Murphy A262 - UK Vintage Radio Repair and Restoration Discussion Forum

Patrick Dixon · 27th Aug 2007, 6:27 pm

I have recently acquired a Murphy A262 which would otherwise have been heading to the tip as part of a house clearance. It actually worked when I (probably unwisely) tried it - which is the main reason why it didn't end up in the tip!

Not knowing too much about these kind of things, I had a look round the web and decided that I needed to replace the wax capacitors and sort-out the dial lamps which didn't work. On removing the chassis from the case I discovered that the dial lamp bulbs were completely missing, so I ordered some new ones and some replacement caps and set about hoovering out the dust. As part of the hoovering process, I removed all the valves and wiped them over with kitchen towel.

When the parts arrived, I fitted the new bulbs and replaced the 3 wax caps, and discovered the first problem. The dial lamp bulbs fouled the tuning pointers (which was presumably why they'd been discarded). Several hours later, I realised that the tuning mechanism had been incorrectly strung resulting in the pointers pivoting back onto the backplate/dial lamps, rather than forward to rest on the glass tuning plate. They would have been travelling in the wrong direction too ...

I also moved the mains voltage tap from 220V to 250V. AFAIK, this set has been in the UK for the past 15-20 years, so I'm not quite sure why it was set thus.

However, the second problem, was that the set no longer worked. I checked and re-checked my work, had a prod around with scope and meter, and it would seem that the IF valve is now dead. It's heater doesn't visibly glow, although it measures a sensible resistance and the voltage reaches the socket correctly. I can see and tune modulated stations (on all 3 wavebands) on it's input grid, but the grid fed from the HT (via a 68K resistor) doesn't appear to be drawing much current as it's sitting around HT too. The output stage appears to work as I can get noises though the speaker by probing it's input/the PU input.

So my question is, is it likely that removing, wiping and replacing this valve has killed it, or am I barking up the wrong tree? I've checked the valve socket and the pins and they all look in reasonable conditions and capable of making contact. What else should I check to verify that this valve is/isn't faulty? If I get a replacement IF valve, which other valves should I also replace as a matter of course (I haven't got a valve tester).

Assuming I can get it working again, the next plan is to replace all the Hunts decoupling caps. A couple of these look really tricky to get to - is it normal to have to disconnect several other things to uncover them, or are there easier ways? It doesn't look like this set was built with maintenance in mind!

The valves in this set are ECC85, 6C9, 6F15, UU9, EABC80, 6P1 and the suspect one is the 6F15. For anyone interested, the A262 schematic is available FOC on the web.

Thanks in advance.

paulsherwin · 27th Aug 2007, 6:43 pm

Welcome to the wonderful world of radio restoration

It's very unlikely that you have damaged the valve unless you have been extremely unlucky. Small signal valves don't always have a clearly visible glowing heater, but if you can measure a reasonable resistance between the heater pins, the heater is OK. Check that you have fitted the valves in the correct holders and waggle them to make sure there isn't a bad connection. Try using some switch cleaner on the pins.

You say you have replaced 3 wax caps. The Hunts Mouldseals are also wax caps and notoriously unreliable

However, this is unlikely to be the immediate problem, unless one of the Mouldseals has broken in two while you were working on the set, which sometimes happens

The most likely cause of this is you have made a wiring error while changing the caps, especially if you are a beginner - we've all done it

You have probably disturbed something around the IF amp. If you can't see what's wrong check the voltages.

More detailed service info with circuit voltages can be ordered from Paul Stenning - click the link at the top right of the page.

Don't replace any valves as a matter of course. There's no point in replacing them unless they're faulty.

Good luck, Paul

Steve_P · 27th Aug 2007, 6:56 pm

First, this set has all the heaters in parallel across one winding of the Mains Transformer, apart from the Rectifier, which is on its own winding.

Like Paul says, it's not always possible to see a glow in Small Signal Valves. I assume you mean V3 by the way. Heater is between pins 1 and 8 - make sure the pins are making contact and if you have a meter, Switch to AC and you should get 6.3 Volts or so across these pins.

