ZX81 Tape loading woes Pt 2.

[falter]

A tape recorder made for a computer (TRS-80) was always likely to be mono, but I thought I'd better ask.

If you use a headphone output either in 'Headphone' or 'Line Out' mode to drive a microphone input directly you have a serious mismatch in levels - the MIC input on a recorder is expecting just a few millivolts from a real microphone, so the ZX81 outputs a similar, tiny signal from its MIC connector. The output from the PC is huge by comparison.

Since the recorder has a 'Line In' socket (sometimes called AUX IN) then using that input (rather than the microphone input) in conjunction with the PC with its output mode set to 'Line Out' would be the correct way to go.

If the recorder didn't have a 'Line In / Aux In' input then you could try making this little level reducer circuit (see image - I kept it small so the forum wouldn't reject it - click on it to see a hopefully more readable version). This will reduce the 'big' output from the PC to the much smaller level expected by the microphone input on the recorder.

Note the use of a stereo plug at the PC end and a mono plug at the ZX81 end. Plugging a mono plug into the stereo output of a PC will short one of its outputs directly to ground, which is undesirable and may upset the audio amplifier or eventually even damage the output in question.

View attachment 30051

The resistor values chosen are pretty arbitrary, the preset (variable resistor) will need to be set quite close to the 'low' end (but not all the way off!) for best results.

For your own setup I would try the PC in Line Output mode and connect it to the Aux In, not Microphone in, of your recorder. I would still make up a lead with a stereo plug at the PC end, for the reason stated above, but it won't need the level reducer resistors.

Thanks guys. I'll try that. But one thing I'm wondering.. I have seen literally half a dozen videos of people loading programs on their zx81s/ts1000s via mp3 player or phone.. how are they able to pull that off?

 

[SiriusHardware]

Thanks guys. I'll try that. But one thing I'm wondering.. I have seen literally half a dozen videos of people loading programs on their zx81s/ts1000s via mp3 player or phone.. how are they able to pull that off?

The main cause of this not working (when it does not work) is insufficient output level from the device in question. You need a minimum level of >2V peak to peak to drive the ZX81's EAR input. MP3 players which run on a single AAA battery are likely to have a particular problem in this respect, but it is still doable. I did quite a bit of experimentation which I reported in detail in your original thread about this here:-

http://www.vcfed.org/forum/showthread.php?50020-ZX81-weird-tape-loading-issues

..So I won't repeat it all here - but my solution for devices with low output was to make a special version of the sound file which made the two stereo output channels work in opposition to each other to produce a larger output signal than could be produced by one channel output on its own.

 

[falter]

I got it!

I plugged into my big PC monitor.. which has a composite in jack.. then hooked the zx81 directly to the speaker output for my main computer. I set the port mode to speakers rather than headphones, cranked the volume to 100%.. and then queued up a wav file in media player. Voila! Didnt have to do any mixing or special cables.



Tomorrow I'll try using this to record to my tape recorders aux port. This setup works but I dont like having all this stuff on my desk. Would like to get it back onto my Commodore monitor.. which is too far away for this pc to reach. At least we know it works now! Thanks guys!

 

[KC9UDX]

I was wondering why you were having problems with volume..

 

[SiriusHardware]

I was wondering why you were having problems with volume..

Very few PCs have the option to output speaker-level audio (ie, output powerful enough to drive passive speakers directly from the headphone socket). None of mine do, they all require active speakers with integral amplifiers.

Some of my very oldest ISA soundcards did have onboard audio power amplifiers which could be enabled for direct output to passive speakers or disabled, providing line level audio out for active speakers instead.

 

[falter]

So I picked up a $14.00 Timex 1000 on ebay and it arrived today. It came with original boxes and included a 16k ram pack, some original game tapes, all the original cables, etc. (including pristine tape recorder cables!) Actually thought it was a great deal.

Applying what I've learned, I decided to try my luck at a 16k game. I plugged the pc speaker output into the AUX port on my tape deck and played the wav file for 3DMaze into it to record it to tape. I then hooked up the TS1000, tape deck and RAM pack and fired it up. I notice the machine takes a bit longer to initialize when you have the RAM pack plugged in. Anyway, 4 min later, it loaded 3D Maze no problem! It went through the intro okay and then I started to play the maze.. which I was able to wander a bit before the whole screen went grey and that was it. I reset, loaded again.. this time hit the CONT button a little earlier in the intro, and again, grey screen. IIRC these RAM packs were prone to failure.. wondering if that's likely what I'm dealing with here. Thoughts? Can they be serviced? I do have tons of RAM chips around.

 

[gertk]

Best way to do a ram upgrade is to do it internally. You can use a 32k SRAM chip and put it in place of the original two 2114 or 2k SRAM chip. Needs a few extra wires (3 as I remember) but then you have a 'non-wobble' and low power upgrade.



