Murphy A 218 blown mains tr. fuse.

[Dick Glennon]

Hi again, I have a Murphy A 218 and the first thing I noticed was that the heat fuse has come away from the mains transformer. There is also some wax on the chassis. Does this mean that the transformer has had it. I wonder what might have caused this to happen. Regards, Dick.

[David G4EBT]

Originally Posted by Dick Glennon

Hi again, I have a Murphy A 218 and the first thing I noticed was that the heat fuse has come away from the mains transformer. There is also some wax on the chassis. Does this mean that the transformer has had it. I wonder what might have caused this to happen. Regards, Dick.

I'm assuming that you've got the 21-page Murphy data (some pages of which are for the U218).
You'll be scuppered without it.

I'd say there are two possibilities:

1) A shorted turn, in which case, yes - the transformer has had it.

2) A fault on the radio has caused excess current to be drawn, but there is a chance that though the transformer has been overloaded and become overheated, it may still be intact.

You don't say whether the radio has been restored or not; whether it's just come into your possession and you haven't turned it on but have examined it and noted the problem; whether the radio was working but has then failed, or whether from a physical examination it looks like it's been 'got at' by the 'phantom dabbler'.

When you say a 'heat fuse', the maker's data doesn't show such a device, which would normally not be a 'fuse' as such, but a bimetallic thermal trip which opens when it gets hot, but resets when it cools down. Have you checked its continuity with a meter on the Ohms range to see if it is intact? (I know that Murphy had a factory in Ireland. Is yours an Irish made Murphy, which might have certain circuit differences?)

As a first step, (assuming the 'fuse' isn't open circuit), check that you have the mains tapping set to 240-260 Volts - not 210-230V, and with the radio unplugged from the mains, check the continuity of the primary winding. If that's open circuit the transformer has had it.

If the primary is OK, check the continuity of the secondary windings:

If the mains transformer secondaries have 'flying leads' rather than tags, I'd suggest that you remove the UU6 rectifier valve, and check for continuity of the HT windings to the anode pins of the valve-holder, which are pins 3 & 5. (If you put your prods into the valve-holder socket, remember that the valve pins number anti-clockwise from above, so for example, pin 3 will be third from the right). The HT winding is centre tapped, each half of which should be approx 110 Ohms, so between pins 3 & 5 you should see 220 Ohms. (Pins 1 & 8 - the UU6 heater winding - if good, would only be very low Ohms).

If either of those windings are open circuit, the transformer has had it.

Assuming all of those windings are good, leave the UU6 valve out of the radio, which will remove the HT load from the transformer, then with the radio plugged in and switched on, check with your meter on the AC Volts range on pins 3 & ground and pin 5 and ground of the UU6 valve-holder to see if you have 240 Volts AC or thereabouts from each half of centre tapped HT winding of the mains transformer.

If you don't have any voltages there, that means the HT winding is open circuit. If the dial lights don't light up, or the valve heaters of the other five valves don't light up, the 4V LT winding of the mains transformer to those valves is suspect. (Assuming of course that the valve heaters are good).

If all the windings are good, and the Voltages are as outlined above, if you leave the UU6 valve out for now, that will remove the HT load on the mains transformer, so the only load will be LT load of the dial lights and heaters of the other five valves. If you leave the radio on for several minutes and the mains transformer doesn't get hot, the melted wax suggests that it's been overloaded, so before putting the UU6 valve back in and switching the radio on, I'd suggest that you look for reasons why that might have happened - for example the electrolytic capacitors C56, (8uF), 57, & 58 (both 16 uf), becoming leaky (in the electrical sense).

The smoothing circuit is an L/C circuit using a choke - not an R/C circuit using a resistor, so you'll need to check the continuity of the choke, which should be 265 Ohms. It's one thing burning out an overloaded HT resistor in an R/C smoothing circuit, but quite another burning out a choke in an L/C smoothing circuit. If the mains transformer and choke are intact, the last thing you want to do is to run the radio with a fault condition which could cause excess HT current to be drawn (EG, faulty C56 & C57), writing off the choke or mains TX. That would render the radio beyond economic repair.

