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

[Marek Doršic]

Hi Paul,

    thanks for your insight. Sadly there is no oscillations on the 18V. 
The problem must be somewhere else. What I just do not understand is, why it starts working for couple of hours when I changed the Q4 PNP transistor. And now all voltages seems to be fine and it did not work. I also today noticed interestingly that touching the Q1 transistor metal case with a voltage probe leads to huge input current changes. While heating-up, when I touch it the current increases from 440mA to 460mA. When the unit was already heated, by touching the transistor case the input current drops from 170mA to only 60mA. I am not capable to understand the circuit right now, but for me this might be the next suspicious part.

The oven heating seems to be OK. On the PCB are pins labeled with C, B, E ( <https://www.dropbox.com/s/i6nq0fbi7p4lrin/IMG_7361.jpeg?dl=0>photo <https://www.dropbox.com/s/i6nq0fbi7p4lrin/IMG_7361.jpeg?dl=0> here) with wires soldered leading directly to the heated core. I measured voltages on these pins together with the input current. See attached graph <https://www.dropbox.com/s/37ck2mlyqgcwd46/Screenshot%202022-05-09%20at%2022.53.45.png?dl=0> below of first 40 minutes while the unit was heating up. I think this pins are the terminals of the heating transistor inside the oven and everything looks good there.  0.44A at 24V = 10.5W and then the current settles down at about 0.17A (4W). This is quite in within spec (should be 3.5W after 2.5 hours warm-up).



   .marek

> On 9 May 2022, at 15:54, paul swed <paulswedb at gmail.com> wrote:
> 
> 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 <mailto: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/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>
>> <
>> 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 <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
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> 
>>>> _______________________________________________
>>>> time-nuts mailing list -- time-nuts at lists.febo.com <mailto:time-nuts at lists.febo.com> <mailto:
>> time-nuts at lists.febo.com <mailto:time-nuts at lists.febo.com>> -- To unsubscribe send
>>>> an email to time-nuts-leave at lists.febo.com <mailto:time-nuts-leave at lists.febo.com> <mailto:
>> time-nuts-leave at lists.febo.com <mailto:time-nuts-leave at lists.febo.com>>
>>>> To unsubscribe, go to and follow the instructions there.
>>> _______________________________________________
>>> time-nuts mailing list -- time-nuts at lists.febo.com <mailto:time-nuts at lists.febo.com> <mailto:
>> time-nuts at lists.febo.com <mailto:time-nuts at lists.febo.com>> -- To unsubscribe send an email to
>> time-nuts-leave at lists.febo.com <mailto:time-nuts-leave at lists.febo.com> <mailto:time-nuts-leave at lists.febo.com <mailto:time-nuts-leave at lists.febo.com>>
>>> To unsubscribe, go to and follow the instructions there.
>> 
>> _______________________________________________
>> time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe send
>> an email to time-nuts-leave at lists.febo.com
>> To unsubscribe, go to and follow the instructions there.
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe send an email to time-nuts-leave at lists.febo.com
> To unsubscribe, go to and follow the instructions there.