ZX Spectrum DC-DC inverter citcuit. How does it work?

[gmc]

I've got a retro ZX Spectrum which has a power supply issue. From the posts I've read the circuit takes the +9V and 5V and has  a DC-DC inverter circtuit to generate -5V, -12V and 12V (For the dram chips)

The problem I'm having is I get 12V and 5V but do not get any negative voltage.

A common issue is TR4 blowing which takes out TR5 as well. In my case TR4, base-emitter was shorted. I replaced TR4 and TR5, powered up and TR4 blew again.  Its possible there is a faulty dram which is causing some issues on the -5V

I've checked shorts between -5 to gnd, and -12 to gnd. All fine there

First I would like to understand how the circuit works so I can figure out why there is no -12/-5.

From what I've read there should be a square wave on collector on TR5 which I'm not seeing.  How exactly is this oscillation setup through the coil with TR4/TR5?

I've tested the resistors/caps/diodes, all working so can't think why its not oscillating.

Next to try and stop TR4 from blowing all the time I need to connect up my current limiting power supply and try and isolate it from supplying the rest of the PCB with -5/-12. 

If I can to cut the track between C46 and C49 and remove D15 would that isolate the -12/+12V/-5V? Would this mess up the oscillator circut?

 

Attached is the DC-DC circuit

Full schematic here: DC-DC circuit at the bottom.
https://spectrumforeveryone.com/wp-content/uploads/2017/08/ZXSpectrumIssue3-Schematics.gif

[Circlotron]

I’d go so far as to say the schematic is wrong.
To get -12V out the collector of the ZTX650 would have to swing -12V below ground, and that just isn’t going to happen. Not only that, the output would be oscillating, not a steady voltage. Something is not right.

[iMo]

FYI
https://retrocomputing.stackexchange.com/questions/2242/how-and-why-did-the-zx-spectrum-use-so-many-voltages

[gmc]

Thanks, just found this writeup on the DC-DC circuit which explains exactly how its supposed to work.

https://www.tablix.org/~avian/blog/archives/2006/05/spectrum_dc_to_dc_converter/

[iMo]

Mind there are at least two dc/dc schematics in the above links..

[massaggi.stefano]

Hi, I know this is an old topic, but maybe someone like me is learning about zx spectrum DC-DC converters and related problems...

I have a zx spectrum which does not boot - it seems dead, and I have first checked all the voltages... and they seems okay, but here comes the question:

sorry, if it's a silly one: but... how can I test with a multimeter the 12VA ?

I mean, it's alternate current, one probe to a 12VA pin which should have that voltage, but the other probe??

It's not like DC current where one probe goes to GND.

Where can I find the phase / neutral?  0v?

Thanks in advance

S.

[gmc]

The 12VA is a confusing label, it really means 12V (Analog supply).
It should measure 12V DC. 

Check that you have +5V, -5V, 12V.If that works it could be a range of other issues that causes it to not boot - one of them being faulty RAM which is a very common issue. Are any of the chips getting hot?

[Mouse69]

This may help