Non-Mutable Instrument repair: 7805 voltage regulator (Solved)

Hello team! I have another question that in no way pertains to this forum, beyond the fact that all synths use electronic components. You’re the most knowledgeable and friendly group I know, so I thought I’d start here. The question:

I have a piece of gear that only sort of powers on, and only sort of works. I flip the switch, and none of the lights come on, but sound will pass through the unit. This led me to believe that it’s just not getting enough juice. I’ve measured the voltage regulators, and the 7805a is receiving approximately 13 Volts, but only outputting less than 2 Volts. If I’m not mistaken, shouldn’t that number be closer to um… 5 Volts? Is this my problem component?

Thanks again for all of your help!

Whats that for a piece of gear? Theres a number of reasons you only have 2 Volts at the Output; bad Regulator, Short somewhere, relative Moon Humidity exceeds Maya Monthly Index etc……

It’s an Electrix Filter Factory. Service manuals are nowhere to be found, so I’m sort of flying blind on this one.

Have you measured resistance between GND and +5V Rail?

13 Volts in and having to regulate that down to 5V means that the regulator will produce heat. Lots of it. Does this thing run off an external transformer/PSU? If so, are you sure it’s the correct one?

What you could do is to try to run it off a bench/lab PSU if you have one. Set the current limit quite low. As long as it doesn’t hit the roof at least there’s no internal short. After that it could be interesting to try to swap the regulator. It’s possible that there are more regulators since it passes sound. This could be indicative that the logic board/components don’t get juice, but maybe the audio parts are seeing something like +/- 9V or similar.

Frank- No. I’m not entirely convinced I know how… noob shame

Jojjelito- the three voltage regulators are all connected to a singular heatsink. The PSU is internal, coming from a 3-pin IEC, a fuse, and the on/off switch. The outputs on the PSU connect to the main board via a five-pin molex type connector. The values I get seem reasonable, but with no service manual, I have no way of knowing what they should be. The other two voltage regulators seem to working fine.

What’s interesting is that one time you’ll switch it on and the audio passing through will be quite low in volume, and the next time it will be a bit louder, and a third time there will be nothing. The guy I bought it from said he hadn’t used it in about five years, and as he was testing it before I purchased it, it crapped out on him. Today when I went to get it, the unit actually did turn on, if only briefly. I have not been able to replicate that beyond just the audio passthrough.

Just measure any point thats known to be GND and any point thats +5V. You could try directly on the 7805:

is the 7805 getting hot?

Okay. Results from measuring resistance between ground and +5 rail: 0.848k Ohms.

That’s much too low, it would mean that at 5V the 7805 would try to output almost 6A, which is way over its rating. Time to go short-hunting… My first suspects would be any electrolytic caps in the power supply section (which have a tendency to crap out over time). Are you able to post a good hi-res pic of that part of the circuit?

Okay. We’re back after replacing capacitors. No change. 7805 is still outputting under 2V. Replace 7805?

Edit: I haven’t checked the heat coming off of the 7805. Do need to do a scientific measurement, or is a “yeah, that’s putting off heat” hand proximity test sufficient?

Heat is when you don’t want to touch it anymore….

lol. I will not be using my digits as probes.

It only hurts for a second. Man up! :slight_smile:

If you skin keeps sticking to the 7805 its definitive to hot.

Okay. We’re back with some temp readings.
7805- 102F (39C)
7808- 120F (49C)
7908- 120F (49C)

I’m not sure what to make of these readings.

IS this after some time running? If so, nothing unusual…. id suggest just swapping the suspicious 7805 - its a 30ct item.

Fixed! I replaced the 7805 and it fired right up.

Thank you, MI crew, for your help!