[time-nuts] Re: OSA-5400 power transistor

[paul swed]

Marek
No diagram included that I can see.
The next comment may be totally wrong since I have nothing to go on.
If the input voltage is 24 V and the supply is 10 V reg at .48A, then
during the initial warm up the transistor easily dissipates 6 watts.  That
would be a power transistor and some form of heat sink to keep the junction
temperature reasonable.
Regards
Paul
WB8TSL

On Sun, May 8, 2022 at 2:14 PM Marek Doršic <marek.dorsic at gmail.com> wrote:

> 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
>
>
>
>
>
>
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