Moog Music Forum

One last question regarding my minimoog restoration:
Some tech before me replaced the tempco resistors on the old oscillator board with regular resistors. I want to correct that. But they are hard to find.

I can get them at the original 3300ppm but with a power rating of only 1/6w - the service manual says 1w though. Would they still work?
Alternatively i can get the with 1w rating at 3500ppm…

Thanks
Jakob

Salomonander wrote: Sun May 04, 2025 11:14 am One last question regarding my minimoog restoration:
Some tech before me replaced the tempco resistors on the old oscillator board with regular resistors. I want to correct that. But they are hard to find.

I can get them at the original 3300ppm but with a power rating of only 1/6w - the service manual says 1w though. Would they still work?
Alternatively i can get the with 1w rating at 3500ppm…

Thanks
Jakob

If the specs call out for a 1W resistor, no a 1/6W resistor will not work. It will likely fail in time, it not immediately. Though I am surprised to see a 1W resistor in the oscillator section. Is this part of the oscillator circuit, or just used for a power supply? I'm not familiar with the minimoog oscillator circuit.

For 3500ppm vs. 3300ppm, yes this could make a difference. The error is compounded with temperature change. For example, a 10 deg C temp rise results in 0.21% error in the resistor value (3300 vs. 3500 ppm). For 20 deg C, that error is 0.43%, due to the 200ppm difference between 3300 and 3500. Imagine this as the difference between using a 0.1% resistor vs. a 0.5% resistor. Both are very accurate but the resistance error is a moving target due to temperature changes.

If the synth is used in a controlled temp environment, it probably doesn't matter which is perhaps why the previous tech replaced them with regular resistors. Once warmed up, it should remain consistent in a controlled environment. But gigging outdoors as the sun sweeps across the stage might be a different story.

Elby Designs has a PDF document that explains solving this issue using a second resistor in series with the tempco resistor. See this PDF:

https://www.elby-designs.com/webtek/dat ... tempco.pdf

For example, using a 1K, 3500ppm resistor, they put a 47ohm 50ppm resistor in series and they get 3345ppm. In case the above link disappears, here is the equation:

Desired PPM = ( (R1 * TCR1) + (R2 * TCR2) ) / (R1 + R2)

Where R1 and TCR1 are the 3500ppm resistor (e.g. 1K ohms, 3500ppm) and R2, TCR2 are 47 ohms and 50ppm.

For a 2K ohm 3500ppm resistor, a 91 ohm, 50ppm resistor is needed and you get 3340ppm. Getting exactly 3300ppm depends on the available 50ppm resistance values.

Because most the voltage is dropped across the larger (e.g. 1K or 2K) resistor, the wattage for the 47 or 92 ohm resistor is very small (e.g. of the order of 5mW). So a 1/8W or 1/4W axial lead resistor is more than enough.

Thanks a lot!
That was very informative. I will go for the 3w 3500ppm. I did read at some places now that the 1/6w might actually work… but its TCR has a tolerance of 5% while the 3w is 1%. I guess i will have a more uniform result across the three oscillators.

One more thing: on all pictures i could find, the tempco resistor’s legs are bent in a very specific way. See picture. I wonder if this actually serves a purpose? Do i need to bend them in a similar fashion?

And maybe you could help me solve the last issue:
I do have some osci bleed, even with all mix switches turned off. I did the vca calibration, but i can still hear it.

In another post i found this:

“The VCA is opened and closed by the current sink Q18 (Minimoog service manual). If there is an audio leakage in your device I would check the base voltage of the equivalent of that transistor in your device first (contour input). If it is not zero, the contour control (envelope?) does not go to zero and therefore does not close the VCA totally and the audio material is not attenuated correspondingly”

Is this correct? Because my q18 base measures -0.78v. I assume this causes the issue? Any ideas what the culprit could be?

The Kelvin force/sense lines from the power supply terminate at the oscillator board. If the board connector flag(s) on any of these lines are open, the power supply malfunctions and causes oscillator bleed in the audio.

You can do a quick test by applying pressure on the oscillator circuit board close to the connector at the location of these lines. Applying pressure may make temporary contact with the faulty connector flag.

The service manual suggests shorting the force/sense lines at the connector, but this defeats an important function of the power supply.

If this is the case, seek the services of a competent tech. Repairing those connector flags requires an expert as they can break easy, and there are no direct replacements.

Salomonander wrote: Tue May 06, 2025 8:37 am One more thing: on all pictures i could find, the tempco resistor’s legs are bent in a very specific way. See picture. I wonder if this actually serves a purpose? Do i need to bend them in a similar fashion?

They are bent like that when the circuit boards were put through a solder flow machine. The bent leads add length to the leads to protect the tempco resistors from excess heat while in the solder flow machine.

If you put a heat clamp on the leads while you solder by hand, you won't need to bend the leads. A heat clamp absorbs heat from the leads.

My Minimoog doesn't have the legs bent like that because it is an early model before they used solder flow machines

Thanks a lot MC! Highly appreciated

And another question: how do people in the eu wire up their power light?
Its 125v and obviously we are running 230v. On my unit there is a 2k resistor and another part that i can not even identify in series (can you?). Is this legit? Does not look like this was done at the factory and the hot lines are not even isolated.

And in case anyone is interested, i was able to get the bleed down a lot by soldering all sense lines on the card edge connectors. These were crimped only and adding additional solder helped.

So i thought i was done. All happy. Until i realized that there is another issue goddammit! I dont know if this has always been the case or if i messed something up, but my keyboard isnt working properly anymore.
When i hold a key and trigger another one, the new note only plays when its a lower key than the one that i was holding before. So going down works as expected. The new note kills the one before. But hitting a higher key doesnt work. The new note only plays when i let go of the one i was holding. I can measure this at pin 2 of the keyboard connector (the one sitting below the left hand controller). Voltage only changes when i go down. On the way up it only changes note when i let go of the old one. Now i have taken out all boards except for the psu and the behavior stays the same. Any ideas? Does this tell me that the error sits somewhere in the basic connections of the keyboard?

Doing some more reading…. Looks like this is normal behavior? Only the reissue is capable of being set to last note priority?

This is called "low note priority" meaning the pitch of a monophonic synth is determined by the lowest note pressed, and is normal in the Minimoog.

Thanks MC,
yes, i was chasing a ghost. Guess thats what happens if a studio tech works on a synth i think all is well now. I do struggle a little with tuning on the range trim pots. They are super sensitive and its almost impossible to get the 3520hz… is this normal behavior? Just wondering as the scale trim pots feel much easier to set.
Thanks

I can barely get this thing tuned… i use a frequency counter and get the pitch very close. But i cannot adjust it perfectly due to the rough trimmers.
So im not capable of getting zero beat. Any adjustment is too much. Whenever i mix signals i get some phasing. Hmmmm…. Does it just never get perfect?
Or should i think about replacing those trimmers with modern multi turn ones?
There are kits for this. Im just worried that once they are installed, one will be never able to tune it with the back cover attached. Which, according to the manual is mandatory. What do you guys say?