I hope these wordy notes might help, unless anyone else has any better ideas.

Good luck with it Dick.

[Dick Glennon]

Hi again, no, I do not have the schematic of the A 218. The schematic I have is for an A 186. Being in Ireland I cannot download anything from Paul’s service data. The temperature fuse is shown at S29 which is in the negative mains coming in. There is also a safety resistor R16 which may have blown as well. If anyone has an A 218 circuit I would be glad to have at whatever it costs. Regards, Dick.

[John10b]

Hi, you have been given some extensive information from David, having service data/ circuit diagram is essential, particularly if you are unfamiliar with Radio. I don’t know why you can’t purchase from Forum perhaps the Moderators can help.
John

[Silicon]

If I remember correctly, the 'heat fuse' makes use of a special low melting point solder.

When the solder softens, a metal contact springs apart and disconnects the mains supply to the primary.

Apparently, You can solder it back into place. I have not needed to do this.

Don't add any extra PCB type solder.

[Simon Gittins]

The A218 doesn't seem to be available from the forum anyway. Perhaps someone could post at least the power supply section.

[ms660]

I can't find the service data for the Murphy A218 in here:

https://www.service-data.com/section.php/148/1/murphy

EDIT: Post crossed.

Lawrence.

[John10b]

You're right, I also took a look and noted it wasn’t listed, on a previous post a few years ago someone was also trying to get information.
John

[David G4EBT]

I'm embarrassed to say Dick that I was quoting from the service data on the A182 - not the A218, which isn't on Paul's disc.

That said, if the A186 has similarities to the A218, they all have centre-tapped HT secondaries, and a smoothing choke, so the diagnostic suggestions I made on the stages of testing the transformer for shorted turns and open circuit windings, apply just the same. Also, about being cautious in applying HT until you've checked out what may be caused the temperature fuse (as it's referred to on the A186 circuit) to have become detached.

Do you know if it is actually a 'fuse' which fails, or a thermal trip which re-sets? Such devices are fairly uncommon in domestic radios, which makes me wonder if it's a Murphy Ireland radio. Bush and Murphy opened factories there, I think to overcome tariff barriers. I say that because if you look at the advert for 1954 Murphy radio models (including the A218) at this link, which I guess were made in Ireland, on the prices it states: 'No Duty':

http://www.murphy-radio.co.uk/murphy...ish_inside.jpg

There might also have been design differences, perhaps due to differing safety regulations between Ireland and the UK.

Or maybe it was a 'Murphy' trait to include a thermal fuse on some radios?

Once more, apologies for the confusion, and good luck in tracking down the service data.

[ms660]

Originally Posted by David G4EBT

I'm embarrassed to say Dick that I was quoting from the service data on the A182 - not the A218, which isn't on Paul's disc.

Originally Posted by David G4EBT

If the mains transformer secondaries have 'flying leads' rather than tags, I'd suggest that you remove the UU6 rectifier valve, and check for continuity of the HT windings to the anode pins of the valve-holder, which are pins 3 & 5. (If you put your prods into the valve-holder socket, remember that the valve pins number anti-clockwise from above, so for example, pin 3 will be third from the right). The HT winding is centre tapped, each half of which should be approx 110 Ohms, so between pins 3 & 5 you should see 220 Ohms. (Pins 1 & 8 - the UU6 heater winding - if good, would only be very low Ohms).
.

In the service info I'm looking at the Murphy A182 doesn't have a center tapped transformer or a UU6 rectifier.

Lawrence.

