Murphy A252 No VHF - UK Vintage Radio Repair and Restoration Discussion Forum

stevekendal · 27th Dec 2011, 1:18 am

Working fine on AM, but not so much as a rustle on FM. Still in the case at the moment and I tried replacing both the B7G valves in the tuner head. Maybe the switch section that shorts out that 470K in V2 anode circuit? Steve.

Mike Phelan · 27th Dec 2011, 10:08 am

How's the HT voltage Steve? FM front ends can pack up first with low HT.

I'd be inclined to check all the voltages in the tuner head and have a look at the switching as well.
Then it's going to be down to seeing how far the FM IF is getting.

stevekendal · 27th Dec 2011, 1:52 pm

Well there's loads of volts at both anodes in the head and the switch seems OK. When I pulled the oscillator valve, the replacement gave an encouraging "chirrup" as it warmed up, but still no signals.

Tim · 27th Dec 2011, 3:43 pm

Change all Hunts moldseal caps particularly the de-couplers round the VHF head , 6C9 and 6F18.Also the 5uf discriminator cap.

stevekendal · 27th Dec 2011, 5:23 pm

Quote:

Originally Posted by Tim

Change all Hunts moldseal caps particularly the de-couplers round the VHF head , 6C9 and 6F18.Also the 5uf discriminator cap.

Thanks Tim, but I'm trying to avoid the "scatter gun" technique and actually find the bad component(s) or fault. Steve.

Tim · 27th Dec 2011, 9:10 pm

Quote:

Thanks Tim, but I'm trying to avoid the "scatter gun" technique and actually find the bad component(s) or fault. Steve.

More fun I suppose! I like to keep my hand in too.

OK especially the de-coupler on the 6C9 screen grid.

I have done a few Murphys from this period, (262 & 272) and both showed gradual improvement with every cap changed. These often howl once the VHF head valves are replaced, due to the above mentioned de-coupler being o/c.

Steve_P · 27th Dec 2011, 9:24 pm

Have you got a signal generator? If you have, does 10.7 Mhz get out OK?

Cheers,

Steve P.

stevekendal · 28th Dec 2011, 2:26 am

No, I don't own a genny at present.

Jonmurphy · 11th Apr 2012, 11:25 am

Did you solve the problem in the end? I've got an a 252 with exactly the same issue. Got everything working bar FM. I also have been trying to avoid ripping out every cap. But so far haven't had so much as a crackle.

Nickthedentist · 11th Apr 2012, 8:14 pm

Whilst I admire peoples' ability to fault find logically, I have to say that Hunts Mouldseal capacitors are so notoriously unreliable that I would change them all anyway first.

If the fault persists, then it's time to engage one's brain. But there's a good chance that it will have vanished, and you can sit back enjoy a working radio that will probably continue to work long into the future, rather than one which may be running suboptimally and prone to failure.

Just my thoughts as an amateur dabbler though, of course.

ThePillenwerfer · 11th Apr 2012, 8:53 pm

I agree, Nick. I take the Bomber Harris approach and have yet to test a removed Hunt's or wax capacitor and it NOT be leaky.

- Joe

Station X · 11th Apr 2012, 10:19 pm

I agree with the sentiments expressed in the last two posts. but I suggest only changing one capacitor at a time and checking performance between each change. Then if you make a mistake in the wiring of a capacitor, you'll only have to check the last one replaced and not all of them.

Herald1360 · 11th Apr 2012, 11:33 pm

That sounds logical, but if the last one you changed didn't fix anything because it wasn't (one of) the fault(s), checking performance after changing it won't prove anything- it still won't work!

Quote:

Originally Posted by stevekendal

No, I don't own a genny at present.

Some sort of battery operated signal injector (maybe a multivibrator using fast devices or some fast logic gates) would help and is easy to knock up. If you change caps from output back towards input stages you can at least check signal still gets through after the change.

peter_sol · 12th Apr 2012, 12:41 pm

We have been here before
I very much doubt that there is "one fault"
If any of the caps are of the waxy type they will need changing.
The problem is that the gain at vhf and vhf I.F. frequencies is reduced if the decoupling caps are faulty including the cathode ones
That is why MW and LW may seem OK

Jonmurphy · 25th Jun 2012, 11:29 am

Got FM working no problem now. Little 500pF decoupling cap tucked away at the back of the middle section of the VHF unit. C8 (Murphy service guide)

Sounds great.

Nickthedentist · 25th Jun 2012, 11:48 am

So, was it a Hunts?

Herald1360 · 25th Jun 2012, 12:00 pm

Did Stevekendal pass the set on then?

Jonmurphy · 25th Jun 2012, 1:29 pm

Yes it was a Hunts!

SteveKendal may very well still have his. I'm afraid I just hijacked the thread.

Nickthedentist · 25th Jun 2012, 3:21 pm

Change all the other Hunts, Jon. Seriously, it will save you a lot of heartache in the long run!

Nick.

