The Sinclair QL Forum

I read, years ago, that stuck pins in a dot matrix print head, can be freed by soaking the front end in WD40. From the images above it looks like a couple of pins are stuck.

Obviously needs to be dried off before use.

Cheers,
Norm.

I gently removed the larger gunk bits with the point of a blade and had dabbed some isopropanol on it. I'd rather try that (less messy) than any kind of thin oil.

There are sprays that can dissolve clumped ink, and I once had the same problem with my Epson 24pin.

Head soaking in a glass of IPA. I don't think the ribbon comes off and don't want to force it, so it's in situ. While it's apart I also removed and dumped the ROM contents. It's virtually the same as the one Urs provided on Dilwyn's site - only difference is the wording of the self-test page, very minor slightly different wording, one has a full stop after 'ver', the other doesn't etc, but it's the same s/w version and the dates are the same (16th May 1985).

Oh and this is the PCB model detail:

Hmmm, soaking it for an hour and then trying it did absolutely nothing. Having looked closer at the dot patterns, it's 4,5,8 and 9 (from top down) that are stuck.

I'll try again, soaking for longer.

If you can get at the other end of the ribbon cable. You could measure the resistance of the coils in the printhead to see if they are OK. I would expect the thick middle one to be the common.

Also I don't know how well the flexible cable has faired over the years, look for any cracks. Think QL keyboard membrane.

The ribbon looks very good quality, but I do wish I knew how to take it off the head, or off at the other end. I suppose I need to undo the 4 screws on the head to see if I can get access to the ribbon. I mean it must come off somehow I assume, as head replacement was something that went on back in the day. I just don't want to end up with loads of little parts that don't go back together well! I was also going to try IPA in an ultrasonic cleaner with the head dipped in that to see if it loosens anything.

Ok, with the back off the head it is possible to access the ribbon connections without everything falling apart. Middle certainly is common. On the lower right there is a gnd to the metal part of the head. And there are 9 other connections for each pin.

Not looking great though. Measured between the common in the centre and each outer pin, 5 of them are around 32 ohms. The other 4 are 1.2 - 1.6M ohms. That's not good is it. Does that mean the little solenoids have died?

Edit: with the head unit completely removed from the printer (I found how/where it unplugs) the 4 that had very high resistances are now O.L.

Looks like the coils have burnt out. Probably driver transistors blown up. You will likely find a row of 9 transistors, or there may be a couple of '4 pack' transistors, and an odd one.

The '4 pack' option sounds like a good bet, as there are 4 blown coils. A bit academic unless you can get a replacement head. If you find blown transistors, there may be other things gone as well.

Thanks Martin,

Further disassembly, this is the complete main PCB. I've found the transistor arrays I think. The 3 in the bottom right area seem to be connected to the 2 black connectors above them which go to the print head (the red and white connectors go to the 2 motors). All 3 transistor arrays are TK3L10Z. Haven't found a pinout for those yet. I don't mind replacing them at all, but like you say, with a dud head (and who knows what else) it's going to need a donor with the same head and that's probably more effort/expense/space/fiddling that I want to get in to!

Shame, as otherwise it's really in nice condition. Expect I'll end up putting it all back together and selling on as parts.