exxos's Atari, Amiga & retro forum

exxos wrote: 23 Sep 2023 15:36

rubber_jonnie wrote: 23 Sep 2023 15:21 Measuring the voltage at the left hand side of D15, I get about 8.5v, but the right hand side is 30....

So the 30v must be coming from elsewhere. 8.5v left side means no current flow if the right side is 30v. 30v can't feedback as the diode blocks it. So sounds like the PSU is mostly working then.

Swapping D19 made no difference, so not that, so where is 30v coming from? It should be 12v...

I tried a boost converter, but the 5v from the Speccy doesn't have the power needed to drive it satisfactorily, and the screen is very flickery, and the highest input voltage is 6v, so I can't use the 11v from the PSU to drive it directly.

It runs perfectly if I drive the boost converter from a bench PSU.

Meh...

What happens is you take out R62 ?

exxos wrote: 23 Sep 2023 17:24 What happens is you take out R62 ?

I haven't tried that, but taking out D15 resulted in R79 going up in smoke!!

I'll replace R79 and put D15 back, then see what happens with R62 out.

Incidentally, since replacing the caps with new, I seem to be up to 70v...

It just seems to be running away. -5v is still fine, but that seems to be controlled by D19, that Schottky diode, which I know is a 5v one, but it made no difference to the 12v rail at all. Spares board is still producing the correct voltages.

:fire1:

I'm just waiting for it to start producing 40000 volts and using it to power a whole city.

Sinclair did say you could run a nuclear power station with a ZX81 so why not?

stephen_usher wrote: 23 Sep 2023 22:17 I'm just waiting for it to start producing 40000 volts and using it to power a whole city.

Sinclair did say you could run a nuclear power station with a ZX81 so why not?

The night is young let me see what I can do...

Ok, so I swapped the coils over and the original works just fine in the spares board, so it isn't the coil. I had already checked the coil for shorts and thought it was OK, but this has fully satisfied my curiosity.

The coil from the spares board when fitted to the + board works just as well at producing 70v as the one I swapped into the spares board, and sorry to say @stephen_usher, no 40KV tonight.

@exxos Lifting R62 changed nothing, but the 70v was still visible at D15.

I can only assume that the transistors are switching at a rate inconsistent with generating 12v. I'm thinking I should compare the frequencies seen on the TR4 base between the two boards.

Or something isn't behaving as a regulator/feedback mechanism for said transistors.

I've replaced all the electrolytic caps with brand new from Retroleum, all the resistors are in spec when I check with the multimeter, the coil is good, D11 & D18 have been swapped for new, D15 & D19 were swapped with the spares board and the spares board works just fine, so they are good.

The only component I have yet to swap from the spares board is D17, so that is next on the agenda, and though I do have some on order, I can swap in the known good one from the spares board for now.

As for the spares board, it continues to run, with the exception of what appears to be a memory issue, which I don't have the spares to fix at the moment, though I could look at the logic ICs first as I probably have those to hand.

I'll probably order the RAM and a diag cart (Always handy to have to hand) from Retroleum, they are quite inexpensive, though I do have a lot of 41256s I could use in theory with a bit of hacking, but I also need 16 pin sockets, so ARRRRGGGGHHH!!!

I'm thinking of changing the title of the thread to 'RJ's adventures in high voltage with a Sinclair Spectrum'.

I guess other Spectrums may have had this issue back in the day and the 4116's and LM1889 would have been 'well done' in cooking parlance.

@rubber_jonnie Have you checked D15 both sides on a scope ? I can't see how you could get from low voltage one side to high voltage the other. It's kinda defies the laws of physics :lol: A diode can't act as a voltage multiplier. The high voltage must be on the left side of the diode.

Are you 100% sure the schematic is correct ?

Pulsing the coil with no load could pulse up to high voltages. So I'd make sure you got a load of 100mA or something on it to keep it stable.

Have you replaced C43 and C49 ?

I could do a simulation of the circuit to see how it's working. Though might be a little unfair as we don't know the specs of the transformer. I guess if your able to measure the resistance and inductance of the primary and secondary, then that might be enough. Obviously turns ratio would have to be guessed. But it might be a 1 to 1 ratio for all we know.

Just my two cents, it might sound stupid, but do you have the original PSU ?

Isn't the polarity reversed ?

HigashiJun wrote: 24 Sep 2023 01:49 Just my two cents, it might sound stupid, but do you have the original PSU ?

Isn't the polarity reversed ?

Yes, it's the genuine Sinclair PSU that came with the machine, and yes, the polarity is revered, so it's centre negative.

The voltage is only "reversed" relative to today's standards. All the equipment I have with an external power supply with a similar plug from the 70s and early 80s has centre negative.

I'm guessing that it was changed so that you couldn't accidentally short the power if the cable was dropped onto something metal.