Repairing ZX Spectrum issue two

[1024MAK]

Well done

You got there

Now you can enjoy it

Mark

[redgatemodels]

Yes, it is very satisfying to resurrect a pcb, my personal battle with my series 2 was long and beset with setbacks. I hope it’s now sorted short of some new 3.5mm sockets and a comp mod. It will not get much use as the series 6 that I bought to prove things is happily residing in the plus case and is intended to be the main “working” example.

Next job is to get those damn microdrives working …….

[Pygmalion]

Thank you for your help and patience.

I would like to ask you a few more questions

1. Is it normal that the ZX Spectrum only sends picture and no sound via the antenna cable?
2. I do not like that the output voltage of the power supply is unbalanced. Would it be a good idea to replace all four diodes and if so, which diodes should I use?
3. I also plan to make a spare set for emergencies. Since ULA has a large heatsink and is completely cool, I assume the weakest points of the ZX Spectrum are the lower memory chips and the linear regulator. Is there anything else I should have on hand as a spare?

[redgatemodels]

I can answer the first one - yes, on a standard spectrum the internal beeper is all you have. As a kid I had a little circuit that outputted the beeper to the rf modulator. I took it off my series 2 when I started to fix it and didn't note where the three wires were soldered too

Best do a comp mod and take the feed from the mic socket to the red of white socket on the TV

[1024MAK]

Next job is to get those damn microdrives working …….

Good luck with that. I don’t do microdrives

Mark

[1024MAK]

1. Is it normal that the ZX Spectrum only sends picture and no sound via the antenna cable?

Yes.

• I do not like that the output voltage of the power supply is unbalanced. Would it be a good idea to replace all four diodes and if so, which diodes should I use?

I’m not sure what you are asking. The mains power supply unit is an unregulated type made up of a thermal fuse (which may be part of the transformer or under it), an isolating mains transformer, four diodes, and one or two electrolytic capacitors. Plus obviously a plastic case and input and output cables. They produce between 9V and 12.5V DC at up to 1.4A. Yes, if any of the diodes have become faulty, replace all of them. Yes, if the electrolytic capacitor(s) are life expired, renew it/them. Or, buy a modern PSU. A 9V type with a current rating of 1.4A or higher will do. But keep in mind most will have the output plug with the wrong polarity. Either get one with the correct polarity, or chop off the existing plug and fit a new one (2.1mm inside dia., 5.5mm outside diameter barrel type, some machines may need the long version).

• I also plan to make a spare set for emergencies. Since ULA has a large heatsink and is completely cool, I assume the weakest points of the ZX Spectrum are the lower memory chips and the linear regulator. Is there anything else I should have on hand as a spare?

DRAM faults are indeed the most common. The 7805 voltage regulator is actually fairly reliable. But they are cheap, so it is worthwhile having a spare.
For other spares, apart from the DRAM, I would list these parts:
TR4 - ZTX651 or ZTX653 (to replace the ZTX650)
TR5 - ZTX751 (to replace the ZTX213)
BC549C - to replace TR1 or TR2 if necessary (these are normally reliable, but they can be damaged if the composite video output is shorted out).

That’s about it (assuming your keyboard membrane is good).

Mark

[Pygmalion]

I have now taken a closer look at the board and compared it with the official circuit diagram.

I discovered this mod connected to the extra stereo audio socket (one pin of the socket is unused), which seems to be a bit nonstandard mod for composite video:

(IP and 5V are also inputs of the modulator)

I also found out that MPS2369 is used instead of ZTX313 for TR6 in "spider" mode and is located on another part of the board.

Also ZTX650 is replaced by BFJ50 as TR4.

Otherwise, everything seems to be pretty standard.

[1024MAK]

Apart from using an old metal can transistor (the BC107 is the older relative of the BC547, of which the BC108 / BC548 and BC109 / BC549 are all part of the same family, the letter as the suffix is just the gain group, B class are better than A class but not as good as C class, not that it makes any difference in this circuit) and slightly different resistor values, it’s a “classic” composite video buffer amplifier design.

Yeah, the MPS2369 has fast switching characteristics like the ZTX313.

Mark

[Pygmalion]

• I do not like that the output voltage of the power supply is unbalanced. Would it be a good idea to replace all four diodes and if so, which diodes should I use?

I’m not sure what you are asking. The mains power supply unit is an unregulated type made up of a thermal fuse (which may be part of the transformer or under it), an isolating mains transformer, four diodes, and one or two electrolytic capacitors. Plus obviously a plastic case and input and output cables. They produce between 9V and 12.5V DC at up to 1.4A. Yes, if any of the diodes have become faulty, replace all of them. Yes, if the electrolytic capacitor(s) are life expired, renew it/them. Or, buy a modern PSU. A 9V type with a current rating of 1.4A or higher will do. But keep in mind most will have the output plug with the wrong polarity. Either get one with the correct polarity, or chop off the existing plug and fit a new one (2.1mm inside dia., 5.5mm outside diameter barrel type, some machines may need the long version).

If you remember, I once had 50Hz harmonics in the 12V line, which indicated that one of the diodes was not working properly.

So I opened the PSU and the part of the board with the diodes is visibly darkened, indicating that the diodes were overloaded. This is strange, they are 1N5391, 1.5A diodes, and when I used my hobby rectifier it showed that the ZX Spectrum draws a whopping 0.7A of current.

[Paul]

I would replace all diodes and the capaitor