Sound and LOAD issues

About original Memotech hardware.
muldjord
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Sound and LOAD issues

Post by muldjord » 07 Oct 2018 15:11

Hi everyone! It's been a while since I've had my trusty Memotech MTX512 fired up. I've recently been getting back into retro machines with the A500, A600 and a C64. But the star of the show is, of course, still my MTX512.

Now here's the sad part. When I wanted to play some games on it today, it turned out that the computer started having some issues. And I was hoping some of your experience might help me troubleshooting this.

When I turn the machine on the display and keyboard work as expected. I get the "Ready" prompt and can type in commands. So that side of things works just fine.

However, the audio side of things are not. From the speakers of the TV I instantly get a constant mid-range tone. A few years back my machine started having issues with sound in games. In particular, when dying in Blobbo and Knuckles, the "fart sound" when the character died stop working. But I was able to load games back then even with the sound issues. But not anymore. When I type in LOAD "" and start playing an audio file into the machine through the jacks on the back, I get nothing.

My guess is that the TI SN76489AN sound chip has gone bad. Do any of you have experienced similar issues who can confirm that a faulty sound chip would make the MTX512 unable to load games from the audio jacks?

One more question: IF it's the TI SN76489AN sound chip that's faulty, would it be ok to switch it with a new chip of the same model? They can be bought on ebay, but I have no idea if these new chips are compatible.

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Dave
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Re: Sound and LOAD issues

Post by Dave » 07 Oct 2018 15:14

Hi Lars,

thanks for posting here - it is a MUCH better place than Facebook for technical queries - and it won't throw my replies away :-)

The sound problem is very similar to one that I fixed here :

http://primrosebank.net/computers/mtx/r ... darren.htm

While you are reading that, I will give a better answer to the other part of your question . . . .

regards
Dave

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1024MAK
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Re: Sound and LOAD issues

Post by 1024MAK » 07 Oct 2018 15:23

The TI SN76489AN sound chip is not connected to the cassette tape interface system. They are separate sub-systems. However, that may not mean that there is not a problem that is common to both symptoms.

Mark

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Dave
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Re: Sound and LOAD issues

Post by Dave » 07 Oct 2018 15:27

Following up on my earlier comment . . .

When you say that "you get nothing" when playing an audio tape, do you just mean that you can't hear it loading, or are you certain that no data is actually being loaded? The sound chip isn't used to actually load the program, but is likely used to generate that chirping you hear when data is being loaded.

The MTX uses ports 3 and 6 for sound and tape I/O. Sound, using the SN76489 uses In 3 and Out 6. The Cassette interface uses Out 3 for data save. Data load is done by the CTC - not the sound chip. The Circuit Diagrams are in the Operator's Manual - available here

http://primrosebank.net/computers/mtx/d ... hPages.pdf

muldjord
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Re: Sound and LOAD issues

Post by muldjord » 07 Oct 2018 15:35

Dave wrote:When you say that "you get nothing" when playing an audio tape, do you just mean that you can't hear it loading, or are you certain that no data is actually being loaded?
When I type in LOAD "" and press enter, I don't get any sound, and nothing is found or loaded. So I jumped to the semi-conclusion that the problems where connected. But it might aswell be that it is an issue with how I play the sound into the machine. It always confused me with the "MIC" and "EAR" indication on the backplane, so I couldn't remember which one to use. I tried them both, but nothing loaded and no "data sound" either. I played the sound into the MTX512 using the minijack on my phone and the Mission Alphatron WAV file played through the Android version of VLC. Tested it with headphones, and the sound came out the jack as expected. But I did use a stereo jack, so that might be an issue.

But after reading your previous link, it very much sounds like it could be the 74LS374N octal flip-flot IC that's faulty. Unfortunately I am no electronic wizard and would not dare to start error checking any of my chips myself. But, I would have no isse trying to swap out the 74LS374N IC (I do know how to solder quite well, and I could put in a socket while I'm at it). Then, when that is done I could try the "DING" key combination to ensure sound is back in working order.

And also I'll probably try and get data into the machine again. I stopped working on it relatively quickly since I assumed it was a sound issue. I mainly thought that maybe the SN76489A was doing som AD conversion for the data loading. But now that I know that is not the case, I will spend some more time trying to get it to work.

Thank you for the input guys, It's much appreciated!

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Dave
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Re: Sound and LOAD issues

Post by Dave » 07 Oct 2018 15:51

Yes, the labelling of "Ear" and "Mic" isn't particularly user friendly.

I guess that Memotech were working on the basis that this was a common way of labelling the jacks on cassette players at the time. The "Mic(rophone)" jack would be the source for saving data on the tape and the "Ear(phone/plug)" jack would be for playback.

From an MTX perspective, the "Mic" port (J3) is sending output FROM the MTX to the tape player and the "Ear" port (J2) is taking input TO the MTX from the tape player.

MTX "Ear" (J2) <------ Tape "Ear"
MTX "Mic" (J3) ------> Tape "Mic"

Of course, when you get the Tape I/O working, you will want to upgrade to CFX or REMEMORizer :D

regards
Dave

muldjord
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Re: Sound and LOAD issues

Post by muldjord » 07 Oct 2018 16:07

I've just ordered some SN74HCT374N and sockets. Looking forward to switching out that IC and see if it fixes the sound issue. Thank you for the MIC/EAR description Dave. It really is an odd choice of labeling, as both EAR and MIC are input channel names in my logic. I do seem to recall my old datasette having the same labels, as confusing as it was.

I will have a look around for a mono jack to see if I can get some data to load even with the sound issue. And I'll look into using the composite out aswell. I actually thought that was some other type of output, so I've never tried it. :D The RF modulator does have issues for me aswell. The image is very washed out and noisy but I just thought it was down to my television.

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Dave
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Re: Sound and LOAD issues

Post by Dave » 07 Oct 2018 18:16

Modern LCD TVs tend not to work too well with the video modulator RF output from '80s computers, the best results are with an older CRT type TV.

Even at that though, those old modulators really do deteriorate with age. Even when they do work, they tend to drift off station as they warm up.

The composite video output is much more likely to give good results. For some reason, on a fair few of the MTXs that I have seen, Memotech had set the video output to monochrome on the video board. If you only get a Black and White picture from the video output, there's a link to change that on the video board

regards
Dave

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1024MAK
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Re: Sound and LOAD issues

Post by 1024MAK » 07 Oct 2018 18:44

I don’t know where it started, but Sinclair use the same labelling for the cassette ports on the ZX80, ZX81 and ZX Spectrum computers. The idea is simple. Link the like named sockets between the computer and the cassette deck.

On the cassette deck, Ear stands for earphone (output to earphone). Mic stands for microphone (recording input). So obviously, on the computer, Ear is the input...!

Most mono 3.5mm jack sockets are incompatible with stereo 3.5mm jack plugs. So much so that the contacts do not line up. This is the case for both the Memotech MTX and the Sinclair computers. So you must use a mono 3.5mm phono jack plug.

The UHF/RF output was always rubbish... even back when the machines were brand new (and CRT televisions had curved screens with rounded sides). Since the vast improvement in television picture quality, our expectations have changed rather a lot...

Mark

muldjord
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Re: Sound and LOAD issues

Post by muldjord » 07 Oct 2018 22:45

Thank you for the clarifications. I remember having a TV back in the 80's that drifted off-channel. It was a pain to work with. I'll be sure to get my hands on a mono-jack. I feel pretty sure that the data-loading will work then.

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