Hello from Canada

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McKlaud
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Location: St Albans, UK

Re: Hello from Canada

Post by McKlaud » Mon Apr 16, 2018 7:24 am

On the output should be +5V +/- 0.25V. Anything above +5.25V means that something is wrong the voltage regulator chip. Take it out and measure a resistance between GND (middle soldering point of this chip) and +5V rail (right soldering point). The reading should be in low kiloohms but definitely more than 1K. If you read the resistance below 1K there is Hugh chance that a short, faulty memory chip or chips are there. Anyway good luck with the repair.
Claudius
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RetroTechie
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Location: Hengelo, NL
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Re: Hello from Canada

Post by RetroTechie » Wed Apr 18, 2018 6:19 pm

McKlaud wrote:
Sun Apr 15, 2018 7:13 pm
Hmm too hot to touch though? It seems to be giving out more then 5V closer to 7
Or measuring wrong - seen often when people (try to) check voltages.

For 78xx regulators like the 7805 in the photo, seen from the front (printed) side, pins from left to right are input, GND, output. So you'd put your voltmeter black lead on the middle pin (or safer/easier: on the metal tab, which is connected to middle pin), and red lead on the right pin (in the photo, the bottom pin of the 3). If result is outside of 4.8~5.2V range, chances are regulator overheated & died @ some point. Although 78xx's are pretty robust & used to running hot. ;) That or some memory chip died & works as a short circuit. Maybe that could be detected by feeling if any of the logic/memory IC's get hot. Or noticeably hotter than their neighbours.

Btw not sure why a 5V regulator is there anyway, with regulated 5V on the edge connector. Maybe they wanted to avoid extra heating of the ZX81's internal 5V regulator, and added that 7805 to move the added heat loss outside the ZX81 case? :?

If you power the ZX81 with a low enough voltage (7~8V regulated DC rather than the 11~12V usually coming from original power supply) or replaced that with a switching 7805 drop-in such that internal heat losses are limited, you could just remove that extra 7805. Like: cut it out, and re-wire its output pin hole (5V) to come directly from the edge connector. If you leave it in there, adding a small heatsink wouldn't hurt! (but beware of creating short circuits). Also (if you can get your hands on some), with a bit of re-wiring you could replace the 8 pcs 64Kx1 RAMs with 2 pcs 64Kx4 bit RAMs. Roughly similar power consumption per IC, but only 2 vs. 8 of them.

Dead logic IC's are possible, but rather uncommon in my experience. Anyway, being single 5V supply (and 64K!) this RAM pack would be a better place to start than a 4116-using 16K RAM pack with multiple supply voltages inside (and crappy circuitry to generate those).

Ogien
Posts: 13
Joined: Thu Nov 28, 2013 12:27 am

Re: Hello from Canada

Post by Ogien » Thu Apr 19, 2018 2:47 am

RetroTechie wrote:
Wed Apr 18, 2018 6:19 pm
McKlaud wrote:
Sun Apr 15, 2018 7:13 pm
Hmm too hot to touch though? It seems to be giving out more then 5V closer to 7
Or measuring wrong - seen often when people (try to) check voltages.

For 78xx regulators like the 7805 in the photo, seen from the front (printed) side, pins from left to right are input, GND, output. So you'd put your voltmeter black lead on the middle pin (or safer/easier: on the metal tab, which is connected to middle pin), and red lead on the right pin (in the photo, the bottom pin of the 3). If result is outside of 4.8~5.2V range, chances are regulator overheated & died @ some point. Although 78xx's are pretty robust & used to running hot. ;) That or some memory chip died & works as a short circuit. Maybe that could be detected by feeling if any of the logic/memory IC's get hot. Or noticeably hotter than their neighbours.

Btw not sure why a 5V regulator is there anyway, with regulated 5V on the edge connector. Maybe they wanted to avoid extra heating of the ZX81's internal 5V regulator, and added that 7805 to move the added heat loss outside the ZX81 case? :?

If you power the ZX81 with a low enough voltage (7~8V regulated DC rather than the 11~12V usually coming from original power supply) or replaced that with a switching 7805 drop-in such that internal heat losses are limited, you could just remove that extra 7805. Like: cut it out, and re-wire its output pin hole (5V) to come directly from the edge connector. If you leave it in there, adding a small heatsink wouldn't hurt! (but beware of creating short circuits). Also (if you can get your hands on some), with a bit of re-wiring you could replace the 8 pcs 64Kx1 RAMs with 2 pcs 64Kx4 bit RAMs. Roughly similar power consumption per IC, but only 2 vs. 8 of them.

Dead logic IC's are possible, but rather uncommon in my experience. Anyway, being single 5V supply (and 64K!) this RAM pack would be a better place to start than a 4116-using 16K RAM pack with multiple supply voltages inside (and crappy circuitry to generate those).
I think the regulator is shot. It starts at 5V but it keeps climbing as the regulator gets super hot even up to 9V.

My power supply is 9.75V so cutting it off may not be the best idea, but the 7805 regulator is a $1.25 at my local electronics store so I will see if I can pick one up before the weekend and see if I can fix it.

I will keep you posted with the result.

Also none of the other ICs get hot.

makeit2day
Posts: 28
Joined: Sat May 10, 2008 2:31 am

Re: Hello from Canada

Post by makeit2day » Thu Apr 19, 2018 5:10 am

If your power supply is at 9.75V you could reduce the voltage down by adding a 1N4001 (50V@1Amp) in series with the input to
your computer.

McKlaud
Posts: 107
Joined: Tue Dec 19, 2017 10:02 pm
Location: St Albans, UK

Re: Hello from Canada

Post by McKlaud » Thu Apr 19, 2018 10:55 am

Any unregulated PSU gives you more voltage than its rated voltage if the load is lower that the nominal load. Make the story shorty, it is uncommon that the ZX81 PSU gives approx. 10.5V when connected to the ZX81. In idle (no load connected) the PSU output voltage can reach 14.5V easily. Bear in mind that these PSU are rated 9V for 700mA load.

There is no harm to the Zeddy being supplied with 10.5V volts at all. All internal logic chips are fed from the 5V regulated rail.The main concern might be heat from the power regulator (7805) as it needs to "eat" more volts i.e. more heat generated.
Claudius
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Ogien
Posts: 13
Joined: Thu Nov 28, 2013 12:27 am

Re: Hello from Canada

Post by Ogien » Thu Apr 19, 2018 5:48 pm

McKlaud wrote:
Thu Apr 19, 2018 10:55 am
Any unregulated PSU gives you more voltage than its rated voltage if the load is lower that the nominal load. Make the story shorty, it is uncommon that the ZX81 PSU gives approx. 10.5V when connected to the ZX81. In idle (no load connected) the PSU output voltage can reach 14.5V easily. Bear in mind that these PSU are rated 9V for 700mA load.

There is no harm to the Zeddy being supplied with 10.5V volts at all. All internal logic chips are fed from the 5V regulated rail.The main concern might be heat from the power regulator (7805) as it needs to "eat" more volts i.e. more heat generated.
Now I am really confused about how to fix this :(

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