ZX81 rebuild

Discussions about Sinclair ZX80 and ZX81 Hardware
Lardo Boffin
Posts: 661
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Sun Oct 08, 2017 11:45 am

I guess at £2.15 with p&p I will take my chances but would rather keep things simple with a proven supplier! Is it possible to test them in any way before fitting them?

What I would really like to do is prove they work before fitting them to the rebuild (to make the trouble shooting easier when I finally build it, switch it on and nothing happens...). I have a slightly trashed test bed zeddy I can use. Would it be possible to desolder one of this zeddys transistors, push one of these into place, bend the legs out to ensure contact and stick them down on the solder side using insulating tape? Would this give a good enough contact for a brief test?

Lardo

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1024MAK
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Location: Looking forward to summer in Somerset, UK...

Re: ZX81 rebuild

Post by 1024MAK » Sun Oct 08, 2017 1:01 pm

To test all the characteristics of a transistor requires some expensive test gear. However, a lot of low cost multimeters now have a simple transistor test socket, the meter will provide base and collector bias, and display the DC gain (HFE) on the display. If the transistor is healthy, the gain should be in the range indicated on the datasheet.

There are also dedicated testers like those from Peak.

If you don't have a meter that has a transistor tester, then you could always put a simple circuit on a plug board. You just need a couple of resistors and a meter that can read mV as well as volts. Example test resistor values for a NPN transistor are 20k or 22k from +5V to the base. 100 ohm (this resistor may get hot) from +5V to the collector. The emitter goes to 0V. If you test and record the voltage across each resistor (from one lead to the other), you can then use ohms law to work out the currents. Then divide and you get the DC gain at that bias (the base current and collector current).

Mark

gozzo
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Re: ZX81 rebuild

Post by gozzo » Fri Oct 13, 2017 3:00 am

oh, what were these eeproms that were bad? and from what supplier? i've bought a fair few but not got round to using them yet...

gozzo
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Re: ZX81 rebuild

Post by gozzo » Fri Oct 13, 2017 3:04 am

and i think that supplier above is where i got some mps2369s from..havent tried them yet..something about the type number printing doesnt look quite right, though..... maybe something else relabelled?!!

gozzo
Posts: 444
Joined: Fri Jul 08, 2011 7:52 pm

Re: ZX81 rebuild

Post by gozzo » Fri Oct 13, 2017 3:09 am

these will almost certainly be genuine, very good supplier.. http://www.ebay.co.uk/itm/MPS2369A-MOTO ... 0653829622 the type number 'print' is totally different, seems to be 'etched' ...

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1024MAK
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Joined: Mon Sep 26, 2011 9:56 am
Location: Looking forward to summer in Somerset, UK...

Re: ZX81 rebuild

Post by 1024MAK » Fri Oct 13, 2017 8:59 am

gozzo wrote:
Fri Oct 13, 2017 3:00 am
oh, what were these eeproms that were bad? and from what supplier? i've bought a fair few but not got round to using them yet...
Read through this thread over on StarDot.

Mark

Lardo Boffin
Posts: 661
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Sat Oct 14, 2017 9:09 am

I have ordered all the bits on the list. Fingers crossed it is all the right stuff!
When it arrives I can start the rebuild... :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

Lardo Boffin
Posts: 661
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Fri Nov 17, 2017 8:07 pm

Some time later...

Some bits! All the bags neatly marked up. Its amazing what you will do to delay having to actually start building this. The sooner I start the sooner I switch it on and nothing happens...

Other bits are lurking in various places.

C233A37B-02B9-4B26-A3CE-0C85F6F2783F.jpeg
C233A37B-02B9-4B26-A3CE-0C85F6F2783F.jpeg (2.62 MiB) Viewed 90 times

PCB prepared for the 16k mod as i don’t have a 1k chip.

3C17947C-70F6-41D8-9631-98CAB4E96C7B.jpeg
3C17947C-70F6-41D8-9631-98CAB4E96C7B.jpeg (3.71 MiB) Viewed 90 times
16C66F07-4F43-4F2A-92BC-8E0621A1AD0F.jpeg
16C66F07-4F43-4F2A-92BC-8E0621A1AD0F.jpeg (3.21 MiB) Viewed 90 times

Lardo
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: 661
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Fri Nov 17, 2017 9:07 pm

Resistors fitted. Who knows I may have even got the resistor packs the right way round!

B8707836-E02E-4153-BCE2-ABE3F5D8CD46.jpeg
B8707836-E02E-4153-BCE2-ABE3F5D8CD46.jpeg (3.5 MiB) Viewed 75 times
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: 661
Joined: Sat Nov 26, 2016 1:42 am

Re: ZX81 rebuild

Post by Lardo Boffin » Fri Nov 17, 2017 9:32 pm

Capacitors
46B5CE28-F8B6-49F8-91D7-A3699847D757.jpeg
46B5CE28-F8B6-49F8-91D7-A3699847D757.jpeg (1005.95 KiB) Viewed 66 times
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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