ZX81 repair and upgrade. Help! I now have a white screen output

[Tezz]

Hi to all,

This is my first post here, I've recently become interested in developing for the ZX81 and thought It'd be a good break from coding first to repair, restore and upgrade a few ZX81's to keep them living on and put them back into circulation. I ended up buying eight along with a 16K RAM pack and some power supplies. Six of those eight ZX81's I bought were functional so I started working on one of the non-functioning boards which was an issue one. It had bad RAM so I removed that and then tested the board with the 16k Sinclair RAM pack. All good, started up to the K with the expected longer wait with the 16k installed. I continued on to building the video conditioning circuit designed by Alastair at ZX ZIGG onto a stripboard and then removed the modulator from the board and took the circuitry out before installing the new circuit. I connected the ZX81 up to the composite input of the only remaining CRT TV in the house (a 100Hz Panasonic TV bought around 1999) powered on and got a black screen. Moved over to another room to take a look at it via a Samsung LCD TV which has composite input, powered on and could just about make out a very dark output with the K so I could at least see that the board was still alive. This board had the early 2C158E ULA's with the date marked 8119, it had a suspect raised heat mark on the surface so I assumed that this ZX81 had been well used in it's day and so was one of the many that became faulty being pushed beyond Ferranti's design. I read online that the damaged ULA's continue to function somewhat but produce a very dark output and not just from the missing back porch which would have been rectified by the new circuit. Rather than pulling chips from the other system for testing I bought a parts board which was fully socketed and had a 2C210E ULA included. I didn't test the parts board as it was missing it's modulator however. As the old ULA was thought to be faulty I snipped all the legs off carefully and then removed each leg individually with the soldering iron and plyers so that I didn't damage the board. I cleaned the board and checked that there was no damage or any bits from the desoldering anywhere on the board and noticed that one through hole ring on the reverse side of board had come away during desoldering somehow even though I was extremely careful. The ring on the top side of board was still present luckily. I went ahead regardless and installed a 40-pin socket as I thought that pin will still make contact from the top side of the board. Plugged in the new ULA and powered the board on, got a white screen and the 16K RAM pack was making an audable buzz I quickly tested the RAM pack again with one of the other working ZX81's and it does still seem to be working, or at least the system powered on with the longer wait for the K. The audible buzzing was still present plugged into the working ZX81 however.

I've read here on the forum that the white output could be a bad connection with the RAM pack although I'd previously cleaned the expansion connector with IPA before starting the repair and the RAM pack was working on this board before I took out the old ULA. I'm not sure if my replacement ULA is functional, I'll have to see if one of the working systems has a socketed ULA so that I can test it, I should really have done that before hand I know. I'm also not sure whether I've made a mistake building the video circuit. Two of the three resistors that I bought, 10kohm and 68ohm were both 5% metal film rather than 1% that was specified. If I understand correctly this means the resistance can be up to 5% +/- of the stated value so I don't know whether this will be an issue. The through hole ring on the back of the board that was missing was ULA pin 26 (D3). Looking at the schematics the data lines are directly wired to the Z80 so ULA pin 26 (D3) goes directly to Z80 pin 8 [D3]? Can I check for continuity pin to pin? I suppose I could add a jumper wire if that is the cause? I've no idea what's happening with the buzzing RAM pack, can anyone advise what that could be?

thanks, any help or advice would be appreciated

[1024MAK]

Hello and welcome

The Sinclair 16k RAM pack contains a DC to DC converter / inverter (a type of switched mode power supply). It could be the coil (high frequency transformer) that is making the noise. ZX Spectrum computers use the same arrangement and early models are known to buzz loudly.

All PCB track connections between the ULA and the Z80 are essential, so yes, you need to test the damaged track. Use a digital multimeter on a low resistance (Ohms) range. Typically use a 200 ohms range (or equivalent range). You are looking for a resistance of zero to 2 ohms. Or use the continuity testing range. If the track / pad is open circuit, repair it with suitable sized (thin) solid core wire.

Using 5% resistors in place of 1% should not make any significant difference in this circuit.

