Micromoog basket case

[armillary]

Here's an update on the basket case Micromoog my son brought home (in pieces). The first problem was that the power supply board was fried. I obtained parts and rebuilt it. Now we have solid +15 and -15 volts. Here are before and after pictures:
http://home.comcast.net/~armillary/Micr ... CN4200.JPG
http://home.comcast.net/~armillary/Micr ... CN4219.JPG

Using the service manual, I hooked up everything and set the user controls to the service settings. Powering up, I get no power LED, no sound and the power supply gets warm quickly (I admit it has larger fuses than it should.) The service manual says "-15V shorted out somewhere." If I test with the main board removed, the power supply stays cool, and if I then jumper the tone wheel module with +15 and -15V the power LED lights. So I'm pretty sure the service manual is correct. I can replace the electrolytics on the main board, and maybe that will fix it. But what I'd like is any advice/suggestions on a method for tracing a short. It's not something I've done before and I'd like to learn. Thanks in advance for your comments.

[MC]

Some things can't be fixed over the internet. Take it to a tech.

[thealien666]

Components that are known to short out over time, sometimes, are Tantalum capacitors. I don't have the schematics in front of me, but there might be some of them on that main board and around the synth.

As for tracing shorts, it is usually difficult to trace, unless there are some signs of localized overheating or visible soldering bridges. One can try and remove suspected parts, and check, with an ohm meter, for absence of shorted out power lines at PSU connector. A very hot to the touch (burning fingers if left on it for more than a fraction of a second) IC will also give a possible indication that it's defective and shorted out, as well as transistors and diodes. (edit: But as Kevin wisely mentions below, using another lab type PSU rather then the original one is always best)

It could also be a short caused by the board mounts, or the external connectors.

Whatever you do, don't plug in your freshly rebuilt PSU until you localize the short. Otherwise you could fry it yet again and possibly do even more damage to other parts of the synth.

If you don't feel too comfortable with starting such an endeavour as to localize a short, which can be a pain and require lots of patience and determination, you could always follow MC's suggestion to take it to a qualified tech...

[Kevin Lightner]

One method I use to locate shorts is to use a digital ohm meter that can read sub-ohms (less than one ohm.)
Some meters have a lo-ohm setting to facilitate this.
You don't need (and shouldn't have) any power on to the synth or supply.

In your example, I would (solidly) hook one probe to the power supply ground and then probe various areas of the board known for -15 volts.
Pin 4 of most opamps, for example.

As you move around the board taking readings, look for the lowest ohm reading you can find.
This can localize the area where a shorted component is in.
Removal or replacement of suspected components can then be done to verify.

While I don't recommend using the stock power supply to look for warm or hot chips, a lab type of supply may be used for this purpose because you can regulate how much current is being drawn.
This avoids the stock power supply from dying and components from blowing up.

[armillary]

Great information. Thank you very much. I have an ohmmeter with a sub ohm range, and a regulated DC power supply with controls for voltage and current. Now I have some ideas to put them to use. Thanks again.

[armillary]

So with what you've taught me, I've removed the main board from the wiring harness completely and am measuring resistances on the main board, starting with plug P1 which is where the +15/Gnd/-15 would plug in from the power supply. From +15 to ground measures about 1.3K ohms. From -15 to ground also measures about 1.3K ohms. From +15 to -15 measures 8 ohms. I take it my short is between +15 and -15, correct?

[Voltor07]

I would say you have a short between +15 and -15V, yes...however, I can not be certain, as I do not have the baseline specs in front of me, and have no experience repairing Micromoogs. 8 Ohms seems to be abnormal, though.

[Kevin Lightner]

With what you've told me, it might be something like an opamp or 3080 (OTA.)
These chips usually have both + and - power hooked up, but no ground.
If it's a rail to rail short, you can still use the same technique to narrow down the short.
While a short might still be located elsewhere, you might consider reading the resistance across each opamp or OTA.
The one reading the lowest may be shorted.
Unhook the PSU completely also. It's not needed here and has diodes across the rails.
Make sure you identify the chip before testing it too as dual and single opamps have different + pins usually.
Single=pin 7, dual=pin 8.

[armillary]

Thanks Kevin. From your earlier post, I began checking the resistance between pins 4 and 8 on the MC1458CP op amps. One was 8.3 ohms, the others in the area were 8.4, and further away they were 8.6 and 8.7. So I removed the one with the lowest value (IC204), put in a chip socket and a new 1458 and no more short! With expert advice it was easy!

I haven't powered it up yet though. I think when I do it will be with the lab power supply set at 125 ma.

[armillary]

Another update on the Micromoog. I hooked the main board up to the wiring harness and powered it up with the rebuilt power supply board. The power LED came on, the power transistors stayed cool, and no blown fuses (using 1/4 amp quick blow because 1/8 slow blow are hard to find. It had 1A in it when I got it). I get no sound at the output, but the oscillator is making a good sawtooth and the waveshaper output works, so it's somewhere down the line or maybe the keyboard isn't triggering. The service manual is great and I'll get to the bottom of the problem. Thanks again for your help!

[armillary]

An update on the Micromoog basket case. Before I rebuilt the PSB, I measured the output. The +15 V side measured 2 volts, and the -15 V side measured -32 volts. Pretty high for ICs, I guess. Since rebuilding the PSB and replacing the electrolytics, most sound/VCF/tuning problems have been resolved by replacing the various 3080 op amps and the 1458 dual op amps. That and the 3046 arrays for the summer circuit and VCF. At this point, there's pretty much no chip but the noise chip that I haven't removed, chip socketed and replaced with a new one. It tunes up and sounds great, except for the one problem that it's had ever since I got it working again: the hanging note. It always plays the last note pressed. Power it up, and it's playing a note. Every note on the keyboard works, but there is no note release, ever. I've studied the keyboard trigger circuit. and pin 7 of IC202B is indeed putting out +13.5 volts even without pressing on a key. So that's one problem. But if I remove IC203, so that IC202 output drops to -13.5 volts, the trigger is off, but a note still plays. (And the trigger is off on IC204, the S-trigger circuit too.) Even with the keyboard trigger circuit at -13.5 volts, a note is still playing. Why? I see the trigger voltage goes to the Source selector switch, and things get very complicated after that. So here's my dumb question. Where does the key trigger voltage go, and what does it do? (I've been reading Delton Horn's manual of design and construction, but the Micromoog keyboard triggers seem to be more complicated than he could envision...) I'm trying, but would love some education. Thanks for your comments.

[armillary]

Sorry, by PSB I meant power supply board, aka power supply unit (PSU).

[thealien666]

I would hazard a guess that the trigger signal goes to the envelope (contour) generator circuit maybe ?
Maybe the 556 timer in that section doesn't start, and is busted ?

[armillary]

Thanks Alain, I remembered the Micromoog block diagram on page 6-3 and it looks like you are right. The Dual Contour Generator. Thanks! I replaced that 556, but I'll check in the area again. Maybe diodes 703 and 704.

[thealien666]

Can you have any kind of attack and decay time before the sustain stage is reached (when powering up the synth with attack level to maximum)?

If not, is switch 601 (contour bypass) in the "ON" position ?

If not, then it could be IC601 (3080) that is maybe stuck open... ?

Or R606 trimpot (VCA balance) that maybe needs recalibration ?