ZX Spectrum DC-DC inverter citcuit. How does it work? - Page 1
Forum section: electronics / beginners

author: gmc
author_rank: Regular Contributor
timestamp: February 03, 2020, 07:52:36 pm
content: 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 -5VI've checked shorts between -5 to gnd, and -12 to gnd. All fine thereFirst 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 circuitFull schematic here: DC-DC circuit at the bottom.https://spectrumforeveryone.com/wp-content/uploads/2017/08/ZXSpectrumIssue3-Schematics.gif

author: Circlotron
author_rank: Super Contributor
timestamp: February 03, 2020, 07:58:50 pm
content: 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.

author: iMo
author_rank: Super Contributor
timestamp: February 03, 2020, 08:11:21 pm
content: FYIhttps://retrocomputing.stackexchange.com/questions/2242/how-and-why-did-the-zx-spectrum-use-so-many-voltages

author: gmc
author_rank: Regular Contributor
timestamp: February 03, 2020, 08:26:20 pm
content: 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/

author: iMo
author_rank: Super Contributor
timestamp: February 03, 2020, 08:51:25 pm
content: Mind there are at least two dc/dc schematics in the above links..

author: massaggi.stefano
author_rank: Newbie
timestamp: January 30, 2022, 05:47:51 pm
content: 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 advanceS.

author: gmc
author_rank: Regular Contributor
timestamp: January 30, 2022, 05:51:37 pm
content: 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?

author: Mouse69
author_rank: Contributor
timestamp: February 01, 2022, 06:50:05 pm
content: This may help