Moog Memorymoog troubleshooting and overhaul thread

[WhinyLittleRunt]

So I've already been digging into my Memorymoog and think I've found the source of my initial problem, which is that the synth freezes / locks up after about 4-5 minutes or so, sometimes less.

I think the power supply is definitely the culprit here- been doing some tests per the service manual and see that my 5v rail is draining. So by the time it drops to like 2-3 volts everything locks up. I made sure to test the power supply pins also with the connector off as it says to see if voltage exists there (pin 12 and 14) and I get nothing. So reading through the power supply notes it lists the potential culprit parts.

Knowing it's silly to repair without a recap I will be doing that as well.

The supply uses LM723N voltage regulators. Obsolete part. I found 1 in a pile of old ICs, but I don't know if it's good. Is there a way to test these (or any IC for that matter) with a scope out of circuit to see if it's working internally? I don't want to just start part-swapping with this synth, not at this caliber.




Also, it lists "P3" in the troubleshooting steps. I don't know what "P3" refers to. The connectors are P13x.

[WhinyLittleRunt]

So it looks as though my 5v rail is no good. Trying to follow the service manual steps is not going so well; I read that the pinout on the PS is different for the Memorymoog from the Polymoog? I guess at any rate I was supposed to measure between pins 12 and 14 for the 5v but that was giving me 0v. So I read it as pins 2 and 4 instead and am showing 16v...

I plan on re-capping the supply. I'm using an infrared thermometer on the supply when it fails and see that the 3300uf cap on the left side is getting very warm, like 105 degrees, when it's failing. I can't seem to pinpoint it though directly to the cap, and it could be diodes too I suppose. Things feel warm to the touch but not insanely hot.

[WhinyLittleRunt]

So doing some measurements it looks like I'm getting 12v on the unregulated 5v line at the bridge. Measuring across the diodes gives me this. I tested to make sure the transformer was putting out the 5v AC on that line first.

I'm also getting somewhere between 7 and 12v at every point up to the two power transistors, TIP41 (Q5 and Q6). I had already removed the sockets from the supply and soldered on some extensions to get them mounted and soldered directly into the supply for better connection. After those, I'm getting nothing else in the 5v circuit. The voltage regulator is getting nothing, the resistors get nothing... I've put a few circles around where I feel like the problem area is.

What's interesting to me though, is on Q5 and 6 as opposed to 1-2 and 3-4, the Collector pin is showing me 12v on both, whereas the 'working' lines are giving me my +/- 15 on the base and emitter. Could that indicate that they're bad? I'm just curious if I'm on the right track with my troubleshooting. The 4700uf cap is measuring correctly across and this has not been recapped yet.

[WhinyLittleRunt]

ALL IS WELL with the Memorymoog power supply. In my closer investigation I was able to see that the solder joint at one of the shunt resistors was cracked which bugs me to death that I didn't see this initially with the supply sitting on the bench under a microscope.

Decided to test it and see if that was all it was and THAT WAS ALL IT WAS !!

I'm now left the synth on for at least an hour and a half, hit the auto-tune somewhat regularly, and it's been keeping very good tune, all 6 voices are tuning up. Controls on the panel all seem to work with no problems... this is very cool!

EDIT: after being left on with the hood up for the 1.5 hours I see now that 2 voices are tuning. Everything is still sounding and playing but only 2 are working with the autotune program. So it looks like I'm likely going to have to replace trimpots and calibrate the voice cards... but that seems to be standard practice on these I think.

[madtheory]

Awesome! You’ve a top class beast there.

[WhinyLittleRunt]

Well, I guess I've gone and done it now....

Seems to always happen; get a good thing going, and then throw the wrench in.

I was starting the full calibration earlier this evening and I slipped my DVM lead clip right in the beginning trying to measure the 15/5 rails and sparked. Then the synth went nuts like it did when the power supply was bad... So, probably foolishly, I spent a couple hours undoing all the cables and whatnot to get the DMUX board out so I can see if I potentially did any damage under the board (or shorted something). While the board was out I did a re-cap of those 4 10uf caps.

Fortunately, when everything went back together I got bootup/lights but no output. Touching the power supply and wiggling it back and forth will cause the entire synth to short and reboot. I tried measuring my 5v rail again and now have an unadjustable 9v coming into the board... is it plausible that I somehow 'back blew out'' the power supply when I shorted that rail? Or at least did some damage to it?

I did recap the power supply the other night which worked out fine, but like a dummy I put the transistor sockets back on the board with new TIP41s and thermal paste/mica pads. I wonder if they are flexing and causing that issue when the supply is touched / bent. This also means I cannot close the lid because everything pushes into the supply and it cuts out.

Any ideas where I can go from here? Aside from taking out the power supply and inspecting it?