Hi With 24V in, 450 ma is 10.8W. That’s a pretty hearty number for warming up an OCXO. That power would not be going through the 18V regulator for a couple of reasons.
Once warmed up, 170 ma at 24V is 4.08W. That’s more than a functioning OCXO should pull at typical lab bench temperatures. 60 ma at 24V is 1.44W. That is a reasonable power for a functioning OCXO on a lab bench. Bob > On May 9, 2022, at 3:32 PM, Marek Doršic <[email protected]> wrote: > > 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@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 <[email protected]> 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 <[email protected] >> <mailto:[email protected]>> 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 <[email protected]> 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 <[email protected] >>> <mailto:[email protected] <mailto:[email protected]>>> 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 -- [email protected] >>>>> <mailto:[email protected]> <mailto: >>> [email protected] <mailto:[email protected]>> -- To >>> unsubscribe send >>>>> an email to [email protected] >>>>> <mailto:[email protected]> <mailto: >>> [email protected] <mailto:[email protected]>> >>>>> To unsubscribe, go to and follow the instructions there. >>>> _______________________________________________ >>>> time-nuts mailing list -- [email protected] >>>> <mailto:[email protected]> <mailto: >>> [email protected] <mailto:[email protected]>> -- To >>> unsubscribe send an email to >>> [email protected] <mailto:[email protected]> >>> <mailto:[email protected] >>> <mailto:[email protected]>> >>>> To unsubscribe, go to and follow the instructions there. >>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] -- To unsubscribe send >>> an email to [email protected] >>> To unsubscribe, go to and follow the instructions there. >> _______________________________________________ >> time-nuts mailing list -- [email protected] -- To unsubscribe send an >> email to [email protected] >> To unsubscribe, go to and follow the instructions there. > > _______________________________________________ > time-nuts mailing list -- [email protected] -- To unsubscribe send an > email to [email protected] > To unsubscribe, go to and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] -- To unsubscribe send an email to [email protected] To unsubscribe, go to and follow the instructions there.