It's wise to replace certain caps as a matter of course. Some sets are not terribly well made out for repairs I know. A little patience please.... You must change C73 by the way.

Valves rarely fail, but I notice that someone has been running this on 220v. This means that yours may have been overrun, so keep it at the higher setting.

Check your wiring thouroughly and also check you havent nudged anything.

Cheers,

Steve P

Patrick Dixon · 27th Aug 2007, 7:13 pm

Thanks for the welcome and the quick responses.

The caps I've changed are C73, C78, C83 (the large wax ones) - all around the output stage. The Hunts ones all look OK visually. I have got 6.3VAC on V3 1 & 8 socket, and I don't think I've disturbed anything around that area of the circuit. V3 sits pretty snug in the socket and wiggling it makes no difference.

I should probably say that I'm an electronic design/development engineer, but I have virtually no experience of valves and vintage circuits, and I'm definitely not immune to making mistakes!

According to the sch, V3/5 (screen grid) should be 75-85V - but is actually ~200V. V3/6 (control grid) has a good-looking signal on it (at + 5-10V IIRC), and V3/2 (anode) has no signal but gives me clicks though the speaker when I probe it. The 120R cathode resistor measures fine. I'll keep looking ...

Steve_P · 27th Aug 2007, 7:24 pm

Give the pins of the Socket a very gentle push together. And clean the pins of the valve and the socket connections. Also re-solder the joint.

R14 might be worth a look.

Cheers,

Steve P

Station X · 27th Aug 2007, 7:33 pm

If V3 pin 5 is reading 200 volts, then either the valve isn't passing any screen current, or there's a problem with the screen dropper resistor R16. Measure the value of R16 with the set off. If it's more than 20% off value replace it. With the set on measure the voltage across R16. I'd expect to see at least 140 Volts. You could also check the valve's heater for continuity.

Also measure the value of the catode resistor R14. This should also have a voltage across it due to the combined anode and screen currents flowing through it.

paulsherwin · 27th Aug 2007, 8:04 pm

Quote:

Originally Posted by Patrick Dixon

According to the sch, V3/5 (screen grid) should be 75-85V - but is actually ~200V. V3/6 (control grid) has a good-looking signal on it (at + 5-10V IIRC), and V3/2 (anode) has no signal but gives me clicks though the speaker when I probe it. The 120R cathode resistor measures fine. I'll keep looking ...

You don't actually say so, but I suspect V3 anode has zero volts. If so trace the circuit back to the HT line. It could be lots of things - dry joint, open circuit R17, open circuit IF transformer etc. Also check C56 (0.01uF between one of the IFTs and chassis, probably a mouldseal) isn't short circuit or very leaky.

Paul

Steve_P · 27th Aug 2007, 8:31 pm

Maybe.

But if there's Volts on the Screen but not the Anode, wouldn't the screen and it's cable inside the valve be glowing a pretty colour?

Cheers,

Steve P

Station X · 27th Aug 2007, 8:38 pm

Quote:

Originally Posted by Steve_P

But if there's Volts on the Screen but not the Anode, wouldn't the screen and it's cable inside the valve be glowing a pretty colour?

Not if the heater is open circuit, or there's bad contact between the heater pins and the valveholder. It's also posible that there's bad contact between the screen grid pin on the valveholder and the screen grid pin on the valve. We'll have to wait until Patrick reports back to know the answer.

paulsherwin · 27th Aug 2007, 8:39 pm

Quote:

Originally Posted by Steve_P

But if there's Volts on the Screen but not the Anode, wouldn't the screen and it's cable inside the valve be glowing a pretty colour?

Maybe

But doesn't that only happen with high power valves? I wouldn't expect any obvious effects with an IF amp (other than it not working of course).

First thing I'd check anyway.

Paul

brunel · 27th Aug 2007, 8:53 pm

Try it with the mains tap set for 220V. If it bursts into life then one or all of the valves may be low emission.

A favourite trick of Bill the bodger is to drop the mains tap to a lower setting.

Brunel

Patrick Dixon · 28th Aug 2007, 3:32 pm

OK, to answer a few of the questions.

