2N6429A equivalent for HP10811 repair

[jpb]

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?

[Daxxin]

Lets try bc414 bc550

[ArcticGeek]

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!

[jpb]

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.

[jpb]

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?

[jpb]

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!