4P resonance problem [SOLVED]

I’m having lower resonance on one of my 4P voicecards. All of them go into self oscillation but this one doesn’t. It does have some resonance so it is working. Also the filter cutoff seems to be a little bit higher.
Checked lots of things, resoldered the whole board twice.
I just can’t find it… help?

All 220pF capacitors are correctly soldered in the right holes?

yup… with the polystyrene caps it’s actually difficult to put them in the wrong holes.

Photo of your board?

Here they are, i feal a bit ashamed since i just tried to clean the solder side with alcohol… wich was a bad idea… but the problem was already there.
I also had the 3.6 v diodes replaced cause i wasn’t 100% sure they were the right ones and couldn’t read it. They turned out to be right anyway… i also tested the board without the diodes to see what that would do to the resonance, i expected it to be really high but it wasn’t.

Have you tried setting the unit to unison/mono mode, with two voicecards (the faulty one and the correct one), and comparing the signal at intermediate nodes (for example each TL072 output) between the two voicecards?

No, will try that…

So, IPA=Bad for your MI board? :frowning:

On all the TL072 outputs (pin1 and 7 ) the signal is normal but there is no resonance tone like on the other cards with the res on 63.

I think the black ‘paint’ started to dissolve… it turned all sticky. Rinsed it with water really quick to get the alcohol off.

remove the LM13700 and measure the current flowing between pin 1 and 8 for minimum and maximum resonance. Compare with a working voicecard.

Ok will try that found something else now… on IC3 pin 7 the signal is same as pin 1… on the other boards it’s more filtered then on pin 1

Then focus on IC3 - check both 220pF capacitor around it. I suspect there’s either a bad solder joint on a ground pin of IC3, or a bad solder joint for a capacitor, so one pole of the filter is actually not operating.

ok just measured the current between 1 and 8 without the LM13700 but i don’t measure anything on the working and on the not working… ? i thought my meter was broken but just tested with a battery and it does see current…

Even when resonance is swept?

i tried it on 0 and 63.

it’s the 4P and pin 8 doesn’t seem to be connected on that one…

resoldered all on IC3 and caps for 3th time but doesn’t make a difference. also checked all continuity and seems to be fine too. Could one of the caps be bad?

Sorry, pin 1 and 6 :frowning:

wanted to work it today again but after switching it on i discover i lost communication between all voicecards and the SD card! …
checked all power rails but they seem ok. strange thing. Otherwise it starts normal.
I tried updating the main board but this didn’t help.