How to determine if my ZX81 works or not?

[sweh]

So I just received a Timex 1000. Plugged it in... and got nothing. Now I dunno if that's just because I've plugged it all in wrong or if the machine is dead.

Are there any tests I can do to determine if the machine is working properly or is broken? eg if I hook a scope up to the video out is there are any pattern I should see?

[1024MAK]

Full(ish) support is available at http://www.sinclairzxworld.com

I assume that you are using the correct PSU. Check the 3.5mm jack as poor connections here are common. You may need to retension the socket contacts with a flat bladed screwdriver.

You should get a white display with a inverted K in the bottom left hand corner.

The only available video output is RF. The modulators are not that stable, so modern colour TV's do not always find the signal (as it is often not at the specified "spot" frequency).

It is possible to modify a Zeddy to output composite video. Some (as in most) mods these days use the modulators RF existing connector to output the composite video (the modulator is normally disabled if this is done).

There was more than one version of the ZX81/Timex ULA. The early ULA versions do not produce the part of the TV signal known as the back porch. This did not matter for black and white CRT TVs. However modern colour TVs often use this part of the video signal to work out how to process the signal. Not all TVs do this. It is not possible to tell until you connect a ZX81/Timex 1000 and try it. The symptoms are a very dark display with very poor contrast or no usable signal at all on RF.

The last version of the ULA produced by Sinclar, the 2C210E does produce a back porch. As a result many more TVs will work with a ZX81 and produce a nice bright picture with good contrast.

The poor picture (resulting from a missing back porch) can only be fixed by 4 methods:
a) Using an old black & white TV
b) Buying and fitting a replacement ULA (type 2C210E or ZX-ULA2)
c) Buying and fitting a ZXVid in the place of the TV modulator
d) Buying and fitting a ZXCCB

The ZX81/Timex 1000 has no sound capability. If you were used to the keyword entry system, one possible way would be to blind "type" SAVE "T" with a mono tape deck (or suitable lead to connect to an audio system) connected to the Mic socket on the ZX81. Then see if the Zeddy saved tones on the tape / makes a noise. Note SAVE is entered with one button press and is NOT typed letter by letter. Also note that the output level is very low. Using a line level audio input is no good.

Also the left side of the keyboard will get noticeably warm after the machine has been on for half an hour or more.

I hope this helps

Mark

[1024MAK]

Are there any tests I can do to determine if the machine is working properly or is broken? eg if I hook a scope up to the video out is there are any pattern I should see?

Yes! The schematic diagram is attached.

Check the +5V supply pins. Then use a logic probe or a 'scope to test the pins on the Z80 CPU.

This thread may help: http://www.sinclairzxworld.com/viewtopic.php?f=7&t=1236

Also repair guides here:-
Servicing Sinclair Computers Part 1
Servicing Sinclair Computers Part 2

Mark

[sweh]

Ah ha. That diagram helped. So although the PSU claims 9.75V my multimeter claimed it was closer to 12V. That could just be my meter not being properly calibrated

So I took the bottom case off and measured some voltages. In particular on the expansion slot I could find the nominally 9V line (again, reading 12V) but there was nothing on the 5V line.

This made me take the risk of undoing the board and carefully looking at the component side. Where I find that the ribbon cable had cracked and broken and there's no connection to the keyboard at all. Worse there's a low level of corrosion across the whole board, as if it's been stored somewhere not condusive to maintaining the electronics.

No wonder I didn't get any picture from it!

[1024MAK]

The typical output from the PSU is 10.5V to 11.5V. It is an unregulated design and will only produce 9V at full rated current.

The ZX81 / Timex 1000 on it's own draws rather less current than this.

It sounds like the 7805 voltage regulator is faulty, or there is a dry joint or broken track.

Broken / cracked keyboard tails are a very common fault. New ZX81 keyboard membranes are available (there are two different labels on a ZX81 keyboard compared to a Timex 1000). See here.

Mark

[Prime]

Ah ha. That diagram helped. So although the PSU claims 9.75V my multimeter claimed it was closer to 12V. That could just be my meter not being properly calibrated

So I took the bottom case off and measured some voltages. In particular on the expansion slot I could find the nominally 9V line (again, reading 12V) but there was nothing on the 5V line.

Best place to check this is at the regulator, middle pin is GND, and the other two pins are input & output so one should be the 9V and the other 5V.

Might also be worth re-flowing the solder (just heat the joint up till the solder melts and re-solidifies should be enough) on the pins of the regulator as it can sometimes become broken.

Cheers.

Phill.

