Replacing +3 PSU - recommendations (2024)

So, the +3 I just collected has a blown F2 fuse (T1.25A) in the PSU - I've ordered some spares.
I checked the voltages and it was only producing -12V - will that fuse knock out both +12V and +5V?

Is it worth looking at building a new PSU? Are the Amstrad supplies likely to fail in a dangerous way?
I know the Meanwell PT-45B has all the required voltages and fits neatly in the case - has anyone got any experience using that one, or would you recommend something else?

Unless the transformer has burnt out it shouldn't be hard to fix. The thing you have to be careful of is the PCBs are pretty terrible quality and the traces tend to crack and need building up with wire.

The +5V regulation circuit is powered from the +12V rail, so losing that would kill 5V too. The bigger question is why the fuse blew in the first place...

My rubbish website including the redrawn Amstrad schematics and the Sinclair Wiki.

http://YouTube.com/specbasstuff

p13z wrote: ↑Sun Oct 06, 2024 11:53 amComponents in mine appear bombproof

Dunno about bombproof, it's quite easy to kill the transistors by shorting the output if you mess about with the PSU powered on, but they're simple replacements.

My rubbish website including the redrawn Amstrad schematics and the Sinclair Wiki.

The Amstrad PSUs do not like the DIN plug being disconnected or reconnected while they are powered up.

Usually ether the driver transistor (T2) has gone short circuit or the main output transistor (T1) has gone short circuit.

But F2 is the fuse for the +12V supply, so the 7812 voltage regulator may have died. With no +12V supply, as Guesser says, there will be no +5V supply.

This web site has the technical details of the +3 PSU

As to a replacement, any modern SMPSU (switch mode power supply unit) that can provide a regulated (within +/-3%) +5V with an output current equal to or exceeding the Amstrad +3 PSU will power the ZX Spectrum +3 up. The +12V is used for some of the analogue video circuitry, the disk drive and the RS232/MIDI and AUX ports. The -12V is used by only by the RS232/MIDI and AUX ports.

If the SMPSU has current ratings significantly greater than the rating of the +3 PSU, it's wise to include suitability rated fuses to protect the ZX Spectrum +3 PCB just in case of a short circuit.

Mark

Thanks, all.

I'll see how it is after replacing the fuse, then.

Is the fuse blackened? Is the glass discoloured? Or has the fuse wire just melted in the middle?

You may want to do some resistance tests and diode/semiconductor junction tests (with it disconnected from the mains).

Then after replacing the fuse, test the voltages before attempting to power up the computer. I keep a spare DIN socket for this, as trying to hold meter probes on the DIN plug pins is far too fiddly.

Mark

Fuse has broken cleanly in the middle.

I'll try checking the semiconductors, and I'll definitely check the voltages before plugging it in to the computer.

Likely the fuse overheated "slowly" (although maybe still fast for the human eye) then rather than a violent break due to a dead short circuit.

But with low voltage circuits it can be difficult to tell for certain.

Mark

I checked the voltage regulators and they seemed OK - no shorts.
Replaced the fuse and the voltages are now all there and look good.

It's alive, and all keys working.
I've not tested the disk drive yet as I had it out examiningthe belt - it looks OK, but probably isn't.
Now I know the computer is working, I'll order a replacement.

It's an 70830 issue 2 motherboard, so could do with an audio mod, and it would be nice to put the audio jack back on and remove the trailing audio cable.