"bitrex" wrote in message
...
On 06/02/2017 09:36 PM, Phil Allison wrote:
bitrex wrote:

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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?



** 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)