Video Problem

[floydbloke]

My father very kindly dug the old Zeddy out of the closet and sent it down to me.

In high anticipation I hooked and powered it up, unfortunately to be presented with a screen that looks like this.

You can just make out the cursor.

When I switch input sources on the TV (tuner card in PC in this case) then back to 'TV', very momentarily the screen looks as it should , but then quickly fades to black as per the attached image.
Fine-tuning the frequency has no effect. The ZX81 is a NZ model, with an option switch for Ch 2/3 at the bottom of the unit, Ch 3 appears not to work at all.
I rudimentarily fitted the composite video mod as per http://www.zx81.de, with the same result except the picture qualtiy was slightly worse. Again though, this time flicking the switch on the bottom from 2 to3 and back would for an instant show the correct grey screen with black k, only to fade immediately.

Any thoughts as to what the problem might be and, more importantly, would it be fixable for not too much cost.

[sirmorris]

Howdy!

This looks like the 'back porch' problem. Early ULAs didn't form their video signal correctly, it was missing the part which a receiver uses in order to set the black level of the image.

This wasn't suck a problem in the old days with old days TVs, but the more moderner the TV, the more picky it is about its signal.

You most likely have two options. Get an old b/w portble TV from a jumble sale/car boot/freecycle or replace the ULA.

Here's some more info:

viewtopic.php?t=54#p349


Good luck!

[floydbloke]

Thanks for the help as always sirmorris, my symptoms certainly look very much the same as the discussions you pointed me at. I'll check the ULA number when I get home tonight to confirm. (My apologies too, I should really have searched a bit harder than taking the lazy way out by just posting a question).

Unfortunately neither of the options you suggest are very appealing in that I don't have the physcial space for another telly, and here in NZ I don't fancy my chances of being able to get hold of another ULA. ZX81's for sale are few and far between, let alone their parts.

I might have a crack at Andy Rae's cirucit though, even with my limited electronics and soldering expertise I might be able to pull this off. On this topic, I'm struggling to get hold of an LM1881N locally, could I use a LMH1981MT instead (National Semiconductor will send me a sample free of charge) and if so what do I need to do with the pins that I don't use. Can I leave them 'floating'?

Also, the BF199 is proving difficult to source, are there equivalents I can use, e.g BC548 or BC639. (apologies again, other than PNP/NPN I really don't have much of a clue about the differences in transistors).

[floydbloke]

Some progress

I've managed to get hold of all the right bits for version 3 of Andy Rea's mod as described towards the end of this thread:
http://forum.tlienhard.com/TS1000/www.t ... 795758.htm

No success with version 3 unfortunately, I just get a black screen. I managed to get version1 going with a pretty horribe picture quality, but I had to use slightly different resistor values for some as that was all I have.

My preference would be to get v3 going and maybe someone can help. I don't have access to a scope, only a cheap multimeter and did a voltage comparison as described in the link above. My Q3 base voltage is only a fraction of a volt nowhere near the 1.93 that Andy achieved. Q3 emitter and Q2 collector voltage are also virtually nil, maybe as a result of the above?? I put my meter on pin 1 of IC1 and it gave a reading of approx. 4.5v DC, measuring the anode side of D2 reads virtually nil also.

I don't mean to sound emotive but I'd be really grateful if anyone can offer any help with this, I'm keen to get this baby up and running.