Diagnosing a ZX81 - solved!

[patters]

This ZX81 has a non-working EAR port which appears to be down to the ULA. I carefully desoldered the ULA (I've done a Z80 successfully on a Spectrum before now) and socketed it. Just now I stupidly managed to insert it upside down in its socket and powered it on.

With the ULA back in the correct orientation the system only starts to a white screen now. What are the odds I have killed the chip? Is it more likely to be some other issue?

I have another working ZX81 with socketed chips, but ideally I don't want to tempt fate and break that one in trying to repair this one.

[BarryN]

This ZX81 has a non-working EAR port which appears to be down to the ULA. I carefully desoldered the ULA (I've done a Z80 successfully on a Spectrum before now) and socketed it. Just now I stupidly managed to insert it upside down in its socket and powered it on.

With the ULA back in the correct orientation the system only starts to a white screen now. What are the odds I have killed the chip? Is it more likely to be some other issue?

I have another working ZX81 with socketed chips, but ideally I don't want to tempt fate and break that one in trying to repair this one.

May we have a moment of silence for the dead ULA...

[1024MAK]

Putting most chips in the wrong way round, or misaligning the pins in the socket is very likely to kill them

The problem is that the semiconductor junctions are not designed to withstand a reverse voltage and pins that end up connected to opposite polarity connections will result in excessive current flow, leading to localised heating of the internal circuit and thus damaging it.

RIP ULA…

With the ULA removed, test each pin of the socket using the resistance range on your multimeter. Test between each pin and a suitable 0V/GND pin. Then repeat but testing each pin to the +5V supply. If you don’t find any short circuits, it *should* be safe to test the board using a known good ULA.

You can also test each pin of the ULA chip (out of circuit) to each of it’s own supply pins (34 & 40). If no shorts are found, then the risk of trying it in a known good board are reduced but not eliminated.

Only you can decide which risk you want to take.

Mark

[patters]

May we have a moment of silence for the dead ULA...

Ah well it was damaged anyway. It wouldn't load from EAR, whether using a TZXDuino or a WESPI (which work fine with my other Zeddy) and I tested the components leading from that input to the ULA. Even tried some other ratings for the capacitor and resistor under guidance from 1024MAK.

Didn't I read in another thread that a ULA has to be functional to even get the white screen though?

[1024MAK]

Didn't I read in another thread that a ULA has to be functional to even get the white screen though?

The ULA needs to be partially functional to get a white screen.

If the ULA survived, then if nothing else has changed (or been damaged), then there may be a problem with a poor connection with the socket. But I’m presuming that you have visually inspected all the solder joints.

You could continuity test each ULA pin (on the pin of the chip) to another node that the PCB track goes to.

Mark

[patters]

Thanks, I'll give that a go. I think there's a chance the ULA might live on, since I only had it powered for a few seconds in the wrong orientation. Not enough time to heat up.

This board was hanging around for months and may have been handled a bit while its ULA was out. So unfortunately it's not as if I can guarantee it was working before today's mishap. I have inspected it carefully for stray solder splashes and for bent components (like had happened earlier in this thread) but didn't spot anything. I do have a spare 32K SRAM so I'm tempted to wire that in and replace the one that's in there. I don't have a RAM pack unfortunately to rule out a RAM problem without soldering.

I had been keeping an eye out for ULAs or knackered old Zeddys on eBay to act as donors but even the worst examples now seem to fetch a minimum of £35 which is too much for a punt.

[patters]

Well it wasn't the SRAM. I guess I need a ULA then (which was the problem all along, so nothing lost).

You could continuity test each ULA pin (on the pin of the chip) to another node that the PCB track goes to.

I did that, but nothing unusual to report.

[bola_dor]

I had a case of a white screen with a (partially) dead ULA.. clearly the Sync generation part was functioning but the computer did work with a new ULA.
Usually a wite screen is a good sign of a working ULA but there are other functions as generating the CLK that will prevent the Z80 toeven start if not working properly. You will need an oscilloscope to test pin14 output. Or Z80 pin6 input...
You did mention before that had some problems with the CLK circuit.. did you re check that part?

[patters]

Yes, at the time I started this thread the timing crystal leg was bent and was touching the resistor leg next to it. Currently no component is touching any other.