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

[Marek Doršic]

    Yes, it is a power transistor with heatsing.
Please find attachned the attachments via dropbox
   
https://www.dropbox.com/s/efzgvs2rh8c76in/Screenshot%202022-05-08%20at%2018.58.10.png?dl=0 <https://www.dropbox.com/s/efzgvs2rh8c76in/Screenshot%202022-05-08%20at%2018.58.10.png?dl=0>
https://www.dropbox.com/s/wd4yrndn4scfzov/Screenshot%202022-05-08%20at%2019.01.46.png?dl=0 <https://www.dropbox.com/s/wd4yrndn4scfzov/Screenshot%202022-05-08%20at%2019.01.46.png?dl=0>

    .marek

> On 8 May 2022, at 21:05, paul swed <paulswedb at gmail.com> wrote:
> 
> 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 <mailto: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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