Superstar 4900 B for possible repair. First look and diagnosing

A first look at a superstar 4900b cb radio that has some issues. Trying to diagnose this radio for a possible fix, or is it too far gone. the customer agreed to doing some diagnosing and parts changing in the hopes of fixing it.

[@MikeReed]

I am impressed how far you got. Sad that whoever owned it before just did not take good care of it. But that seems to be the fate of allot of radios. I do hope you get it to work.

[@dgafkucky]

I remember superstar 4900b it looks like a kenwood 570

[@slingerrr9548]

Hello, According to the documentation the voltage on the positive lead of C252 that you replaced should read + 24 volts in receive and transmit mode because that goes to the Emitter of Q65 ( KTA1940 ) and the collector should be 10 volts in RX & TX. Anyhow the schematics do show a 3300Uf @ 25vdc. Maybe they used what they had as a source at that time at the factory. There feeding the voltage right off the bridge rectifier straight to that power regulator transistor.
They do have a voltage regulator IC14 78l10 which is a voltage regulator for the rest of the radio. Max in 25v Max output is 10.4 volts according to spec sheet
The KTA1940 is the same as a 2Sb754 (PNP) they commonly use in all radios for the power regulator circuit. And you are correct those schematics really are awful to follow. I wish you good luck on that adventure for repair. Take care !

[@pulsarcb]

You must be mistaken or there's an error on the documentation. That filter cap gets 13.8 volts. That's exactly what's measured there now. These radios are known for bad power boards. When they go bad, it will show sometimes 20 volts at that cap.

[@slingerrr9548]

@@pulsarcb Ok thanks for responding. The Repair Memorandum pdf documentation is incorrect. I hope your making progress and take care.

[@stevepolkinghorn2883]

Is there a 13.8 vdc plug on the back? Use that for your initial diagnosis so you aren’t chasing the multiple problems all at once.

[@pulsarcb]

The rear plug is 3 pin header style. But I did bypass the rectifier board side in the video. You can see the alligator clips to the dc power supply.

[@stevepolkinghorn2883]

Oh, okay. I missed that part!