Not enough power?

Discussions about Sinclair ZX80 and ZX81 Hardware
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Lardo Boffin
Posts: 799
Joined: Sat Nov 26, 2016 2:42 am

Not enough power?

Post by Lardo Boffin » Sun Sep 09, 2018 11:09 pm

The zeddy that came in the bundle with my Memotech keyboard is in lovely condition, produces a reasonable picture even on my full HD lcd screen and even has the till receipt stapled to the shop guarantee card. So rather than refurbish it I have just changed the keyboard membrane (which was beyond trimming) and will sell it on (I really have enough tribbles zeddies now) as a nice collectors piece.
It works fine but if I plug in both a ZXPand and ChromaSCART I just get a dark screen and nothing going on. If I plug in just one or the other it works fine. Both work fine combined on other zeddies.
If I plug in the Memotech RAM pack that came with it and the ChromaSCART I get the same.
Is it struggling for power with the old regulator? (The engines cannie take it cap’n!)
ZX81 iss 1 (bugged ROM, kludge fix, normal)
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: 799
Joined: Sat Nov 26, 2016 2:42 am

Re: Not enough power?

Post by Lardo Boffin » Sun Sep 09, 2018 11:09 pm

This was tried both with the UK700 PSU that came with it and a good quality modern one.
ZX81 iss 1 (bugged ROM, kludge fix, normal)
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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Andy Rea
Posts: 1480
Joined: Fri May 09, 2008 2:48 pm
Location: notts UK

Re: Not enough power?

Post by Andy Rea » Thu Sep 13, 2018 1:26 pm

Clean edge connector power supply pins, try beefier PSU, if still no good try chging 7805 in zeddy.

If none of that work I'm stumped.

Regards Andy
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

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1024MAK
Posts: 2059
Joined: Mon Sep 26, 2011 10:56 am
Location: Looking forward to summer in Somerset, UK...

Re: Not enough power?

Post by 1024MAK » Thu Sep 13, 2018 2:25 pm

Due to the amount of power required for those expansions, the 9V and 0V/GND circuits / conductors (including connectors) between the PSU and the expansions must be good. Inspect, clean, test (with the power off, use a good multimeter on the 200 ohms range, circuit resistance must be low, 0.1 ohms or lower).

Before changing the 7805, check that (1) making sure that the nut and bolt holding the heatsink and the 7805 are tight. If you have any heatsink compound/paste, remove the heatsink, clean off any existing compound (if any was used), smear a new thin film where the 7805 is pushed up against the heatsink, then reassemble.
And (2) the solder joints are good (if necessary, resolder them by removing the old solder, cleaning the pads and pins, then resoldering).

Mark

Lardo Boffin
Posts: 799
Joined: Sat Nov 26, 2016 2:42 am

Re: Not enough power?

Post by Lardo Boffin » Thu Sep 13, 2018 2:43 pm

Thanks all. I have tried a modern PSU and it made no difference.

I wanted to keep it as original as possible due to its condition so don’t really want to change the 7805 if possible. I was mostly curious as I have never had one before that didn’t work with the two expansions connected.

I will check out the connections etc. and ensure the nut and bolt are tight.
ZX81 iss 1 (bugged ROM, kludge fix, normal)
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
Andy Rea
Posts: 1480
Joined: Fri May 09, 2008 2:48 pm
Location: notts UK

Re: Not enough power?

Post by Andy Rea » Thu Sep 13, 2018 8:28 pm

i thought of one other thing... is the Z80 socketed ? pretty sure both Chroma 81 and ZXpand use M1 signal it has been know for some Z80's to have very weak M1 line....

just a thought

Regards andy
6 x ZX81, 1 x TS1500 , 1 x +3e, 1 x timex 2040 printer, 1 x timex 2020 cassette deck, siclair printer and some spectrum

Lardo Boffin
Posts: 799
Joined: Sat Nov 26, 2016 2:42 am

Re: Not enough power?

Post by Lardo Boffin » Thu Sep 13, 2018 10:55 pm

I had put this on eBay last night with the comment that I could replace the regulator and caps if the new owner wanted me to.
It sold this afternoon and the new owner wanted me to.
So I have replaced the regulator (the nut was extremely tight) and the caps and now it runs both fine. I guess it was the regulator then!

Thanks all.

Lardo
ZX81 iss 1 (bugged ROM, kludge fix, normal)
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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