Minimoog D 'Emphasis' fault.

[ClockworkRobot]

Hi, so the Emphasis knob on my model D no longer works.. it has no effect.
I pulled out board 4 and checked the knob was functioning and had continuity to the PCB socket. No problems. Also checked my +10 and -10 volts - ok. There's also a line on the filter board marked +15v.. it appeared to have something more like 20v on it
Most of the electrolytic caps on the board have been replaced and i can't find any obvious continuity problems so can anyone suggest what i should look at next?
Thanks.

[thealien666]

Have you checked the regen cal trimpot for proper operation (R73) ?

[ClockworkRobot]

Yeh tried that. (is that R609 on the schematic?)).

[thealien666]

R73 is the trimpot.

The resonance (emphasis) circuit is rather simple. The output of the ladder filter, from the output buffer, is sent back to the ladder filter input through the emphasis knob of the front panel. If the filter operates properly, but doesn't resonate, the only things that could prevent that signal from reaching the ladder filter could be C10 (10mf), or R73 (1K trimpot), R14 (emphasis pot on the front panel), poor electrical contact on edge connector for board #4 (pins 6 and 7), R76 (330 ohms resistor). I repeat that this is assuming that the filter operates properly, but doesn't resonate when the emphasis knob is turned up.

I would clean the edge contacts first, then check for possibly broken wires leading to, and coming from, R14 (emphasis pot on the front panel) to the bottom socket connector for board #4.

[ClockworkRobot]

i must be looking at a sightly different schematic to you as the numbers you're using don't match:
http://www.fantasyjackpalance.com/fjp/s ... ics-04.gif
But I think i follow you all the same.
I'm pretty sure it's a problem on the PCB as i've checked and re-checked the wiring, socket and cp pot. I've even used a crocodile clip to connect pins 6 and 7 directly on the board.
At the moment my moog plays although the sound is very quiet.

Attack, Decay and Sustain work although those pots are very poor so it's a bit jumpy.

The cut off frequency does little except make the sound fade from low to nothing as you tun it left to right.
Similarly the Amount Of Control knob just makes the sound grow more quiet.
Emphasis knob does absolutely nothing.

So i wonder if the problem is in the filter circuit its self all those transistors.

[thealien666]

First, you're looking at very early version of the schematics for the Minimoog filter only (with different parts numbering). Board 4 actually contains the filter, the VCA, the overload lamp driver, and the A440 reference.
Here are the right schematics for board #4 inside the Mini:
http://www.fantasyjackpalance.com/fjp/s ... -schem.gif

Second, what I wrote before was assuming that only the resonance wasn't working properly. But from what you describe, the filter itself seems to not behave normally. And maybe some other parts of the synth, too ?

I would strongly suggest taking it to a qualified tech that could have a look at it, as it seems to have more serious problems than just emphasis not working...

Good luck.

[ClockworkRobot]

Thanks. didn't really want to go looking for someone else to have a shot at it but maybe that's what it needs.
I did consider buying one of those repro '4' PCBs and if it worked, using it to compare with my one.. then hopefully selling it on if i find the problem.