I’m checking voltages on a Micromoog after recapping the power supply. The power supply puts out a perfect -15 and +15V without the main board connected. With everything connected and working, the voltages read -15 and +14.86V. The Micromoog functions pretty well. I suspect something leaky somewhere, not an outright short, but I’m just a hobbyist who could use some advice on how to chase this down. (Please don’t say take it to a tech, I’m trying to learn something here.) Thanks in advance for your help!
14.86 V is probably within specifications and nothing to worry about, it’s only about 1% off from 15 V. If you see it drop by 5-10% then it is definitely time for some servicing.
If it gets worse then there are two possible scenarios for low voltages. First is the power supply is unable to supply enough current for the load (the rest of the circuitry). The other scenario is the power supply is functioning fine but the load is drawing more current than it is supposed to thus pulling down the power supply voltage. It can be tricky to determine which is the issue. I usually start with replacing electrolytic capacitors on the power supply and then measuring voltages again. Sometimes on the power supply the active circuitry like transistors or voltage regulators need replacement too. If none of that fixes the problem then I begin checking out the other circuit boards for failing capacitors or faulty semiconductors (ICs and transistors).
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That was manufactured in 1979. Has it been recapped? Have the voltage regulators been changed? I routinely swap PSU components on any device from the 70s, 80s, and now 90s. I’ve had to rebuild several now from the early 2000s. If there is one place to start on your learning adventure, this is it. Learn to identify obviously failing components. If one looks bad, replace all. Learn to remove components safely. Learn how to shop for suitable replacement components.
As stated above, it’s probably not sufficient sag to keep it from working correctly, or cause harm, but aging components pose other risks, both from leakage and running out of spec. I’ve had PSUs so out of spec that xformers were screaming out in pain, instantly resolved with new components.
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Hello,
if its been recapped, have a look a the main power doides (1n4004) and see if they all match on the multimeter diode setting.
There is also the resistors around the TIP voltage regulators. they should all be as rated, otherwise replace if they don’t meet their resistance rating.
The schematic copy I have is blurry, but there is also a 2K voltage adjust trimpot past the power regulator, it might be able to also bump up the voltage to to +15
cheers
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Thanks Travis, erica_austin and midnightvisions for your comments . It’s been 10 years since I recapped the 2 electrolytics in the power supply, but at that time I also replaced the PS transistors (a TIP29 and TIP30), all the PS diodes (qty 6 1N4004) and the 1468. Then I replaced the electrolytics on the mainboard. I socketed and replaced a bunch of ICs. That got it working, but it’s always had issues with oscillator stability and range. When I get frustrated with it I put it away for a year or two and work on more simple projects.
Just recently I found a great Micromoog schematic created from scans of the Service Manual foldouts by a gentleman named Dave. It’s been really helpful in tracing the control voltage inputs to the oscillator, which seems to be where the remaining bugs in my Micromoog are. Right now I’m focused on the Oscillator Octave Step, which bumps up the CV an octave at a time as you rotate the Octave rotary switch (section 4.2.1.4 in the Service Manual). The 2’ octave is perfect, but when I switch down to 16’ the tone is sharp, and there’s not enough adjustment in the OSC OCT trimpot to correct it.
Anyway, here’s the schematic that Dave posted: