ql printer repair woes

[NormanDunbar]

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.

[Chr$]

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.

[RalfR]

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

[Chr$]

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:

[Chr$]

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.

[Martin_Head]

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.

[Chr$]

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.

[Chr$]

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.

[Martin_Head]

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.

[Chr$]

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.