Behringer VT1951 Tube Ultra-Q repair

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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?

Re: Behringer VT1951 Tube Ultra-Q repair

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Check R8, R224, C142 and ic2.  



Re: Behringer VT1951 Tube Ultra-Q repair
 bitrex wrote:

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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  

Re: Behringer VT1951 Tube Ultra-Q repair
On 06/02/2017 09:36 PM, Phil Allison wrote:
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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.

Re: Behringer VT1951 Tube Ultra-Q repair
On Saturday, June 3, 2017 at 1:39:48 PM UTC+10, bitrex wrote:
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** 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


Re: Behringer VT1951 Tube Ultra-Q repair
On 06/02/2017 11:52 PM, Phil Allison wrote:
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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.

Re: Behringer VT1951 Tube Ultra-Q repair
On 06/02/2017 11:52 PM, Phil Allison wrote:
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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.




Re: Behringer VT1951 Tube Ultra-Q repair
bitrex wrote:

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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  

Re: Behringer VT1951 Tube Ultra-Q repair
On 06/04/2017 05:58 PM, Phil Allison wrote:
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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.

Re: Behringer VT1951 Tube Ultra-Q repair
On 06/04/2017 06:25 PM, bitrex wrote:
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inputs, rather


Re: Behringer VT1951 Tube Ultra-Q repair



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


Re: Behringer VT1951 Tube Ultra-Q repair
On 06/04/2017 05:58 PM, Phil Allison wrote:

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Fixed, cause was crudded up/flaky solder joints on the interconnect  
cable header coming from the tube board to the solid state section.

Re: Behringer VT1951 Tube Ultra-Q repair
 bitrex wrote:

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** I guess that is the nearest thing to "thanks" I'm gonna get ....  


.....  Phil  

Re: Behringer VT1951 Tube Ultra-Q repair
On 06/06/2017 04:24 AM, Phil Allison wrote:
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You're right, that was insensitive of me. Thank you, Phil - if I ever  
have a chance to visit that part of the world I'll know where to take my  
stuff.

Accents sound kinda funny, though. We all talk like this here:

<
https://www.youtube.com/watch?v=bgfDALzpfJU


Probably sounds wicked retahded to anyone else.


Re: Behringer VT1951 Tube Ultra-Q repair
bitrex wrote:

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 >  
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 ** So you are the opinionated talking bear - right ?  



.....  Phil  

Re: Behringer VT1951 Tube Ultra-Q repair

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



Re: Behringer VT1951 Tube Ultra-Q repair


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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.....


Re: Behringer VT1951 Tube Ultra-Q repair
snipped-for-privacy@tubes.com wrote:

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** Incapable of using Google ??  

 You seem to be a very lazy shithead.  




.....  Phil  

Re: Behringer VT1951 Tube Ultra-Q repair
On 06/03/2017 11:54 PM, snipped-for-privacy@tubes.com wrote:
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You put your sick beats in one end and they sound dope-er coming out the  
other

Re: Behringer VT1951 Tube Ultra-Q repair



On 06/03/2017 11:54 PM, snipped-for-privacy@tubes.com wrote:
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You put your sick beats in one end and they sound dope-er coming out the
other


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


I like that.  :)


Gareth.  


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