[time-nuts] OSA-5400 power transistor

[Marek Doršic]

I would like to get your thoughts on my problem with OSA-5400 oscillator.

I have on old unit, which is somehow broken. I was told it was overpowered with voltages up to 32V (standard supply voltage is 24V) and even sourced with reverse polarity supply power.

When I first powered it up, it draws only 2mA. I replaced what I supposed was a broken 10V voltage reference (how wrong I was), with a 10V zener diode and voilà , I had a nice steady 5MHz, 14dB signal. But only for couple of hours and then it died again. So I reverse engineered the schematics below and the part in question (Q4) is what I suppose a PNP power transistor. A bought a bunch of different types available. Solder in an 2N2905A and powered the unit. The heater went on, the unit drew 480 mA after power up but the output signal was still only some noisy 2mVp-p. After a few minutes the transistor went broken and the heater and everything went off.
Then I put there a BC160-10. This seemed to be good choice. The unit worked again normally, with nice output signal, but again only couple of hours and then the signal was lost.
But all the voltages at test pads remain as labeled on the PCB. The thermistor output pins on front panel are always 2 Ohms. This part in heated core of the unit is probably already broken.

Do you have please any thoughts, what can be wrong with the unit or what kind of power transistor should be used (Q4). The original part has gold plated leads and TO-39 package.
The resistor values are only indicative measured with multimeter while soldered in.

   .md