ZX Spectrum - recommended upgrades?

Trevor_B
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Re: ZX Spectrum - recommended upgrades?

Post by Trevor_B »

After being plugged in for an hour, the colour stayed the same - yellowy green.

Is there anything I can check so I can start pulling together a shopping list of parts? I've only got the basic tool - a multimeter - so checking for pulses on chips is a bit beyond my current capability.

Trev
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1024MAK
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Re: ZX Spectrum - recommended upgrades?

Post by 1024MAK »

With a completely white screen (including a white border and no text *) test the DC voltage on pin 2 of IC14 (LM1889). Use pin 5 for your 0V/GND black probe connection.
Then do the same for pin 4.
Report back your results including all displayed digits that your meter shows.

* use something like:
10 CLS
20 GOTO 20
Assuming you do this after power on.

Mark
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Trevor_B
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Re: ZX Spectrum - recommended upgrades?

Post by Trevor_B »

Hi Mark,

Thanks for helping. Using the multimeter these are the voltages;

Pin 2 - solid 4.93v - no variation
Pin 4 - solid 3.99v - no variation

Trev
Trevor_B
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Re: ZX Spectrum - recommended upgrades?

Post by Trevor_B »

Hi Mark
Sorry to bug but what's next on the list to check?

Trev
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1024MAK
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Re: ZX Spectrum - recommended upgrades?

Post by 1024MAK »

Trevor_B wrote: Wed Mar 24, 2021 9:49 am Thanks for helping. Using the multimeter these are the voltages;

Pin 2 - solid 4.93v - no variation
Pin 4 - solid 3.99v - no variation
For a blank white screen, these voltages should be closer to one another, either pin 2 being a bit lower, or pin 4 being a bit higher.

For comparison, on a issue 1 board that I tested (which has manual colour adjustments), I get 4.61V on pin 2 and 4.54V on pin 4.

Is the ULA in a socket? If yes, do you have another ULA to try? Only swapping the ULA is the next easy step.

If this does not help, with the power off, test these resistors:
R56, 1kΩ
R63, 1kΩ
R74, 10kΩ
R75, 10kΩ
R76, 1kΩ
R77, 1kΩ
R78, 470Ω

Use the appropriate resistance range on your multimeter.
Note some of the results may be affected by other components when tested in circuit.

Mark
ZX81 Variations
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.
Trevor_B
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Joined: Thu Nov 05, 2020 12:31 am

Re: ZX Spectrum - recommended upgrades?

Post by Trevor_B »

That's annoying.

The ULA is socketed but I don't have a spare, and it looks like the UK stock of VLA82's and Nebula replacements is empty. I'll see if I can get an old stock original off ebay or something.

I checked the resistors - all were within 1% of the values.

So that's about as far as I can go until I try a different ULA?

Trev
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1024MAK
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Re: ZX Spectrum - recommended upgrades?

Post by 1024MAK »

No, if you don’t have a ULA, the next step is to renew TR8 and TR8 (both BC184, NOT the alternative lead out version BC184L). The far easier to get BC549B are an alternative with the correct lead out.

Then if the problem remains, renew IC14 (LM1889).

If after all this, the problem is still there, then it’s time to try another ULA.

Mark
ZX81 Variations
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.
Trevor_B
Posts: 101
Joined: Thu Nov 05, 2020 12:31 am

Re: ZX Spectrum - recommended upgrades?

Post by Trevor_B »

I'd pre-empted your response and bought the transistors - BC184s - and the LM1889 (based on your initial post). I know my luck when it comes to these things, so thought I'd be pre-armed, just in case. I'd rather hoped to not have to use them as I hate desoldering IC's, but if needs must.

So - first off, I swapped the transistors. No joy - screen stayed yellowy green. Voltage at pins 2 & 4 same as previous.
I then swapped the LM1889 - and the screen stayed yellowy green, same voltage as before.

I've ordered a second hand - but confirmed working - ULA from ebay.

..and I still haven't swapped the capacitors. This is all pre-amble to "the upgrade".
hellraiseruk
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Re: ZX Spectrum - recommended upgrades?

Post by hellraiseruk »

Might be an idea to replace C25 and C65, as those are instrumental in the colour signal if I remember correctly...

JD
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