Murphy A 186 mains transformer gone O/C

[Dick Glennon]

Hi again, The mains transformer has gone O/C on the Murphy A 186. It’s valve line-up is V 1=6C9, V2=6F15, V3= 6LD20, V4=6P25, and V5=UU9. I have a good transformer from another set . It’s valve line-up is V 1=ECH42, V2=EF41, V3=EBC41, V 4= EL41, and V5=EZ40. Do ye think that this transformer could be substituted for the one gone O/C. Your thoughts would be very much appreciated. Regards, Dick.

[Dick Glennon]

Sorry I posted in wrong section, Dick.

[Cobaltblue]

Moved.

Cheers

Mike T

[Dick Glennon]

I first posted this in success stories. Sorry if I misled you. My question is that having gone O/C would it be possible to change the mains transformer in the Murphy A 186 whose valve layout is, V1=6c9, v2=6f15, v3=6ld20, v4=6p25 and v5=uu9, with one from another set whose valve line-up is v1=ech 42, v2=ef41, v3=ebc41, v4=el41 and v5=Ez40. Any advice would be very much appreciated Thanks, and apology again, Dick.

[Herald1360]

If the HT and LT total currents for the substitute transformer's set are the same or greater than the A186 you should be OK unless the HT voltage ends up significantly higher.

[Nickthedentist]

Dick, could you tell us the make and model of the potential donor set, please?

[David G4EBT]

The Murphy A186 uses a full-wave rectifier on a separate heater winding from the rest of the valves.

The HT winding required for the UU9 is centre-tapped, 225-0-225V.
The heater winding for the UU9 needs to be 6.3V @ 0.58A or above.

A separate 6.3V heater winding is required for the other 4 valves, the total current of which amounts to 2.33A.

Unless your donor transformer can supply the HT for the UU9, which I doubt, and unless it has two separate 6.3V heater windings, it would not therefore be suitable.

Just a point, are you sure that the original mains transformer is a dud?

It does have a thermal fuse, (referred to as a 'temperature fuse') as seems to be fairly commonplace with Irish-made Murphy radios. If the radio has drawn excess current due to fault condition and the mains transformer has overheated, that will have gone open circuit and thus, the primary of the transformer will appear to be open circuit. I'm not sure if it is resettable, or sacrificial.

Presumably you've got the service data? The Murphy data is 17 pages - the Trader Sheet it 4 pages. Quite a lot of tubular paper caps. Don't know if you've done anything at it yet, but at the least, the audio coupling cap should be replaced.

Hope that might help a bit.

Good luck with it Dick.

[turretslug]

At 0.45A and 1.1A heater current respectively, the 6C9 and 6P25 are both quite greedy for valves of their class, the ECH42 and EL41 counterparts being rated at 0.23A and 0.7A. The other 3 valves all take comparable currents (6F15 vs. EF41, 6LD20 vs. EBC41, UU9 vs. EZ40)- in other words, the substitute transformer will have to supply about another 0.6A more heater current than its original design called for. Not a huge overload, but not ideal if it's a bit marginal on cooling and capability anyway. It would be best to check on the HT power requirements, too, if that is also somewhat higher, then looking for a transformer with better ratings would be wise.

When assessing current and power parameters in a substitute transformer, don't neglect illumination bulb requirements- in a 6.3V parallel heater set-up, bulbs are often 0.3A types (sometimes less, occasionally more). A radio may have just one or as many as four bulbs, making nearly an amp of difference.

Good luck with it.

[ms660]

According to a quick calc. using the service info, the current (RMS) flowing from the HT winding will be approx. 72mA (load current*1.1)

Lawrence.

[David G4EBT]

Originally Posted by turretslug

When assessing current and power parameters in a substitute transformer, don't neglect illumination bulb requirements - in a 6.3V parallel heater set-up, bulbs are often 0.3A types (sometimes less, occasionally more). A radio may have just one or as many as four bulbs, making nearly an amp of difference.

Good luck with it.

Good point.

In addition to the 2.33A required for the four valves, there are two dial light bulbs, each requiring 0.3A, so that's near enough 3 Amps in all. Then the rectifier needs a separate 6.3V winding. Apart from the fact that the mains transformer needs the two separate 6V3 heater windings, it also need 255-0-255V HT, and as Lawrence point out, the HT current is 72mA, then maybe a margin on top of that, making it say 80mA.

I doubt that Dick's donor transformer meets those requirements.

I still think he needs to check whether the mains TX really is duff, or just has an open circuit thermal fuse.

Hopefully, the smoothing choke is still intact.

[Dick Glennon]

Hi again, The transformer that I have is from a Pye P 53, thanks again, Dick.

[Leon Crampin]

If the rectifier fitted is an EZ40, a separate heater winding is not required as its Vh-k (pk) rating is 500V. Most sets using this rectifier employ a common heater winding with the receiving valves. I haven't checked the UU9 data sheet for h-k rating, but it is listed as an EZ40 equivalent. The EZ40 is electrically the same as an EZ80 - I've yet to see one of these fail h-k.

