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

[paul swed]

Marek
Thanks. I have the schematic and can now see that its a 18V regulator. So
thats only 3 watts. Its a classic differential regulator so it can accept a
wide range of transistors because the circuit has quite a bit of gain. If
your transistor is being destroyed then potentially there is an oscillation
in the circuit.
A scope on the +18 should tell you.
Other then that the current should start high at .46 amps as you mention in
by 20 minutes should drop down to 46 ma as a guess. If it stays high the
ovens overheating and as you are concerned perhaps a bad themistor.
Let us know how you are doing.
Regards
Paul
WB8TSL

On Mon, May 9, 2022 at 9:11 AM Marek Doršic <marek.dorsic at gmail.com> wrote:

>     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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