EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: jpb on May 13, 2019, 09:04:01 pm
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About a year ago I repaired an HP10811 and was doing a final voltage check and managed to blow up Q5 which is a 2N6429A.
I replaced it with another NPN in a TO-92 and get a distorted waveform - I then put the repair on hold while I moved house and more recently ordered some approximate equivalents (a multicomp MPSA06) and a BC108.
The MPSA06 is a disaster, it has too much gain I think and overloads things so the frequency drops to 1.7MHz!
The BC108 is equivalent to the other generic NPN I used and is the right frequency but is distorted.
I've just ordered some 2N6429A off ebay but I suspect that they are probably not genuine.
Has anyone found a good replacement when repairing a 10811?
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Lets try bc414 bc550
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The original 2N6429A is a transistor that has very high gain, specified from 500-1300 according to the datasheet. The MPSA06 hfe is between 100-500, so it is quite a bit lower gain than the original. The MPSA18 transistor has high gain like the original part, so I would recommend a MPSA18 - they are readily available from Digikey and Mouser.
Good luck!
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Thank you both.
The seller of the 2N6429As I bought on ebay cancelled the order today as he didn't have them so it will definitely need to be a substitute.
I'll get them from digikey but unfortunately I have to order £33 + VATs worth to get free postage so I may need to collect some other stuff together first (or be extravagant and buy myself some other goodies).
I'll buy a selection and see which works best.
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I've made some progress but Q5 doesn't fix the issue.
The signal at the base of Q5 is a nice sine wave of the correct amplitude but the output on the collector is a much lower magnitude sine wave (it should be roughly the same magnitude).
I'm now suspecting the final stage with Q9. It may be the inductor/capacitors that are part of the stage but all the ones I've checked so far have been ok so I suspect it is Q9 itself but I can't find any info on it.
The part number in the manual is :
TRANSISTOR NPN PD.600Mw FT6600m142 28480 1554-0833
The transistor itself has
~612
54-
0833
printed on its face.
Does anyone have any info on this or can even tell me which of the many numbers corresponds with the device?
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After removing and testing yet more components, I now think it is the inductor L7 which seems to have gone open circuit.
I didn't realise that inductors died like that - I generally suspect capacitors of going bad.
Need to order yet more digikey parts - this is getting expensive!