Spectrum 128 +2 grey loading issue

acsi
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Re: Spectrum 128 +2 grey loading issue

Post by acsi »

Yes. With the tape deck disconnected I get

Pin 34 = 0.5v
Pin 35 = 0.4v

and the ULA test passes saying EAR stays Low
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1024MAK
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Re: Spectrum 128 +2 grey loading issue

Post by 1024MAK »

Looks like time to investigate the tape deck then. Alas, I’m not going to be much help with this, as I’ve never needed to get into detail with any of mine (other than the more usual stuff that we’ve already talked about).

I do have an Amstrad service manual. But I think a copy is also online.

Mark
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acsi
Posts: 47
Joined: Tue Apr 23, 2013 12:03 pm
Location: New Zealand

Re: Spectrum 128 +2 grey loading issue

Post by acsi »

Thanks for your help Mark.

I did a few tests to try isolate it to a certain area. I hooked up an external tape deck to GND and Pin 5 of the LA6324 and also tried with a cassette adapter. Both gave the same results.

In doing this though I did notice that the sound level is variable (after a lot of use its to the point where the red and blue stripes vanish and come back) rather than constant so possibly incorrectly I have ordered a replacement LA6324 and I'll let you know how I get on.

Mike
acsi
Posts: 47
Joined: Tue Apr 23, 2013 12:03 pm
Location: New Zealand

Re: Spectrum 128 +2 grey loading issue

Post by acsi »

Update

Replaced the LA6324 on the tape deck PCB. Sound from the tape is now constant and the border stripes are more stable however I still get a reset or tape loading error with the occasional success (usually when the machine is cold).

I went back to testing with the Divide plus when the machine was warm and get almost constant tape loading errors now with it when loading from .tap.

My next step (rightly or wrongly (probably wrongly knowing me)) is to replace the ULA. Luckily vRetro is local (country wise) so I've got a vla128 and a turbo sound 8192 replacement on the way (was thinking about doing this anyway).

Hopefully this will get it working properly.
acsi
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Joined: Tue Apr 23, 2013 12:03 pm
Location: New Zealand

Re: Spectrum 128 +2 grey loading issue

Post by acsi »

Well that's interesting. It failed loading Jetpac after the sometime after the loading screen (Blue and Yellow stripes stopped). Further tries fails to display constant blue and red stripes (tried adjusting the azimuth and only get an occasional glimpse).
So tracing the circuits from Pin 34 of the ULA to the cassette interface and I can see the 680ohm resister. Pin 34 of the ULA to Pin 5 of IC11 and I can see the capacitor and 68 ohm resister. However I can get no connection between pin 34 of the ULA and R102 (soldered underneath across C48) or C48.
R102 and C48 are connected to GND on one side but I cannot find a trace leading from the other side of them?
acsi
Posts: 47
Joined: Tue Apr 23, 2013 12:03 pm
Location: New Zealand

Re: Spectrum 128 +2 grey loading issue

Post by acsi »

So I made the missing connection between R102 and Pin34 of the ULA. I now get the red and blue and Blue and Yellow stripes and successfully loaded Jetpac once. Every other attempt failed with a reset towards or at the end of loading.

My Divide now fails to work and if it does then the directory is messed up.

I'm thinking I'm on the wrong track and something else is wrong. I'm running it through a soak test with my diagnostics rom but so far it's passing fine.

Any ideas on what else I should check?
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1024MAK
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Re: Spectrum 128 +2 grey loading issue

Post by 1024MAK »

Which issue board do you have?

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
acsi
Posts: 47
Joined: Tue Apr 23, 2013 12:03 pm
Location: New Zealand

Re: Spectrum 128 +2 grey loading issue

Post by acsi »

It's an Z70500 issue 3.
acsi
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Joined: Tue Apr 23, 2013 12:03 pm
Location: New Zealand

Re: Spectrum 128 +2 grey loading issue

Post by acsi »

Well this is turning into a mission and in normal fashion for me it'll be the last chip or thing I change that gets it working.

I've ordered a ZXMUX to replace the 40058 chip which I suspect is the culprit and have removed the old one and soldered a socket in place for the ZXMUX however putting the old one back in to test I now just get garbage on the screen so either the 40058 failed further in the process or a ram chip has decided to fail completely.

Now to wait a few weeks for the postie. I'm determined to get this thing working though.
acsi
Posts: 47
Joined: Tue Apr 23, 2013 12:03 pm
Location: New Zealand

Re: Spectrum 128 +2 grey loading issue

Post by acsi »

So a few more hiccups with me dropping the glass tube from my desoldering iron and smashing it (replacement took a few weeks to arrive) and then accidentally destroying the regulator (Now replaced with a Traco one).

I've desoldered the 40058 chip and put in a socket as well as socketing the ram. However either I've messed up the soldering on the socket or the actual cause of the problem has gotten worse as now I just get garbage on the screen (This was after removing the 40058 and testing with it in the socket). Initially I thought it was just the 40058 not liking the heat from desoldering but even with a new ZXMux it is the same.

I've tested all the ram chips (and swapped with others) and swapped out the Z80 and ULA. I've even removed and tested IC6 and IC16 (Might try IC15 next) but no joy.

I think I now need to compare the signals on my 48K to try and see what's not working right.
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