VR28A and VR28B are ganged. With VR28 all the
way counterclockwise, there is similar but inverted waveforms at the anode
connections of R4 and R6. As VR28 is rotate clockwise, the signal at R4
anode decreases while the signal at R6 anode connection increases.

That's the channel that appears to be working OK. There's almost no signal
at all on the anode connections of the other channel, for any position of
VR27A/B, and the channel goes quiet as the pot is rotated clockwise.

There's normal-looking signal at pin 8 of both IC1C and IC2C for each
channel, the op-amp phase inverter, though.

Check X10-X18 cathode ground wire, especially if it is flat IDC cable
(common problem in behringers)

 

On Sat, 3 Jun 2017 20:32:57 +0200, "megamusic" <nema@nema.nema> wrote:

That's the channel that appears to be working OK. There's almost no signal
at all on the anode connections of the other channel, for any position of
VR27A/B, and the channel goes quiet as the pot is rotated clockwise.

There's normal-looking signal at pin 8 of both IC1C and IC2C for each
channel, the op-amp phase inverter, though.

Check X10-X18 cathode ground wire, especially if it is flat IDC cable
(common problem in behringers)

Just curious. What is that thing. I looked at the photo but could not
read the details on it. Is it a preamp or what?
Having both ICs and Tubes is kind of unusual too.
Looks kinda cool, whatever it is.....

 

[Phil Allison]

olds...@tubes.com wrote:

Just curious. What is that thing.

** Incapable of using Google ??

You seem to be a very lazy shithead.




...... Phil

 

[bitrex]

On 06/02/2017 11:52 PM, Phil Allison wrote:

On Saturday, June 3, 2017 at 1:39:48 PM UTC+10, bitrex wrote:
On 06/02/2017 09:36 PM, Phil Allison wrote:
bitrex wrote:

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


I have one of these on my bench that I'm repairing:

https://www.globalmediapro.com/att/a/0/1/a/a01aq8/lbx.jpg

Currently the only problem is a weak left channel when the "Warmth" knob
is increased. When the "Warmth" control (VR200, bottom left of the
schematic) for that channel is fully counterclockwise signal passes
fine, however when it's brought up the channel goes almost completely
dead, and the meter barely deflects.

http://imgur.com/a/JMosx

I've traced the problem down to the circuitry associated with R4 and
that tube section; with no signal applied the DC voltage across the
anode resistors of all tubes sit at approximately half the 48V supply;
however with the bad channel's "Warmth" control is brought up the
average DC voltage across R4 collapses to around 200mV, while the others
remain about the same.

In hopes of a quick fix I replaced the cathode bypass capacitor C38 and
coupling capacitor C77, but no change in the symptoms. Tried several
different sets of tubes and also no change. Suggestions?



** Use your scope FFS.

The halves of each 12AX7 are driven in reverse phase and the plate outputs fed to a differential stage. With sine wave input, the waveforms on each plate should be similar amplitude with one inverted.

If your post is correct this is not the case with your unit.

Suspect a dud section in one of the op-amps.


..... Phil


Better schematic image:

http://imgur.com/a/5RtGH

Here's what the scope shows: VR28A and VR28B are ganged. With VR28 all
the way counterclockwise, there is similar but inverted waveforms at the
anode connections of R4 and R6. As VR28 is rotate clockwise, the signal
at R4 anode decreases while the signal at R6 anode connection increases.

That's the channel that appears to be working OK. There's almost no
signal at all on the anode connections of the other channel, for any
position of VR27A/B, and the channel goes quiet as the pot is rotated
clockwise.

There's normal-looking signal at pin 8 of both IC1C and IC2C for each
channel, the op-amp phase inverter, though.


** You have made a start and you story has changed.

You need to scope the X7 tube grids in the bad channel.

Plus recheck all DC voltages too.


.... Phil

Checked the DC voltages at quiescent across all the relevant resistors
for both channels and and here's what I find:

R3: 26.1

R4: 24.5

R5: 22.6

R6: 22.3

R7: 1.08

R8: 1.01

R9: 0

R10: 0

R11: 0.94

R12: 0.91

R13: 0

R14: 0

Scoped the tube grids in the bad channel and they're getting signal just
fine; with a 1V P2P sine wave as input the grid signals for the bad
channel look identical to the good channel as the "Warmth" controls are
rotated.

There's zip on the plate resistors, though. DC voltages remain the same.
Checked all the tube pins for continuity to the board and they check
out. Checked all the ground leads for continuity and they check out
fine, too. It seems like there wouldn't be a problem there anyway if all
the bias voltages look correct, anyway.

IF C36/C37 were bad I believe I'd still be getting some signal. Only
thing that makes much sense to me at this point is a fault with IC1B.

