Help with debugging MKIxErica S&H/Noise
So I have constructed the module per instructions, everything worked on the breadboard.
It seems that the S&H part itself is working (when I connect another input I can sample that, slew is also working), but that the noise generating circuit is not generating any noise.
However, if I touch the transistor that is the noise source with my finger it suddenly does work (found out while trying to measure using oscilloscope). I've checked the joints and they are OK.
Does this mean the transistor is broken, or could it be something else?
Posted Oct 17, 2022, 9:09 PM UTC
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It's likely that the transistor has a broken leg based on your description.
In that case I think I was unclear. I mean that I was touching the contacts on the pcb, not the top of the transistor. When I touch the top nothing happens.
Is this from a kit with a PCB or your own point to point build?
Still seems likely that the transistor is the issue. They are relatively easy to overheat when soldering. Also, some transistors are noisier than others which is why there is often an adjustable gain setting for the noise level.
It's the pcb from the Erica synths kit. I assume the specific transistor was chosen to be adequately noisy. Anyway I agree that trying a simple replacement is probably the easiest way forward. Now I just need to source one.
Well ... ordered a whole bunch of the BC548's since I don't need anything else at the moment, and I seem to encounter them in different circuits a lot.
Finally got around to replacing the transistor.
Unfortunately this didn't fix things. Still has the weird behaviour where if I touch one of the legs the circuit suddenly works. Which leads me to believe that the stuff after the transistor is fine. It must be before ... (which consists only of a single 100K resistor to +12V and the base going to ground. Worst case I can but that bit on a tiny perfboard and parasite it on there.
Are the PCB traces intact? If there are breaks you can likely add a jumper wire from the transistor to the pot.