[Nanozeugma]

From what I've seen, the A186 appears to be closest to the spec of the A218. It sports a centre tapped ht secondary, feeding a UU9 and has a heat fuse in the primary.
It has variable bandwidth/selectivity, coupled to the waveband switch as does the 218.
A round top magic eye is fitted (6M1/EM34.)
Page 12 even annotates differences between Welwyn and Dublin built variants, which include the Dublin build having only a two core mains lead.
Comparing the valve complement might support this...
The 186 has 6C9, 6F15, 6M1 (or EM34,) 6LD20, 6P25 and UU9.
As regards the heat fuse having opened and the TX having dribbled wax, it *may* have survived the overload if you are lucky.
Adequate advice regarding things to check having already been covered, there would be no point in duplicating it.

[Silicon]

What is the valve line-up of the A218?

I have an Irish built A212 in Ireland and that has a heat fuse and metal rectifier.
I can't see any information about resetting the fuse on the service instructions.

The Murphy pamphlet describes the A218 as a 3 waveband, 6 valve luxury table radio incorporating a magic eye.

[David G4EBT]

Originally Posted by ms660

In the service info I'm looking at the Murphy A182 doesn't have a center tapped transformer or a UU6 rectifier.

Lawrence.

That's correct Lawrence, my apologies.

I'll try 'best of three': It was the circuit of the A128 from which I was actually quoting.

Dick said he had the circuit of the A186, which it's said is similar to the A218 which he's searching for. If that is so, both sets have a centre-tapped transformer; both have what Murphy call a 'temperature fuse'; both have an HT choke, and both have three electrolytics in the HT for the reservoir and smoothing, which - if leaky, could lead to excess HT current being drawn.

The title of Dick's thread is of course 'Blown Mains Transformer Fuse' which infers that he's tested it, and it is open circuit. I've discovered from the Trader Sheet on the A186, that it is 're-settable' but it's not straightforward. I've attached a clip below which outlines the process.

If it's desired to retain that feature, maybe a thermal fuse might be a better bet, such as these sort of things:

https://uk.rs-online.com/web/p/therm...B&gclsrc=aw.ds

https://cpc.farnell.com/multicomp-pr...kaAnhpEALw_wcB

Though the A128 I mistakenly referred to doesn't have a 'temperature fuse' and has a different valve line-up, it has all of the other features in the power supply circuitry to the A186 and to that extent, the diagnostic suggestions I made will hopefully prove useful. Namely, check that the mains transformer windings are intact, then apply power with the rectifier valve initially removed to check if the transformer stays cool off load. Then before fitting the rectifier and applying HT, investigate what fault conditions might have caused the transformer to have overheated, notably the electrolytics.

As to the A186 circuit which Dick says he has, in addition to the 17-page Murphy data, there is also a 4-page Trader sheet (1154), which helpfully lists the anode and screen currents. For V1 - V4, the anodes total 52.7mA, the screens 15.5 mA, so 68.2 mA in all. That enable the HT current to be monitored to see if it's more or less in line with expectations.

Even without the elusive A218 datasheet, hopefully the A186 will be of some assistance and Dick has enough information to make a start.

Incidentally, in addition to the 'temperature fuse' the A186 data also states:

"Those parts of the circuit which derive their HT through R22, (2K2 1 Watt) are protected by the ability of that resistor to become open-circuit if it overheats. It is mounted on the smoothing choke, (L22) well away from other components". It says to replace it with the same device.

Pic 1: A186 clip of circuit annotated.
Pic 2: A186 Temperature fuse.

I don't want to wear my welcome out, so I don't think there's anymore that I can usefully add.

[Station X]

How difficult can it be to reverse the power supply section of a valve radio and do a few continuity checks?

[Dick Glennon]

Hi again,The valve line-up of the A 218 is as follows 6C9, 6F15, 6LD20, 6P25, EZ40 and the magic eye is 6M2. Regards, Dick.

[Nanozeugma]

Originally Posted by Dick Glennon

Hi again,The valve line-up of the A 218 is as follows 6C9, 6F15, 6LD20, 6P25, EZ40 and the magic eye is 6M2. Regards, Dick.

Valve lineup the same as the A186 then...
EZ40=UU9, 6M2=EM34.
Have you made any progress with your set?