27th Dec 2011, 1:18 am	#1
stevekendal Heptode Join Date: Mar 2004 Location: Kendal, Cumbria, UK. Posts: 625	Murphy A252 No VHF Working fine on AM, but not so much as a rustle on FM. Still in the case at the moment and I tried replacing both the B7G valves in the tuner head. Maybe the switch section that shorts out that 470K in V2 anode circuit? Steve. __________________ Knobs and tubes rule ok

27th Dec 2011, 10:08 am	#2
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: Murphy A252 No VHF How's the HT voltage Steve? FM front ends can pack up first with low HT. I'd be inclined to check all the voltages in the tuner head and have a look at the switching as well. Then it's going to be down to seeing how far the FM IF is getting. __________________ Mike.

27th Dec 2011, 1:52 pm	#3
stevekendal Heptode Join Date: Mar 2004 Location: Kendal, Cumbria, UK. Posts: 625	Re: Murphy A252 No VHF Well there's loads of volts at both anodes in the head and the switch seems OK. When I pulled the oscillator valve, the replacement gave an encouraging "chirrup" as it warmed up, but still no signals. __________________ Knobs and tubes rule ok

27th Dec 2011, 3:43 pm	#4
Tim Dekatron Join Date: Jan 2003 Location: Bradford on Avon, Wiltshire, UK. Posts: 3,310	Re: Murphy A252 No VHF Change all Hunts moldseal caps particularly the de-couplers round the VHF head , 6C9 and 6F18.Also the 5uf discriminator cap. __________________ "Nothing is as dangerous as being too modern;one is apt to grow old fashioned quite suddenly."

27th Dec 2011, 9:24 pm	#7
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Murphy A252 No VHF Have you got a signal generator? If you have, does 10.7 Mhz get out OK? 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 Dec 2011, 2:26 am	#8
stevekendal Heptode Join Date: Mar 2004 Location: Kendal, Cumbria, UK. Posts: 625	Re: Murphy A252 No VHF No, I don't own a genny at present. __________________ Knobs and tubes rule ok

11th Apr 2012, 11:25 am	#9
Jonmurphy Retired Dormant Member Join Date: Apr 2012 Location: Leeds, West Yorkshire, UK. Posts: 33	Re: Murphy A252 No VHF Did you solve the problem in the end? I've got an a 252 with exactly the same issue. Got everything working bar FM. I also have been trying to avoid ripping out every cap. But so far haven't had so much as a crackle.

11th Apr 2012, 8:14 pm	#10
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,844	Re: Murphy A252 No VHF Whilst I admire peoples' ability to fault find logically, I have to say that Hunts Mouldseal capacitors are so notoriously unreliable that I would change them all anyway first. If the fault persists, then it's time to engage one's brain. But there's a good chance that it will have vanished, and you can sit back enjoy a working radio that will probably continue to work long into the future, rather than one which may be running suboptimally and prone to failure. Just my thoughts as an amateur dabbler though, of course.

11th Apr 2012, 8:53 pm	#11
ThePillenwerfer Octode Join Date: Mar 2011 Location: Sheffield, South Yorkshire, UK. Posts: 1,453	Re: Murphy A252 No VHF I agree, Nick. I take the Bomber Harris approach and have yet to test a removed Hunt's or wax capacitor and it NOT be leaky. - Joe __________________ "Experts are only good at one thing: explaining why something will not work!"

11th Apr 2012, 10:19 pm	#12
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Murphy A252 No VHF I agree with the sentiments expressed in the last two posts. but I suggest only changing one capacitor at a time and checking performance between each change. Then if you make a mistake in the wiring of a capacitor, you'll only have to check the last one replaced and not all of them. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

12th Apr 2012, 12:41 pm	#14
peter_sol Octode Join Date: Aug 2003 Location: Welwyn Garden City, Herts. UK. Posts: 1,906	Re: Murphy A252 No VHF We have been here before I very much doubt that there is "one fault" If any of the caps are of the waxy type they will need changing. The problem is that the gain at vhf and vhf I.F. frequencies is reduced if the decoupling caps are faulty including the cathode ones That is why MW and LW may seem OK

25th Jun 2012, 11:29 am	#15
Jonmurphy Retired Dormant Member Join Date: Apr 2012 Location: Leeds, West Yorkshire, UK. Posts: 33	Re: Murphy A252 No VHF Got FM working no problem now. Little 500pF decoupling cap tucked away at the back of the middle section of the VHF unit. C8 (Murphy service guide) Sounds great.

25th Jun 2012, 11:48 am	#16
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,844	Re: Murphy A252 No VHF So, was it a Hunts?

25th Jun 2012, 12:00 pm	#17
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Murphy A252 No VHF Did Stevekendal pass the set on then? __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

25th Jun 2012, 1:29 pm	#18
Jonmurphy Retired Dormant Member Join Date: Apr 2012 Location: Leeds, West Yorkshire, UK. Posts: 33	Re: Murphy A252 No VHF Yes it was a Hunts! SteveKendal may very well still have his. I'm afraid I just hijacked the thread.

25th Jun 2012, 3:21 pm	#19
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,844	Re: Murphy A252 No VHF Change all the other Hunts, Jon. Seriously, it will save you a lot of heartache in the long run! Nick.