2 Oldies from under the dust. Can we fix them??

[Yobe]

Hi all,

Pulled 2 ZX81 from a cabinet. They are not originally mine. 1 (only the unit) I got from a school cleanup, claimed it doesn’t work.
Another one, unit, power supply and 16K, I bought a long time ago from EBay for low price, so almost sure also has issues. But never really had time to look into both.
Included some first pictures. As expected many screws missing, so has been worked on before.
I don’t have a old TV/monitor but current one has RF connection. Hooked another working device onto it to set the channel (no manual mode) but when connecting both of the ZX units, nothing happens on the screen....

I know ULA is the critical part. Don’t have my scope with me now, but I assume, after checking my 5v circuit, I better have a look to the ULA.
Best place to start is check if I have a clock signal. Right? (What pin was that again?)

Lucky both have similar ULA with socket . One from 81 and another from 82.

Anything else to check on the ULA?

Lets see where this adventure takes me

Greets

PS. Yes, 2 red wires are mine. Long time ago I used it to supply power.

[Yobe]

Ok,

At least I see power on the board and 5v on pin40 of ULA.
Hoped there was a power issue, but no luck...

Suppose better get my scope once for next analyse.
Hopefully soon as quite busy with other stuff. Sorry...

PS: as I get no image at all, could this also be related to ULA? Or should I see at least something?

[1024MAK]

The first thing to say is that as the modulator is completely analogue, it’s unlikely that they will transmit on the exact correct frequency. It’s even less likely that any two will transmit on the same frequency. Hence you have to tune each one in individually.

Even if the ULA is faulty, if the modulator is working, you should be able to see a change in the static shown when the ZX81 is switched on or off.

If the ULA is working, but either the RAM, ROM or Z80 is faulty, then most of the time you will get a completely white screen. But note that this is with a CRT or LCD TV that does not mind the sync signals not being present.

Often if the ULA has partly failed, it’s possible to get a black screen.

The most useful pin to test is pin 6 on the Z80, this is its 3.25MHz clock signal. If there is no clock signal, then it’s possible that the ULA is faulty.

If the clock signal is okay, what signals do you get on the following pins of the Z80 CPU?
Be very careful, if you short the probe between adjacent pins you may damage one or more of the chips.

Pin Function
16 /INT (should be pulsing)
17 /NMI (should be pulsing)
18 /HALT (should be pulsing)
19 /MREQ (should be pulsing rapidly)
21 /RD (should be pulsing rapidly)
22 /WR (should be pulsing)
24 /WAIT (should be pulsing)
25 /BUSRQ (should be high)
26 /RESET (should be high, except for a very short time after power is applied)
27 /M1 (should be pulsing)
28 /RFSH (should be pulsing)

Then test address lines A14 and A15. These give an idea of which memory the CPU is accessing and if it is trying to generate a TV display.
You can also test the remaining address lines A0 to A13 and the data lines D0 to D7. A lack of activity is suspicious.

Mark

[Yobe]

Me happy, based on the scope readings both devices should work.
I also see a video signal, although not sure if ok.

So I really need to get composite out now. Ok if I do the simple bridge of the ULA output? At least I should see something...

Now, my most important question: if both ULA’s work, what is the best “preventive action” I can do to keep them alive as long as possible. Add a heatsink? Or an mini fan?
Really wanna keep them going

Would you replace the 7805 to the xxx(forgot reference now). Or keep original?

Any good ideas on “cleaning” the power socket/tape? Sometimes bad contact.

Hopefully I can soon get an screen.

Thanks Mark for the ref to the pins

[Yobe]

But both flat cables look really bad. Will need new keyboards...

[Lardo Boffin]

Replacing the 7805 with a modern efficient regulator will produce a lot less heat in the case.

In my experience this keeps the whole computer a lot cooler.

The ULA does not really seem to generate a lot of heat.

[Yobe]

But I thought that ULA mainly fails because gets too hot?

[Moggy]

Me happy, based on the scope readings both devices should work.
I also see a video signal, although not sure if ok.

So I really need to get composite out now. Ok if I do the simple bridge of the ULA output? At least I should see something...

Now, my most important question: if both ULA’s work, what is the best “preventive action” I can do to keep them alive as long as possible. Add a heatsink? Or an mini fan?
Really wanna keep them going

Would you replace the 7805 to the xxx(forgot reference now). Or keep original?

Any good ideas on “cleaning” the power socket/tape? Sometimes bad contact.

Hopefully I can soon get an screen.

Thanks Mark for the ref to the pins

No doubt mark will be along to explain far better than myself the course of action you should take next but stop worrying about the ULA getting hot. I have yet to meet anyone in nearly 40 years who can prove measurably or scientifically that without any doubt what so ever that heat killed their ULA.
They just guess that heat was the problem without really looking into it.

The biggest ULA killer is failing RAM packs (Memotech especially), Interfaces plugged in to the back of the computer incorrectly due to a missing locater peg or interfaces attached or detached whilst powering up.

The two simplest things you can do to reduce heat in side the case is..

1) use a decent regulated power pack capable of outputting 7.5 volts with a rating of at least 1.5 amps,you would be surprised at how effective this method can be. The only caveat is that certain interfaces like the Memotech RAM pack require at least 9 volts which their internal circuitry increase to the 12 volts needed for the RAM chips. If not using such a RAM pack then all should be good.

2) Fit a switching regulator which runs very cool even with the original Sinclair power pack. There are various threads on the forum relating to this if you wish to seek them out.

As for fan cooling yes it can be done because I've done it but trust me that considering how easy the other two options are it's not really worth the effort and I only did it, at the suggestion of another forum member, as a proof of concept.

[1024MAK]

I hate it when Moggy has the crystal ball on full power... (it’s all that fur rubbing against it generating a high voltage static field)

So I really need to get composite out now. Ok if I do the simple bridge of the ULA output? At least I should see something...

You need a buffer transistor amplifier, as the ULA output is not designed to directly feed a composite video input on a TV or monitor. Something like this.

Now, my most important question: if both ULA’s work, what is the best “preventive action” I can do to keep them alive as long as possible. Add a heatsink? Or an mini fan?
Really wanna keep them going

Would you replace the 7805 to the xxx(forgot reference now). Or keep original?

Use a buffer transistor amplifier to protect against short circuits on the video output.
I recommend reducing the amount of heat in the machine. The series pass regulator, the 7805 is the main source of heat, so replace it with a modern DC-DC converter. You can use a Recom, a Traco Power or a MuRata type. See this topic.

Any good ideas on “cleaning” the power socket/tape? Sometimes bad contact.

Use Isopropyl alcohol (IPA) to remove any grease or oil and other general dirt. If the problem is tarnished contacts, use some IPA and then keep plugging in and unplugging the plug lots of times. If that does not work, carefully use a small flat bladed screwdriver with some IPA to try to clean the contacts. If it’s just a contact that is not making tight/firm contact, you can try pressing it down at the top of the side (with no plug in it) by using a flat bladed screwdriver. If none of this helps, it’s still possible to buy new sockets of the same type.

Mark

[1024MAK]

But I thought that ULA mainly fails because gets too hot?

The design of the ULA is such that it is intended to run hot. This is 1980s technology. Very low power chips came along later...
And for some reason, people think that if it’s too hot to touch, something must be wrong. Yet they don’t do that with valves (tubes), conventional filament light bulbs, or internal combustion engines...

The internal operating temperature of a semiconductor chip can be up to between 100°C and 125°C

Mark