Hi
This is what i got when i connected the output of a 30 volt centre tapped power transformer and read various input pins of the transformer under test.
The transformer under test was still in circuit , but unpowered.

Photo 1--------Applied voltage to the output of PUT-------24.03V

Output voltages from primary of PUT

2--------4-5--------34.97V
3--------3-5--------32.25V
4--------2-5--------1.876V----?
5--------1-5--------1.865

What do you make of these? PUT being in circuit may corrupt the test , may have to unsolder grrr!

Best regards

 

The voltage readings do provide some good news, which is that the transformer is not burned up. It is telling me that the problem is a high resistance in series with the primary winding so thatnot nearly enough current can flow to provide power to the amp. That points even more towards it being that failed temperature fuse device. So now you have adequate reason to openthe transformer and to bypass that thing. OR take it to a transformer shop and have them do it.

 

Hi.
A747 , pin 1 is used for 1 of 4 mains voltage selections.
The schematic i have is all i can find on the net , so i am flying blind to a degree.
The next move for me is to price a replacement transformer.
This is what i think is needed.

Toroidal transformer with these properties
1300VA power (assuming 0.5 power factor)
110 V centre tapped output
When costed , then the owner can decide the next move.------replacement , burial or heart surgery1

What do you guys think.
Best regards.

 

Hi.
A747 , pin 1 is used for 1 of 4 mains voltage selections.
The schematic i have is all i can find on the net , so i am flying blind to a degree.
The next move for me is to price a replacement transformer.
This is what i think is needed.

Toroidal transformer with these properties
1300VA power (assuming 0.5 power factor)
110 V centre tapped output
When costed , then the owner can decide the next move.------replacement , burial or heart surgery1

What do you guys think.
Best regards.

If the transformer is dead it might not be the cause but rather a victim of something else. If possible, I would substitute a variac as input to the PS section of the amp, or a bench supply after it and make sure you are not dealing with something downstream that is the cause.

 

Y is certainly correct about that!!! Substituting that much smaller transformer that was used for checking should allow testing for short circuited components, although it may not be able to power enough for amplification. But a 110 volt secondary is excessive by any analysis.

And now, based on the voltage readings in post #52, One more check.. Disconnect the 3 connections from the secondary side and connect the mains feed to terminals #4 and #5. With no load you should be able to measure a voltage at the secondary that will be close to the voltage needed for a replacement transformer. That will be a very useful thing to know.
And I am thinking that a 1300VA transformer will be so much larger that it will not fit in the space available. So your replacement should be about the same dimensions as the present one. I base this on the fact that the present transformer lasted for quite a few years before failing.

AND, as Y mentioned, check the other parts. I would start by checking the power diodes to see if one or more are failed shorted.

 

Certainly failed capacitors will cause a failure, BUT this amp also shows a fuse located in the mains power connection. So examining that fuse to see if it is the correct value can provide a useful clue. And that is an easy thing to check. And I still suggest powering the transformer with no load connected to see what voltages are found on the secondary. If those voltages are close to what is expected then there are not shorted turns, and that provides an incentive for trying to repair the present transformer, which will save a whole lot of money!

 

And time.

Hi
747 , i don't have a local motor/ transformer shop , i'm 100+kms from a city that might help.

Y and MB2 , i chose 110 V secondary based on the voltage required +/- 49.5V (no signal) as per schematic.
KVA was a guess as i don't know the PF.
Max power 650 Watts @ .5 = 1300KVA.
Found one that might do the job , but gave that away when i saw the price----A$ 450----ouch!!
The fuses are okay.

The attached photos show secondary V outputs with output connected and unconnected from external circuitry.
I hope these will give clues and i await your responses.
Best regards

 

I asked for one voltage reading, which was the secondary with no load, nothing connected. I did not request pictures of meters connected to ????
The reason for my request was that it appeared that the transformer primary had a high resistance in series with it, which would be possible if that internal device had developed some high resistance breakdown condition when the mains voltage was applied. In the one photo the range selector is set on a current range, and that presents a very low resistance across the meter terminals. That will certainly not report much voltage from a high impedance source.

 

I asked for one voltage reading, which was the secondary with no load, nothing connected. I did not request pictures of meters connected to ????
The reason for my request was that it appeared that the transformer primary had a high resistance in series with it, which would be possible if that internal device had developed some high resistance breakdown condition when the mains voltage was applied. In the one photo the range selector is set on a current range, and that presents a very low resistance across the meter terminals. That will certainly not report much voltage from a high impedance source.

Hi
Hope this is the reading you wanted.
240 Volts connected to pins 4 and 5.
Voltage output of pins 9 and 10 (unloaded)
Best regards

 

Looking back at post #52 it seems that the voltage with 3 and 5 powered should have been more. , since it was 32+ volts between those pins when the secondary was powered. How about trying 240 volts between 3 and 5 and measuring the secondary volts. All I need is numbers, not pictures. And use the AC volts setting. This must be done withno other connections to the secondary.

 

Looking back at post #52 it seems that the voltage with 3 and 5 powered should have been more. , since it was 32+ volts between those pins when the secondary was powered. How about trying 240 volts between 3 and 5 and measuring the secondary volts. All I need is numbers, not pictures. And use the AC volts setting. This must be done withno other connections to the secondary.

Hi
Primary 240 Volts across pins 3 & 5

Secondary Volts as follows:
pin 9 to 10------2milliVolts
pin10 to 11-----2milliVolts

Best regards

 

OK, nothing here makes any sense. This means that it is time to remove the transformer and gently, carefully, open it up. No saw or other destructive tools. Those transformers are not usually encapsulated, and this one was not dipped in tar. So careful surgery should be possible. That will reveal the actual condition.