DC-DC mod query

[dfzx]

I have an issue 3 board which has a faulty 12V supply (reads a wobbly +1.2V). -5V works correctly.

I've never tried to fix a DC-DC problem before, so I thought I'd start from first principles. The board has had a DC-DC mod of some sort, so I've had a good look at it and taken notes. Here's an annotated image:



I was following the modification described here. The mod on my board seems similar, but not quite the same. In particular, the 2 hand soldered diodes I've marked with green dots don't seem to correspond to anything in the documented mod. Do I misunderstand, or is this a variant with a couple of extra diodes?

I'm not quite sure how to proceed. I could just dismantle this lot. I think I might have to because I can't physically get to TR4 and TR5 in order to test or replace them. Nothing moves or bends! I can feel with a fingertip that one of them gets hot. On the other hand, most of this seems to be related to the -5V supply which is working fine, so I don't really want to pull it all apart if that's not necessary.

Advice please?

[drdivago]

Hi,

I follow the instructions in this video: https://www.youtube.com/watch?v=AbIJza7ElrE

Ciao
Leo

[dfzx]

That's an excellent video, but his final work doesn't look much like the board I have!

I'm still working on it...

[1024MAK]

So, over the lifetime of the Spectrum, there were various changes to the circuitry including that of the DC-DC converter section. The website/blog you linked to is mine BTW.

On your board, you may have either an earlier version or it has just been implemented differently. The service technicians that did the alterations did not always do it the same way.

I know it's hard to tell, but if TR4 is getting hot, one of the 4116 (or equivalent) "lower" DRAM chips may have failed and is now drawing excessive current. Hence dragging the +12V rail down.

I'll have a closer look at your photo later when I'm on a PC.

Mark

[dfzx]

The website/blog you linked to is mine BTW.

Yes, I thought so. Thanks for doing that.

I wanted to tell you that the schematic has two D15 diodes on it, which has rather puzzled me. I think one of them is actually the new D17 which is one of my green dotted diodes in my image?

I was also a bit puzzled by the "C78/C80" pairing, which I originally thought was 2 capacitors, but now appears to be one capacitor with an odd double-barrelled name. Or possibly alternative names, for some reason.

[dfzx]

I know it's hard to tell, but if TR4 is getting hot, one of the 4116 (or equivalent) "lower" DRAM chips may have failed and is now drawing excessive current. Hence dragging the +12V rail down.

I'll have a closer look at your photo later when I'm on a PC.

This is the current working version of my image:



I desoldered the board end of what I think is D15. (The point labelled "D15 moved".) I reconnected it using a bit of wire so it's functionally the same but now with a bit of room to access those 2 transistors.TR4 is definitely getting hot; it's still not easy to tell with TR5 but I think that one's OK.

I've actually taken to exploring the original circuit on an unmodified issue 3 board I have, trying to figure out how it works and how this broken one differs. I'm still working on that.

[dfzx]

To complete the story, in case anyone finds it in a search, I got the DC circuit working.

I desoldered the cluster of components, then rejoined them all spread out so I could get access to those underneath. I then printed out the circuit shown here, and went through it point by point, ticking off each junction of each component as I confirmed each one with a continuity meter. I was thoroughly confused by the way that schematic shows moved and removed components in parallel with the ones they replace, but once I'd worked out what it was saying I was able to confirm the mod on my board is in fact the one on the diagram with the exception of C47 which has been removed but not replaced on my board.

I replaced TR4 and TR5, then I found a couple of errors in the implementation someone had done.

First, C79 had been fitted the wrong way round. The capacitor had burst. It didn't seem to affect anything, the -5V circuit was still working and nothing changed when I replaced it.

Second, diode D15 had been fitted the wrong way round. This is very wrong, and led to this alarming looking trace from the collector of TR4:


free jpg

Um, yes, that really does show 60 volts! I think with D15 being the wrong way round, it blocked the spike from the coil from escaping into the smoothing and regulating part of the circuit and the voltage just built up at that point.

With everything separated out and the DC-DC circuit fixed, it looked like this:



Further investigation showed the lower RAM was not working, which I know how to fix, but then I tried putting the ULA into a working Spectrum and discovered the ULA wasn't generating the screen image correctly. So I've marked as beyond economical repair. The Z80 might be worth something in a few years time.