Behringer VT1951 Tube Ultra-Q repair
On 06/04/2017 06:25 PM, bitrex wrote:
On 06/04/2017 05:58 PM, Phil Allison wrote:
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.
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
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