Murphy 310 - EHT fault? - UK Vintage Radio Repair and Restoration Discussion Forum

TNC · 12th Nov 2009, 6:32 pm

This is the second Murphy 310 restoration in a row - but they do give great results!
This fault occurs as the line drive begins to develop and the EHT starts to rise. The line-drive starts around 160v, the EHT rec. starts to conduct ...and the screen begins to light up. Nudge the HT up a little and an intermittent arcing (a rapid clicking sound) starts at the aquadag/chassis contact point (spring). It's quite a little firework display...it fizzes along the line of the contact spring.
Raise the HT a little higher and suddenly the EHT dies, with the rec. having a fairly blusih ring at the bottom of its envelope.
To get the HT back, power down and repeat the exercise.
I haven't started the re-cap yet - so could this be related to faulty cap(s)?...my limited experience says it's unlikely but I would be grateful for an expert opinion.
Many thanks,
Trevor.

Steve_P · 12th Nov 2009, 6:48 pm

Take the whole lot off, discharge the tube, clean it all and put it back. Make sure that the earths are OK. Then replace the caps, the EHT lead and cap. Then do a capacitor change.

Cheers,

Steve P.

dazzlevision · 12th Nov 2009, 6:51 pm

Hello Trevor,

Is the U26 EHT rectifier, or the CRT gassy?

Regards,

Dazzlevision

murphyv310 · 12th Nov 2009, 7:05 pm

Hi Trevor
As dazzlevision says is it the U26 or the tube?
Fit a new U26 first if it still glows purple take off the CRT anode cap and run it again, there is always the possibility theres a flashover inside the lopt, if there's no blue glow inside the U26 with no anode cap on the tube it does sound like the CRT being gassy but one other thing could be a heater cathode short in the CRT giving excessive beam current although this doesn't explain the sparking in the dag spring
Good luck
Trevor

Boom · 12th Nov 2009, 7:38 pm

What would happen if the U26 started passing ac due to the blue glow? Might that cause the sparking? Just a guess..

HamishBoxer · 12th Nov 2009, 8:17 pm

Is there a Visconol cap in the EHT lead? Other than that is EHT breakdown somewhere.

David

TNC · 12th Nov 2009, 8:49 pm

Thanks guys for all those helpful thoughts. I did fit a replacement U26 and the result is similar...next I will disconnect the EHT cap from the CRT and see what the U26 does.
Btw, after reading some of the 'other Trevor's' posts I have wondered about a H/C short in the tube...but I guess I could get around that with a separate heater feed to the CRT...but the comment about a gassy tube is a bit of a worry...there is not a great deal of visibility in the CRT neck to see anything amiss...I can see the heater glow and that looks normal. What sort of reaction might one expect to see in a gassy tube?
HB - there is no visconal cap on this set.
David - would that blue glow allow AC to pass?
I'll report back - thanks.

Trevor.

Boom · 12th Nov 2009, 9:06 pm

Quote:

Originally Posted by TNC

David - would that blue glow allow AC to pass?
I'll report back - thanks.

Trevor.

I dunno. I'm guessing Trevor and don't have any experience with that set but on the occasions that I've seen a blue glow in the rekky it hasn't been the tube so I'd try a few other things first. Take the cap off and see what if any spark you get to chassis. Ac will hiss and dc will crackle.

If the heaters are glowing normally I wouldn't have thought the tube was gassy.

murphyv310 · 12th Nov 2009, 9:17 pm

Hi
DC doesn't always crackle it can also hiss in damp surroundings its called corona discharge often called "spraying" by engineers.
As David says you should get a spark from the EHT cap to deck. A new U26 really discounts the line stage though, try tapping the CRT neck when the faults on to see if there are any flashes, If the CRT is duff these 17 inch 70 degree tubes are common.
Cheers
Trevor

Heatercathodeshort · 12th Nov 2009, 9:38 pm

Hello Trevor,
You may find the EHT rectifier heater winding is breaking down to the core as the voltage builds up. This will cause all the symptoms you describe. Worth a check. Regards, John.

murphyv310 · 12th Nov 2009, 10:32 pm

Quote:

Originally Posted by Heatercathodeshort

Hello Trevor,
You may find the EHT rectifier heater winding is breaking down to the core as the voltage builds up. This will cause all the symptoms you describe. Worth a check. Regards, John.

