Pretty well nearly finished a rebuild on this radio and was letting it run for a while. Despite a new set of resistors and capacitors the Tasma just stopped.

No audio at all so back to the bench. Soon found the audio output transformer primary had gone open circuit which was rather disappointing after so much work.

6M5 output valve, according to the circuit diagram from Kevin Chant the o/p transformer is a 7,000 ohm primary and probably 8 ohm secondary. The print on the circuit diagram is a bit hard to see.
Have temporarily put in a 7K to 3.5 ohm standard output transformer which has restored the radio back to life for the moment.

If someone has the correct Tasma transformer would like to make a purchase. It would be nice to have a original transformer in place.

The photo of chassis has the original transformer with a matchbox as a reference for size.

Please PM me if you can help out with this request.

Thanks

Photos uploaded.

How about you put the new transformer inside the old case or wont it fit ?

The speaker won't be 8 ohms unless it's a much newer replacement. It will be 3.5 ohms. So you are OK.

Yes, good idea to pot the replacement transformer in the case of the old one.

Do you have access to an oven to melt it out?

That you are prepared to clean afterwards to get rid of the smell?

On the subject of oven cleaners, some advice from my son (who is a chef and so should know)

Always choose the cleaner with the scariest safety warnings on the label. Don't waste your time with anything else!

Repotting the replacement transformer into the original case may be the way to go. Have done a pitch repot and its a messy process but can be done.

May as well give the case a squirt of black paint to make it look pretty while doing the job.

Thanks for the comments.

The only potentialy messy bit is getting the old one out & I have never had an issue with that. Disposable metal food trays can be useful.

A piece of metal strip from harware as a base plate between the existing case and the speaker will act as support for the new transformer. A heat gun can can be of advantage & allows the old transformer to be towed out.

Do not re use the bitumen as a lot of that like Australian diesel, has a high sulphur content and I believe that with that and its insulation properties; That this contributes to them failing. In many cases that potting mix is not a total fill.

Do make sure the new transformer cannot rattle. I have mounted them, as have others, (especialy if the transformer is live frame) inside the pan.

Hot melt glue is a compatible and safe potting material for this kind of job. 4 or 5 sticks should be enough.

I use it for re-stuffing paper caps for a job that you want to look original.

I try not to over do bogging them in. This based on the fact that either the sulphur in the bitumen corrodes the copper, or its the heat retention from being tarred, but not featered that seems to cause transformers and coils that were done that way, to have a high attrition rate.

Watch the wires, I have seen a few cut through by the can.

Think I will repot in hot melt glue and discard the old bitumen. Pouring with rain so will have to wait for the messy task of easing the old transformer out of the case. Best done outdoors.

The substitute transformer will easily fit into the case and there is a metal strip which will help keep it in.

Thanks for remarks.

Hot glue is an excellent potting agent, I've used it for many projects.
One doubtful event does come to mind when I sealed a minuture flux gate magnetometer in a small glass jar. The FLM failed within days.
Reason unknown, and I often wonder if the glue heat weakened those tiny surface mount components.?

I hope you can do the rewind-rebuild but if rebuilding doesn't work out there is always...

https://ozvalveamps.org/optrans.htm

There are also other websites which discuss this usage.

Don't try to use a glue gun. Melt it in an oven.

Be VERY careful with it to avoid burns which are particularly nasty because the stuff is endothermic. Much like a steam burn. It gives up a LOT of energy when it solidifies.

Liquid insulin tape may work?

In the end used a heat blow gun to heat up the case. Put the case in a old cooking pan and let the heat gun run for a while until the pitch melted. Scooped most out with a screwdriver and after a struggle pulled the transformer out of the case.

A messy task for sure.

Rewinding is completely beyond my resources and in any case the transformer fell to pieces as it came out.

Washed out the case with mineral turps and repainted. Actually the only replacement transformer on hand is 5K to 3.5 so only used a minimal amount of hot glue to hold it in place.

Will keep looking for a 7K to 3.5 ohm transformer.

First time I have used a hot glue gun and can see they are handy things to have.

EDIT: Occurred to me to convert the o/p stage from a 6M5 to 6BQ5. The 6M5 was not a particulry popular valve while there are oddles of 6BQ5's. Even new ones from guitar amp suppliers can be bought.

Another option I have seen for removing the tar from the old transformer case is to freeze the transformer and then give the case a good belting and the innards will crack and come out.
Probably still difficult but less messy.