Micromoog Emphasis & tone not working?

[rekem1000]

Hi, I have a micromoog that goes dead when switched to tone mode, also the emphasis is not doing anything.

looking at the schematic I changed the transistor array ic 502 but no luck, everything else seems to test ok on the scope but i can't find the problem. any help or guidance would be much appreciated.

thanks

[Kevin Lightner]

Make sure the switch is ok. It has many contacts.

Past that, Q512 and Q513, the emphasis trims, PCB cracks or connector problems might be the culprit(s.)

Good luck,
Kevin

[rekem1000]

Thanks Kevin!
Tone now passes signal after cleaning the switch but still no resonance.
Unfortunately all those other parts checked out ok, it might be a PCB crack as it's hard to tell with the green foil flaking away as it does. In your experience can the opamp be the culprit, i was thinking the next one down the circuit, ic 601 LM3080. I don't have any replacements for that so can't change it now but I can't see any signal getting to it on pin 2 or 7 so I guess the problem is probably somewhere else.

[Kevin Lightner]

IC601 is an OTA. It's not a standard opamp.
It's part of the VCA.
If it didn't work, you probably wouldn't hear anything.
A 3080 forms a VCA by using an inverted voltage to its pin 5.

You might instead considering checking out IC501, also a 3080 OTA.
Btw, 3080s are available if you look around, but many parts sellers no longer carry them.
Most manufacturers no longer make them.
I have some in both round and DIP styles if you need one or two tho.

How did you verify those transistors were good?
They're 2N3392 NPN transistors, fwiw.
You might also consider testing the resonance pot.
This can be done with either a basic DMM meter or even just shorting it using jumper clips.
I always suspect parts that move first before I suspect ICs.
Connectors and their wiring also.
ICs DO fail, but more often there are physical problems instead.
The traces don't break easily on Micros, but they can crack where pots, switches and connectors are soldered in.

I'd check or swap those trannys, make certain all contacts on that filter switch are good, verify that trims 525 and 526 are ok and then lastly try testing or replacing chips.
Past that, all I can suggest is taking it to a tech or tracing things using a scope.
However I don't know where you are, so I'm not sure who might be able help in your area.
I am also ok with people in or around my area coming here and doing the work themselves in a shop well equipped with both parts and tools.
You could then do the job yourself and would have a tech (me) available to help out should you have a question or get stuck.
Best I can offer, but I'm in the Californa high desert mountains. Not so close to anybody.

K

PS: Some people may be shocked to find that one of the most popular component sellers, Mouser.com, no longer carries the common 741 opamp. This is the opamp used in most Minimoogs and sooo many other synths.
One can still find them around, but many of these parts are becoming more and more scarce as time goes on.

[MC]

Feh, no one cried when LM301s became obsolete. Plenty of pin compatible 741 replacements out there. The one exception I ran into was the 3046-based minimoog card, there was one circuit where only the 741 would work. Back then they designed around the output stage of the substrate so it was also dependent on which maker of 741s.

However with everything going to SMT it is getting harder to find DIP parts.

[Kevin Lightner]

Feh, no one cried when LM301s became obsolete. Plenty of pin compatible 741 replacements out there. The one exception I ran into was the 3046-based minimoog card, there was one circuit where only the 741 would work. Back then they designed around the output stage of the substrate so it was also dependent on which maker of 741s.

My experiences differ.
I've never found any make of 741 that doesn't work in a Mini.
TI, NS, SGS, Kyocera, Fairchild, etc, etc.
Every make of 741 I've ever tried has worked fine in Minis.

However, one cannot replace all 741s in a Mini with other opamp types.
There are 5 locations on the 3046-based Mini that can be replaced with better opamps.
My favorite is the LT1006. But one can't put better opamps like this in every 741 location on the Mini.
I've found that 3 locations must have 741s and nothing else.
But I've not found any make of 741s that didn't work in any location on the 3046-based Minis.

The Polymoog, however, doesn't appear to like any CD4046 make except Motorola.