Another poorly ZX81...

[Rink]

Hi guys, I'm back another with another broken ZX81.

No doubt the ULA or CPU will turn out to be dead, but for now I'm dealing with an apparent lack of voltage. The power supply is fine and the socket is making a good connection. But, measuring across the socket shows a only minuscule voltage. Same across the 7805 (barely anything on the input so nothing on the 5v output). I switched the 7805 for a known good one (made a right pig's ear of it to be honest) but no change.

I'm tending to think this might be a dead cap something which is draining the power from the circuit. I have suitable replacements but wanted to see what you all thought before attempting that. Plus, I need to buy a new soldering iron first anyway.

Cheers fellas.

Edit: While I'm here, have you ever heard of a modulator failing before the rest of the computer? My friend has an 81 which has a signal going into the modulator (don't know if it's a good one mind) but none coming out. I suggested removing it and going for a simple transistor composite mod.

[PokeMon]

Check your power supply and what's going in the machine.
Could be a loose/bad contact of power supply connector or shortcut somewhere on the board.
There are only 2 electrolytic caps (one for reset signal) on the board - so a dead electrolytic cap should be visible and I think should get warm when short connecting inside. What about the power supply ? Does it get hot ? Did you try the one from your other ZX81 ? Did you measure current going into the ZX81 ?

[Rink]

Power supply remains at a normal temperature; as does the 7805. I haven't tried a different supply but have tried that supply with a different ZX81 which works fine and probing around inside reveals expected voltages in expected places. Not so on the poorly one.

Is it only electrolytic capacitors that are likely to die? I was going to start with those and replace C2, C4, C8 and C9. But I guess a short could be just about anywhere so this could be fun.

[1024MAK]

Power supply remains at a normal temperature; as does the 7805.

When you say normal temperature, do you mean room temperature (not warm) or normal operating temperature (warms up after being on for a while)?
And when noting the temperature, are you leaving the PSU connected to the mains supply?
If yes then the normal characteristics of a transformer are that the magnetic laminations warm up anyway, regardless of load.

It sounds more like there is an open circuit fault on the Zeddy power socket, either the contact or the solder joint(s) (possible "dry" joint).

If it is a short, there are only three practical ways of finding it:

• Visually looking for metal and solder that could be shorting between tracks/traces/component lead-outs/wires/pins.
• With the PSU disconnected, testing using a meter on resistance (ohms), and if the meter detects a unexpected low resistance, removing each suspect component one at a time until the short disappears.
• Renewing each suspect component one at a time until the computer works

As there are only three components connected to the +9V line (the power input socket, C3 a 22uF 16V capacitor and the 7805 voltage regulator) and you have already changed the regulator, this only leaves C3 or a solder splash. Check the track/trace for the +9V line to the edge connector as well.

Mark

[Rink]

Good point - instead of 'normal' I should have said that I can't feel any heat from them.

This morning I replaced the electrolytics C3 and C5, made no difference but did give me a chance to try a new soldering iron which I'm much enamoured with. Just gone back to a regular one after years of using a gas powered iron.

My multimeter couldn't register any current draw... Weird.

Visually I can't see anything wrong - it all looks pretty good in there so I guess you're right Mark, going to have to start pulling stuff out (starting with the jack socket) and see what happens.

Has anyone bought a ZX81 in the last five years which actually worked? Lol. Every one I get my hands on seems to have a problem somewhere.

[Paul]

have you ever heard of a modulator failing before the rest of the computer?

Yes, definitely. These modulators were often thrown out because of malfunctions.
Anyway, exchanging the contents of the Modulator agains a ZX81CCP is a good idea as nowadays tv's have tuning problems. Most have composite input that is working well.
Or one of Andys ULA's which have a very good video output and making the mod that disables the modulator.

Has anyone bought a ZX81 in the last five years which actually worked? Lol. Every one I get my hands on seems to have a problem somewhere.

And yes, there must be some users that obtained a working zeddy as I gave away three in the last two years
I'm also constantly working on the zeddys, but most work immediately after exchanging 7805 against Recom, Z80 against CMOS version, ULA against Andy's and Keyboardmembrane against a new one. If you attach a ZXPand there is not much left inside the Zeddy thats could prevent working . I wonder why I never had a problem with a dry cap, maybe because they do not shortcircuit but just don't have the named capacity?
I have to add that a lot of board that come are across are missing the membrane (molex) connectors, but Rich happily supplies those too.

[PokeMon]

Has anyone bought a ZX81 in the last five years which actually worked? Lol. Every one I get my hands on seems to have a problem somewhere.

Yes - last year I bought two. One is working properly, the other one had a defective ULA. The video out is not working but rest seems to work okay.
If I buy over Ebay I try those were is the start screen ("K") shown or confirmed that it starts with a K.

If anybody sells the ZX81 with
"no time to test",
"no monitor to test",
"no power supply to test",
"no cable to test",
"don't know how it works, have two left hands"
you can be sure it's definetely not working.

[Rink]

have you ever heard of a modulator failing before the rest of the computer?

Yes, definitely. These modulators were often thrown out because of malfunctions.
Anyway, exchanging the contents of the Modulator agains a ZX81CCP is a good idea as nowadays tv's have tuning problems.

I just saw the thread about those - looks very interesting. Are they available yet? Weighing up my options.

[Rink]

If anybody sells the ZX81 with
"no time to test",
"no monitor to test",
"no power supply to test",
"no cable to test",
"don't know how it works, have two left hands"
you can be sure it's definetely not working.

Hahahaha. If I buy any computers on eBay these days I just assume they'll need fixing. I think that's part of the appeal for me - certainly with the ZX81 anyway. Charming machine, but I'm not clearly not going through all of this just to play Manic Miner.

[Rink]

Well... Despite my feelings to the contrary, it looks like the problem was as simple as a faulty power input jack. Took that out, bodged on another mono socket for now and got voltages in all the right places. Bodged on a composite mod and I can now see that wonderful K.

Thanks for the advice chaps. Once again, you've been a great help.