Hi,
I was wondering if anybody could shed some light on a problem I have with an old PSU, everything was working fine on it but know one of the voltage lines has gone abnormally high. It should be +12Vdc but I am getting a reading of +21Vdc. I have checked the diodes, resistors and caps in the circuit and all appear fine, there is also a MC1723 voltage regulator which I have changed, it has got me stumped as to what can be pulling the voltage up.

There are also +5V, +24v and -12v supplies on the same unit which are all fine.

Any pointers would be appreciated, Thanks

 

There should be a pass power transistor in the supply line. This is probably shorting.

Photos of the unit, model number and circuit schematic would help us to better help you.

 

What is the secondary(s) AC output?
If you reverse-engineered the circuit for the MC1723, post it here.
Max.

 

Is there a load on the line?
Even linear PSUs need a load to regulate the voltage. Switching PSUs tend not to provide a voltage to an open circuit, whereas linear PSUs always have a voltage on the line, which tends to be high on an open circuit.

 

Thanks for quick replies, attached is the schematic, I have put sticky notes on the PDF relating to the problem circuit.

In answer to the questions,

1, Yes there is a power transistor TR1 (TIP3055), this has been changed.

2, TX Secondary output is 14Vac

3, I am testing with no load, the other two PSU's I have here are reading 12Vdc (or there abouts) not under load.

 

Pics of unit

 

Pics of unit

In the picture (ending 953), there is a red wire that looks like it got disconnected from the large capacitor. Is that right?

 

It is at the moment as I dropped it off to test cap, it's C15 on schematic.

Also C10 & D13 & D14 are dropped of for testing, normally they are all connected up

 

TR1 looks to be a 2N3055. Going back to Mr. Chips post #2. That would be the pass power transistor. IC2 looks to be the 723 driving that transistor. I would check TR1 for a shorted emitter collector.

Ron

 

pin 10 of IC2 should be at around +12 volts. if it is working. and there are 2 2n3055's as well as 2 723 regulators. one for +5 and one for +12.

 

Owww! I missed the other 2N3055.

Ron

 

Just remember that 3055's come in two main packages, (TO3, TOP3 and so on) and so get the right one.

If you have TO3 then a 2N3772 might be a better chioce of replacement.

 

If the resistors R16, R17 and VR1 are correct, you should have a little over 7V on pin 4 for a 12V output. With 21V on the output, the voltage would be about 12.5V.

What is the voltage on pin 4?

 

I'll measure voltage tomorrow when I'm back in the garage.

I've been trying to work out how the 723 works as the data sheet doesn't tell you much.


Thanks

 

Is this not enough?

 

View attachment 83559

Is this not enough?

Thanks, the data sheet I have doesn't have all that info on.

 

Well been playing about tonight, measured the voltage on pin 4 and nothing.
When the fault first occurred I changed the 723 and I added a turned pin header for future ease of change of chips, on closer inspection the joint on pin 4 looked a little dry so I re-flowed it.

It is no happily sat there with 7.1v on pin 4 and 12v on the output. So would nothing on pin 4 cause this? It appears so.

Thanks to everyone for comments, going to give it a good run now to make sure fault doesn't re-appear.

 

Well been playing about tonight, measured the voltage on pin 4 and nothing.
... on closer inspection the joint on pin 4 looked a little dry so I re-flowed it.

It is no happily sat there with 7.1v on pin 4 and 12v on the output. So would nothing on pin 4 cause this? It appears so.

Since pin 4 is labeled as the Inverting Input, the operation would definitely be affected if it was floating (but I wouldn't know if you would get overvoltage or undervoltage).