Behringer VT1951 Tube Ultra-Q repair

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

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.

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?

Reply to
bitrex
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Check R8, R224, C142 and ic2.

Reply to
megamusic

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** 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

Reply to
Phil Allison

Better schematic image:

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.

Reply to
bitrex

** 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

Reply to
Phil Allison

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.

Reply to
N_Cook

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.

Reply to
bitrex

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.

Reply to
bitrex

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

Reply to
megamusic

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.....

Reply to
oldschool

** Incapable of using Google ??

You seem to be a very lazy s*****ad.

..... Phil

Reply to
Phil Allison

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.

Reply to
bitrex

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

Reply to
bitrex

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

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I like that. :)

Gareth.

Reply to
Gareth Magennis

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

Reply to
bitrex

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** 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

Reply to
Phil Allison

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.

Reply to
bitrex

inputs, rather

Reply to
bitrex

inputs, rather

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

This is more common than you may think.

Gareth.

Reply to
Gareth Magennis

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.

Reply to
bitrex

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