Dot matrix printer: finding where the smoke came out

Discussion in 'Technical Repair' started by Speculatrix, Mar 30, 2018.

  1. Speculatrix

    Thread Starter New Member

    Mar 30, 2018
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    I just first say I'm a beginner and (keen but not experienced) amateur at electronics.

    I have a 35 year-old Epson MX-80 dot matrix printer that I'm trying to bring back to life. A while back it started issuing thick smoke plus a bit of crackling. I powered it down straight away. My suspicion was dried-out caps even though the main caps on the power supply looked fine. To be on the safe side I chose to replace them anyway. But having taken the old caps off the board and tested them, they are responding perfectly well – my DMM says they're holding the rated capacitance.

    Also, the smoke residue on the underside of the Epson's upper case somewhat suggests the smoke may have come from the transformer. I've stared and stared at the damn thing but can't see any obvious issue.

    So my inclination is to do the following:
    • Without putting the main circuit board back in, apply power to the printer and see if the transformer starts smoking.
    • If it doesn't, start probing the cables coming from the transformer to see what voltages they're providing.
    But before I do that, can I ask the greybeards around here – am I nuts? Is this a sensible course of action?
     
  2. ericgibbs

    Moderator

    Jan 29, 2010
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  3. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    I have an old signal generator that one day produced a lot of white smoke. It took me a while to find the culprit - the interference suppressor connected across the mains input. The case was cracked but it certainly didn't look like it had produced all that smoke.

    Have a look for such a supressor.
     
  4. Speculatrix

    Thread Starter New Member

    Mar 30, 2018
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    Yeah, I think I know what you're referring to. The mains comes in and is routed to a separate board with three caps – two 4700pF and one 0.1µF – the main switch and a fuse. Then the wires route straight into the transformer. This small board looks in fine fettle and is some distance from the smoke residue.
     
  5. Speculatrix

    Thread Starter New Member

    Mar 30, 2018
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    In the end I managed to find technical manuals via Archive.org - no signing up necessary. And I've found the page that tells me what voltages I should expect coming out of the transformer.
     
  6. ericgibbs

    Moderator

    Jan 29, 2010
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    hi Spec,
    Will be interested to hear what was the cause of the problem.?
    E
    Dumped a couple of MX printers in the recycle centre last year, the recycle centres can be a cheap source of spares for old gear.
     
  7. Speculatrix

    Thread Starter New Member

    Mar 30, 2018
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    Good thinking - I'll keep an eye out.

    I decided to go ahead and power up the transformer (without the logic boards in the machine). There was no smoke.

    The transformer has four outputs which I measured. To my surprise, they weren't dead.

    Two of them – 9V AC for the logic circuitry and 25V AC for a stepper motor – measured fine. However, the 10V AC supply for the other stepper motor actually measured 17V AC (admittedly without a load). And the nominal 12V AC supply for optional interfaces (which I don't have) measured 9.4V.

    Could it be a short in the secondary windings causing one to measure low and the other high? Mind you, given that something in this machine gave out smoke, I can't trust this transformer anyway...

    BTW, I've started blogging about this here: https://mansfield-devine.com/speculatrix/2018/03/epson-mx-80-resurrecting-an-old-friend-part-1/
     
  8. ericgibbs

    Moderator

    Jan 29, 2010
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    hi,
    May sound a little 'corny', but many burning components make their own unique smell.
    Transformers smell like burning varnish.
    E
     
  9. Speculatrix

    Thread Starter New Member

    Mar 30, 2018
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    It was more than a year ago that the actual smoking happened, so my memory of the smell is probably not to be trusted either! And yeah, it's taken a while for me to get around to doing this, but you know what it's like with projects...
     
  10. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
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    If there is a shorted turn then the transformer will get hot even with no load. Just try leaving it on for a while and make sure it stays cool.
     
  11. danadak

    Well-Known Member

    Mar 10, 2018
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    Looking at components with a magnifying glass also can be useful to
    ID parts that have had thermal stress, localized change of color, cracking.

    Close in nose test can be quite useful to locate something that got hot.

    Testing caps can be a challenge. Bulk caps, like large electrolytics, may have
    vent plugs, if that exhibits leakage or physical change, can indicate an issue.
    Or bulging cans. ESR testing a good practice, but that is not an HV test, also important for
    breakdown testing.

    Also there have been production errors in Tantalum's affecting specific
    lots, specific vendors in the past. Google this.

    https://en.wikipedia.org/wiki/Capacitor_plague

    Regards, Dana.
     
  12. Speculatrix

    Thread Starter New Member

    Mar 30, 2018
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    I didn't know about ESR testing (still so much to learn). But I do have a new function gen so I tried the caps (as well as known-good ones to compare) and got an ESR of about 0.5ohm, probably less.

    BTW, I found a different section of the service manual which suggests that the voltages from the transformer are all correct. But, of course, I was measuring them without a load.
     
  13. danadak

    Well-Known Member

    Mar 10, 2018
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    Any discoloration anywhere on PCB Board ?

    Regards, Dana.
     
  14. Speculatrix

    Thread Starter New Member

    Mar 30, 2018
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    No, and I've looked at it very closely. The smoke stain on the underside of the case top tells me whereabouts the smoke came from, which is why the transformer is still the main suspect. I could conceivably have been the caps, but I'm pretty much ruling that out now. And there's nothing else in that neighbourhood.
     
  15. danadak

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    Mar 10, 2018
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  16. Speculatrix

    Thread Starter New Member

    Mar 30, 2018
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    Many thanks for the links - I'll check them out.
     
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