Good luck, and please keep us informed

Mark

[Tezz]

Thanks Mark

I'll test the connections in the morning between the ULA pins and the Z80 pins with the multimeter like you suggested. I started using wire stripped from cat5 cable as jumper wire but I didn't find it easy to work with and I found the insulation melts extremely easily when soldering the connections so I've now bought some pre tinned breadboard jumper wires. I'm not sure what the gauge is but hopefully it will be sufficient if I need to fix the connection between the ULA and Z80 pin.

I'm relieved to hear that the resistors I used in the video circuit should't be a problem. I'll try to read up with regard to the RAM pack.

thanks for your help and advice, I'll post an update tomorrow.

[mrtinb]

There are many threads in this forum with help for broken ZX81s. I got help here: viewtopic.php?f=7&t=1851

I was advised to use a digital probe to find the problem.

[Tezz]

Thanks Martin, I had a read through that thread. I will buy a logic probe to help diagnose these problems, that will be helpful.

If the ULA turns out to be faulty, I guess any of the NOS ULA's are all sold by now? I'd rather buy a working ULA instead of pulling one from another working board which would defeat the purpose. I've already bought some heatsinks to help to keep them alive for the future. I'm indending also to install a modern DC power converter instead of the old linear 7805 to bring the temperature down. Hopefully that should see them into a longer future.

[mrtinb]

I have a working ZX81, and a broken. In the thread Create ULA for €5 I have project going where I'm trying to create the ULA with a CPLD. On this hobby project there is not much progress at the moment.

[Moggy]

I've already bought some heatsinks to help to keep them alive for the future.

FTFY.
To help insulate and keep in all that lovely ULA heat for those cold winter evenings.

ARRRGGGGHH NOOOOOOOOOOOO!!

Mark, Karl we have another "heat-sink-in-a-closed-system" user help me burn him with fire and beat him with sticks.

Sorry Tezz it's an in joke TOTALLY at my expense.

Welcome to zeddy world and top respects for buying eight at a time.
There is a ton of excellent advice,knowledgeable folk and reading here and,at the risk of embarrassing one guru in particular, can I say that Marks' clear concise and to the point advice is invaluable re knackered zeddies and he has bailed me out more than once and will do the same for you I'm sure.

The DC converter jobby?
Best money you could ever spend for cool running.

At this point Moggy slinks off like a gibbering buffoon muttering to himself..
"He puts the heat-sinks in the furnace and melts them down He puts the heat-sinks".....

[Tezz]

Some progress this morning.. I checked with the multimeter between pin 26 of the ULA to pin 8 of the Z80 @ 200 Ohm range, display showed a higher figure quickly dropping and holding at 0.065. I cleaned the expansion connector again with IPA which did take some more grime off. connected the system and 16k RAM pack up to the TV composite and got the same white display. I went through the other working systems to see which if any were socketed, found one with another older 2C158E ULA date marked 8120, swapping that ULA into the board I then got a very dark display with vertical lines. I then tried the 2C210E ULA in the working board and that looks to be faulty. I put the working ULA back into the working board to re-test and that is still working fine so it seems I have a problem with my board and a faulty replacement ULA. One thing I didn't mention is that I haven't soldered the modulator case legs back to the board yet so it's just sitting in the holes. I wanted to make sure my video circuit was working before putting the worlds supply of solder back onto the legs I notice that one of the modulator legs goes to the boards ground plane, as an amature I don't know whether that will prevent it from working? I'm not sure how to progress now.

I guess the the heatsinks I bought were not such a good idea then. One of the systems I bought had a large hole drilled into the top of the case over the ULA so I guess the previous owner was concerned too I bought another case for that one. I did actually consider the heat building up inside the case but I thought that it would still be beneficial to dissipate the ULA heat. At least with the 7805 out of the way there should be a lot less heat building up in the case.

[Moggy]

Hi Tezz.

Please ignore my ramblings as I'm the forum klutz as for heat-sinks go for it,it's just something the guys like to beat me over the head with when I go off on one.

Get the DC converter it really does make a difference and trust me when I say the guys here will have you up and running in no time at all.


Regards.

Moggy.

[1024MAK]

@Tezz - darn it!
You've woke the sleeping cat up!



Mark