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LOAD "" function error - 48k (Rubber key) Issue 2

Posted: Fri Jun 08, 2018 10:02 am
by FatTony66
Hey guys!

Weird problem....I obtained a faulty 48k which would only display random pixels in green with black stripes. After working a lot of hours on it I finally got it to boot into the command line with the screen going back to its original colour! I replaced:

- All elec capacitors
- Membrane
- All the 4116's (added sockets so I dont waste another afternoon cutting IC's out)
- 7805, TR4 and TR5

So all is great, eagar to play a tape, typed in LOAD"" and.......literally nothing! Cleared Grey screen, no flashing Red/Cyan border, but with the functionality to BREAK and return to command line. Obviously it doesn't even attempt to read the tapes.

Am I being an idiot and missing something obvious here? Or is it another hardware fault?

Re: LOAD "" function error - 48k (Rubber key) Issue 2

Posted: Fri Jun 08, 2018 11:17 am
by 1024MAK
If BASIC works, that's at least 99% of the hardware working normally.

When you enter LOAD "" the computer will wait to hear a tape signal. If it hears nothing, the border will stay white. If it hears anything, the border will change colour. If it hears tones that are in the frequency range it is expecting, you will see coloured horizontal stripes in the border. See these you tube videos:

First off, the tape interface sockets on Sinclair computers are named after the connectors on the cassette tape deck that the connecting cables should be connected to. So Ear on the computer is an input and Mic is an output. Check how you have connected it up.

Second, where there are separate Ear and Mic connectors, these are mono 3.5mm jack sockets. They will only work with mono 3.5mm jack plugs. Stereo 3.5mm jack plugs don't work.

Third, the volume output of the mono casssete decks available when Sinclair designed the ZX computers are capable of being very loud. Modern audio outputs to 'headphone' sockets are limited. So are often not able to produce a strong enough signal for the Sinclair computers. I recommend a set of mains powered PC amplified speakers that have a headphone output socket. You will need appropriate connection leads.


Re: LOAD "" function error - 48k (Rubber key) Issue 2

Posted: Fri Jun 08, 2018 1:19 pm
by FatTony66
Brilliant info there, thanks dude!

As a kid I always remember the Red/Cyan alternating flashing when LOAD function was run, but one thing I didn't consider until reading your reply is the Cassette Deck we had would always be connected to the Ear/Mic ports (Tape Deck electrical noise triggering the alternating flashing??), so this looks like an easy issue... I simply overlooked the obvious!!

I will have another go this evening and report back!

Another observation was the TV signal output. I understand theres a composite mod available but I want to keep it original with the tuner. The signal to the TV is bloody noisy! Worse than my NES with a scrumpled-foil UHF socket.
Would replacing the transistors potentially improve this?

Re: LOAD "" function error - 48k (Rubber key) Issue 2

Posted: Fri Jun 08, 2018 1:59 pm
by 1024MAK
Hi FatTony66

Welcome to our lovely forum.

The trouble with the TV RF/UHF arial output, is that it was not very good even back in the 1980's. See note below.

The simplist composite modification is to disconnect the input wires to the modulator, remove the lid of the modulator, disconnect the output wire. Then thread a new (insulated) wire into the modulator, one end connecting to the video out pad on the Spectrum PCB, the other end connecting to the modulator output connector. This bypasses the modulator. This is completely reversible if done carefully.

Alternatively Paul can supply SPECTRA interfaces which have a RGB SCART enabled socket (for prices and availablilty click here).

It's hard to say if replacing any thing will improve the quality of the picture. Is it okay in black and white? Maybe you could post a photo showing the problem. I assume you have tied fine tuning?

Note: the TV RF/UHF arial output offers the worst possible quality picture. The output video signals from the ULA chip are a special form of component video signal. To get a colour composite video, the component video signalS are sent to a colour encoder chip. The output of which is then mixed with the monochrome video signal from the ULA. This resulting signal is then sent to the modulator. The modulator limits the bandwidth of the signal, and uses it to modulate a high frequency signal from an analogue oscillator. The result is what is present on the modulators output socket. The TV then has to reverse all this modulation, mixing and encoding to get back to a signal suitable to display on screen. Each stage limits the signal and introduces noise. So it's not very surprising that the result is poor.


Re: LOAD "" function error - 48k (Rubber key) Issue 2

Posted: Sun Jun 10, 2018 12:36 am
by Lardo Boffin
Just to say the SPECTRA produces a superb picture. Almost emulation good and no dot crawl. And obviously as it plugs in the back there are no mods whatsoever required for it.