Audio amp power transformer question

Thread Starter

spuddo

Joined May 2, 2013
45
Hi ,
With the transformer out of circuit i got these readings:

Primary :
Pin 5 to Pin 4 L = 16 Hs ,
R could not register a reading. ?
Secondary :
Pin 9 to pin 10 L = 17 mHs
R = 100 mOhms
Pin 10 to pin 11 L = 17 mHs
R = 100 mOhms
Pin 9 to pin 11 L = 74 mHs
R = 500 mOhms
This seems to indicate the transformer is OK (famous last words).

When wired up and fired up back in the power supply , these are the readings :

Primary :
Pin 4 to pin 5 240 Volts

Secondary :
Pin 9 to pin 10 0 Volts
Pin 11 to pin 10 0 Volts
No smoke , no fuses blown.

This seems to indicate the transformer is kaput.
Any thoughts.
Thanks for listening.
 

Attachments

BobTPH

Joined Jun 5, 2013
3,316
Why do you think the readings are inconsistent? When I calculate the expected voltage for the secondary from the inductances, I get 7.8V,. Not exactly 10, but not way out of whack..

Bob
 

Thread Starter

spuddo

Joined May 2, 2013
45
Why do you think the readings are inconsistent? When I calculate the expected voltage for the secondary from the inductances, I get 7.8V,. Not exactly 10, but not way out of whack..

Bob
Thanks for your reply.
The readings for the output were 0Volts , not 10Volts
 

Thread Starter

spuddo

Joined May 2, 2013
45
There may be an internal fuse in the transformer. You can sometimes get to them by removing the frame.
thanks for your reply.
There is a fuse shown in the schematic , but as i could read 240 Volts and measure 16 Henrys i considered the primary was OK.
 

BobTPH

Joined Jun 5, 2013
3,316
Inductance is the square of the turns, so the turns ratio is the ratio of the square root of the inductances.

Tr = sqrt(0.017) / sqrt(16)

Vout = Vin * Tr

Bob
 

sarahMCML

Joined May 11, 2019
39
Hi ,
With the transformer out of circuit i got these readings:

Primary :
Pin 5 to Pin 4 L = 16 Hs ,
R could not register a reading. ?
Secondary :
Pin 9 to pin 10 L = 17 mHs
R = 100 mOhms
Pin 10 to pin 11 L = 17 mHs
R = 100 mOhms
Pin 9 to pin 11 L = 74 mHs
R = 500 mOhms
This seems to indicate the transformer is OK (famous last words).

When wired up and fired up back in the power supply , these are the readings :

Primary :
Pin 4 to pin 5 240 Volts

Secondary :
Pin 9 to pin 10 0 Volts
Pin 11 to pin 10 0 Volts
No smoke , no fuses blown.

This seems to indicate the transformer is kaput.
Any thoughts.
Thanks for listening.
What do you get between pins 2 and 5? You may be able to ignore the internal fuse and rely on the external one.
 

BobTPH

Joined Jun 5, 2013
3,316
Not sure how he got inductance reading of an open circuit. Come to think of it 16H for the primary and no resistance reading seems wrong. Perhaps the 16H was just the max reading if whatever he is using to measure it?

Bob
 

Thread Starter

spuddo

Joined May 2, 2013
45
Thanks for your replies , sorry for the delay , i come from a land down under---you go to bed , i get up.
Pin 2 was the original connection for neutral.
In a previous question to this forum i was advised to use pin 4 , made sense , now we have the fuse in circuit.
In the schematic it is a little blurry and looks like pin 2 and pin 4 are connected , that appears to be a draftsman typo and doesn't make sense.
First picture: primary pins 4 & 5 L= 16.9 H secondary pins 9 & 11 R = 910 mOhms.
Second picture: primary pins 4 & 5 R = ? ( no milli's) secondary pins 9 & 11 L = 74 mHs.
Best regards
 

Attachments

anniel747

Joined Oct 18, 2020
1,122
Thanks for your replies , sorry for the delay , i come from a land down under---you go to bed , i get up.
Pin 2 was the original connection for neutral.
In a previous question to this forum i was advised to use pin 4 , made sense , now we have the fuse in circuit.
In the schematic it is a little blurry and looks like pin 2 and pin 4 are connected , that appears to be a draftsman typo and doesn't make sense.
First picture: primary pins 4 & 5 L= 16.9 H secondary pins 9 & 11 R = 910 mOhms.
Second picture: primary pins 4 & 5 R = ? ( no milli's) secondary pins 9 & 11 L = 74 mHs.
Best regards
Yes pins 2 & 4 are connected by the fuse so should be near 0 Ohm.
 

