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ZX81 keyboard help

J

jakk

Member
Joined
Aug 24, 2007
Messages
21
Hi. I am putting together a ZX81 kit. I have it fully assembled and I just need to install it in the case. Right now, I am attempting to connect the keyboard to the pcb connectors. The problem is, I can't get the keyboard tails to plug into the connectors, they refuse to go in. Is there some trick I should be using?
 
Hi. I tried looking for a notch to slide up, but I did not see anything. That sounds like a zif socket, but I don't think that these connectors are zif connectors. How would I go about using tweezers to put the ribbon cables? I'm kind of at a loss, I don't understand why this is so hard. The cables should go right in, but they refuse to.
 
jakk said:
Hi. I tried looking for a notch to slide up, but I did not see anything. That sounds like a zif socket, but I don't think that these connectors are zif connectors. How would I go about using tweezers to put the ribbon cables? I'm kind of at a loss, I don't understand why this is so hard. The cables should go right in, but they refuse to.
Click to expand...

No, just grab the end of the connector where the cable is supposed to slide in, and pull it gently towards you. If it's the kind I'm thinking of, the little end piece of the connector should slide about 1/8" forward, then you slide the cable in and push that end piece back, which locks the cable in.
 
Hi. I really can't find a part to pull out. Here's a picture of one of the connectors(yes,my awful soldering skills can also be seen...).

Picture

Here's another good example of what it looks like:

Picture two
 
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They look like plain connectors without locks. Instead of trying to insert the whole width of the cable try tilting it a little bit just to get a corner inserted and then straighten it out to persuade the rest of the cable; usually works for me when they're tight.

Good luck!

m
 
Well,I was able to get the ribbons in. It wasn't easy,though. I had someone else hold the pcb while I inserted a plastic shim along with the ribbon. This engaged the springs and pushed them back. I then fed the cable in and removed the shim. Now, I can't get a video signal(I must sound like an idiot..). The board is getting power because the heatsink for the regulator gets hot. Is there some way I can measure video output with a multimeter to make sure it's putting out the 60hz us signal and not the 50hz uk one? I did solder a wire for r30 like the manual said to select the us signal. Thanks for everyone's help so far.
 
No,this is the first time I tried it. I know it's getting as far as the voltage regulator because the heatsink gets warm. So,either some solder joint is bad, a component came bad, or I'm not tuning it correctly. That's why I was wondering if I could probe the video output somehow. If there's some place I can probe it, that would tell me if it's my tuning or not.
 
I doubt you could do it with a multi-meter. You'd need a scope to see the waveform and adjust for the correct amplitude.

You might want to check the output of the voltage regulator and see if it's the right voltage. If not, something "downstream" is drawing too much current. With an added heatsink, the regulator shouldn't be more than warm.
 
Is there any sign that the computer is actually working and it's just a video problem, or is it actually completely dead as far as you can tell?

m
 
I just probed the power supply(+lead on the tip and - behind the black ring), and it indicates 11v! Is this my problem, and did I fry my zx81?
 
Probably not, although it sounds a little high if the regulated voltage is 5V and would explain why the regulator's running hot. Is this the recommended adapter for that kit? And is that 11V with the computer connected or not?

What matters is the voltage on the regulator output; if it's a standard 3-terminal device one pin should be the 11V, another 0V, and the third 5V.

m
 
Ok. One is about 6.9v and the other is about 11.8v. The power supply is the one the kit came with.
 
Is the third one 0V? Are there numbers on the regulator chip? Are you sure your meter is correct?

If it should be 5V and is really 6.9V that's not really a good thing. It's pretty unusual though, since a regulator will do its darndest to keep the output where it's supposed to be, but when it fails it usually shorts or opens completely.

Are any of the other chips unusually warm?

m
 
The schematic shows the supply voltage as 7 to 11V, so 11.6 is definitely on the high side. Surprising that they'd ship an 11V supply, 9V is usual for something like this.

m
 
Ok. Ofter looking at the datasheet for the 7805 regulator, I can measure 4.95v at the output and 11.9v at the input. I was probing the wrong leads before. I can't get a voltage across anything else,though. I'm going to flow a bit more solder in the regulator leads and see what that does. I also noticed that the wire coming from the output lead of the regulator does not seem to be connected to anything. There are two holes in it for component mounting, but there are not components there. As soon as I find my good digital camera, I will post a picture of what I am talking about.

edit:Ok,here's the picture. The output of the regulator is in the bottom right. There is one big green line going from it, and then two holes with nothing in them. I'm not sure if something is supposed to go there, the manual does not mention anything. If anyone else has a zx81 and could post pictures of the same area, that could be helpful.

Picture

Update: I probed it a bit more, using the ground of the regulator as a reference for the voltage. I am detecting 3-4v throughout different parts of the pcb, I have been able to get a voltage on a least one pin of every ic socket, so the pcb is getting power throughout, at least in some parts. I'm thinking it's either me not knowing how to tune the tv or I wired the rf modulator. I've got an old tv circa 1987 that I'm going to try. The instructions indicate that I should be getting a signal on channels 30-40. I don't think it's like a timex 1000 where the tuning is done to channel 2 or 3. I believe the jumper wire just selects 60hz. If anyone has any additional info, that would be appreciated. I wish I had a scope to see if I'm getting output.


Update 2: When I plug in the zx81, the picture on the old tv goes from solid white static to a rolling/wav pattern, so I think some sort of signal is being produced. However, I cannot see any hint of a k prompt.

Update 3: Possibly more good news. My multimeter can measure frequency. When I put it in the video port, I get a solid 60hz measurement. So, at this point it would appear that the computer is possibly functioning ok. however, I still can't seem to get a picture. If it was outputting a pal signal, should I stilll be able to see a k prompt?

Update 4: Ok,I discovered that I had managed to solder the rf output in the wrong spot. I redid it to the correct spot, but I still don't get a picture. However, the ULA gets hot after only 30 sec or so of running..is my ULA shot? That would explain why I'm getting no video signal.

Final update: The person I bought the kit from sent me a new ULA free of charge as they have experienced a failure with about 5% of the kit ULAs. I received it today and swapped it out with my old one. I now own a working ZX81!
 
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