ZX81 Tape Woes...

[Rink]

Hi all, I'm hoping to pick a few brains.

A friend of mine has lent me his recently acquired ZX81 which he's never really used since he doesn't have a tape player for. I've tried loading tapes from an iPhone and a PC (running WinTzx) both directly and through a Xenyx powered mixer (built-in pre-amps etc) but I can't get this thing to respond to audio input. BASIC works fine, and if I amplify the signal, I can hear that saving also works.

The ZX has had a video mod to make it work with LCD TVs btw.

After typing LOAD "", the display switches to a white screen with diagonal black lines (about ten of them) which appear to crawl slightly. However, no matter what I do with regard to audio input (e.g. No input, no cable plugged in, or after testing a variety of volume levels), that screen is all I see until I unplug the power.

I've unscrewed the unit and checked the contacts on the EAR socket. Seems ok - there's a bit of continuity between the poles but so has the socket for MIC. There's also a little bit of discolouration on some solder points there (but not black) so i'm wondering if something has fried?

Can't see any kind of cable problems - there's a good loud signal being delivered to the EAR socket even without running it through the pre-amp first.

Any ideas would be much appreciated.

R

[RetroTechie]

Welcome here, Rink!

Glad you have that ZX81 working - some tips:

• Tape input (& output) is mono. Using a cable with either stereo or mono plugs on both ends may not work if the output equipment is stereo. Optimal is a stereo -> mono cable where the signal is connected to 1 audio channel (eg. the left) only.
• ZX81 'wants' a TTL level signal, and typical audio signals are weak in comparison. In many cases (portable MP3 player, cheapo tape recorder, PC headphone output) it works when you simply dial up the volume to around 70% (or even 100%). With a stronger signal or some kind of amplifier in between, that might be too much. Try settings in between like 30%, 50%, 70% etc, each time resetting the ZX81 & re-type LOAD "" before each attempt. The loading procedure can also be interrupted by pressing space (BREAK), but that may not always work (especially if there's no signal, or the ZX81 already read something but garbage data).
• That loading screen you describe sounds normal. The pattern should change as soon as the ZX81 picks up a signal, but what is 'good' or 'bad' is hard to tell if you don't have experience with ZX81 tape loading. Unfortunately the load process doesn't give you much feedback on what program is found or being loaded...
• If you get tape loading errors, the setting is better than if you saw no response at all.
• Don't open up the unit if not absolutely necessary & it still works - unless you've replaced the keyboard membrane, the keyboard connectors are very fragile.

It's important to know when the program starts, and if/how the loading pattern changes when it 'hits' the ZX81. Therefore if you can monitor (with your ears!) the signal that's input while attempting to load: do so! The point is to determine at which volume setting the ZX81 begins to respond to audio input, and then dial up the volume a bit further (say, +/- 20% above that minimum level).

I've unscrewed the unit and checked the contacts on the EAR socket. Seems ok - there's a bit of continuity between the poles but so has the socket for MIC. There's also a little bit of discolouration on some solder points there (but not black) so i'm wondering if something has fried?

Possibly. Can you upload a close-up picture of the area? If you own a multimeter, you could grab the ZX81 schematic & test continuity for the connections from tape input -> C10 -> R33 -> ULA pin 20.

Unfortunately sometimes the ULA has a damaged tape input, and that can only be fixed by replacing the ULA (= a hard to source part). In my personal experience I've hardly ever seen that though - maybe I've just been lucky...

[Rink]

Hi RT, thanks for the welcome and for the tips.

At various points, I've been able to use headphones and listen in. Generally everything seems good - a period of silence (does the ZX81 wait for exactly five seconds of it?) followed by all the screeches sounding pretty normal. I've tried every volume level I can coax out of the equipment I have (the break tip is amazing, thanks) but still can't get the screen to change at all.

Is it best to type LOAD "", press return and then start the tape; or enter the command, start the tape and then press return?

I'm going to (carefully) take it apart again tomorrow and take another look at the socket just to be sure, and I'll take a photo of the solder points then. Would sell my granny for an oscilloscope right now. Although I just rang a mate and got him to dismantle his broken Zx81 (torn ribbon connector funnily enough, amongst other problems) and he says the solder points in the same area are a little brown also... So maybe this is a red herring.

