what does this modification do ??

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jojo
Posts: 129
Joined: Mon Mar 16, 2020 5:25 pm

what does this modification do ??

Post by jojo »

a bit puzzled here ,

i got an iss. 2 and an iss.4a with diodes and a resistor were i supposed to find the jumpers for the erpom selection ;

iss. 4a ;
IMG_3066.JPG
it does not see the upper ram ( al components verified ) -> noticed that the memory selector is set for oki and H whilst ti is mounted .
also it is not booting from its own NEC rom , does boot from the retroleum test card .

rigth now the iss. 2 is only displaying stripes ,
and does also not boot from his own rom (eprom ) ,neither does it boot from the retroleum test card.


can anyone shine a light on this ??


jojo
Zx81-16k exp.- Spectrum 48 - Spectrum 128- Interface 1 - DIVide - Zx flashcard- Zx printer - Ram turbo-Velleman interface system-Currah microspeech -Ql - 512k exp.- Ql printer - Cst floppy interf.- Double 3.5 floppy- Minerva - Qimi-and much more .
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1024MAK
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Re: what does this modification do ??

Post by 1024MAK »

I’ve not got time to work out what’s actually going on. But for the H/N links and how to modify the board to take a EPROM using two signal diodes and a resistor, see here.

The two larger “rectifier” diodes, are part of the modification of the DC/DC converter circuitry. See my blog page here for the last variation.

The “upper” DRAM jumpers have to be correct if 32K bit DRAM chips are being used.

Where did you get all these “tinkered with” machines, from Del Boy’s Trotters Independent Traders car boot sale? :shock: :? :lol:

Mark
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jojo
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Re: what does this modification do ??

Post by jojo »

Hi mark ,

Those machines are the last left overs from a 40 year hoarding , my wife wants to get space in the attick/garage .

So now i am starting with the most stubborn speccies which i set aside becouse of time-wasting in those days.

It is a shame to throw them away .

the 4A in the picture has now jupers across the N positions ( it has a rom ) and boots on his own power , albeit only 16K .
Although it is a 48k machine , the upper ram is not found.

i verified all components , both ttl and ram and these are all ok .

so that one remains on the desk .
Zx81-16k exp.- Spectrum 48 - Spectrum 128- Interface 1 - DIVide - Zx flashcard- Zx printer - Ram turbo-Velleman interface system-Currah microspeech -Ql - 512k exp.- Ql printer - Cst floppy interf.- Double 3.5 floppy- Minerva - Qimi-and much more .
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1024MAK
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Re: what does this modification do ??

Post by 1024MAK »

Which “upper” DRAM chips does it have?
And how are the TI/OKI/L/H/3/4 jumpers currently set?

Alternatively, have you tried a set of fully working 4164 DRAM chips?

Mark
ZX81 Variations
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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.
jojo
Posts: 129
Joined: Mon Mar 16, 2020 5:25 pm

Re: what does this modification do ??

Post by jojo »

yes , tried ti 4164 , and the ttl from a working unit

jumper to ti , and 3 or 4 should not matter .

but unfortunately , "no upper ram found "


ram has been taken from another speccie , and has been put back without errors
Zx81-16k exp.- Spectrum 48 - Spectrum 128- Interface 1 - DIVide - Zx flashcard- Zx printer - Ram turbo-Velleman interface system-Currah microspeech -Ql - 512k exp.- Ql printer - Cst floppy interf.- Double 3.5 floppy- Minerva - Qimi-and much more .
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1024MAK
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Re: what does this modification do ??

Post by 1024MAK »

Using your ’scope and during the time that either a diagnostic ROM is trying to test the ‘upper’ RAM, or better (because you have more control) a BASIC program that continuously PEEKs and POKEs an address (any address) above 32K, test IC23 pin 3 or pin 4 (/RAS) and IC23 pin 6 (/CAS) to see if the DRAM is even being selected.

If there are pulses on the the above two lines, check the /Write line on IC23 pin 11.

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.
jojo
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Joined: Mon Mar 16, 2020 5:25 pm

Re: what does this modification do ??

Post by jojo »

sh... , sh...

scope just died out .............

any hameg hm-1007 specialist around ??

no power despite 240 present at input , suspect part of the psu went up in digital heaven .
Zx81-16k exp.- Spectrum 48 - Spectrum 128- Interface 1 - DIVide - Zx flashcard- Zx printer - Ram turbo-Velleman interface system-Currah microspeech -Ql - 512k exp.- Ql printer - Cst floppy interf.- Double 3.5 floppy- Minerva - Qimi-and much more .
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1024MAK
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Re: what does this modification do ??

Post by 1024MAK »

Sorry, have never had to repair a ‘scope… (apart from clean / service switches).

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.
avrovulcan
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Joined: Sun Nov 03, 2019 9:56 am

Re: what does this modification do ??

Post by avrovulcan »

Check the power supply to see if the input filter caps have gone. I had a similar problem with my HP logic analyzer. The Rifa branded caps on the mains input had exploded.
Normally not terminal, but they will need replacing as if they have gone they will be shorting out the input.

HTH

Regards
Andy
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1024MAK
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Re: what does this modification do ??

Post by 1024MAK »

Yes, it does not matter what equipment it is, a Rifa capacitor can cause all manner of problems. Including blowing an internal fuse…

Other common PSU problems are electrolytic capacitors failing, dry solder joints, faulty bridge rectifier / diodes.

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.
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