ZX81 Tape Woes...

[Rink]

This is really helpful stuff, thanks guys. If I ever get this damn thing working and returned to its owner, I may have to buy my own. I'm kinda growing attached to it through all the stress - Stockholm syndrome maybe.

Unfortunately my friend is away this week - so I'd better wait before soldering anything in there. I'll try a few of the other suggestions first, as soon as time allows. Can't remember where he bought it from - may have been the people who did the video mod, but it seems strange that they didn't notice it couldn't load tapes.

Cheers.

[1024MAK]

ZX81 tape circuit S.png

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From your photo it looks like C10 has never been fitted.
Without C10 I don't see how the audio signal can reach the ULA tape input on pin 20.
And I cannot see any other capacitor anywhere else that would do the same thing (couple the input signal from the Ear socket to R33).

If you do not wish to solder in a capacitor (that costs less than 50p) then you could rig up a temporary test set-up using some test leads fitted with clips (e.g. Maplin Quick Hook-up Test Lead N90AP £5.99 or Probe to Croc Clip Leads FE23A £5.99).
I suggest clipping the leads to R34 and R33.

Any of these capacitors will do:-
BX00A Disc 0.01uF 50V £0.39
WX77J Ceramic 10000 £0.29
RA44X Monores Cap 0.01uF £0.39

I have quoted Maplin parts simply because you should be able to get them in most Maplin stores. If you have another supplier near you (or you mail order) then they will most likely be cheaper. Also note that the Maplin web site is rubbish these days, for the capacitors use the scroll bar on the right...

Mark

[1024MAK]

Oh and can you post a better picture of the area where C10, C11 and C12 are marked on the PCB (just behind Mic socket) as it is hard to tell if C12 has been fitted in the normal position.

Mark

[Paul]

Maybe my info leads to nothing. But last weekend I was shocked when I found out that a few resistors in my issue one Zeddy were missing, one of them the one charging the reset capacitor (R35?). I wanted to add them and when I wanted to get the hole free of solder I turned round the board and found those missing resistors mounted on the bottom side
There were some more parts on the bottom side (capacitors and diodes), did you have a look there? Sinclair was some kind of creative in their production.
Maybe for completion you could add a picture of the bottom side?

[XorA]

Erk was that the board I sent? Ill have to check out the rest as they were messier than that one!

[RetroTechie]

Maybe for completion you could add a picture of the bottom side?

From your photo it looks like C10 has never been fitted.
Without C10 I don't see how the audio signal can reach the ULA tape input on pin 20.

Rink's photos already show top and bottom of where C10 should be, and it clearly isn't there (and it shows the tape input not going anywhere else, apart from R34 to ground). So there's no way for the tape signal to reach the ULA pin, which explains why loading patterns don't respond. Who cares about C11/R27 as long as they're not connected to the tape input circuit.

Adding C10 back in is virtually zero-risk (10 nF is a very small capacity so it wouldn't do any permanent damage even if soldered between the wrong signals), so because it's clear that's the problem I'd just do it without asking that friend first. You could also cut the capacitor's leads to length, and bend them to sit like a spring in those holes, without actually soldering the part (just for testing, don't leave it like that!). Or borrow any of C2, C4, C8 or C9 (47 nF, not optimal but should work I think) for a quick 'n dirty test.

[Rink]

I just want to make sure he's fine with it in case something happens like... tearing the keyboard connector, burning something etc. if it was mine I'd be straight in there with the solder.

Clips are a good idea though.

Hoping to do a bit more on it tonight so will let you know how it goes.

Edit: forgot to thank Mark for finding Maplin part numbers for me - I hate their web site so you've probably saved me about half an hour. Cheers mate.

[Rink]

Yep, I think we can be pretty sure the audio input is not reaching the ULA. I just tried sending the signal direct (at a very low level) to pin 20 and immediately the screen started changing. Unfortunately I couldn't hold the wires steady enough to test a full load.

Will go find a cap and report back.

Thanks again chaps.

[Paul]

Rink's photos already show top and bottom of where C10 should be, and it clearly isn't there

So my guess my info would not be helpful is true.

[Rink]

So my guess my info would not be helpful is true.

Definitely interesting and amusing though. To me anyway.