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Strangely intermittent fault

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throbscottle

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The laser printer decided to do its fault again, after working well for a few months since I re-soldered one of the EHT driver transistors. Same symptoms, intermittent fault causing variously, a speckled stripe across a page, missing random colours, occasional blank pages. Eventually it settled down to producing blank pages.

So this time I re-worked the entire EHT board - re-soldered every single joint. Put it back together - great, it started working again!

But, couple of days later, back to faulty behaviour again. Seems to manifest randomly, doesn't look thermal. Clean out of ideas. Is it possible for one of the driver transistors to manifest an internal fault in this way?

TIA
 
There are a couple of headers, one is many data/control lines (I presume) and one is red and black, so I guess DC power. All the other connections are via spring contacts from the various EHT voltages to the parts they supply, which were what I paid attention to when the fault first appeared because I thought they might be dirty. The board is ridiculously simple to remove - 4 screws and those 2 headers, nothing in the way.

I suppose I can check for a cracked wire at those headers. Shame I can't swap the whole board - not prepared to try and buy one in case it isn't the true fault.

Or perhaps one of the header female connectors has got loose, since they've been pulled off a few times now.

Or maybe I should re-work the board again. I dunno.

Thanks for the response anyway :)
 
The laser printer decided to do its fault again, after working well for a few months since I re-soldered one of the EHT driver transistors. Same symptoms, intermittent fault causing variously, a speckled stripe across a page, missing random colours, occasional blank pages. Eventually it settled down to producing blank pages.

So this time I re-worked the entire EHT board - re-soldered every single joint. Put it back together - great, it started working again!

But, couple of days later, back to faulty behaviour again. Seems to manifest randomly, doesn't look thermal. Clean out of ideas. Is it possible for one of the driver transistors to manifest an internal fault in this way?

TIA
Beginning to sound like that - the thermal shock of the re-solder remaking an internal connection on the transistor's die?

If you can get to them, try mechanically tapping the transistors (and ICs, for that matter), one at a time with the printer ON and a job in the queue.
 
Hmmm, plastic stick time. The through hole components are on the inward facing side, have to be reached between the EHT carrying springs...
I did try tapping the board whilst it was printing blank pages - didn't make any difference though. Maybe it needs to be whilst it's working correctly.

My other thought is that maybe one of the output transformers has an internal dry joint. They seem to contain a relay so there could be allsorts in there.
 
Tentative "Yaaaaayy, nailed it, woohooooo!" After all these months that it's been plaguing me...
Used a hair dryer to locate the faulty transistor. Tested when cold, all trannys seemed ok, sure enough printed test page OK. Heated up suspect with hair dryer, printer didn't print but suspect (previously had a dry joint found on earlier attempt at repair) looked OK. Turned out to be a different one, I checked Vbe twice and got two different readings as it cooled down. So that was a 2SB1417, which I can't find a vendor for, so I put in a 2SA1469 (board pull) from my junk box, seemed like a good substitute, faster and a little bit beefier.
So it printed the config page allright, time will tell whether it continues to work when Mrs wants to print knitting patterns and recipes (and when I want to print stuff to take just before I go to work...)
 
Just hoping I actually got it right this time. Still tentative...
 
Well, just printed engineers script - these things usually kill it! All pages have things on them that I want to be there. No blank pages, no speckldey lines. Looks like yellow toner is running out though - it's been leaking like mad so no surprise.
 
the OP said:
Is it possible for one of the driver transistors to manifest an internal fault in this way?

I've taken apart TO-3 transistors and have seen all sorts of bad connections. Internally, on some packages, the wires are ultrasonically wire bonded. Nothing fancy when you have done it before by hand, but fanatical to get it right.

Thru-hole vias can sometimes be a problem, so this is stuff you can't see. Corrosion from leaky caps can be an issue too.
Connectors almost always have issues. Remember that they have a maximum number of mating cycles. Heaven's forbid, if they bury components between the layers. This is possible today.

I just recently read of a 30 Kw device for electric cars being totally 3D-printed and Mansions being 3D-printed as well, The Start-trek replicator isn't too far off.
 
But what do you do with all that worthless gold when you've got your latinum out???
 
Use it in fake semiconductors, dodgy pcb's and poor mating connectors to sell to you lot, obviously :p:D
Or I could just leave it somewhere secure and watch the fun as folks try to steal it :woot:
 
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