Hi Guys,
I've tried Kevin Chant's site and no luck with any Music Masters at all.

I'm chasing chassis model A525 if anyone can help.
It's my family doctor's radio.
I can see an electrolytic exploded inside it,
but some other caps look bloated as well.

Thanks, Brek.


ps. Should mention valve set, only slightly different to my first radio:
5Y3GT, 6V6GT, 6SQ7GT, 6K7G, 6J8GA.

A526 did not appear until 50's and has minature's. A525 not seen.

Is that valve combination posted for the A525 that you have?
Is there a valve chart in it?
The circuits appearing in AORSM's do not use 6K7. Most have 6U7 which is slightly different & most Oz sets used 6U7 (stroppy swine at times). 6K7 can be a metal type.

Some of the Monkeys that masqueraded as repairers, did some horror things. Exploded electrolytic caps are what can happen if the set has been left a long time unused & then powered to see if it works, without checking things.

Set has to be after 1937 if 6J8 is the correct valve and probably SW if it is. Where did the number come from?

It may be consistent with an older circuit.

Marc

It looks close if not identical to my A520, the schematic for that is called A5 Ammended.

I got the A525 chassis type from the chassis stamp.
It has only had one visible repair which is a changed electrolytic,
which exploded again

Had a note left in it dated 1979 describing the PSU circuit.

Factory stamp on the choke (ROLA) is dated 1949.

There is no valve sheet, it's just how it's populated.

It came back ok, was close enough to my A520 to do the caps off.
Was missing speaker and piggy back former, but I had a spare 5K audio tranny here.

I got this note from December 1979:
Image Link

He replaced two 8MFD caps with a single 16MFD cap.
Problem is the 8MFD caps only share their positive terminals.
I guess it must have got the radio going anyway,
and funny, he used one side of a multi-section capacitor,
and left the other legs doing nothing.

Here I am decades after that, and the 8MFD cap he left in there exploded!

The original diagram doesn't look right. From what I understand of these power supplies the second electro should have been after the choke and was probably originally the larger of the two, 8 and 16 would sound about right. It might have worked but without the second stage of filtering it would have had a lot of hum unless there's another electro after the choke that's not shown. I'm no expert on this but I suspect it would introduce a ripple into the bias voltage as well.

If we reference to the circuit of the A5, it is back biased That means that the first cap negative goes to the CT and the one after the Choke, chassis.

A 175 Ohm resistor takes the CT to chassis. Having the caps reversed, will kill them, putting them both on the opposite side of the choke to the rectifier will cut the voltage.

Modern caps have no surge voltage on them. Too far underrated can cause the caps to pop. Note A5 circuit has 525V. That is the surge voltage. Use 500V caps. The surge occurs prior to the heater tubes coming into conduction.

The 5Y3 may be shorted, but normally the RCA type just self destruct especially if you run them horizontally (No no).

I would compare the circuit to the A5 & see if it is basically the same. From memory 6K7 will not blow things up in a 6U7 hole, but they are not of the same characteristics and the set will not run properly.

Do check resistors, the grid stopper and grid leak on 6V6 and plate resistors on first IF have a high attrition rate.

Marc

He omitted one of the 8μF caps altogether.
Funny stuff.

It's all done before I posted this.
It was close enough to the A5 schematic to just do things that way.