Spectrum 128K +2 Power Supply Buzzing and over voltage

Question

I've seen a similar question to asked here but but I'm not sure it fully addresses the issue I'm having. I'm using a PSU the same as this one from the 128K +2

According to this site, this PSU should be compatible with my rubber key 48k (issue 4A). However when I turn on the PSU it makes a disconcerting buzzing noise - it sounds just like a tattooist's needle! The voltage of the dc jack measures around 12.5 volts on the multimeter. I've read that these PSUs are often overvoltage until a load is placed on them. When plugging this PSU into my rubber key speccy, checking the contacts around the DC input on the board gives a reading of between 11v-12v. I'm concerned this could damage the speccy? Opening the PSU to inspect it I can't see any leaking capacitors or any obvious signs of damage. Anyone have any ideas?

EDIT Here's a link to a video showing the buzzing PSU

Answer 1

Buzzing can have two basic reasons:

One (luckily the unlikely one) is electrical current jumping an air gap in the PSU causing intermittent shorts through an arc. It's unlikely you could have measured a stable output voltage in this case, but it's still worth checking carefully. This case is dangerous and a fire and electrical hazard!

The other (more likely one) is just a mechanical annoyance: The metal sheets or the copper windings (or some other parts) in the transformer have losened over time and start vibrating with the AC frequency. In this case, check for loose parts (espcially the PCB on top of the transformer) and try and fasten them. If this doesn't help, check the transformer. It's normally held together with long screws you might need to tighten. If that doesn't help either, you can try a generous spraying of the sheet packet and windings of the transformer with special isolating lacquer

.

This is not quaranteed to help, but might (glues the single components of the transformer together and stops the vibrations). Make sure you get a product that is certified as an isolating lacquer spray for electric components - don't use any old paint you might have!

With regards to the voltage:

The linear regulator in the ZX Spectrum can easily "eat" the 12.5Volts. No reason to bother, it will simply transform the rest into heat. If you do bother, however, you can replace the voltage regulator by a switching one, like a Traco

Answer 2

Welcome to the Retrocomputing Stack Exchange. First of all, if you have any reason at all to suspect the PSU, it's not advisable to connect it to the machine until further tests.

Buzzing could be just due to 50hz vibrations corresponding to the line frequency in the mounting or inside the transformer itself and is not necessarily critical, albeit annoying. You can maybe feel the vibrations if you put your hand on it. Maybe a mounting can be tightened, maybe there is nothing to do about it.

Buzzing can also be caused by arcing due to fx a loose connection on the primary side, which is worse (from a safety standpoint). The sound probably wont be as regular and you might be able to smell ozone in that case. Try to power it up outside its case (being careful not to touch anything on the primary side or to put it on something conductive) - it may help to locate the origin of the noise. Posting an image of the insides could assist people commenting on the issue as well.

I have not serviced this particular version of the Sinclair PSUs, but if they are like the ones I have they only contain the transformer, rectifier diodes and a smoothing capacitor. In that case an over voltage failure mode of the PSU is unlikely. If it contains active regulation, that's a failure point that could fail with over voltage - but as I recall, the regulation is ultimately done with a 7805 inside the Spectrum itself (that regulator can still fail with age, but that's another story as long as the focus is the PSU itself).

It is normal that a voltage measurement without the anticipated load is higher than the nominal voltage. 12-15V for a 9V supply sounds quite normal. But if you loaded it with say around 1A, 12V sounds a little on the high side - but the Spectrum's regulator should be able to deal with it. (Of course, the higher the voltage it has to dissipate the hotter it will get and the more strain is put on it and the shorter its lifespan may ultimately be.)

TL;DR - If there is no active regulation in the PSU, the voltage coming from it is likely fine. (That doesn't mean that it may not need service, fx degraded capacitors could lead to excessive ripple and noise/stability issues, but in general that it is not a condition that will damage the connected device.)