What did I get myself into? Murphy A40C.

[Phonosandradios]

Originally Posted by Katar83

Well, that sucks, cobbled up some capacitors in series to replace the dead 8uf cap, plugged in new UU5 valve, powered it all up and everything started glowing nicely but not a sound from speaker.

I have this 10" celestion speaker that originally came out of a Murphy A172R radiogram. I am not sure what the specs are but have attached some photos. If it is any good to you its yours FOC. There are some cone issues which would be easily repairable. Somewhere I have an identical one which is in better condition which I could see if I can find if you are interested.

EDIT: looking at the service notes for the A172R radiogram they give the spec of each of the two speakers used in the set as being 3 ohms. Send me a PM if interested.

[PJL]

For the replacement field coil you need to make up the 400 ohm 16W resistor and I uggest mounting it on a bracket off the original wooden speaker mounting board with the bolt holes. Using the chassis to dissipate 16W would raise it's temperature.

[Katar83]

PMed for the speaker, thanks! Should match this set nicely!

Already have correct resistor and I'll mount it on that little bracket where the current resistor sits over L31 on the transformer board and see how hot it gets. It's 50W, size wise its pretty much an equivalent to the current one so should fit in nicely and should not go too hot either.

[Mr 1936]

Hi

There are two main approaches to power resistors. One is a metalclad type designed to be bolted to a heatsink so as to increase its surface area. The bigger the heatsink, the smaller the temperature will be above its surroundings. If you leave out the heatsink and just mount in free air, a 50 Watt type must be derated to 10 Watts and will run reasonably warm at that dissipation.

The other type is made of materials that can stand a high temperature e.g. ceramics. Smaller surface area but much higher surface temperature so must not be too close to anything that would be damaged by heat.

[Katar83]

Little update and further questions. Got the resistor replaced and both caps on the transformer board(C63 and C64), that's all good now.
Received replacement Murphy 3 ohm speaker from Phonosandradios(thanks!), plugged that in and we have sound. Volume though was still bit low so decided to tackle panel 6 which turned out to be a complete mess and it took me a little while to figure out what previous person working on this tried to do here. Lots of extra wires and wire leftovers were removed, single 50V/50uF hunts cap was replaced with two 16V/100uF(original 75uF caps missing entirely), while in there I've also replaced both carbon resistors as although they were within spec they looked like they were proper fried many times.
Switched the radio back on and we have proper volume, the initial crackling at LW made me jump
Had a look at both AC4/PENs and I think I've got one of them faulty as cathode voltage on one of them only goes up to approx 6.2V, the other is fine at 8V.
Two questions now.
1. Am I right thinking that this will need a replacement?
2. Are 16V/100uF caps ok to use instead of original 75uF or should I order the latter? I take it 16V is plenty enough?

[PJL]

Have you checked the cathode resistor values?
The service data says 8V so one is a bit weak but not enough to cause major problems.
The 100uF will be OK.

Have you sorted the replacement for the field coil and what is the HT voltage now? Have you compared the valve voltages with those in the service data?

[Katar83]

Yes, both resistors are ok and within spec and cathode voltage follows socket where ac4/pen is plugged in.
I've ordered some 82uF caps just in case but good to know that slightly lower voltage is fine.
HT voltage is 373V so a bit high, should I slightly increase value of the resistor?
Also might be slight measurement error, my analogue Simson multimeter shows just over 360V.
I'll have to get a heatsink for that resistor anyway, after 10minutes it's boiling hot.
Not checked other valves just yet, will do tomorrow.

[Katar83]

Originally Posted by PJL

Does the set have a 240/250 high AC mains tap?

Forgot to mention earlier - yes it does.

[PJL]

Do check the voltages before you leave it powered up for long, particularly the AC4/PENs. I guess the 373V is on the UU5 cathodes which should be 360V?

[Katar83]

Checked voltages and they are clearly all over the place.
I get approx 2V on the anode of V12, which is probably why I get absolutely nothing on SW, I'll investigate tomorrow.
I'll start recapping and inspecting everything over the weekend but if there is something specific I should start with, let me know!
Thanks all!

PS,
Yes it's 374 on the cathodes of rectifiers, then on one side of resistor I get 362.

[Katar83]

Was just wondering, I'm guessing that my cheapo DMM will be showing higher voltages and I should probably stick to analogue one which is showing HT tiny bit over 360V?