I used a SMD 32kB SRAM chip on a small carrier board to make the extra connections easier.

 

[SiriusHardware]

Good to hear the Aux-In dodge worked for making your own tapes.

Ah, yes, RAM packs.

Because the ZX81 edge connector was tinned with solder rather than gold plated the edge connector 'fingers' had a tendency to oxidise, so the contact between those and the external add-ons wasn't always as wonderful as you could have wished for - the upshot was that sometimes the slightest movement of the machine - difficult to avoid when you were pressing the rather unresponsive keypad - would momentarily disconnect one of the address, data, power or control lines, and CRASH. Especially heartbreaking if you had just typed upward of 100 lines of BASIC in and not had a chance to save it. Ways of solving this basically involved fixing the casing of the RAM pack more firmly to the rear of the ZX81, whether with blu-tack, glue, or, my favourite, bolts.

I quite like the previous suggestion of using a modern high capacity SRAM and fitting it internally - it wouldn't be historically correct, but who wants historically correct instability?

Having looked at the photo I imagine you could, alternatively, do it with a DIP device and just take the extra pins to where they need to go via flying leads. Unfortunately it's only this simple on the Timex machine, originally fitted with a wide bodied 2K SRAM. The UK 1K version's memory came in the form of 2 narrow DIP memory ICs.

The ZX81 designer's original concept for the add on RAM had it blending into the rear of the ZX81's case and extending the wedge shape further back, not stuck on the back like an ugly vertical rectangular blob. Some manufacturers of third party RAM add ons (notably Memotech) did make an effort to make their product blend more aesthetically with the good-looking wedge design of the basic unit.

 

[falter]

Thanks guys.

After experimenting a bit more, it appears the TS1000 I have here needs the tape volume at full blast in order for a game to work properly. Any lower and the game may partially load and then crash. With the tape turned up to 10 (max), I was able to load 3DMaze and get killed by that *!@#(ing Rex about 12 times with no grey screen of death.

I'd love to do that internal RAM upgrade on the unit that is in my 3rd party case w/real keyboard. In addition to a much better keyboard, it also has been modified to go direct to composite video. The fellow that made the case didn't provide space for the RAM pack though, unfortunately. And it's a ZX81, which I read above cannot be upgraded easily that way? (It's a North American Zx81.. not UK if that makes any difference). I have a parts TS1000 I might just try to mod and sub into that case. With better keys I feel like I might have a chance to even things up with Rex.

 

[SiriusHardware]

It's a North American Zx81.. not UK if that makes any difference). I have a parts TS1000 I might just try to mod and sub into that case. With better keys I feel like I might have a chance to even things up with Rex

I should probably clarify. If your ZX/TS originally had 2K of memory, it used a wide bodied RAM chip. Although a 32K chip of similar width is a bit longer, quite a lot of the pin designations are the same so you can plug the larger chip with its lower pins pushed into the socket originally occupied by the 2K chip. The pins hanging out in space beyond the top edge of the original socket can then be wired to where they need to go. (It might also need some work doing with the chip select signal to enable the memory across its whole range, not just the first 1K or 2k. I can't remember how it goes at the moment).

On a machine originally only fitted with 1K the memory is usually in two narrow bodied memory chips so there's no direct physical pin for pin relationship between the pins on the new chip and the socket holes on the two sockets. You can still do it, but you have to wire all of the relevant pins - address, data, RD, WR, CS, etc to the relevant socket holes on the memory sockets originally occupied by the two memory chips.

My second ZX81 is also in a third party 'real' keyboard enclosure - dkTronics, in my case, and it actually has aN INTERNAL 'bus board' which redirects the edge connector on the ZX board to a cutout in the rear of the enclosure, but about half way there, there is a vertical bus 'breakout' connector onto which a standard RAM pack can be fitted internally. Unfortunately the PCB is only supported by the connector itself and is not otherwise braced, so it suffers from exactly the same 'bump sensitivity' as the original arrangement. I keep meaning to arrange some extra rigid support for it so it can't move, but there is always something else which needs doing first...

 

[SiriusHardware]

Best way to do a ram upgrade is to do it internally. You can use a 32k SRAM chip and put it in place of the original two 2114 or 2k SRAM chip. Needs a few extra wires (3 as I remember) but then you have a 'non-wobble' and low power upgrade.

View attachment 30165

I used a SMD 32kB SRAM chip on a small carrier board to make the extra connections easier.

Assuming you routed the address, data, RD, WR, CS, ...(OE?) pins to the obvious places, did you have to do anything with the address decoding, or does the machine just test the whole range for RAM and configure itself for however much write / readable RAM it actually finds?