 

[bitrex]

On 06/03/2017 11:54 PM, oldschool@tubes.com wrote:

On Sat, 3 Jun 2017 20:32:57 +0200, "megamusic" <nema@nema.nema> wrote:


That's the channel that appears to be working OK. There's almost no signal
at all on the anode connections of the other channel, for any position of
VR27A/B, and the channel goes quiet as the pot is rotated clockwise.

There's normal-looking signal at pin 8 of both IC1C and IC2C for each
channel, the op-amp phase inverter, though.

Check X10-X18 cathode ground wire, especially if it is flat IDC cable
(common problem in behringers)


Just curious. What is that thing. I looked at the photo but could not
read the details on it. Is it a preamp or what?
Having both ICs and Tubes is kind of unusual too.
Looks kinda cool, whatever it is.....

You put your sick beats in one end and they sound dope-er coming out the
other

 

[Gareth Magennis]

"bitrex" wrote in message news:VFZYA.6917$vu6.2174@fx30.iad...

On 06/03/2017 11:54 PM, oldschool@tubes.com wrote:

On Sat, 3 Jun 2017 20:32:57 +0200, "megamusic" <nema@nema.nema> wrote:


That's the channel that appears to be working OK. There's almost no
signal
at all on the anode connections of the other channel, for any position
of
VR27A/B, and the channel goes quiet as the pot is rotated clockwise.

There's normal-looking signal at pin 8 of both IC1C and IC2C for each
channel, the op-amp phase inverter, though.

Check X10-X18 cathode ground wire, especially if it is flat IDC cable
(common problem in behringers)


Just curious. What is that thing. I looked at the photo but could not
read the details on it. Is it a preamp or what?
Having both ICs and Tubes is kind of unusual too.
Looks kinda cool, whatever it is.....

You put your sick beats in one end and they sound dope-er coming out the
other


************************************************************


I like that.


Gareth.

 

[bitrex]

On 06/04/2017 04:13 PM, Gareth Magennis wrote:

Just curious. What is that thing. I looked at the photo but could not
read the details on it. Is it a preamp or what?
Having both ICs and Tubes is kind of unusual too.
Looks kinda cool, whatever it is.....


You put your sick beats in one end and they sound dope-er coming out the
other


************************************************************


I like that.


Gareth.

I'm pretty sure "The Dopeifier" would be a pretty good name for a signal
processor product; sadly the major manufacturers don't seem to ask my
opinion on these matters much

 

[Phil Allison]

bitrex wrote:

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

** You have made a start and you story has changed.

You need to scope the X7 tube grids in the bad channel.

Plus recheck all DC voltages too.



Checked the DC voltages at quiescent across all the relevant resistors
for both channels and and here's what I find:

R3: 26.1

R4: 24.5

R5: 22.6

R6: 22.3

R7: 1.08

R8: 1.01

R9: 0

R10: 0

R11: 0.94

R12: 0.91

R13: 0

R14: 0

Scoped the tube grids in the bad channel and they're getting signal just
fine; with a 1V P2P sine wave as input the grid signals for the bad
channel look identical to the good channel as the "Warmth" controls are
rotated.

There's zip on the plate resistors, though. DC voltages remain the same.
Checked all the tube pins for continuity to the board and they check
out. Checked all the ground leads for continuity and they check out
fine, too. It seems like there wouldn't be a problem there anyway if all
the bias voltages look correct, anyway.

IF C36/C37 were bad I believe I'd still be getting some signal. Only
thing that makes much sense to me at this point is a fault with IC1B.

** The only way to explain your results is that there is a low impedance path between the plates of the tube - maybe just after the coupling caps.

So not faulty components, a PCB issue.

Cleaning the area with a good PCB solvent is a sensible first step.


..... Phil

 

[bitrex]

On 06/04/2017 05:58 PM, Phil Allison wrote:

bitrex wrote:

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


** You have made a start and you story has changed.

You need to scope the X7 tube grids in the bad channel.

Plus recheck all DC voltages too.



Checked the DC voltages at quiescent across all the relevant resistors
for both channels and and here's what I find:

R3: 26.1

R4: 24.5

R5: 22.6

R6: 22.3

R7: 1.08

R8: 1.01

R9: 0

R10: 0

R11: 0.94

R12: 0.91

R13: 0

R14: 0

Scoped the tube grids in the bad channel and they're getting signal just
fine; with a 1V P2P sine wave as input the grid signals for the bad
channel look identical to the good channel as the "Warmth" controls are
rotated.

There's zip on the plate resistors, though. DC voltages remain the same.
Checked all the tube pins for continuity to the board and they check
out. Checked all the ground leads for continuity and they check out
fine, too. It seems like there wouldn't be a problem there anyway if all
the bias voltages look correct, anyway.

IF C36/C37 were bad I believe I'd still be getting some signal. Only
thing that makes much sense to me at this point is a fault with IC1B.