Indeed
One reason why I fit TV20 EHT sticks to Murphy's
Trevor

HamishBoxer · 13th Nov 2009, 11:12 am

U26 glowing blue if a good one is normally excess current not AC

David

Heatercathodeshort · 13th Nov 2009, 7:32 pm

Hello again Trevor,
I am a bit confused here. I presume you are still using a U26 eht rectifier. If you are it must be that heater winding breaking down. Have you tried a stick with just the + end going to the crt with the heater winding isolated? J.

TNC · 13th Nov 2009, 8:12 pm

Hi John,

Yes, I am still using the U26 - I have been busy today so no time to try disconnecting the EHT supply from the CRT...I will try and check it out later tonight and report back!
atb

Trevor
PS - I don't have a TV20 stick so will have to post a request soon!

TNC · 14th Nov 2009, 1:22 am

Well a little late night experimentation yielded results.

Disconnect the EHT cap from the CRT - no arcing/noise - usual line o/p frequency sound.

Remove heater feed to the CRT and substitute a bench p/s - link out heater leads with a 35ohm 5w, reconnect the EHT/CRT cap and switch on...result - good looking raster!

The question it leaves is - is it an EHT heater short...or is it a CRT heater/cathode short?...and does it matter? In other words for originality, it would be nice to keep the original EHT rectification which can be done by fitting a separate CRT heater tranny - alternatively, the CRT heater feed could be restored and a TV20 stick fitted to the lopty. I would welcome thoughts on that.
I guess there is another consideration...will an H/C crt short also affect picture performance - I didn't have time to feed a signal so that will have to wait until Sunday.

Anyway, thanks for the comments so far - most helpful.
Trevor.

Heatercathodeshort · 14th Nov 2009, 12:46 pm

This is a bit of an odd one Trevor. The Mazda CRM172 tube used in the Murphy V310 did not suffer from heater cathode shorts as did the earlier CRM121 series. It is very rare indeed. A H/C short will result in a flooded white screen with no picture and a very heavy load on the EHT system resulting in arcing around the dag spring. It was only designed to pass around 100 microamps and with uncontrollable peak white would probably exceed this greatly. The capacity to earth of the cathode connection must be very low indeed and the use of an ordinary 12.6v transformer or power supply will result in a very smeary picture due to loss of H.F. picture content. Special transformers were wound for this purpose with a separate secondary winding well spaced from the primary.
You will probably get a picture with the heater powered from your power supply but it might be a poor one. Regards, John.

murphyv310 · 14th Nov 2009, 7:03 pm

Hi
Yes I agree here with John the CRM172 never really suffered with heater cathode shorts but from what you say it does sound like thats the problem.
I have a V320A the 21inch fringe version it uses a CRM212 and it has a HK short, with mine I fitted a 12v transformer from a wall wart it give 13v which I am happy with but was rather smeary, fortunately the murphy's have cutouts for other valve bases so I fitted another valve base and fitted a 6/30L2 with both triodes strapped together and wired it as a cathode follower this cured the poor LF responce and restored the poor sync. Its easy to do Anodes to HT, I used the the HT point where the video amp takes it HT from, Grid to the connection where the crt cathode goes to the video amp circuit (its thro a choke) cathode thro a 12k 1 watt to chassis cathode of the crt to the cathodes of the new 6/30L2, wire both sections in parallel and fit the heater in the chain somewhere where there is about 60-70vac.
Cheers
Trevor
Mine had been OK for about a year or so although I would still like to get a better tube
I'll draw the cct and post it here if it helps

TNC · 15th Nov 2009, 4:28 pm

Thanks again for v. helpful comments.
Before making any mods. I plan to carry out one last test - to restore the original CRT heater feed and then route the EHT AC away from the U26 to a high-voltage diode and thence to the CRT. If that is OK then I would surmise that the short is somewhere in the EHT heater feed - if not then definitely it's an HK short in the CRT...and before I do that I will just check performance with a signal...and look for "smearing".
Am away for a few days from tomorrow so may not be able to update until next weekend!
Thanks all,
Trevor.

TNC · 15th Nov 2009, 6:24 pm

Last update for a few days...

Just applied a signal to the Murphy in its current "set-up" i.e. separate, isolated 12v DC feed to the CRT heater and regular EHT arrangement with a U26 rec. It gives an excellent response - strong contrast, no smearing and it's a good tube. (apart from pre "cap-change" cramping and cogging)

So I am guessing the problem must lie in the EHT heater circuit - some form of leakage there??

Trevor.

TNC · 2nd Dec 2009, 10:34 am

Forum member Norman Raeburn has been kind enough to let me have a couple of TV20 EHT sticks so I have been able to move this restoration on...
(thanks Norman!)

