Quad 44 tape input hum

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18-May-2017 02:04 PM

steveapple

steveapple

Posts: 1

My Quad 44 preamp s/n 21907 seems to have developed hum on the two tape input, issue 3. Perhaps it was always there but, because of not using them for a long time, I had not noticed. I upgraded the unit several years ago with the Dada Electronics kit including replacing all electrolytics and op amps. The mother board is issue 5. The analog switch ICs were replaced a long time ago with NXP4066B.
The hum which becomes obtrusive as the volume control is increased, is totally absent on all other inputs even at full volume except for the two tape modules. Both tape modules behave the same. The problem seems like an earth loop, but I cannot find one. There is no hum when interchanging the tape modules with Radio or Aux modules into the tape positions, and the tape module hum is present when the module is plugged into the radio or aux positions. So the problem does not seem to be with the motherboard and is definitely associated only with the tape modules.
The hum decreases as the input sensitivity DIP switches are closed, becoming noticeably less when the all are closed, presumably due to the decreased gain of the transistor amp. I disconnected the grounded ends of resistors R15, 17, 19, 23 and reconnected them temporarily with wire direct to the motherboard connector earth pin 4 to see if there was a grounding loop problem. No change. I disconnected the output resistor R29 to see if that was the source. No change.
I now believe the problem might be ripple on one or both the power supplies. But without a 'scope I cannot check. I doubled the two PS smoothing capacitors to 2000 uf. No change.
I tried insulating the metal connector mount and leaving the screws out so there is no direct earth connection to the chassis. No change.
I am now at a loss as to where this hum is coming from.
I can only assume it is possibly PS ripple being amplified by the transistor input amp's gain. Or is it?
Anybody else had this problem and if so did you find a solution? Many thanks, Steve.
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20-May-2017 04:08 AM

EJP

EJP

Posts: 1412

PSU caps and everything else are clearly non-issues. You should be looking for a physical fault on the Tape board itself/themselves, maybe an error you perpetrated on both. Check the soldering of the earth wire from the RCA socket to the PCB, and from the PCB to the earth socket that goes to the motherboard. Check track continuity all the way in between. You may also need to add decoupling across the op-amps, pins 4 to 7, or 4 to ground and 7 to ground, whichever you prefer, or from the top and bottom supply rail sockets to centre ground at the motherboard end of the PCB.
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