** The only way to explain your results is that there is a low impedance path between the plates of the tube - maybe just after the coupling caps.

So not faulty components, a PCB issue.

Cleaning the area with a good PCB solvent is a sensible first step.


.... Phil

I was thinking an internal short between the inverting and non inverting
sections of the op amp, but yeah, that wouldn't really explain the
symptoms as it would still present a high-impedance load thanks to R150
and R126.

 

[bitrex]

On 06/04/2017 06:25 PM, bitrex wrote:

On 06/04/2017 05:58 PM, Phil Allison wrote:
bitrex wrote:

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


** You have made a start and you story has changed.

You need to scope the X7 tube grids in the bad channel.

Plus recheck all DC voltages too.



Checked the DC voltages at quiescent across all the relevant resistors
for both channels and and here's what I find:

R3: 26.1

R4: 24.5

R5: 22.6

R6: 22.3

R7: 1.08

R8: 1.01

R9: 0

R10: 0

R11: 0.94

R12: 0.91

R13: 0

R14: 0

Scoped the tube grids in the bad channel and they're getting signal just
fine; with a 1V P2P sine wave as input the grid signals for the bad
channel look identical to the good channel as the "Warmth" controls are
rotated.

There's zip on the plate resistors, though. DC voltages remain the same.
Checked all the tube pins for continuity to the board and they check
out. Checked all the ground leads for continuity and they check out
fine, too. It seems like there wouldn't be a problem there anyway if all
the bias voltages look correct, anyway.

IF C36/C37 were bad I believe I'd still be getting some signal. Only
thing that makes much sense to me at this point is a fault with IC1B.



** The only way to explain your results is that there is a low
impedance path between the plates of the tube - maybe just after the
coupling caps.

So not faulty components, a PCB issue.

Cleaning the area with a good PCB solvent is a sensible first step.


.... Phil


I was thinking an internal short between the inverting and non inverting
sections

inputs, rather

 

[Gareth Magennis]

"bitrex" wrote in message news:sN%YA.58378$EH.37489@fx40.iad...

On 06/04/2017 06:25 PM, bitrex wrote:

On 06/04/2017 05:58 PM, Phil Allison wrote:
bitrex wrote:

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


** You have made a start and you story has changed.

You need to scope the X7 tube grids in the bad channel.

Plus recheck all DC voltages too.



Checked the DC voltages at quiescent across all the relevant resistors
for both channels and and here's what I find:

R3: 26.1

R4: 24.5

R5: 22.6

R6: 22.3

R7: 1.08

R8: 1.01

R9: 0

R10: 0

R11: 0.94

R12: 0.91

R13: 0

R14: 0

Scoped the tube grids in the bad channel and they're getting signal just
fine; with a 1V P2P sine wave as input the grid signals for the bad
channel look identical to the good channel as the "Warmth" controls are
rotated.

There's zip on the plate resistors, though. DC voltages remain the same.
Checked all the tube pins for continuity to the board and they check
out. Checked all the ground leads for continuity and they check out
fine, too. It seems like there wouldn't be a problem there anyway if all
the bias voltages look correct, anyway.

IF C36/C37 were bad I believe I'd still be getting some signal. Only
thing that makes much sense to me at this point is a fault with IC1B.



** The only way to explain your results is that there is a low impedance
path between the plates of the tube - maybe just after the coupling caps.

So not faulty components, a PCB issue.

Cleaning the area with a good PCB solvent is a sensible first step.


.... Phil


I was thinking an internal short between the inverting and non inverting
sections

inputs, rather



************************************************************



I would not rule out a PCB issue if I were you.

This is more common than you may think.



Gareth.

 

[bitrex]

On 06/02/2017 06:50 PM, bitrex wrote:

I have one of these on my bench that I'm repairing:

https://www.globalmediapro.com/att/a/0/1/a/a01aq8/lbx.jpg

Currently the only problem is a weak left channel when the "Warmth" knob
is increased. When the "Warmth" control (VR200, bottom left of the
schematic) for that channel is fully counterclockwise signal passes
fine, however when it's brought up the channel goes almost completely
dead, and the meter barely deflects.

http://imgur.com/a/JMosx

I've traced the problem down to the circuitry associated with R4 and
that tube section; with no signal applied the DC voltage across the
anode resistors of all tubes sit at approximately half the 48V supply;
however with the bad channel's "Warmth" control is brought up the
average DC voltage across R4 collapses to around 200mV, while the others
remain about the same.

In hopes of a quick fix I replaced the cathode bypass capacitor C38 and
coupling capacitor C77, but no change in the symptoms. Tried several
different sets of tubes and also no change. Suggestions?

Alright gents, I carefully inspected the PCB and noticed the solder
joints on the header where the tube board plugs in to the solid state
section looked a bit cruddy. After a good scrubdown with acetone and
re-flowing the joints all the waveforms look good and we're back in
business.