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Trying to troubleshot 1960s Panasonic transistor radio

Skogas

New Member
Hello,

I am trying to resurrect an old Panasonic Transistor Radio that has an extremely low volume (and possibly other issues that I'm not aware of yet :) Have replaced all Electrolytic Capacitors but it didn't seem to solve the problem.

When doing some measurements on the final output stage and comparing it to what the schematic says - the numbers seem quite odd (also I don't really know what I'm doing :)

If I read it right, the two 2SB324 power amp transistors should be getting 0.12v at the base, 0v at emitter and 6v at collector.
However then I probe the contacts marked B/C/E on the board for these according to what schematics says, I get:

TR10
B = 5.78v
E = 4 - 4.6v (floats continuously)
C = 0.02v


TR4
B = 4.1 - 4.5v (floats continuously)
E = 4 - 4.6v (the emitter is shared so it's the same as on TR10
C = 0.02v

Would it be right to guess that these two transistors are bust, and if yes what would be an appropriate modern day equivalent, or could the be somewhere else completely?

Thank you

Screen Shot 2022-08-08 at 9.57.24 pm.png


Screen Shot 2022-08-08 at 9.52.51 pm.png
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I can recall the turmoil and confusion when troubleshooting these new transistor devices. We were taught with tube circuits, which had wide tolerance, but still easy to troubleshoot because of the wide differences in tube terminal voltages. Only a voltmeter was needed. Then these current devices came out. They were only mentioned in "tube" school. But a few years in the navy cleared that up. They put transistors to work much quicker than commercial electronics. But solid state has always been more difficult to trouble shoot. I've seen EE graduates spend hours and days trying to troubleshoot solid state. No money can be made that way. Then to top it off, we went disposable....just change out the whole circuit. ...( maybe because no one could repair them).

You seem a bit confused about electronics servicing?.

Solid state repairs were no more difficult than valve ones, if you understood how the circuits worked - presumably the ones who struggled were valve repairers/, who couldn't make the transition.

The exact opposite happens now - people can't repair valve circuits, so many really nice valve guitar amps (even very new ones) get thrown out because the 'supposed' engineers who work for the music stores can't repair valve gear. I've repaired numerous nice ones for people, with very little effort, where they have been condemned bu multiple music stores.

The reason for 'board replacements' wasn't (and isn't) that they couldn't be repaired, but simply that it's much cheaper for the manufacturer not to have to keep thousands of spares, print expensive service manuals, and run expensive training courses.

Unfortunately domestic electronics is so cheap now that repairing has all but disappeared.
 

BR-549

Member
Alrighty, I'm sure your right, I just wish that electronic servicing was the only thing I was confused about. Haven't been out in the world for quite awhile. My bench work ended in the 90s. I just play now. I didn't mean to mislead anyone. Pardon me.
 

Les Jones

Well-Known Member
Most Helpful Member
The voltage on the base of the output transistors will be defined by the stage that drives them. As you have not shown the rest of the circuit we can't suggest possible causes.

Edit. I had not noticed that we were on the second page of replies. I now see that you have provided the full circuit. I suggest checking the resistors that set the voltage on the centre tap of the driver transformer secondary.

Les.
 
Last edited:

schmitt trigger

Well-Known Member
Most Helpful Member
Germanium transistors, boy does them bring back fond memories!
I recall that power devices had a color dot, indicating the “hfe bin”. If one of the transistors in a push pull stage went bad, you would have to replace them with matching pairs.
 

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