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PAL 200MDX P/S transistor replacements

Discussion in 'Amplifiers' started by Aztec, Mar 20, 2017 at 9:59 PM.

  1. Aztec

    Aztec
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    Hey guys,

    I just aquired this old mobile tube amp. It's a PAL200MDX.

    Unfortunately I think in my excitement I ended up killing it. I seem to have killed the two transistors that drive the transformer. They are unmarked but the schematic says they are PE250 AKA EF250. They are stud style transistors with a bolt on the collector. I can't find any info on these. They are PNP types. I'm wondering if anyone here has any info about these or can suggest a replacement.

    Here is the schematic.

    http://www.firestik.com/Instl-Art/PAL200MDX-dc.pdf

    When I was setting it up for testing I placed the amp on top of something metal. This shorted the collectors of the two drive transistors on the bottom against the case. Apparently negative from the battery and the amps chassis ground are supposed to be isolated as the transistors are isolated from the case with mica insulators. Schematic doesnt show that but on the amp those two grounds are isolated.



    When I keyed it up it made a squealing sound but after that it kinda stopped. When I key it up now it stills squeals a tiny bit and puts about 40 Volts on the plates of the tubes but that's about it. Transistors are testing a little bit weird. Not totally shorted, still conduct in one direction on certain terminals but voltage drop reads around 100mv. Weirder still is that even though they seem to be shorted, when I key the amp up they don't get hot at all which I would expect if they where totally dead. I read somewhere that early power transistors of this type where germanium, maybe that's why I have such a low voltage drop reading? Who knows.

    I would love to get it to work again. Stupid mistake I made. Though in my defense this thing seems extremely vulnerable with those transistors being so exposed like that. With this being a mobile amp I imagine it was often at risk of being put against the metal floor or dashboard of a car.
     

  2. Robb

    Robb
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    Yup

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  3. Aztec

    Aztec
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    Don't think that's them. These come in an old TO-36 package they look late 60s early 70s. They are germanium and PNP. That's all I know.

    Anyone know what the plate voltage on these tubes should be?
     
    #3 Aztec, Mar 20, 2017 at 10:37 PM
    Last edited: Mar 20, 2017 at 11:11 PM
  4. nomadradio

    nomadradio
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    The transistor numbers are "house" numbers, custom-printed on them for a customer buying them in bulk.

    Pretty sure they should be type 2N4048, rated at 60 Amps, 30 Volts and 170 Watts, so long as the heat sink is big enough.

    Those were prone to fail for no good reason, or from overheating. A shorted tube could overload them all by itself. I would suggest you simply assume that the 450-Volt filter capacitors have shorted and replace them before installing expensive transistors. If the HV filter caps are original, they have no excuse to still work this many years down the road. That kind of capacitor can short, overload the transistors and then chemically "heal" the short when you go to check them with a meter. The next time high voltage is applied, the cycle repeats.

    Normal plate voltage is around 800 Volts DC in this amp, pretty sure.

    Those transistors are a bit like the tubes that amplifier uses. When they stopped making them the prices shot upwards. Would be cool if some design genius figured out a way to use five-dollar MOSFETs in that circuit.

    73
     
  5. Aztec

    Aztec
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    Well I fixed it kinda. And yes I figured out how to make it work with mosfets. I originally intended to rebuild the circuit as is. However even with modern day transistors it still wouldn't be efficient or stable. What I did was build a CD4047 oscillator circuit that drives two IRF3205 mosfets. The mosfets are rated higher than the original transistors and are much smaller. I kinda didnt think it would work unless I paired up more mosfets but it seems to be doing just fine.

    I can't get a reading of the plate voltage while transmitting as an RF loaded plate connection isnt safe grounds for my digital meter. Right now the oscillator has variable frequency as I don't know what frequency the inverter used to use. It's a laminated type ransformer not ferrite. .

    At around 500hz or above I get roughly 330-350 volts on the filter cap which is around 700V B+ past the doubler. At below 450hz I get 450 and up to 500 volts. Which is around 1000V. I havent gone below say 300hz. However at this lower frequency the current consumption goes way up, the pilot light dims.

    In summary below 450Hz more voltage but higher current draw. Above 500hz a little less voltage but also less noticeably less current draw. Not sure which is the right one.

    It would be nice for anyone to give me some input on the frequency theyve heard these inverter things work on. Again this one has a laminated transformer not a ferrite one so I would expect a lower frequency. I had a ferrite transformer amp once and it seemed to work at around 1khz or so.

    I need a jumper to hook up my wattmeter, when I do I'll try and report my findings about inverter frequency.

    Ill share the mod once everything is finished.
     
    #5 Aztec, Mar 22, 2017 at 8:54 AM
    Last edited: Mar 22, 2017 at 9:46 AM

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