My Issue 2 continuing problems and Issue 6s fixed

redgatemodels
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Re: My Series 2 Died

Post by redgatemodels »

1024MAK wrote: Thu Sep 07, 2023 2:58 pm
Unless your board has an intermittent connection (socket, dry joint, solder splash, cracked/broken PCB track or similar)
Mark
Spot on so it seems :)
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1024MAK
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Location: Looking forward to summer in Somerset, UK...

Re: My Series 2 Died

Post by 1024MAK »

Most definitely worthwhile modifying it so that you have a composite video output.

The colour problem may well be the TV not liking the non-standard video signal. It’s already known that some converters/upscalers can’t cope with the Speccy!

If you have, can borrow, beg another TV, definitely worth it to save a wild goose chase.

When you have a working keyboard, type in

Code: Select all

PRINT PEEK 23732 + PEEK 23733 * 256 
If you get 65535, the machine has 48K bytes of RAM available to BASIC.
If you get 32767, the machine either is a 16K or the “upper” RAM has a fault.
Any other value means a RAM fault.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor 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 being good this year.
redgatemodels
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Joined: Tue Sep 05, 2023 11:56 am

Re: My Series 2 Died

Post by redgatemodels »

Cheers Mark,

I have dug out our old caravan flat screen tv and it allows me to set the input to PAL not SECAM, this gives me a yellow display, although quite glitchy. Hopefully I can now tune the output.

It is 48k, with an aftermarket upgrade, possibly Watford Electronics back in the day.

Both upper and lower ram check out fine on the Retroleum diagnostics rom
redgatemodels
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Re: My Series 2 Died

Post by redgatemodels »

Here's my Issue 2 PCB, with a close up of the mod board that came with the 48K upgrade
DSC_0406 (2).JPG
DSC_0407 (1).JPG
Note offending ribbon cable heading off to the 9 way D socket.....
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1024MAK
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Re: My Series 2 Died (but now resurrected)

Post by 1024MAK »

Yeah, the issue two board was limited in which 32K DRAM chips it could use. The adapter board was one of a small number of solutions so that OKI DRAM chips could be used.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor 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 being good this year.
redgatemodels
Posts: 173
Joined: Tue Sep 05, 2023 11:56 am

Re: My Series 2 Died (but now resurrected)

Post by redgatemodels »

Still can't get a display with the right colours etc, I'm beginning to think that the ULA might not be working correctly .....
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1024MAK
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Re: My Series 2 Died (but now resurrected)

Post by 1024MAK »

Generally speaking, ULAs mainly die completely or work normally. Only occasionally do they suffer from partial failures.

The ULA outputs three video signals, a ‘monochrome’ Y signal (pin 17) and two colour difference signals (U and V, pins 15 and 16)

All three are analogue signals.

If you lift one end of C65 (22 µF) this will disconnect the output from the colour encoder chip (IC14, LM1889). Then you should get a monochrome video output.

Issue one and issue two boards don’t have automatic colour calibration circuitry. Hence in order to get the colours correct, presets VR1 and VR2 have to be carefully adjusted.

Not sure if you have enough of your keyboard working, but here is a colour bar program:

1 REM ZX Spectrum colour bars
10 LET a=22528
20 FOR b=1 TO 80
30 FOR c=0 TO 7
40 POKE a+c,c*8
50 NEXT c
60 LET a=a+8
70 NEXT b

Even if the colours are not all correct, are there eight different colours?

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor 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 being good this year.
redgatemodels
Posts: 173
Joined: Tue Sep 05, 2023 11:56 am

Re: My Series 2 Died (but now resurrected)

Post by redgatemodels »

keyboard is fine, I'll give the program a go :)
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1024MAK
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Re: My Series 2 Died (but now resurrected)

Post by 1024MAK »

Here's a pic of the results in an emulator...
Colour bars prog using POKE.png
Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor 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 being good this year.
redgatemodels
Posts: 173
Joined: Tue Sep 05, 2023 11:56 am

Re: My Series 2 Died (but now resurrected)

Post by redgatemodels »

does this count as 8 different colours?
DSC_0437.JPG
The glitching on the screen might be this cello flatscreen tv, don't get this on my big lounge LG 40"
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