[Katar83]

Quick update, we have SW!
Inspected panel 1 and found R52 open circuit, currently don't have a 15k resistor on hand but have a rheostat so that was adjusted to 15k and soldered in and not only got all voltages on the SW H.F Amp but also managed to find 3 SW stations, a french one(?), Radio 5 and Radio Caroline?
So now confirmed all bands work I can start recapping.
Voltages across couple valves that I've checked also went down and are a bit closer to what they suppose to be.

[Techman]

Originally Posted by Katar83

but also managed to find 3 SW stations, a french one(?), Radio 5 and Radio Caroline?

SW (short wave)? You mean MW (medium wave)

[Katar83]

Meant S.W(short wave), MW works fine too(couple stations), LW best in this set(I think 3 stations with BBC best)
By the way, are 2W resistors ok to use in this set?

[PJL]

2W resistors are rated for 500V and will work most places.

[Mr 1936]

Hi

To the best of my knowledge neither BBC Radio 5 nor Radio Caroline use any shortwave transmitters.

The nearest to you are R5 from Postwick (Near Norwich) on 693 kHz, and Radio Caroline from Orfordness on 648 kHz. Both at the lower frequency end of Medium Wave.

Where on the SW dial do you receive them ?

[Katar83]

I wouldn't be able to tell you as I didn't see the dial with the chassis on its side and me trying to find station with one hand and checking voltages with the other and I'm not even entirely sure it was radio five and radio caroline, the latter I think might have been european radio since the guy specifically mentioned that its close to midnight when it was ~22:57 here so clearly an hour ahead and when I tuned in he said radio caroli...(and it sounded like caroline).

The other radio sounded like the guy was saying radio five before switching to Bruce Springsteen song

The third one was definitely a french station or at least they talked french at the time.

The bandwave switch was definitely set to S.W and the bulbs were lit on the side dial(set to somewhere in the middle), I do get MW stations as well and as mentioned LW seems to be working best, even with a short incorrect aerial. I'm amazed that even in its current condition, BBC on LW doesn't have that specific LW hum to it like every other radio I ever listened to on LW.

Oh and the gain was significantly improved every time I was checking voltage between chassis and AC/TH1 Hexode anode with my multimeter.

[Cathovisor]

It's important to remember how the A40C works on shortwave, seeing some of the puzzled responses on here.

When the set is switched to short-wave, a broadband converter is brought into operation. The centre frequency is set by the subsidiary dial to the right of the main scale and has a bandwidth of about 1MHz.

Now, the output of this converter is fed into the set at the AC/TP as though it was switched to medium wave and you can then tune across this broad spectrum of signals provided by heterodyning a chunk of the SW bands. So it's a sort of, but quite, a dual-conversion set. The aerial is disconnected from the MW/LW front end and connected to the converter's input, whilst the converter output is now connected as above. See the attached.



Of course, if you go dangling a wire on or near the AC/TP you will get MW breakthrough even though it's set to SW.

[Katar83]

I'm still non wiser when it comes to SW operation with this radio in a way I'm not sure which dial I'm supposed to mainly use here as both seem to be working when SW is selected. I can switch to SW and will tune in and out of stations by using both dials which is quite confusing. I've managed to find a number of stations today, from some chinese(?), romanian, french, spanish even polish radio 1 on LW. This is on a two meters long wire antenna from my Bush radio and which sits on the garage floor.

I had a crack at the panel 1 this evening and that was a complete mess too, only one capacitor was still ok and it was a 25nF waxie, go figure! Even silver micas are bad at around 50% rated capacity, this will take me a while as it appears I'm going to have to replace these too and I barely have any here. For now I've added a few ceramic in parallel to bring it back to spec.
Of course only one screw holding the panel came out and the other had to be drilled out and a new ground added, while in there also replaced further electrolytics, which were completely shot.
It also became clear I'll have to clean up bandwave switch as the contacts are dirty and possibly the SW tuning capacitor as just knocking on them will often tune in and out of a station. Not sure yet, how I'm going to do it, I'll have a look in the morning.
Got few more questions.
Does it matter if I use some ceramic caps instead of polypropylene axial ones? I've got quite few ceramic here.
What would be a correct and a good antenna for this radio? Could someone provide an example?

[B.Taylor]

The only way I can see you are confused by shortwave operation is that the indexing mechanism is not working when you turn the right hand tuning knob. When the knob is turned there should be very positive positions for each SW band displayed in the little window. If the knob is just turning smoothly all the way around it would cause confusion. If this is the case then mechanism needs cleaning.

Bill