Sinclair ZX80 / ZX81 / Z88 Forums

That type of board is called SRBP (Synthetic Resin Bonded Paper) and as you can see, it discolours when it gets/has got hot. Be very gentle when desoldering, as the board may now be more fragile due to the affect of heat on it in the past.

The diodes may be okay, but as they don’t cost very much, as Paul says, it’s a good idea to replace all four. And the capacitor as well (use a 105°C type, they should have a longer life).

When fitting the new diodes, bend the legs in a slight “S” or “?” shape so that the body is about 5mm or more above the surface of the board (but do check that there is enough space for this in the case). That reduces the amount of heat going directly to the board. But don’t bend the leads very close to the case of the diode.
Mark

What’s the thing that I’ve arrowed?

The cable should go from the PCB out through the case via the grommet.

Mark

Oh, forgot to say, yes, a ZX Spectrum without expansions typically draws between 0.6A and 0.7A. Some expansions can significantly increase this. Sinclair primarily supplied a 1.4A rated PSU to cater for the ZX Printer.

Mark

1024MAK wrote: ↑Fri Oct 27, 2023 10:35 am IMG_7132.jpeg
What’s the thing that I’ve arrowed?

The cable should go from the PCB out through the case via the grommet.

Mark

I do not know. I thought it was an ancient fuse, but since its resistance is 1.5Ω, it could be a high power resistor reducing the voltage on the ZX Spectrum. Even with this device, the voltage on the ZX Spectrum is still 10V, which is 1V above the nominal value.

What the heck is a grommet?

https://en.wikipedia.org/wiki/Cable_grommet

With this type of grommet, it’s primarily purpose is to act as a cable restraint, to try to stop the cable from being pulled out of the case. It’s secondary purpose is to help protect the cable where it exits the case, to add extra mechanical strength, to reduce the likelihood of the cable breaking where it exits the case due to being bent at a sharp angle. And lastly, the PVC insulation in the cable may react (a chemical reaction) with the ABS plastic of the case, the grommet acts as a barrier.

Back to the “thing”.

Looking at that better photo, it looks like there is thin wire wrapped around the outside. It may be some type of fuse and it’s been blown at some time. Then someone has added the wire. If it has a resistance of 1.5Ω, it may be fuse wire.

Low current fuses and low current fuse wire can have a measurable resistance.

Anyway, this is most definitely not a standard or original part. Someone has added it. This may explain why the circuit board is so badly discoloured where the diodes are. I suspect at some point in the past, the output has been shorted out, overloading the PSU. This would cause the transformer and the diodes to get hot (a lot hotter than normal).

Alternatively, this is some kind of homemade resistor, with a resistance of 1.5Ω, it will drop about 0.9V when a current of 0.6A is passing through it. Hence slightly reducing the amount of heat that the 7805 voltage regulator in the ZX Spectrum has to dissipate through the heatsink.

Although there are better ways of doing this.

In the past, some have added two diodes (wired in series) between the PSU and the the ZX Spectrum to reduce the voltage. Each diode drops between 0.6V and 1V (depending on how much current is flowing).

Now that modern DC/DC converters, also called switching regulators are available, one of these can replace a 7805. Because modern DC/DC converters have an efficiency of better than 85%, they don’t produce any significant amount of heat, so don’t need a heatsink. Examples are the pin compatible devices from Recom or Traco Power.

However, because they are efficient, they use significantly less power. Less current is drawn from the PSU. With less current being drawn, the output voltage of the unregulated PSU will be higher. If it goes above about 11.8V, there is a risk that the original Sinclair DC/DC converter circuitry (TR4, TR5, the coil and the other associated components) may not work correctly. The result is that the -5V supply to the 4116 (or equivalent) DRAM goes out of specification, or is lost completely.

Later versions of the Sinclair DC/DC converter circuitry were subject to changes in design to overcome this problem. Hence the different circuit design in later issue boards.

Mark

If this "thing" is not original, then I am definitely interested in replacing it with a diode or two.

I also bought a couple of TSR1-2450s from AliExpress and am impressed with how well they work and how cold they are at 0.5A. However, I have not decided if I will actually employ them yet.

I want to restore the ZX Spectrum to its original state as much as possible. I have already bought a new case; in the 1980s the ZX Spectrum was transferred to a huge (and ugly) case with a real keyboard made in Yugoslavia because the membrane of the original keyboard had problems.

I will almost certainly change the output to composite and keep a reset button as a small, black, non-intrusive push button. If the lower DRAM keeps dying, I will replace it with a homemade replacement explained here https://spectrumcomputing.co.uk/forums/ ... p?p=128452. And maybe I will replace the 7805 with a TSR1-2450 if the computer gets too hot. But reading your comment, in the latter case I will have to use several diodes to bring the voltage down below 11.8V. Or maybe cheap step-down circuits like this... https://www.aliexpress.com/item/1005001768198398.html

Be careful with the cheap step-down circuits, some are not particularly reliable apparently. I’m only going on what some other people have said though.

Mark

1024MAK wrote: ↑Sun Oct 29, 2023 12:31 am Be careful with the cheap step-down circuits, some are not particularly reliable apparently. I’m only going on what some other people have said though.

Of course, I will test it on high currents first before using it for ZX Spectrum.

And I think the risks are relatively low, aren't they? The step-down input voltage is already small enough not to destroy ZX Spectrum.

Pygmalion wrote: ↑Mon Oct 30, 2023 10:32 am And I think the risks are relatively low, aren't they? The step-down input voltage is already small enough not to destroy ZX Spectrum.

If you are continuing to use a 7805 in the Spectrum, there is no risk to the ZX Spectrum if the step-down converter fails (no matter how).

If the 7805 is replaced with a DC/DC converter/switching regulator, the risk is relatively low, the concern would be that I described earlier.

Mark

After playing a bit with my ZX Spectrum, I noticed some problems.

1. The computer is unstable. After a while it freezes or resets. At first the problems were very consistent, with thick purple vertical lines, so I stopped them by replacing the bit 0 lower memory DRAM memory. Now the problems occur much less frequently and more inconsistently, so it is difficult to identify which lower DRAM memory as the culprit. I guess the only solution is to replace all DRAMs with a DIY lower DRAM replacement, as described in https://spectrumcomputing.co.uk/forums/ ... p?p=129038.
2. Sometimes R Tape loading error appears. I assume this is related to the tape drive. I suspect that the drive belt is worn and have already ordered a new one that is a bit tighter.
3. The composite image is not perfect - there are "ghost letters" around the actual letters. Is this normal or can it be improved?

Since it's inefficient to just build a board for the lower DRAM replacement, I am thinking of building other DIY accessories for the ZX Spectrum as well. Do you have any good ideas? For example, I found this tutorial https://github.com/konkotgit/ZX-external-ROM for a DIY ZX diagnostic tool, but there is no information on how to program the ROM. Does anyone know of any other good tutorials for DIY ZX Spectrum accessories?