Quad 33 popcorn/crackling in 1 channel

[fastfolkert]

I recently got a Quad 33 for almost free, to be used with my 303.
It sounds fine, everything works, however, there’s a faint “popcorn” noise to be heard when you put your ears against the speakers. The noise is independent from volume, and inaudible when I play music.

When I change the position of the M12017 amplifier boards, the problem changes too, to the other speaker. Therefore I think I narrowed it down to one of these M12017 boards.

Firstly, I renewed C401, C405 and C406. That did not solve the problem, and I put them back.

I checked the supply rail and saw nothing strange there, I would be surprised since the issue is strictly in one channel.

Any suggestions what to check next? Any “known Quad 33” issues that sounds like mine?

Thanks!

(this question was also posted in vintage radio forum)

 

[turk 182]

C400 and Tr400 would be next on my list...other than that if you have access to a scope to narrow down where the "popcorn" is coming from and substitution is the only thing you can do...try subbing all the BC 109's one at a time.

 

[ejp]

I would replace the transistors on those boards, and on the Tape board too for good measure, also the 220K resistors on that board. Use BC549.

 

[fastfolkert]

Thanks for the replies, I will start changing the 3 BC109s on the M12017 board.

But why would it be good to change something on the tape board? The problem “stays” with one of the M12017 boards.

(best wishes, by the way 😉)

 

[ejp]

Because they're the ones that usually go noisy first. Precautionary.