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Thread: Electrophone PSA 123 circuit ?

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    Default Electrophone PSA 123 circuit ?

    Hi All,

    After 34 years if service my old GME power supply has given up the ghost. I am going to attempt a repair as it looks the size seeable for an old bloke with a size 10 soldering tip. My circuits have long since been disposed of & I could re-invent the wheel or be fleeced for $120 for a new one. but it would be nice to fix something for a change.
    Many thanks in advance.



Look Here ->
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    Thanks gents; I keep an eye open for access.

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    Quote Originally Posted by Softek2 View Post
    Thanks gents; I keep an eye open for access.

    Apols - google keep switching things up ~ you should be good to go

    edit: to avoid any further disappointment ;

    Last edited by wotnot; 11-01-21 at 10:50 AM.

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    Firstly; Thank you for a circuit that is a good size to read & no multi legged IC's. I will spend some time over the weekend with the multimeter. There is 13.6VDC on the output & it will run a receiver (350ma). Just wont take a load. I will fish about & try to find an acceptable load for testing. Many thanks once again & I will keep everyone posted when finished.

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    No worries ~ had a power outage here this morning thus couldn't attend to this until now --- the circuit schematic is taken from the service manual, which you will no doubt find useful ; I'm pretty sure the following link will take you to that ;


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    I will make a guess.
    RV4 The current limit adjust pot is dirty/open circuit.

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    I will make a guess 2n3055 phukt
    reject religion for the planets safety

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    Quote Originally Posted by hinekadon View Post
    I will make a guess 2n3055 phukt
    As it still regulates the voltage, the only way the TP3055 could be faulty would be Collector Base open circuit. Unlikely.
    As RV4 sets the current limit, if the wiper has gone open circuit the current limiting will kick in early.

    Fun guessing.

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    disagree politely when a 2n3055 goes legs up it will become a resister therefore allows a small amount of current thru to tht out put and can be measured as such as thruput from emmiter to collecter without base current /voltage
    reject religion for the planets safety

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    Quote Originally Posted by hinekadon View Post
    disagree politely when a 2n3055 goes legs up it will become a resister therefore allows a small amount of current thru to tht out put and can be measured as such as thruput from emmiter to collecter without base current /voltage
    Different experience. They are usually collector emitter short circuit. Occasionally base emitter open circuit.
    Collector emitter open circuit would let the driver become the main voltage and current path still allowing voltage regulation.
    Still think it is a fault in the current limiting section.

    Still fun guessing.
    Last edited by Reschs; 16-01-21 at 08:51 PM.

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    Still fun guessing.
    It's a guessathon? Awesome! Let me spin the bottle...and......and...... thermal fuse has failed and turned into a resistor

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    Quote Originally Posted by wotnot View Post
    It's a guessathon? Awesome! Let me spin the bottle...and......and...... thermal fuse has failed and turned into a resistor
    Actually, because of Q3's base emitter junction, the 5 Amp fuse blown would explain it.

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    I like guessing games.. I'll have a spin of the bottle and say it is a user error. lol

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    G'Day All; Just a quick update. I did look at (R4) & alas it checked out ok. gave the wiper a wiggle, used a light 35W globe for load. BTW the 2n3055 checks ok. When I load the supply first it goes to O/load then gradually comes up to about 10.6V. I checked Q1,2 &3 in circuit with everything looking good. replaced C3 (just in case). that was the end of my hour on the job (& my patience), I will return !!!. How hard can it be? there is only a hand full of bits on the board. I get my replacement glasses next Tuesday so I'll be looking ata dry joint or something obvious then.
    Just out of interest I bought a new PSU for $120 off the web so no pressure any more, it does however, make me feel a bit sheepish.
    Stay tuned.

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    One of the rectifier diodes D1 or D4.
    Also check the soldering around them and the transformer.
    Then check the soldering everywhere. I usually re solder everything on old linear regulated supplies that get hot, usually doesn't take longer than 5 minutes.
    Sorry for the late response.
    Last edited by Uncle Fester; 20-01-21 at 11:54 AM.
    This era of thoughtless consumption must end so we can encourage a world of creative geniuses rather than consumer idiots.


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    Quote Originally Posted by Softek2 View Post
    When I load the supply first it goes to O/load then gradually comes up to about 10.6V.
    Umm...eh? What is 0/load? I might be tempted to read that as 'overload' but the output is only protected by an over-voltage crowbar circuit, and there is no over-current/over-voltage indicator ~ /me confused

    So I suppose that was meant to come out as...'when switched on and a suitable load connected to output, the voltage ...which you previously stated did seem regulated @ 13.6VDC up to 350mA ... drops to 0volts and gradually rises to 10.6v' ?

    If that were so, me being me, 'at a glance' I would consider the regulation circuit & pass-transistor were all working as expected, and it's the voltage frontend folding over.

    In that regard, Uncle Fester's check the diodes pointer is good sound juju, and if you found one of these bad, that'd be par for the course ~ as in, given the time in service already rendered, I would have already condemned the bulk input caps (C1 & C2) and have replaced them (along with the rest of the electrolytic caps) merely because they're cheap as chips, and 34years of service is a long way into expected working lifetime ; time for new ones.

    If a diode was shot, I'd blame C1 & C2 for that too... but after replacing those 2 caps & checking diodes ok, I'd be disconnecting the transformer, and feeding the board DC from the other PSU you've bought, and see if it works ; it's a quick sanity check =)

    I mean, after all, I was the one guessing at thermal fuse ~ when old transformers are involved, I like to eliminate them as a variable asap.

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    Success! . first dry joint -ve side of C1 been o/c for ever. My mistake was to assume that the Britax revolving beacon showing 35W (max) was in fact 55W trust me to not use the correct load. After finding an appropriate set of resistors to load everything up properly. I still had the current protection symptom. Poking about with a small screw driver I finally noticed the fuse might be a bit warm. It was in fact a resistor. Replaced it & found it is faulty (sort of) heated it up removed the endcaps one at a time & resoldered them individually. So presto it works. Still don't understand how that relates to the output voltage varying. This is not the end of the story cause I still can't believe the symptoms. Thanks to everyone for encouraging support. Wish I was 20 again; I knew everything back then.

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    Happy to lose the guessing game, and sit back and watch Uncle Fester win that one.... and yourself of course, good quick fix, so win win ; you can never have too many (working) power supplies =)

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