Murphy played very safe with the early all-glass rectifiers, hence the separate winding. Probably wise as their preferred supplier was Mazda....

Load your replacement transformer with the intended HT and LT loads and check the heater voltage and temperature rise. If the voltage is within 5% of 6.3 and you can hold the core after an hour's operation it will be OK. A further check would be to compare the weights of the two transformers.

Another option is to fit LED or 6.3V 0.1A dial lamps.

Leon.

You could save another 1/2A or so of heater current by using a 6V6 as an output valve - plug in replacement with some loss of gain. Purists might squirm...

[Half a Mullard]

The Pye P53 has a common secondary for the rectifier and other heaters but does have a centre tapped secondary for the full wave rectifier. If the primary is open circuit I would echo David’s comment about the thermal fuse. Well worth checking. I have one of these Murphy sets stored away in the loft but if memory serves it is quite a substantial transformer, very Murphy style.

[Mr Moose]

Hello,

From the service data the power consumption of the Murphy 186 is 52 watts, whereas that for the Pye P53 is 40 watts.

The combined anode current for the four valves of the Pye 53 is 38.9ma.

From the Mazda valve data the Vh-k(max) is 300V but only for vibrator power supplies so for this purpose it should have a separate heater supply.

My data shows the Murphy A186 as having a 6M1 Magic Eye. Does your set have one ?

So this information would suggest that the Pye P53 transformer would not really be suitable without circuit modifications unless Pye have used an overrated transformer in the P53.

Yours, Richard

[Dick Glennon]

Hi again,yes the 186 does have a magic eye. I think I might wait and see if I can come across a similar set with with a good mains transformer. Thanks again for all help, Regards, Dick.

[Ed_Dinning]

Hi Dick, it may be possible to rewind, or you can save some power by using Si rectifiers.

If no success can you post a dimensioned picture and I'll see if I have anything suitable (but this may take a while as we will be going away shortly)

Ed

[Techman]

I've got this same model and I have to confess that I hadn't noticed that the transformer had a thermal fuse fitted until it was mentioned here and I looked again at the circuit. As far as I know this model isn't known for cooking its mains transformer, so the fact that it has gives a strong indication that something else has gone seriously wrong in the set to cause it to go. So perhaps I'm stating the obvious by suggesting that the fault within the set that caused it to blow in the first place ought to be found before even thinking about fitting a replacement transformer.

Having found the fault that caused the overheating, I'd then be looking to repair that thermal fuse. Transformers like this tend not to just go open circuit, but rather get very hot and fizzle with some smoke and dripping tar or wax, which I'm guessing yours hasn't done, having been protected by that thermal fuse device, so I'd be looking at trying to fix that first.

Unless the set is a mint example, or has special sentimental value, then I wouldn't waste a lot of money on it. There's plenty more out there to be had for not very much!

[Leon Crampin]

The A186 was slightly special in that it has a bandpass tuned front end with a 3 gang tuning capacitor, variable selectivity and good audio. A good specimen is well worth preserving, I think.

Leon.

[David G4EBT]

Originally Posted by Techman

I've got this same model and I have to confess that I hadn't noticed that the transformer had a thermal fuse fitted until it was mentioned here and I looked again at the circuit. As far as I know this model isn't known for cooking its mains transformer, so the fact that it has gives a strong indication that something else has gone seriously wrong in the set to cause it to go. So perhaps I'm stating the obvious by suggesting that the fault within the set that caused it to blow in the first place ought to be found before even thinking about fitting a replacement transformer.

There are two versions of this and many other models of Murphy radios. Namely, the Welwyn (UK version) and the Dublin (Irish version). Back in time, the Irish government imposed tariff barriers on the importation of UK manufactured goods into Ireland and offered financial incentives for inward investment. Murphy considered it worthwhile to set up a factory in Ireland. ('Anglo-Irish Trade Wars', outside the scope of the forum).

It seems a reasonable assumption that Dick's radio is an Irish made one.

There are various differences between UK and Irish versions of the same models of Murphy radios, perhaps to meet differing regulations, and the use of temperature fuses appears to be one such difference. The fuse is resettable with a soldering iron but requires special low temperature solder. There is sufficient solder for it to be re-set several times (I clean solder free iron and no flux should be used).

The whole idea of the temperature fuse is of course to safeguard the transformer should an overload arise and the temperature rise above a certain temperature. That's what makes me sceptical as to whether Dick has assumed that if the primary is showing open circuit, it's burnt out. Unless I've missed it, I've seen no indication as to whether or not Dick's tested the temperature fuse.

As you rightly say, the correct diagnostic procedure would be to investigate possible fault conditions which might have caused an overload.

Pic 1: Part circuit of the A186 showing the temperature fuse.
Pic 2: Information on resetting the temperature fuse.