What did I get myself into? Murphy A40C. - Page 4 - UK Vintage Radio Repair and Restoration Discussion Forum

Phonosandradios · 28th Jan 2023, 9:58 am

Quote:

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 · 28th Jan 2023, 10:07 am

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 · 28th Jan 2023, 10:37 pm

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 · 29th Jan 2023, 8:04 am

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 · 1st Feb 2023, 7:38 pm

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 · 1st Feb 2023, 10:08 pm

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 · 1st Feb 2023, 11:42 pm

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 · 2nd Feb 2023, 12:22 am

Quote:

Originally Posted by PJL

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

Forgot to mention earlier - yes it does.

PJL · 2nd Feb 2023, 10:00 pm

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 · Yesterday, 12:23 am

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 · Yesterday, 9:00 pm

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 · Yesterday, 11:21 pm

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 · Today, 1:07 am

Quote:

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 · Today, 1:42 am

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 · Today, 12:12 pm

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

Mr 1936 · Today, 1:32 pm

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 · Today, 2:49 pm

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.

28th Jan 2023, 10:07 am	#62
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,722	Re: What did I get myself into? Murphy A40C. 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.

28th Jan 2023, 10:37 pm	#63
Katar83 Tetrode Join Date: Feb 2021 Location: Thetford, Norfolk, UK. Posts: 58	Re: What did I get myself into? Murphy A40C. 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.

29th Jan 2023, 8:04 am	#64
Mr 1936 Hexode Join Date: Mar 2017 Location: Romsey, Hampshire, UK. Posts: 385	Re: What did I get myself into? Murphy A40C. 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.

1st Feb 2023, 7:38 pm	#65
Katar83 Tetrode Join Date: Feb 2021 Location: Thetford, Norfolk, UK. Posts: 58	Re: What did I get myself into? Murphy A40C. 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?

1st Feb 2023, 10:08 pm	#66
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,722	Re: What did I get myself into? Murphy A40C. 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?

1st Feb 2023, 11:42 pm	#67
Katar83 Tetrode Join Date: Feb 2021 Location: Thetford, Norfolk, UK. Posts: 58	Re: What did I get myself into? Murphy A40C. 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.

2nd Feb 2023, 10:00 pm	#69
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,722	Re: What did I get myself into? Murphy A40C. 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?

Yesterday, 12:23 am	#70
Katar83 Tetrode Join Date: Feb 2021 Location: Thetford, Norfolk, UK. Posts: 58	Re: What did I get myself into? Murphy A40C. 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. Attached Thumbnails

Yesterday, 9:00 pm	#71
Katar83 Tetrode Join Date: Feb 2021 Location: Thetford, Norfolk, UK. Posts: 58	Re: What did I get myself into? Murphy A40C. 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?

Yesterday, 11:21 pm	#72
Katar83 Tetrode Join Date: Feb 2021 Location: Thetford, Norfolk, UK. Posts: 58	Re: What did I get myself into? Murphy A40C. 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.

Today, 1:42 am	#74
Katar83 Tetrode Join Date: Feb 2021 Location: Thetford, Norfolk, UK. Posts: 58	Re: What did I get myself into? Murphy A40C. 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? Attached Thumbnails

Today, 12:12 pm	#75
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,722	Re: What did I get myself into? Murphy A40C. 2W resistors are rated for 500V and will work most places.

Today, 1:32 pm	#76
Mr 1936 Hexode Join Date: Mar 2017 Location: Romsey, Hampshire, UK. Posts: 385	Re: What did I get myself into? Murphy A40C. 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 ?

Today, 2:49 pm	#77
Katar83 Tetrode Join Date: Feb 2021 Location: Thetford, Norfolk, UK. Posts: 58	Re: What did I get myself into? Murphy A40C. 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.