Flaky ZX81

[Lardo Boffin]

I have an issue 3 ZX81 which is not switching on reliably.

I get one of three states:

A grey screen (switched on but nothing happening)
A white screen (but no cursor)
A normal boot

I have done the following:

Replaced the voltage regulator
Socketed all the chips (except RAM) and put in known working ones
Replaced all the electrolytic caps

Any suggestions whats next?

I only switched it on once before working on it and got a K so don’t know if I have done something or it was like it anyway.

I have tested it with a Chroma 81 (to rule out the RAM) and it did exactly the same.

[Paul]

Did you measure the capacity of the new Reset electrolyte capacitor?
Try if increasing to a larger capacitor (parallel to existing) does change the behaviour.

[Lardo Boffin]

Thanks.

I used the same value capacitors that were already there - a 1uF and 22uFs. The problem was there before I recapped it.

Is the 1uF the reset cap?

Also could it be a power supply issue? I ask because I have just finished doing a 16K RAM mod to another issue 3 and that is doing weird stuff. It always switches on fine but after about 5 seconds goes weird either losing the K or showing random stuff on the screen. Both have had the same type of voltage reg fitted, bought from the same place at the same time.

When I get chance I will refit the original regulator back to one of them and see if it makes any difference.

[PokeMon]

When normal boot happens - does it work properly then ?
Is there maybe a cold or broken solder joint ?
What happens when slightly knocking on the board or the chips with a screwdriver or similar ?

[1024MAK]

C5, a 1µF electrolytic capacitor, along with resistor R15 (220kΩ) form the reset circuit for the Z80.

However, if either of those components were faulty, it would not explain a ‘grey screen’. If the Z80 is not running, or is processing junk instead of the ROM program, normally you get a white screen.

If you have a digital multimeter, you can test to see if the +5V supply is within spec (4.75V to 5.25V).
Although you either need a very expensive digital multimeter (one that can measure mV AC at least as high as the DC-DC converter operates at, and which can ignore the DC value) or a good oscilloscope to check for noise and ripple with a DC-DC converter.

Have you tied each of the chips from the suspect boards in a different (known good) ZX81 board (one at a time)?

Mark

[Lardo Boffin]

Thanks Mark.

I have tested the voltage at the Z80 and ULA (cheapo multi-meter) and both are around 4.95 to 4.97v.
Except for the RAM none of the chips in this zeddy are original now.
All are known good working chips but I will try some more when I get chance.

[Lardo Boffin]

I had to print out some letters to Santa so managed to have a look. (I asked for a Jupiter Ace )

After switching on and off about 10 times it booted to a K and seems to work fine.

[Lardo Boffin]

And runs The Gauntlet OK.

Tapping chips / board when it is up and running doesn’t have any effect.

[Lardo Boffin]

I have just power cycled about 20 times with the ‘spand plugged in and it switched on fine every time.

Does this point to RAM? I thought that the Chroma’s on board RAM would do the same effective test?

[PokeMon]

I would be carful with the Chroma Interface when using other components than the original.
It is very timing sensitive. Better use a plain RAM expansion when you are unsure about the internal RAM.

What happens after startup with a manual reset instead of powering off ?
You could just use any tactile switch or hold the cables together, RESET and GND (can simply short the 1uF capacitor).
I saw a switching regulator on your board, maybe need some time to get stable output ?