Flaky ZX81
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Flaky ZX81
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.
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.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Re: Flaky ZX81
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.
Try if increasing to a larger capacitor (parallel to existing) does change the behaviour.
In theory, there is no difference between theory and practice. But, in practice, there is.
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- Posts: 2183
- Joined: Sat Nov 26, 2016 2:42 am
Re: Flaky ZX81
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.
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.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Re: Flaky ZX81
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 ?
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
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Re: Flaky ZX81
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
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
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Looking forward to summer later in the year.
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp
Standby alert
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb
Looking forward to summer later in the year.
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- Joined: Sat Nov 26, 2016 2:42 am
Re: Flaky ZX81
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.
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.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
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- Joined: Sat Nov 26, 2016 2:42 am
Re: Flaky ZX81
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.
After switching on and off about 10 times it booted to a K and seems to work fine.
Last edited by Lardo Boffin on Sat Nov 07, 2020 12:49 pm, edited 1 time in total.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
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- Posts: 2183
- Joined: Sat Nov 26, 2016 2:42 am
Re: Flaky ZX81
And runs The Gauntlet OK.
Tapping chips / board when it is up and running doesn’t have any effect.
Tapping chips / board when it is up and running doesn’t have any effect.
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
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- Posts: 2183
- Joined: Sat Nov 26, 2016 2:42 am
Re: Flaky ZX81
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?
Does this point to RAM? I thought that the Chroma’s on board RAM would do the same effective test?
ZX80
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
ZX81 iss 1 (bugged ROM, kludge fix, normal, rebuilt)
TS 1000 iss 3, ZXPand AY and +, ZX8-CCB, ZX-KDLX & ChromaSCART
Tatung 81 + Wespi
TS 1500 & 2000
Spectrum 16k (iss 1 s/n 862)
Spectrum 48ks plus a DIVMMC future and SPECTRA
Re: Flaky ZX81
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 ?
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 ?