R14 measures 125R (120R)
R17 measures 6K (4K7)
R7 measures 45K5 (33K)
R16 measures 71K1 (68K)

V3 Screen Grid is 259V
V3 Anode is 237V
V3 Cathode is 0V
V3 Control Grid is ~0.5V + signal

HT is 264V which means that R16 is dropping a few volts only, and the screen grid isn't drawing much current.

I've now also replaced all the Hunts Wiremolds (including C56) bar the 2 around V2 (C33, C34) which are a pita to get to, but without any improvement to the non-radio aspect of the set.

So I still suspect V2 (although it does get warm - so the heater must be doing something), but why it should suddenly go like this I don't understand.

Station X · 28th Aug 2007, 4:36 pm

If V3 cathode measures zero volts with respect to earth then V3 is passing hardly any anode and screen current. Does the valve have heater continuity and does it get warm?

Patrick Dixon · 28th Aug 2007, 4:37 pm

Yes & Yes.

Steve_P · 28th Aug 2007, 4:37 pm

V3 - It's a little suspect to me too. Try (as a temporary measure!) doing what Brunel suggests above.

Either way, V3 is very suspect to me!!!

Cheers,

Steve P

Station X · 28th Aug 2007, 4:48 pm

Ok. I'm stumped. It's extremely rare for a valve to have zero emission.

A really stupid question I know, but it's all I can think of. Have you got the valves in the right sockets?

Edit. Sorry Steve P and anyone else following this thread. I managed to reply to Patrick's post before I approved it, so you won't have seen post 14.

Patrick Dixon · 28th Aug 2007, 4:58 pm

Quote:

Originally Posted by Station X

Ok. I'm stumped. It's extremely rare for a valve to have zero emission.

A really stupid question I know, but it's all I can think of. Have you got the valves in the right sockets?

That's definitely not a stupid question - but yes I have double checked! IIRC I took them out one at a time anyway.

I've ordered a new 6F15 from Valve & Tube supplies, and describing the symptoms to the gentleman on the other end of the phone, he said it sounded like the valve had lost it's vacuum. So I suppose it's possible that the act of unpluging the valve may have broken a seal somewhere? With those Mazda sockets there isn't much opportunity to do much more that pull the valve straight out by the glass envelope, so I guess it is just one of those things. I should have a new one on Friday, so in the meantime I'll have to figure out how to get to those last two Hunts Wiremolds.

Station X · 28th Aug 2007, 5:01 pm

I hope that works for you, but loss of vacuum generally results in the filament burning out when exposed to air. That would give no heater continuity and the valve wouldn't get hot.

Patrick Dixon · 28th Aug 2007, 5:04 pm

Quote:

Originally Posted by Steve_P

V3 - It's a little suspect to me too. Try (as a temporary measure!) doing what Brunel suggests above.

Either way, V3 is very suspect to me!!!

Cheers,

Steve P

Yes, I did try that too - no improvement.

Thanks for all the suggestions, I can see that fixing these things could easily get a bit addictive - especially if you get one with a nice case.

Although this isn't the prettiest of sets, it's not in bad nick, was born in the same town as me (Welwyn Garden City), and is only two or three years older!

Patrick Dixon · 28th Aug 2007, 6:35 pm

OK, I've pulled the valve again and the heater is now OC - so it looks like the additional period of testing has burnt it out.

