I’ve been having some problems with some circuits and decided to check the supply voltage to find 24VDC. I’ve had this power supply for over 25 years and did have some circuit cards connected to this power supply fail about 6 or 7 years ago, but I can’t remember if I checked the power supply voltage at the time. I know the voltage will be a little high until a load is applied, but I just don’t remember it being this high.

Anyway I pulled off the cover to see if there might be an adjustment someplace and there isn’t anything. There are a couple of large resistors on the output of the transformer and wondering if they might be going bad. I haven’t actually measured the resistance and I don’t have a schematic, but I just wanted to run it by someone who knows this stuff before I start tearing it apart.

 

Is it a linear supply or a switching supply?
What is the AC output of the transformer?

 

can you post a picture of the nameplate that shows what the output voltage should be?

 

Its common for an unregulated supply to output voltage higher than its rating when unloaded..

 

The photo doesn't really show enough to be able to tell a whole lot. Need the other side, maybe the top of that circuit board, name plate and rear.

 

Is it a linear supply or a switching supply?
What is the AC output of the transformer?

The transformer is AC output and uses a fullwave bridge inside the enclosure for DC. I don't know about a switching supply. It is a Motorola power supply for 12 volt radios.

 

can you post a picture of the nameplate that shows what the output voltage should be?

and

 

The photo doesn't really show enough to be able to tell a whole lot. Need the other side, maybe the top of that circuit board, name plate and rear.

 

Well I wanted you to tell me what the AC output voltage of the transformer was. If the supply is not a switcher which seems likely given it's age and application then 24V on the output would be a bad bad thing. There is a failed component that is allowing the input to the regulator to pass through to the output. If the AC output is about 18VAC, then multiplying by 1.414 is 25.5VDC from which we subtract the bridge drop of 1.5 V to get 24VDC out of the bridge rectifier.

 

Firstly, check your voltmeter against a know voltage source such as a 9V or 12V battery.
If the meter checks out ok then it looks like the pass transistor is shorting.

 

Firstly, check your voltmeter against a know voltage source such as a 9V or 12V battery.
If the meter checks out ok then it looks like the pass transistor is shorting.

I don't know which one is the pass transistor. Is it one or both of the big ones in the huge heat sink shown in the last photo?

 

Here is service info for that unit. Hams have been using retired commercial TwoWay Radio stuff for years...

 

I don't know which one is the pass transistor. Is it one or both of the big ones in the huge heat sink shown in the last photo?

You have two in that unit, the big ones on the heat sinks marked M9639.

Remove both transistors and measure the resistance from collector to emitter.

 

Here is service info for that unit. Hams have been using retired commercial TwoWay Radio stuff for years...

WOW that's fantastic! I can't thank you enough MikeML The schematic is a big help and I think I can now make some checks.

 

You have two in that unit, the big ones on the heat sinks marked M9639.

Remove both transistors and measure the resistance from collector to emitter.

Thanks I think I'll be able do that much

 

You might as well improve it while you fix it...

 

You might as well improve it while you fix it...

Oh now that's exciting. Thanks again