Power supply problem need help

[Techno1]

I have a 13.5 volt power supply that is putting out the voltage but the amps are not there under a load the voltage will drop way down.
What part should I look for to be bad or would it be 1 of the output transistors.
Have not had time to check it out yet.

thanks

 

[sp5it]

I sense... bad power supply

 

[S&W357]

1.) What is PS rated for?

2.) How big of a load are you putting on the power supply?

3.) Welcome to the forum !!

 

[Techno1]

Howdy

No name on supply or tag for specs I measured the volts 13.77 out if i put a small load the volts will drop to zero.

the transistor on back im sure is the output says Astron on it so maybe an astron power supply. First time cover ever been off I looked for parts to be bad -burned or the caps bulged and nothing. Rectifier has 23 volts going out of it to the regulator board. The pc board is mounted upside down so i hope I can get a schematic for it. I have had it for years and some where it lost its amps.

Here are some pictures.

 

[Techno1]

Looks like the output transistor is rated for 15 amps so I would say its a 10amp power supply

 

[Techno1]

here is some more pictures

 

[Captain Kilowatt]

According to the picture you posted, it clearly says 7.5 amps continuous and 10 amps maximum intermittent peak. I'm thinking the regulator is shot.

 

[Techno1]

Well I just went down to my shop and checked something i saw on youtube.
The 2n3771 regulator was not getting warm stone cold So I removed it and added some heat grease on the mica insulator and tightened the 2 little sockets that the regulator plugged into. Now the regulator was getting warm, so i hooked up a cobra 29 to it and transmitted and watched the voltage and 13.78 with no drop.
I did check the 2n3771 and checked out good. So i guess a common problem with the little pin sockets get loose over time. His video on his supply had 4 regulators for a 35 amp Astron supply and 1 of the regulators was not getting warm like the others so he removed and checked it and was ok just that the pin sockets on his were a bit loose also. So I hope an easy fix. thanks for the replies.

 

[Captain Kilowatt]

Glad it worked out but FYI that 2N3771 is called a pass transistor, not the regulator. The regulator is the LM723 IC chip. It can handle only very small current, so it drives the 2N3771 that handles all the heavy current.

 

[Techno1]

So im back with the Astron power supply.
Seems the volts are there 13.7 volts but when a load like keying a cobra 29 it sucks the voltage down I replaced the 2n3771 and the LM723 with same problem.
So now im suspecting the RCA29/TIP29.
The regulator board looks fine no burnt spots and any obvious problems I know looks aren't everything.
Just wondering what else it might be? If i hook the cobra 29LTD all is good until I key the radio and it will dim out like the amps are not there . Its a 7-10 amp power supply. Ill post the schematic and see if someone can shed some light on what might cause this to happen.

 

[nomadradio]

Way back in the bad old days when the economics supported repairing this kind of power supply, we learned to pull every semiconductor part out and test it first. If it checked okay, just put it back. Any resistors that looked overheated got changed, and any bad diodes/transistors/chips replaced. The problem with changing just one part, powering up and seeing what happens is that a new part can get clobbered if another bad part is still in place.

Bottom line is that finding one of these with "just one" failed part turns out to be an exception, not the rule.

73

 

[Greg T]

I'd just buy a Megawatt 30A and call it a day.

 

[Especialista B4A]

Interesting schematic.
Try to check the PS with another radio and see if the same symptom occurs.

It may seem strange, but the problem may be in the radio, for example, the final amplifier transistor in the radio may be shorted out and so, the overcurrent sensor in the PS (basically, LM723 + 2N3771) will drop the output current/voltage to minimum for safety.

Another test would be to measure the voltage at the base of 2N3771 with the radio turned on and then keying the radio. if there's no problem ahead of the PS, both readings must be pretty close (if not the same)

If both are different, try the same on PIN10 of LM723. It's the output to the voltage/current control. if there's nothing wrong there, maybe you want to pull out RCA29 and test it.

Disclaimer: Do it at your own risk. It's just my two cent's, since I'm a little bit rusty in analogics.

 

[Techno1]

I have tried 3 radios and the same problem.
I just ordered the RCA29/TIP29 and will try that. The power supply is on a shelf and wired to a binding post block. So a pain and will check part when it gets here next week. The other 2 parts I replaced cost less than $8.00 and the RCA29 is 2 for $1.79 so this will be the last part as I have checked the resistors and diodes all seem good. Thanks for the reply.

 

[SuperLid]

Did you check the pass transistor emitter resistors? They are usually sand wirewounds, one in series with each emitter, but I have seen other types in Assbomb power supplies. They are almost always less than one ohm.