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:

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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.


** 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.