If the ULA did have a dodgy tape input, would this not also affect the video output? Although I guess there must be division between the two functions in there somewhere.

All this for a cable eh?! I've never played with a ZX81 before - it's not exactly winning me over. Hahahaha.

Cheers.

[RetroTechie]

Is it best to type LOAD "", press return and then start the tape; or enter the command, start the tape and then press return?

The ZX81 has to be listening when the screeches start - it isn't listening until after you've pressed enter (ehm.. NEWLINE ). So LOAD "" -> enter -> start tape is the proper sequence. It detects automatically when the program starts; a combination of proper volume setting & specific frequency signals.

Second that wish for an oscilloscope - I would want to have a look at 2 points: at the EAR socket, and at the ULA pin. On other machines I'd do a quick type-in to see if that tape input bit flips logic level, but strangely ZX81 BASIC lacks instructions to read/write Z80 I/O ports. A simple continuity check goes a long way though - if you measure directly to/from an IC pin, that can even detect a bad IC socket contact. If not bad circuit trace or damaged ULA (unlikely), it's either a cable or volume issue (likely).

If the ULA did have a dodgy tape input, would this not also affect the video output? Although I guess there must be division between the two functions in there somewhere.

Video & tape output (MIC) share an ULA pin (unbelievably this works btw), tape input (EAR) goes through another -dedicated- ULA pin. So it could be dead with video output & SAVE working fine.

All this for a cable eh?! I've never played with a ZX81 before - it's not exactly winning me over. Hahahaha.

Yeah... funny how back in the 80's, this was fun & games for the masses, and these days operating a ZX81 is considered more or less 'hardcore'...

[sirmorris]

Isn't there an R/C filter on the input? That would explain any continuity between hot & cold.

I can't think of anything to add here apart from a further reinforcement of the encouragement to be careful when disassembling

Good luck
C

[1024MAK]

I'm going to (carefully) take it apart again tomorrow and take another look at the socket just to be sure, and I'll take a photo of the solder points then. Would sell my granny for an oscilloscope right now. Although I just rang a mate and got him to dismantle his broken Zx81 (torn ribbon connector funnily enough, amongst other problems) and he says the solder points in the same area are a little brown also... So maybe this is a red herring.

If the brown is on and or around the solder, it could be flux residue.
Is this a Issue ONE (typically the PCB is green in colour with the issue written in the solder mask on the board near the modulator) or a issue THREE ZX81 (typically the PCB is red/pink in colour)?

Mark

[gozzo]

If the ULA's tape in port IS duff, would it be possible to rig up an 'external' tape in port, decoded to the correct address, with an open-collector buffer (or maybe a normal 3 state type?) to the required 'bit', there's a similar idea on the 'net for a keyboard (zx81 bkb v3, I think.??). External to the ULA ,I mean, not the machine, maybe could be built small enough to fit within the case!

[Rink]

Video & tape output (MIC) share an ULA pin (unbelievably this works btw), tape input (EAR) goes through another -dedicated- ULA pin. So it could be dead with video output & SAVE working fine.

Aaah. That makes sense. Cheers.

Yeah... funny how back in the 80's, this was fun & games for the masses, and these days operating a ZX81 is considered more or less 'hardcore'...

I first started programming because the Acorn Electron (my first computer) I had struggled to load most games off tape... It's amusing to me that after all these years I'd be battling the same demon. But I still find the old machines much more fun to tinker with and write code for than the soulless IBM clones. I sort of enjoy iOS programming but I rarely do it on my own time.

@Mark - pretty sure it's green (definitely not pink or red) but I'll check for an issue number etc.

[gozzo]

Just to confuse the 'issue' (!) more, I have several ZX81's both issue 1 and 3 versions, and in both cases, some have red solder resist, some have green !! I think they used what they felt like! Plus I must also warn about being careful when disassembling/reassembling - removing and reinserting the membrane 'tails' - and don't be tempted to squirt contact cleaner or similar on them thinking that will ease them going in - it won't and you will end up with a crumbling membrane - guess how I found out

[Rink]

Ok, since you've all terrified me when it comes to opening this thing up, I'm going to leave that as a last resort and first try every other audio playing device I have access to.