27th Aug 2007, 6:27 pm	#1
Patrick Dixon Hexode Join Date: Aug 2007 Location: Bristol, UK. Posts: 480	Murphy A262 I have recently acquired a Murphy A262 which would otherwise have been heading to the tip as part of a house clearance. It actually worked when I (probably unwisely) tried it - which is the main reason why it didn't end up in the tip! Not knowing too much about these kind of things, I had a look round the web and decided that I needed to replace the wax capacitors and sort-out the dial lamps which didn't work. On removing the chassis from the case I discovered that the dial lamp bulbs were completely missing, so I ordered some new ones and some replacement caps and set about hoovering out the dust. As part of the hoovering process, I removed all the valves and wiped them over with kitchen towel. When the parts arrived, I fitted the new bulbs and replaced the 3 wax caps, and discovered the first problem. The dial lamp bulbs fouled the tuning pointers (which was presumably why they'd been discarded). Several hours later, I realised that the tuning mechanism had been incorrectly strung resulting in the pointers pivoting back onto the backplate/dial lamps, rather than forward to rest on the glass tuning plate. They would have been travelling in the wrong direction too ... I also moved the mains voltage tap from 220V to 250V. AFAIK, this set has been in the UK for the past 15-20 years, so I'm not quite sure why it was set thus. However, the second problem, was that the set no longer worked. I checked and re-checked my work, had a prod around with scope and meter, and it would seem that the IF valve is now dead. It's heater doesn't visibly glow, although it measures a sensible resistance and the voltage reaches the socket correctly. I can see and tune modulated stations (on all 3 wavebands) on it's input grid, but the grid fed from the HT (via a 68K resistor) doesn't appear to be drawing much current as it's sitting around HT too. The output stage appears to work as I can get noises though the speaker by probing it's input/the PU input. So my question is, is it likely that removing, wiping and replacing this valve has killed it, or am I barking up the wrong tree? I've checked the valve socket and the pins and they all look in reasonable conditions and capable of making contact. What else should I check to verify that this valve is/isn't faulty? If I get a replacement IF valve, which other valves should I also replace as a matter of course (I haven't got a valve tester). Assuming I can get it working again, the next plan is to replace all the Hunts decoupling caps. A couple of these look really tricky to get to - is it normal to have to disconnect several other things to uncover them, or are there easier ways? It doesn't look like this set was built with maintenance in mind! The valves in this set are ECC85, 6C9, 6F15, UU9, EABC80, 6P1 and the suspect one is the 6F15. For anyone interested, the A262 schematic is available FOC on the web. Thanks in advance.

27th Aug 2007, 6:43 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Re: Murphy A262 Welcome to the wonderful world of radio restoration It's very unlikely that you have damaged the valve unless you have been extremely unlucky. Small signal valves don't always have a clearly visible glowing heater, but if you can measure a reasonable resistance between the heater pins, the heater is OK. Check that you have fitted the valves in the correct holders and waggle them to make sure there isn't a bad connection. Try using some switch cleaner on the pins. You say you have replaced 3 wax caps. The Hunts Mouldseals are also wax caps and notoriously unreliable However, this is unlikely to be the immediate problem, unless one of the Mouldseals has broken in two while you were working on the set, which sometimes happens The most likely cause of this is you have made a wiring error while changing the caps, especially if you are a beginner - we've all done it You have probably disturbed something around the IF amp. If you can't see what's wrong check the voltages. More detailed service info with circuit voltages can be ordered from Paul Stenning - click the link at the top right of the page. Don't replace any valves as a matter of course. There's no point in replacing them unless they're faulty. Good luck, Paul

27th Aug 2007, 6:56 pm	#3
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Murphy A262 First, this set has all the heaters in parallel across one winding of the Mains Transformer, apart from the Rectifier, which is on its own winding. Like Paul says, it's not always possible to see a glow in Small Signal Valves. I assume you mean V3 by the way. Heater is between pins 1 and 8 - make sure the pins are making contact and if you have a meter, Switch to AC and you should get 6.3 Volts or so across these pins. It's wise to replace certain caps as a matter of course. Some sets are not terribly well made out for repairs I know. A little patience please.... You must change C73 by the way. Valves rarely fail, but I notice that someone has been running this on 220v. This means that yours may have been overrun, so keep it at the higher setting. Check your wiring thouroughly and also check you havent nudged anything. Cheers, Steve P __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

27th Aug 2007, 7:13 pm	#4
Patrick Dixon Hexode Join Date: Aug 2007 Location: Bristol, UK. Posts: 480	Re: Murphy A262 Thanks for the welcome and the quick responses. The caps I've changed are C73, C78, C83 (the large wax ones) - all around the output stage. The Hunts ones all look OK visually. I have got 6.3VAC on V3 1 & 8 socket, and I don't think I've disturbed anything around that area of the circuit. V3 sits pretty snug in the socket and wiggling it makes no difference. I should probably say that I'm an electronic design/development engineer, but I have virtually no experience of valves and vintage circuits, and I'm definitely not immune to making mistakes! According to the sch, V3/5 (screen grid) should be 75-85V - but is actually ~200V. V3/6 (control grid) has a good-looking signal on it (at + 5-10V IIRC), and V3/2 (anode) has no signal but gives me clicks though the speaker when I probe it. The 120R cathode resistor measures fine. I'll keep looking ...

