ZPSDX -- high-pitched whine only in tape mode

[midwayfair]

It's a high pitched whine, but not super high pitched, and VERY loud. It sounds almost like clock noise, but it ONLY appears in tape mode. It happens as early as IC2 (extemely faint -- it could actually just be bleed); it's of moderate volume at the output of IC3, and then it's quite loud at pit 9 of IC4 and even louder (as you might expect) at pin 8 of IC4. It's so loud that I can actually pick it up with my probe an inch away from the TL072!

All my voltages are correct; I've checked my values and they are all correct. I've replaced all chips and even tried substitutes.

What's truly baffling is that it doesn't exist ANYWHERE in the circuit in modes other than Tape. So somehow switching modes is introducing noise earlier in the circuit than the rotary switch. How?

I'm really not sure how to being trouble shooting something that doesn't make any electronic sense.

[madbean]

If you remove IC4 altogether, and select the Deep or Shimmer modes does the whine go away?

And, not to belabor the point: are you positive R31, R32, R33, R36 are the correct values? Any of those could cause oscillation if they are off by an order of magnitude...perhaps.

[midwayfair]

If you remove IC4 altogether, and select the Deep or Shimmer modes does the whine go away?

And, not to belabor the point: are you positive R31, R32, R33, R36 are the correct values? Any of those could cause oscillation if they are off by an order of magnitude...perhaps.

I think I found it: Lugs A and B were connecting somehow. I've removed the daughter board and I'll see if I can get it working from here.

[madbean]

Here's a closeup to the trace layout for the daughter board. I'd check to make sure there is no error with the traces on the PCB, just in case. They could be connected on the main board too, maybe through a solder bridge somewhere.

[midwayfair]

Here's a closeup to the trace layout for the daughter board. I'd check to make sure there is no error with the traces on the PCB, just in case. They could be connected on the main board too, maybe through a solder bridge somewhere.

Hah. I was going to e-mail you for this, but I decided it was quicker to just scrape away some plastic and check continuity on the traces.

It's fixed --- it looks ugly since I had to run wire to the pins, but it works now. (Finally.) Just have a few more mods to do and then I can start thinking about boxing it.

[jimilee]

Here's a closeup to the trace layout for the daughter board. I'd check to make sure there is no error with the traces on the PCB, just in case. They could be connected on the main board too, maybe through a solder bridge somewhere.

Hah. I was going to e-mail you for this, but I decided it was quicker to just scrape away some plastic and check continuity on the traces.

It's fixed --- it looks ugly since I had to run wire to the pins, but it works now. (Finally.) Just have a few more mods to do and then I can start thinking about boxing it.

Was there a defect in the board?

[midwayfair]

Was there a defect in the board?

No, I probably soldered the daughter board too close to the main board and ended up with solder bleeding into neighboring pads.