SQ80 Overheating

So the power supply on my SQ80 has been overheating. After a while the whole keyboard will lock up with no response to either the keys or panel buttons/sliders. If I restart it, it will work for a short time before overheating again, and the overall time it takes to lock up depends on the ambient air temperature. During the summer it takes about twenty minutes, during the winter it rarely happens.

I already intend to replace the original EL caps on the PSU so that the voltage regulators don’t have to work as hard, but I was also considering replacing the regulators themselves as well as the thermal compound between them and the heat sink. Has anyone else tried this on the SQ80 or another board? Will something like arctic silver or mx-4 CPU thermal compound work? Any other suggestions for overheating issues?

I’ve considered attaching some small fans to the heat sink as well, but that’s a last-resort scenario.

If you can borrow a thermal imaging camera (highly recommended for circuit diagnosis) then you can see what is causing all the heat.

Linear voltage regulators emit excess input voltage as heat, so it may be they’re getting too much input voltage. The thermal compound may be on too think or have dried out, but I don’t think that’s the issue.

You can replace the regulators for switch mode regulators, these don’t emit heat but you have to some at high switching rate to avoid noise. Probably not recommended for a synth with so much analogue circuity.

That’s a cool idea, to use a thermal imaging camera!

It’s a common approach for spotting bad RAM chips on old 8-bit micros. In a row of identical chips the bad one usually glows.

I imagine it would work on analogue synths with rows of identical voice chips or transistors.

A low tech option is to use an IR thermometer.

Thanks for the advice! Unfortunately, the low-tech option may be what I have to go to for the time being, short of just outright replacing everything. I can’t think of anyone in my area that would even HAVE an IR camera, let alone would let me borrow or even rent it.

Camera is about £300 to buy, depending on model. But an IR thermometer will give you a temp reading without making contact. Those are £10 tops.

is your 5v rail getting pulled low after a while? say to 4.5v - foldback?
On some power supplies Ensoniq used a kludge with two (or a single) resistor to squeak some more current out of the 5v regulator.

I’m testing a Traco SMPS (switch-mode) 5v 2a and will let you know. It doesn’t need a heatsink, its more efficient and it won’t cause additional noise as you retain the 12v +/- for the audio rails, CEM chips etc.

hi,
Traco SMPS TSR 2-2450 works and you can remove those shunt resistors… Runs far cooler and it is more stable.

I don’t have an SQ-80 power supply to look at right now, but to clarify, you are replacing the 7805 regulator with this TSR 2-2450, and then simply removing some resistors from the PCB? Which specific resistors are you removing?

Thanks for the tip!

hi Sam,

  • yes i am replacing 7805 with a Switch Mode ‘tiny’ voltage regulator substitute - of the correct size. You also remove the screw that bolts the 7805 to the heatsink as this is not required
  • depending on the SQ80 power supply there will be two 51R resistors in parallel or 101R across, where there markings for two 51r resistors. On the PCb they are marked as R1 and R2. usually, by this point the resistors may look slightly charred.

The TSR-2-2450 puts out lots of amperes so you do not need the shunt design Ensoniq used as a kludge to extract more amperes out of the 7805 and shift the extra heat away from the 7805 package.

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