Hi folks,
Could anyone with an original Moog 904B HPFilter make a couple of checks for me please…?
I have built two of these myself and although they work perfectly - I am getting ‘thumping’ on the output and it’s passing audio from the control inputs.
Cheers,
Tom
Are Q13, Q17, Q21, Q25 a matched set? Q14, Q18, Q22, Q26 matched?
You are aware that the test circuit from the service manual for matching PNPs is incorrect, yes?
If any of them are unmatched that is where the feedthrough comes from. The 904B has no trimpot for reducing feedthrough.
Thanks for your comments MC - it’s actually sorted.
The notes on the schematic are misleading though… it’s not Q’s 13/17/21/25 & Q’s14/18/22/26 that need matching. The wording could be better there.
They actually need matching on the pairs which work together: 13/14 - 17/18 - 21/22 - 25/26.
I used the hfe function on one of my DVM’s and not that Moog circuit, but I am aware of the error.
Also C19 is wrong… it should go to ground and not the -6v rail.
I also ‘upgraded’ the circuit with a slight addition. I a/c coupled the audio amp and also put in a trimmer to clamp the base of Q27 to zero.
I think there was a factory upgrade which added a couple of caps on the rear of the board around Q28, which I also performed.
It’s all working perfectly now… 
Great that you got it sorted out.
Sheesh, the 904B schematic sure is loaded with errors…!