I’ve just completed a careful build of my Anushri, and it is a truly lovely thing, but I’ve hit a troubleshooting snag that I could use some help with!
Basically my fuzz circuit does not output anything. I have a vcf input signal at pin 3 of the LM13700N, but nothing doing at pins 8 or 5. If I flip the fuzz switch, I get no audio from the synth which is as expected since the fuzz output pin 8 is quiet as a mouse. I have checked for continuity and incorrect shorts to ground on all the LM13700 pins and it looks OK, it gets -5v on pin 6, +5 on pin 11, and is grounded ok at pins 4 and 13 which checks out too. Checking with my multimeter, all the associated resistors (R46-R50) seem to be soldered in OK too, and there’s -5v on the other side of R50 as per the circuit diagram. I also swapped the LM13700 out with a new chip just in case, but no dice. Scratching head doesn’t help either, though I tried that lots too.
When I power the unit up, pin 8 (fuzz output) appears to my multimeter as if it is shorted to ground (connected to an audio amp it is super quiet too). With power off, it does not.
No doubt this is something simple, and I wonder if it has anything to do with the other OTA amplifier on the chip that seems to be involved with the VCA circuit, but I thought someone out there in Mutable land could point me in the right direction?
Can you confirm that the synth works normally when the fuzz is off?
Could you post a picture of the board centered around IC4?
Yes the synth all seems to work normally with fuzz off. Well I’m having fun with it, so I guess I’m doing that bit right
I’ve attached some pics of top and bottom side around IC4 - there’s a dodgy looking connection on the bottom of the board with one of the legs on c14 (bad desoldering job by me when I accidentally put a 100nF cap in) but it’s soldered in well enough on the topside.
Any clues appreciated!
I don’t see anything wrong here… Maybe it’s a bad connection near the switch (SW10) or the connectors (J16 on the top board ; J7 on the main board).
Have you tried 'listening" to IC4 pin 8 (beware, the signal level will be high!).
Thanks for taking a look at my pics
The switch and connector joints check out fine, I just have no signal going to them from pin 8 of IC4
IC4 pin 8 is completely silent (it behaves as if it is grounded when power is on, but not with power off), so it must be something in the IC4 circuit I figure. Should I be able to measure anything at pin 1 or 5 of IC4 (before IC4BUF1)? They have a little noise, but no audio signal to speak of and not as quiet as pin 8.
I did replace IC4 just to be sure, but the problem is still there…
I seem to have fixed the fuzz, but I’m not sure what was the fault.
I did make a wrong assumption that there should be fuzz output at pin 8 of IC4 all the time, because with the fuzz switch down, I think this pin is effectively grounded.
When I was initially testing things, I’m sure there was no output from the box with fuzz switched on, but at some stage while re-seating the IC or scratching about, I must have fixed a bad connection around IC4 or around J7 on the control board but I continued testing over and over looking for output at pin 8, but with the switch down, I was never gonna get anything… FYI, the original IC4 was fine.
Feel a bit silly now, but I hope this insight helps someone.
It’s Victory Candy time!
Same problem here. Turns out the longest IC chip was loose (different type socket which doesn’t use a death grip). Actually fell out while I was searching for the problem elsewhere. Jammed it back in and fixed the fuzz problem as well as the erratic LED problems when powered on. This might be the cause of some peoples “loose connection somewhere” problem as well.