Diagnosing tape loading issues

Discussions about Sinclair ZX80 and ZX81 Hardware
patters
Posts: 85
Joined: Thu Mar 11, 2021 12:34 pm

Diagnosing tape loading issues

Post by patters »

My Issue 1 ZX81 works perfectly when tape loading from my Maxduino device with latest v1.64 firmware, stated to have improved ZX81 reliability.
My Issue 3 however will not load anything using the exact same setup (even the audio cable).

My LCD TV does continue to show the wobbly picture during loading and on the working machine I can see certain distinct wobbles in the pattern as information is loading, which I simply don't see on the non-working machine. Even power cycling the TV after the program should have loaded does not help. The screen stays de-synced until I hit BREAK then it returns to normal - so it's not crashing - it's as if there's no input at all going to EAR.

Where do I start with troubleshooting this? AFAIK the levels on a Tzxduino/Maxduino are appropriately loud, and besides there is no adjustment possible. What is this likely to be, a failed component, ULA trouble? Tracing back from the EAR input there's a resistor R34 and then ceramic capacitor C10, then resistor R33, then to pin 20 on the ULA. All joints seem fine but I notice that C10 seems to have some strange gloop around its base (pictured below).

Is that normal - glue perhaps, or has it failed? Is this the sort of damage that could occur by someone leaving the power jack plugged into the EAR socket for instance? If it has failed what is this cap's intended value? I can't read it without bending it over and risking snapping it.

C10 possible leak.jpg
Last edited by patters on Sat Apr 03, 2021 7:58 pm, edited 1 time in total.
Moggy
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Joined: Wed Jun 18, 2008 2:00 pm

Re: Diagnosing tape loading issues

Post by Moggy »

Wouldn't have thought any damage caused by leaving power power plug in EAR socket as capacitors don't pass direct current. There is a very very slim/debatable chance that some AC ripple could find its way through the cap' or if some one repeatedly very quickly shoves the power plug in and out of the EAR repeatedly creating "pseudo" AC or modulated DC so as to get through the cap'.

Either scenario totally unlikely and in my experience ULAs either work or don't and as you have picture and keyboard both working would think the problem lies elsewhere.

The cap and resistors are just a filter circuit to tailor the incoming waveform as far as I'm aware and may be different values from the issue 1 so that could be worth checking. Bare in mind these circuits were meant to work with humph kicked out by the old shoe box type cassette devices,whose earphone sockets could can even power an extension speaker and modern devices tend not to have as much kick and yours may be just getting by with the issue 1 but not strong enough for the issue 3.

A better picture of the "gloop" might help too although I suspect it is some form of sticky usually found around the base of electrolytic caps' for stability as I've never known ceramic caps' to leak.
patters
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Joined: Thu Mar 11, 2021 12:34 pm

Re: Diagnosing tape loading issues

Post by patters »

I noticed a ceramic cap in the TV modulator which I removed had a crusty white corrosion all over it, so I wouldn't rule out a cap failure. What bothers me is that there's so little to go wrong here. And as you said, it's fairly unlikely a ULA would be in working order for everything except audio input. The continuity from each component to the next is fine too, going all the way from the EAR socket to the ULA.
Moggy
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Joined: Wed Jun 18, 2008 2:00 pm

Re: Diagnosing tape loading issues

Post by Moggy »

You will find that white powdery stuff on the caps' of just about any Memotech module or RAM pack various it's just oxidisation nothing more I find it all the time on the stage amps I have serviced over the years. As I said the old shoe box players kicked out a reasonably hefty current enough to power an unpowered loud speaker.

The Tzxduino/Maxduino you are using if memory serves uses an LM386 as its output which depending on the type used, on a good day with a decent voltage will kick out as little as 250ma which isn't even line level and the top 386 type just scrapes by and produces nothing like the level of the older devices.