MisterBill2

Joined Jan 23, 2018
8,780
Given the symptom, a resistance check of each winding would be the first step. Given the failure mode, reading 240 volts across the primary indicates that the mains connections and power switch are OK, and tells nothing about the transformer.
The clue is in the schematic, where a thermal fuse is shown inside the transformer. These are usually the cause of an open primary circuit. On many occasions this device is accessible from the outside of the transformer, and it can often be seen by looking at the edge of the windings on the mains connection side. I have been able to carefully cut the paper over the device and remove it, or just installing a jumper across the open device, and then closing up the paper covers. It is very important to chech the rest of the system after this fix to see if there is really an overload due to some failed part, or is it just a poor quality thermal fuse. Once the problem is fixed it is important to have an external fuse in the mains feed to avoid a problem if an overload happens again.
Poor quality thermal fuses are much cheaper than having an external fuse, which is the reason that they are so common. The equipment makers do not care at all that the cost of a replacement transformer will prevent repair of the equipment.
 
It doesn't even have to be a thermal fuse. It just there to prevent people from bypassing stuff. I had a can opener that had a fuse just under the paper. The transformer had a lump that sould not be there, so I peeled some away and there was a small regular fuse.

The fuse in your diagram looks out of place and I have no idea why you looked at inductance first.

I only worked on one amp that had a primary to case short. I didn't see it at first because I use an isolation transformer. It wasn;t repaired for the customer.

I moonlighted for an outfit that did consumer electronics repair. Strange way we became friends. I was in the store and an amp came back and I offered to take a look at it. That was the start of a nice relationship. I did their audio amps that were not simple. I'd picked them up. They would order schematics and parts and I'd fix after work and after class. Win-Win. They now totally switched gears and are doing IT network stuff. They had a strong background in Satellite TV, the large dishes.

One particular AV amp came back to their shop about 3x for the same problem. An amplifier IC went bad. Turns out the turn-on transient exceeded the maximum rating of the IC power. Easy to fix.

Another amp was interesting. A disco owner borrowed a high power 200W amplifier from a relative and it broke. The amp was taken to five shops and they all refused to work on it. I have never seen components that "puddled" on the PCB. i.e. metal film resistors melted. Nonetheless, he wanted it fixed at any cost.

After fixing it, I did add thump suppression. The output connectors were totally shorted.

A good proportion of the repairs were the large amplifier STK modules in Fisher amplifiers.

Another issue that was consistent was the use of 1 to 10uF electrolytic capacitors for coupling.

Very few amps had a DC adjust. All had bias adjust.

They did a lot of VCR repairs and had to quarantine some because of roaches. They found a sandwhich in one VCR.
I worked on an amp full of cat hair because the cat liked to lay on the warm amp.

At work, I did some electronic servicing. Not a lot, but some. Since we had about 8 vacuum gauges, I stocked parts for those.
We had a bunch of DCR series switching Sorenson power supplies, that ended up with the same failures like replace electrolytics. I maintained x-y recorders. They required maintenence, like replacing calibration pots and lubricating the slidewires, replaceing the electrostatic hold downs. I did a lot of modifying stuff, so the problem would not occur again.
I trouble-shot the DEC LSI-11 systems to the module level.

Then there was the high power, high voltage stuff like 0-100kV power supplies for an X-ray diffraction system, a 1000 W tube RF transmitter for an RF sputtering system and the 0-15 kV 1.5A power supply for a CV-8 ebeam evaporator power supply.

At the other end of the spectrum, I built systems to measure femtoamps of currrent.

I built some solenoid drivers.

I spent a lot of time designing I-V converters. My flagship was a 4-terminal +-100 mA FS, with 3 more decade lower ranges at +-10V out that was capable of biasing +-10V. It had suppression of +-50 mA. It was intended as an AC I-V converter, a job it did very well. DC performance was good, but was supposed to be better.

There was one design, I would have done things differently. It was a 24 channel relay driver. I should have supported the board from the sides of the rack case so the relays could be replaced without major disassembly. I built things as appliances, so they never really got scrapped.
 

MisterBill2

Joined Jan 23, 2018
8,780
The only thing that I found a primary to case short on was the very burned power trans of a tube type Monarch brand receiver.It burned because the 2 amp line fuse had been replaced with a 15 amp automotive fuse. The root cause was a plate to cathode hort in an IF amplifier tube. They found a burned up cathode resistor and replaced it with a ten watt one so that it would not burn up again. Just a bit of overload on the B+ line. So then the line fuse would pop, o it was replaced by the 15amp fuse. THEN the power trasformer burned up and the moke convinced them it was all over. Then it came to me. It ended because the new transformer would cost more than the whole receiver.
 
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