Total Recall no modulation

[Martan]

Hi all,

My Total recall doesn't modulate. I get echoes and it biases properly. I get varying voltages in the modulation clock section, but no mod. The only voltage that seems odd is pin 1 IC9, whcih reads 0 when it should be varying.

Things I've checked/tried:
Verified resistor color codes in the mod section
swapped out the 4558 (IC9), 4047, and both BBDs for known working ones
Replaced the mod pot
checked the mod switch (works electrically but has no effect on circuit)
no continuity across caps in that section
compared resistor voltage drops with a working Total recall
Removed IC9 socket
diode seems to be electrically sound
reflowed all joints in that section
Cleaned back of board with flux remover

Looking for more ideas.

Thanks,

Marty

[Martan]

pic

[Martan]

pic

[thomasha]

Pin 1 is the output of one of the OpAmps. Check if it's grounded, or if there is a cap with wrong orientation somewhere nearby.

Interesting that the signal varies at pin 7. How strong is it? Around which value is it oscillating?

I once had some weird behavior with weak LFO but it was a problem with the supply pins. One of them was not properly connected and the bias of the OpAmp was completely wrong, so that it was hitting the rail.

[Martan]

Hi Thomas,

Pin 1 isn't grounded, but does read 0V. It does this on my Total Recall taht works as well. Pin 7 fluctuates between -6.9 to -8.1V.

I keep getting a weird reading on pin 3 of the mod pot. Mainly its 0 but if I press harder I get -7ish

[danfrank]

Sounds like a bad mod pot. See if the TR modulates when you press down on the pot, like when it would read 7v

[Martan]

Thanks Dan,

I switched out the mod pot and still no luck. I can hear the tiniest hint of mod if I let things ring for a while. I get no mod at the delay time pot. Shouldn't this be where it has its effect on the sound? maybe that cap network there is my problem.

[Martan]

The problem evolves. I replaced all of the caps around the delay pot, and now I can tell that the mod circuit is working, but there is very loud constant audio from that area. If I try to bias pin three on IC six, it is an incredibly loud solid audio tone. The pitch is controlled by the mod pot. If I even get the audio probe near those caps, I can pick up the audio. I've since replace those caps with another round of new ones, and replaced ICs 8 and 9 but the problem remains. Any thoughts?

[thomasha]

That seems a though one.
It's almost like a short of the LFO to the audio path or something. Could you post some pictures of your build, that could help spot some weird solder points or maybe an inverted cap?

[Martan]

I wonder if I have a wrong Cap value in there that is bringing the clock down into audible noise. Will check in the morning

[Martan]

OK, so I traced down the siren to the delay pot:

edited with new info:
Replaced that and it's quiet. Now my 4047 pin 1 reads 0V and pins 2 and 10 read -15.2. Voltages seem fine on the 4047 socket if the IC is removed. So does the delay pot (-15.2V). I get delay on pin 3 of the bbd but it is very weak. Have to find the next issue