Yes they may work as advertised but looking at the electronics involved they may be more than adequate for something a bit more sophisticated like the c64 or even the spectrum which in my experience never needed a big level to load but for something as crude as the 81 I think these devices are just scraping by.
Last edited by Moggy on Sat Apr 03, 2021 4:32 pm, edited 1 time in total.
patters
Posts: 85
Joined: Thu Mar 11, 2021 12:34 pm

Re: Diagnosing tape loading issues

Post by patters »

I have had zero success on ZX Spectrum with any tzx player over the years since emulators were new (be that on the PC, the iPod, numerous different phones, with mono cables, supposedly with volume limiter disabled etc.). However my Maxduino works with every old device I've tried (48K Spectrum, 128K Spectrum, Issue 1 ZX81). This Issue 3 is the odd one out. I'm fairly skeptical that volume is the issue.

One way I can test I suppose is to wire up the working WESPI I have to this machine since it interfaces with the EAR input. I'll see what 1024MAK's advice is before I do that.
Last edited by patters on Sat Apr 03, 2021 8:18 pm, edited 3 times in total.
Moggy
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Re: Diagnosing tape loading issues

Post by Moggy »

In which case I'll leave it to Mark.
patters
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Joined: Thu Mar 11, 2021 12:34 pm

Re: Diagnosing tape loading issues

Post by patters »

Moggy wrote: Sat Apr 03, 2021 4:25 pm Yes they may work as advertised but looking at the electronics involved they may be more than adequate for something a bit more sophisticated like the c64 or even the spectrum which in my experience never needed a big level to load but for something as crude as the 81 I think these devices are just scraping by.
Not sure I agree - the same tape recorders were used with ZX81s and ZX Spectrums after all. I remember setting the volume to about 7 or 8 out of 10 on the WHSmith tape recorder for our Spectrum back in the day. It certainly wasn't at the low end.
Moggy
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Re: Diagnosing tape loading issues

Post by Moggy »

I said I'd leave it to Mark as all you seem to do is gainsay anything I offer (have you checked to see if there is a difference between issue 1 and 3 input circuits?) but trust me on this one when I say that with 50 years experience of making/servicing and designing audio equipment anything that uses a 386 at 5volts for, its audio out put is scraping by compared to the shoebox cassette types. and the "cleanness" of the signal is probably doing it more favours than out put level and lack of input level on this particular issue 3 shouldn't be so glibly discounted especially for a Tzxduino/Maxduino that before having upgraded firmware was documented as kicking out extreamly weak signals for ZX80/81 use.


You've admitted that your spectrum didn't have the cassette volume running flat out whereas ZX80s and 81s almost always were run at top volume (player type dependent of course) so again wonderful as they are the modern devices just don't have the same kick

If you posses a set of small powered PC speakers try feeding the signal into that then run a lead out of the mono socket some of these speakers have for the left speaker to connect to, into the zeddy, this way you can control the volume of the speakers which should have more than enough thump..

Just things to try rather than just dismiss.
patters
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Joined: Thu Mar 11, 2021 12:34 pm

Re: Diagnosing tape loading issues

Post by patters »

Moggy wrote: Sat Apr 03, 2021 5:11 pm especially for a Tzxduino/Maxduino that before having upgraded firmware was documented as kicking out extreamly weak signals for ZX80/81 use.
It's on the latest firmware, as mentioned in my first post.
patters
Posts: 85
Joined: Thu Mar 11, 2021 12:34 pm

Re: Diagnosing tape loading issues

Post by patters »

I'm sorry if you feel I'm being deliberately contrarian, but I don't have a tape recorder to test, or PC speakers with an output, and I can't adjust the output of the Maxduino so that does rather limit pursuing the volume suggestion. I posted here to get a consensus view of what could have gone wrong, so let's see what some other voices reckon. Can ULAs become progressively more 'deaf' with age for instance? Or is their failure all or nothing? It just seems particularly odd that it works on one ZX81 and not the other.
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