[sweh]

OK, so the regulator is an MC7805. Something a little like this: http://tv.yoreparo.com/foros/files/mc7805ct.jpg

In this I get a 12V difference between the middle and the left. I also get 12V between the left and the right. I get no voltage between the right and the middle; I do get a 1.5kOhm resistance between those two pins though.

[MartinB]

This is the basic in-circuit schematic of your regulator and you should be able to see what to expect from this. Sounds like the 7805 isn't working but the only word of caution is that often these could appear to be doing nothing if there's zero current being drawn from the regulated side but it's very unlikely for there not be even a tiny passive load. If it was working, apart from the obvious I/O voltages, you'd also potentially see between 4v to 7v across the two active pins (left/right.)

Additional thought, check (with power off) that there isn't a short-circuit between the output pin and ground (0v) - your measurements suggest that might be the case or if not externally, that the chip has become internally shorted. This would cause the the 7805 to shut down and give what you're seeing without there necessarily being any smoke....

[sydney]

How to determine if my ZX81 works or not?

Does the door stay open?
I'll get my coat.

[danielj]

How to determine if my ZX81 works or not?

Does the door stay open?
I'll get my coat.

[jonb]

Oh it's all well and good for you posh kids to take the mick out of our ZX-81s, what with your posh BBC Micros and the like! But we were hand assembling Z-80 machine code and POKE-ing it into memory long before your expensive Beebs were off the drawing board!

[1024MAK]

If you view the 7805 so that you can read the type number, with the metal tab upper most, the left hand leg is the input, the middle leg and the metal tab are 0V/ground and the right hand leg is the output.

Replacements don't cost much.
When fitting a replacement ensure that the aluminium heat sink is firmly attached to the 7805 regulator.

Mark

[Prime]

Oh it's all well and good for you posh kids to take the mick out of our ZX-81s, what with your posh BBC Micros and the like! But we were hand assembling Z-80 machine code and POKE-ing it into memory long before your expensive Beebs were off the drawing board!

Been there, hand assembling it from the character set listing at the back of the manual (ZX81, but was also in the Spectrum and probably ZX80 too), which conveniently listed all the Z80 mnemonics, includeing the modified ones.

Cheers.

Phill.

[jonb]

Yep, and it worked too. I got a speaker to beep via a parallel port card by pulsing one of the lines. So I thought, I wonder if I can make it do two tones using a nested loop? I worked on that one for a good few hours, but stupidly didn't save it before running. It crashed, and I gave up. Darn!

[danielj]

I've poked hex into a REM statement on a ZX81 I also have two. Only one is being used to hold a door open

d.

[1024MAK]

None of my retro gear will fit under a door... My doors are correctly sized
But Beeb PSU's are heavy enough to stop a door blowing shut if needed

Also unless someone is in attendance to watch the lovely warm glow of any fire, it is good practice not to wedge doors open

Mark

[Prime]

Also unless someone is in attendance to watch the lovely warm glow of any fire, it is good practice not to wedge doors open

You obviously don't have cats.....I guess we could fit cat flaps internally

Though to be fair normally when we are out most of the upstairs doors are closed.

Cheers.

Phill.

[pkersey]

(Please disregard this message in case you know about all this.)

Timex PSU's outputting 12V aren't that abnormal. They're expected to drop voltage as soon as some load is on them and even if not, the regulator should do its job. There's a number of Sinclair compatible equipment around the world designed like that.

I'm not sure how US equipment would behave in a UK telly through RF, so there's a chance that the computer is working but you get no perceivable image.

TS-1000s are meant to work anywhere between US Channels 2-4, so you shouldn't expect to get a picture on channel 36.

[1024MAK]

This what they look like when they get too hot

And cats are incompatible with my chief EUP tester...


Mark

[sweh]

I'm not sure how US equipment would behave in a UK telly through RF, so there's a chance that the computer is working but you get no perceivable image.

TS-1000s are meant to work anywhere between US Channels 2-4, so you shouldn't expect to get a picture on channel 36.

Well, I'm in the US (moved here in 2001) with US TVs

[sweh]

If you view the 7805 so that you can read the type number, with the metal tab upper most, the left hand leg is the input, the middle leg and the metal tab are 0V/ground and the right hand leg is the output.

Replacements don't cost much.
When fitting a replacement ensure that the aluminium heat sink is firmly attached to the 7805 regulator.

Mark

Hmm, I might see about doing this. But because the ribbon cables are broken this might turn into an expensive fix-up job... and I never even had a ZX81 BITD, so I'm not sure I want to devote the energy.

Alternatively it could turn into a good hardware learning exercise with minimal downside (if I blow it up, no real loss ).

Thanks for everyone's input!