10.7MHz IF transformer, Murphy 502

[Impecunious]

Having worked on a couple of these sets I've obviously become over confident and complacent. Loosening of slugs in the coils has usually worked by inserting an allen key and then warming with soldering iron, replacing key with trimmer tool and then unwinding. However this particular set was so gummed up that I had to heat to the extent that the plastic former must have become distorted and so when I came to eventually drill out the core, the former broke into so many pieces that repair is unfeasible.
My questions.
1) Will any 10.7 IF transformer do or will it have to be one matched to the 10F18 that drives it ?
2) Many sets are MW, LW FM with dual function IF cans. Could I strip the 10.7 MHz coil out of one of these and mount it inside the original can?
Is there another solution ?
Diagram of the IF coil attached.

Thanks
David

Just a point of interest. When I first got the set I tested all the valves and they were all very, very "tired". On testing the triodes in the UCC85 they each had an anode current of below 1.5mA ( should be about 12mA) yet in the set it still oscillated and pulled in the stations. When exchanging for a new UCC85, little or no difference. Anyway after that surprisibg result I went ahead to try and realign and ended up in the position described in the opening paragraph.

[Sideband]

Originally Posted by Impecunious

Just a point of interest. When I first got the set I tested all the valves and they were all very, very "tired". On testing the triodes in the UCC85 they each had an anode current of below 1.5mA ( should be about 12mA) yet in the set it still oscillated and pulled in the stations. When exchanging for a new UCC85, little or no difference. Anyway after that surprisibg result I went ahead to try and realign and ended up in the position described in the opening paragraph.

This is where valve testers don't always tell the truth! The real proof is running the valves in actual operating conditions which most testers don't do.

[Techman]

Originally Posted by Sideband

This is where valve testers don't always tell the truth! The real proof is running the valves in actual operating conditions which most testers don't do.

That is so very true!

[Impecunious]

Perhaps it's also a tribute to the Murphy circuitry that a triode with such low emission can be persuaded to oscillate. Thanks for those comments about the results of valve testers, In future I'll not be so quick to chuck out those valves that are well below spec.
D

[Techman]

That's right, valves that test low on a tester can often still work perfectly well in a set.

It's a fact that the UCC and ECC85 valve has a hard life and ceases to function before any of the other valves causing no FM reception. However, your test has proved just how low that particular valve can be while still able to function. It's mainly these signal valves that still function perfectly well, even when giving low results on a tester. There are some exceptions, such as the UABC and EABC80, where one of the ratio detector diodes within this valve going low compared to its companion can cause distortion on FM.

[Impecunious]

So, any ideas what toi do with this Murphy IF Txformer?
All suggestions gratefully received.
Thanks David

[Ed_Dinning]

Hi David, not too many turns bad a standard type of former, so you may get lucky and find a similar former to rewind one

Ed

[Impecunious]

I've just been reminded what a fantastic sense of satisfaction one can get from this hobby.
I found just one coil in my collection which looked like it might just do. It had the same diameter and length of former and 40 turns and 20 turns on the primary and secondary. The original coil had 70 and 35 turns.
I was surprised it was this way around. Is it usual for IF transformers to effectively step down the voltage?

Anyway, I fitted it and of course it didn't have the correct resonant frequency. By fitting it in circuit, putting a variable capacitor across each winding and using a siggy gen and oscilloscope, I could see what capacitance was needed across each winding. It turned out to be 120pf across primary and 60 pf across the secondary. So I fitted fixed caps of these values. The slugs were then used to fine tune the coils to 10.7MHz. Set is working wonderfully. Bliss. I thought I was going to have to start looking around for IF coils.

[agardiner]

Well done. What a great result.

[vinrads]

Satisfaction is something money can't buy well done getting it sorted .Mick.

[boiss]

One of the worst radios I have ever been asked to look at was a sort of kit radio that was sold here in Spain. It was a 4 valve superhet. I never took more than 25 euros (as pocket money and to buy caps etc) to repair any fault provided that the problem did not include valves or transformers. There was no O/P on Mw or FM. Voltage wise everything looked OK. Closer inspection revealed chewed up IF tranformer cores, slight heating made no difference I had to slowly drill out and remove the cores in bits. I found on ebay some new ferrite cores, Unfortunately the formers had been damaged and the cores could not be set correctly. I contacted the owner and asked if he had any spare 4**KHz/10.7MHz IFs and he sent me a couple. The cores needed replacing as well but the formers were OK. Mounting them, the wiring needed to be modified. With only 1 IF amp stage there wasn't a lot of gain to go around and there were also probs with the steerable ferrite rod and the FM foil antenna.
It was possible to tune the IFs but they were not well suited to the valves.
The set worked but sensitivity left a bit to be desired. Total time spent on the set about 5 afternoons, a few Euros for caps. Less than 1 euro per hour. It was interesting but a good lesson in not accepting anything that has been got at ever again.
M