ZX81 PSU repair

[jarweb]

Hi

I have a ZX81 PSU (UK 700) which has a fairly high output with no load, and this actually fluctuates between 14V to as high as 18V

When connected to a ZX81 it might work OK for a while but eventually the machine powers off, maybe up to half an hour or less.
When I check the voltage from the PSU it has dropped to around 2.5V. If I switch it off for a while it will usually go back to the previous value when switched back on.

I realise I could get a cheap modern PSU instead but wondering if anyone would know what the issue could be with this one ? Would be good to repair if it could be done cheaply. There aren't many components on the PCB but don't know the values and how available they are now. Probably not worth it if it's actually the transformer at fault I would imagine ?

Thanks

[1024MAK]

It's not unusual for the output to be between 11V and 16V when the PSU is not connected to a load (the ZX81). The output voltage should fall to between 10.5V and 12V approximately when an unexpanded ZX81 is connected.

The components inside are:

• Transformer,
• Thermal fuse (may be inside the transformer or on the PCB under it),
• A bridge rectifier made up of individual diodes (four for the types I've seen inside),
• One or two electrolytic capacitor(s)

I can't give exact details as Sinclair used a number of companies to supply these. And they vary slightly between different manufacturers.

The type number should be written on the diodes. And the value will be printed on the capacitor(s).

The biggest problem is normally a break in the low voltage cable. Either at/near the 3.5mm plug or where the cable exits the PSU case. The PVC insulation sometime keeps the broken copper strands so they touch some of the time.

If it's not the cable, it could also be a poor or dry solder joint. Especially for the transformer connections.

After that, the diodes and the capacitor are the most likely components to cause the problem you are describing and replacements are not very expensive.

Any of the 1N400X range of diodes are suitable (1N4001 through 1N4007).

Any replacement capacitor should have a voltage rating of 25V, not 16V (as typically found in these PSUs). And I recommend using a value of 4700μF. Modern capacitors are smaller than these early 1980s types, so shouldn't be too large.

Don't buy from a an online marketplace unless the trader is a known good one. The cheap diodes may not be reliable and they have thin leads which hinder the cooling of them. Capacitors may not be good quality or may not be the value that they are marked as.

Some examples of some UK700 9V 0.7A (700mA) PSUs:

• PCB marked "310005". Thermal fuse in transformer. Four 1N4001 diodes. Two 1000μF 16V capacitors.
• PCB marked "CARCO" "DPL82-20". Thermal fuse on PCB. Four 1N4001 diodes. Single 4700μF 16V capacitor.
• PCB marked "XITRAN LTD 8110 ISS 3" " TELEX 418469G". Thermal fuse on PCB. Four 1N4001 diodes. Single 2200μF 16V capacitor.

As you can see, there is a bit of variation.

They all are held together with three screws. But often double sided foam sticky pads have been used to pack out the space between the top of the transformer and the lid. These may make getting the lid off more tricky.

Mark

[Moggy]

Another to add to the list, Mark in case you don't have it.
( click to magnify.)

[jarweb]

Hi Mark
Thanks for the reply.

I should have said I've already re-flowed the solder joints and touched up any that looked dry. Made no difference. Hadn't checked the cable but will do. Didn't think it would be that as it can be just sitting untouched and the voltage drops out.

I can see the value of the capacitor (1000uF) and there's only one. But wasn't sure about the diodes - will probably need to get one out to check.

There's also something running the width of the PCB under the transformer which has tape over the top. It's hard to see underneath but don't know if this is the fuse but it shows as open circuit with a meter.

The PCB is stamped "Adapters & Eliminators Ltd, 14 Thames St, Louth, Lincs". Which is the one Moggy has posted since I typed this up

Cheers

[1024MAK]

There's also something running the width of the PCB under the transformer which has tape over the top. It's hard to see underneath but don't know if this is the fuse but it shows as open circuit with a meter.

That's the thermal fuse. It should read as a short circuit. Unless it's been replaced by something else.

Basic functionality of the diodes can be done with a suitable multimeter using either the diode test function or the 200 ohm resistance range.

It is possible to get replacement thermal fuses, indeed I have bought some in the past for the Uk1400 PSU. But I forget the details.

For testing purposes only you can fit an insulated wire. But it can't stay like that, as it's a fire risk.

Mark

[jarweb]

Hi
Looks like it might have finally just packed up. After leaving overnight, powered on today and it's only showing 4.5V without being plugged in.

Need to check the cost of components to see if it's worth taking the chance that only a capacitor and/or diodes will fix it.

Doubt it's even worth trying to sell as faulty.

Thanks guys

[1024MAK]

The main point: if the transformer is okay, the rest can be fixed.

The transformer primary winding will be somewhere between 200 ohms to 600 ohms. The secondary will be less than ten ohms IIRC.

Mark

[jarweb]

Thanks Mark

[jarweb]

Hi
Hope I'm not too late with a follow up question on this.
I replaced the thermal fuse with, hopefully, something similar.
I also replaced the capacitor with a higher value one as mentioned by Mark. So the 1000uF 16V one has been replaced by a 4700uF 25V one.

Supply seems to work for now but I've noticed that, when it's been off and switched back on, it now shows around 13.9V and slowly increases to roughly 14.4 over several minutes. I didn't notice it doing this before, when it was actually working.

Is this normal ?
Thanks

[1024MAK]

It's not unusual. The unloaded output voltage will vary. It depends on the actual mains voltage, the temperature of the diodes (warm diodes conduct better) and on the (very, very small) leakage current flowing through the capacitor.

Mark