27th Aug 2007, 7:24 pm	#5
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Murphy A262 Give the pins of the Socket a very gentle push together. And clean the pins of the valve and the socket connections. Also re-solder the joint. R14 might be worth a look. Cheers, Steve P __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

27th Aug 2007, 7:33 pm	#6
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: Murphy A262 If V3 pin 5 is reading 200 volts, then either the valve isn't passing any screen current, or there's a problem with the screen dropper resistor R16. Measure the value of R16 with the set off. If it's more than 20% off value replace it. With the set on measure the voltage across R16. I'd expect to see at least 140 Volts. You could also check the valve's heater for continuity. Also measure the value of the catode resistor R14. This should also have a voltage across it due to the combined anode and screen currents flowing through it. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

27th Aug 2007, 8:31 pm	#8
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Murphy A262 Maybe. But if there's Volts on the Screen but not the Anode, wouldn't the screen and it's cable inside the valve be glowing a pretty colour? Cheers, Steve P __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

27th Aug 2007, 8:53 pm	#11
brunel Heptode Join Date: Sep 2005 Location: LEEDS.......North of the River Aire. Posts: 872	Re: Murphy A262 Try it with the mains tap set for 220V. If it bursts into life then one or all of the valves may be low emission. A favourite trick of Bill the bodger is to drop the mains tap to a lower setting. Brunel

28th Aug 2007, 3:32 pm	#12
Patrick Dixon Hexode Join Date: Aug 2007 Location: Bristol, UK. Posts: 480	Re: Murphy A262 OK, to answer a few of the questions. R14 measures 125R (120R) R17 measures 6K (4K7) R7 measures 45K5 (33K) R16 measures 71K1 (68K) V3 Screen Grid is 259V V3 Anode is 237V V3 Cathode is 0V V3 Control Grid is ~0.5V + signal HT is 264V which means that R16 is dropping a few volts only, and the screen grid isn't drawing much current. I've now also replaced all the Hunts Wiremolds (including C56) bar the 2 around V2 (C33, C34) which are a pita to get to, but without any improvement to the non-radio aspect of the set. So I still suspect V2 (although it does get warm - so the heater must be doing something), but why it should suddenly go like this I don't understand.

28th Aug 2007, 4:36 pm	#13
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: Murphy A262 If V3 cathode measures zero volts with respect to earth then V3 is passing hardly any anode and screen current. Does the valve have heater continuity and does it get warm? __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

28th Aug 2007, 4:37 pm	#14
Patrick Dixon Hexode Join Date: Aug 2007 Location: Bristol, UK. Posts: 480	Re: Murphy A262 Yes & Yes.

28th Aug 2007, 4:37 pm	#15
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Murphy A262 V3 - It's a little suspect to me too. Try (as a temporary measure!) doing what Brunel suggests above. Either way, V3 is very suspect to me!!! Cheers, Steve P __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

28th Aug 2007, 4:48 pm	#16
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: Murphy A262 Ok. I'm stumped. It's extremely rare for a valve to have zero emission. A really stupid question I know, but it's all I can think of. Have you got the valves in the right sockets? Edit. Sorry Steve P and anyone else following this thread. I managed to reply to Patrick's post before I approved it, so you won't have seen post 14. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

28th Aug 2007, 5:01 pm	#18
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,289	Re: Murphy A262 I hope that works for you, but loss of vacuum generally results in the filament burning out when exposed to air. That would give no heater continuity and the valve wouldn't get hot. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

28th Aug 2007, 6:35 pm	#20
Patrick Dixon Hexode Join Date: Aug 2007 Location: Bristol, UK. Posts: 480	Re: Murphy A262 OK, I've pulled the valve again and the heater is now OC - so it looks like the additional period of testing has burnt it out.