ZX81 rebuild

Discussions about Sinclair ZX80 and ZX81 Hardware
Lardo Boffin
Posts: 714
Joined: Sat Nov 26, 2016 2:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Sat Dec 09, 2017 9:15 am

1024MAK wrote:
Sat Dec 09, 2017 12:54 am
Don't Panic is a phrase on the cover of The Hitchhiker's Guide to the Galaxy...
Hence is the standard response to any situation... :lol:

Mark
Clearly I forgot my towel.
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

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1024MAK
Posts: 1914
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX81 rebuild

Post by 1024MAK » Sat Dec 09, 2017 6:02 pm

Lardo Boffin wrote:
Sat Dec 09, 2017 9:15 am
1024MAK wrote:
Sat Dec 09, 2017 12:54 am
Don't Panic is a phrase on the cover of The Hitchhiker's Guide to the Galaxy...
Hence is the standard response to any situation... :lol:

Mark
Clearly I forgot my towel.
Available here and here apparently ;)
<Galactic trading rules dictate that Earthlings must trade at their own risk>

Have you got any further? I'd be tempted to remove the RAM and ROM chips and make a "NOP" generator and put this in the ROM socket. You could then use your logic probe to confirm that each address line is active at different frequencies.

A NOP generator is simply eight resistors mounted on a DIL socket (or DIL header if you are feeling rich). The value of the resistors is not critical, any value between 330 ohms and 1k should be fine. Wire each resistor between a data line (D0 through D7) to the 0V/GND pin so that when the DIL socket is inserted into the ROM socket on the ZX81 board, the correct connections are made. These two web sites may help you: linky1 and linky2.

Mark

Lardo Boffin
Posts: 714
Joined: Sat Nov 26, 2016 2:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Sun Dec 10, 2017 10:53 pm

1024MAK wrote:
Sat Dec 09, 2017 6:02 pm
Lardo Boffin wrote:
Sat Dec 09, 2017 9:15 am
1024MAK wrote:
Sat Dec 09, 2017 12:54 am
Don't Panic is a phrase on the cover of The Hitchhiker's Guide to the Galaxy...
Hence is the standard response to any situation... :lol:

Mark
Clearly I forgot my towel.
Available here and here apparently ;)
<Galactic trading rules dictate that Earthlings must trade at their own risk>

Have you got any further? I'd be tempted to remove the RAM and ROM chips and make a "NOP" generator and put this in the ROM socket. You could then use your logic probe to confirm that each address line is active at different frequencies.

A NOP generator is simply eight resistors mounted on a DIL socket (or DIL header if you are feeling rich). The value of the resistors is not critical, any value between 330 ohms and 1k should be fine. Wire each resistor between a data line (D0 through D7) to the 0V/GND pin so that when the DIL socket is inserted into the ROM socket on the ZX81 board, the correct connections are made. These two web sites may help you: linky1 and linky2.

Mark
Top towel links! Although sad they used the modern Marvin...

I have not had chance to do much this weekend - combination of work (actual paid stuff) and family.

However I did take all the ICs out and put them in another board - it worked fine and brought up a K.

So there is definitely a problem with the board not the ICs. If that narrows it down...
Also I had got the whole thing working with one of the original ZTX313s before going back to the MS2369 experiments. Oh how I long for those halcyon days! Well, 5 minutes. :?

So my conclusion is:-

I bodged something while playing with transistors, either creating a short somewhere (most likely with solder, although I cannot see any) or by physically damaging a component somehow.

I have given the solder side a good few scrub downs with a toothbrush in order to try to get rid of any stray solder but I have found some really small flakes even after doing this. Although I have not found any where they would cause issues. It may be time to get the magnifying glass out again... :cry:
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

Lardo Boffin
Posts: 714
Joined: Sat Nov 26, 2016 2:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Mon Dec 11, 2017 10:21 am

Found some very small solder strands (after much holding the board up to the light at odd angles to see if anything glinted) and removed them. No difference. :evil:
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

User avatar
sirmorris
Posts: 2821
Joined: Thu May 08, 2008 5:45 pm
Location: oxon, uk

Re: ZX81 rebuild

Post by sirmorris » Mon Dec 11, 2017 11:21 am

Rule the third:
'there's always one more'
--=== ZXpand+ ... A more moderner all-in-one upgrade solution for ZX81 ===--
* SD/MMC storage * 32K Hires-capable RAM * AY sound chip * Joystick port * Serial *
* BASIC enhancements - plug & play * Reset button * Auto-boot facility *

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gammaray
Posts: 433
Joined: Sun Apr 17, 2016 2:44 am
Location: Texas

Re: ZX81 rebuild

Post by gammaray » Mon Dec 11, 2017 4:27 pm

Is there a ZTX313 at TR1 position too? I would use a ZTX313 on both TR1 & TR2.

As Wilf wrote me once "...check the pin out...(again)"

Good luck!
Finish what you start. Truth is over the mountaintop or six feet under.
4 x TS1000, 2 x TS1500 , 2 x timex 2040 printer, 1/2 x timex 2020 cassette deck, ZXPAND+AY, ZeddyNET, ZXBlast..., unfinished Spectrum...Still no life!

Lardo Boffin
Posts: 714
Joined: Sat Nov 26, 2016 2:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Mon Dec 11, 2017 5:28 pm

sirmorris wrote:
Mon Dec 11, 2017 11:21 am
Rule the third:
'there's always one more'
Wise words!
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

Lardo Boffin
Posts: 714
Joined: Sat Nov 26, 2016 2:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Mon Dec 11, 2017 5:29 pm

gammaray wrote:
Mon Dec 11, 2017 4:27 pm
Is there a ZTX313 at TR1 position too? I would use a ZTX313 on both TR1 & TR2.

As Wilf wrote me once "...check the pin out...(again)"

Good luck!
Also wise words!
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

User avatar
gammaray
Posts: 433
Joined: Sun Apr 17, 2016 2:44 am
Location: Texas

Re: ZX81 rebuild

Post by gammaray » Mon Dec 11, 2017 9:21 pm

Good thing I am a spectator coaching the game from general seating! I know it takes more effort and brains than I have to actually do it. :cry:
Finish what you start. Truth is over the mountaintop or six feet under.
4 x TS1000, 2 x TS1500 , 2 x timex 2040 printer, 1/2 x timex 2020 cassette deck, ZXPAND+AY, ZeddyNET, ZXBlast..., unfinished Spectrum...Still no life!

Lardo Boffin
Posts: 714
Joined: Sat Nov 26, 2016 2:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Tue Feb 06, 2018 11:54 am

I had come to the conclusion that diagnosing this was way beyond my meagre skills and equipment so I sent it off to Mutant Caterpillar Games be looked at.
They spend a number of hours trying to figure out what was wrong and in the end identified the 6.5 mHz resonator as being at fault! Apparently it was not consistent / accurate enough. When a different resonator was used it booted to the K. When the resonator in mine was put in another zeddy it caused the same issues.
So there you go!
I do feel better that it was not an obvious gaff on my part but that does lead to how do I find an unused resonator that works? :shock:
ZX81 iss 1 (bugged ROM, kludge fix, normal, kit)
Iss 3 with ZXVid, another in nearly perfect box
TS 1000 iss 3, ZXPand AY, ZX8-CCB & ZX-KDLX
TS 1500
TS 2000
Spectrum 16k (iss 1 s/n 1499)
Spectrum 48k
Toastrack + DIVMMC future

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