I therefore reverted the CRT heater supply back to the heater chain and fitted the TV20 EHT stick...result - success...no dag spring arcing!
Which, I think, goes to prove that there is some form of short/leakage between the EHT heater-coil/choke assembly and the LOPT case. Anyway I have now removed (cut) the EHT rec. heater supply lead from the LOPT.

By the way, I was pleased to find that the centre hole in the EHT rec. valve base required only minimal drilling to enlarge it sufficiently to allow the rec. stick to push through and be held tight - which also made the soldering connection at that end much easier to achieve - in all, a neat solution.

So it's back to the recapping - so far the time-bases have been done but the IF/RF remain. The CRT is excellent but I am getting cogging - or maybe it's better described as tearing - when the contrast/brighness is lowered. Most pronounced on Test Card but also occurs on programme. I will take a photo and post asap.

Thanks for help/input thus far!

Trevor.

12th Nov 2009, 6:32 pm	#1
TNC Hexode Join Date: Apr 2003 Location: Westcliff-on-Sea, Essex, UK. Posts: 386	Murphy 310 - EHT fault? This is the second Murphy 310 restoration in a row - but they do give great results! This fault occurs as the line drive begins to develop and the EHT starts to rise. The line-drive starts around 160v, the EHT rec. starts to conduct ...and the screen begins to light up. Nudge the HT up a little and an intermittent arcing (a rapid clicking sound) starts at the aquadag/chassis contact point (spring). It's quite a little firework display...it fizzes along the line of the contact spring. Raise the HT a little higher and suddenly the EHT dies, with the rec. having a fairly blusih ring at the bottom of its envelope. To get the HT back, power down and repeat the exercise. I haven't started the re-cap yet - so could this be related to faulty cap(s)?...my limited experience says it's unlikely but I would be grateful for an expert opinion. Many thanks, Trevor.

12th Nov 2009, 6:48 pm	#2
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Murphy 310 - EHT fault? Take the whole lot off, discharge the tube, clean it all and put it back. Make sure that the earths are OK. Then replace the caps, the EHT lead and cap. Then do a capacitor change. 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...?

12th Nov 2009, 6:51 pm	#3
dazzlevision Dekatron Join Date: Sep 2003 Location: Near Swindon, North Wiltshire, UK. Posts: 3,621	Re: Murphy 310 - EHT fault? Hello Trevor, Is the U26 EHT rectifier, or the CRT gassy? Regards, Dazzlevision

12th Nov 2009, 7:05 pm	#4
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,422	Re: Murphy 310 - EHT fault? Hi Trevor As dazzlevision says is it the U26 or the tube? Fit a new U26 first if it still glows purple take off the CRT anode cap and run it again, there is always the possibility theres a flashover inside the lopt, if there's no blue glow inside the U26 with no anode cap on the tube it does sound like the CRT being gassy but one other thing could be a heater cathode short in the CRT giving excessive beam current although this doesn't explain the sparking in the dag spring Good luck Trevor __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

12th Nov 2009, 7:38 pm	#5
Boom Retired Dormant Member Join Date: Jan 2007 Location: Westbury, Wiltshire, UK. Posts: 2,451	Re: Murphy 310 - EHT fault? What would happen if the U26 started passing ac due to the blue glow? Might that cause the sparking? Just a guess..

12th Nov 2009, 8:17 pm	#6
HamishBoxer Dekatron Join Date: Aug 2007 Location: W.Butterwick, near Doncaster UK. Posts: 8,935	Re: Murphy 310 - EHT fault? Is there a Visconol cap in the EHT lead? Other than that is EHT breakdown somewhere. David

12th Nov 2009, 8:49 pm	#7
TNC Hexode Join Date: Apr 2003 Location: Westcliff-on-Sea, Essex, UK. Posts: 386	Re: Murphy 310 - EHT fault? Thanks guys for all those helpful thoughts. I did fit a replacement U26 and the result is similar...next I will disconnect the EHT cap from the CRT and see what the U26 does. Btw, after reading some of the 'other Trevor's' posts I have wondered about a H/C short in the tube...but I guess I could get around that with a separate heater feed to the CRT...but the comment about a gassy tube is a bit of a worry...there is not a great deal of visibility in the CRT neck to see anything amiss...I can see the heater glow and that looks normal. What sort of reaction might one expect to see in a gassy tube? HB - there is no visconal cap on this set. David - would that blue glow allow AC to pass? I'll report back - thanks. Trevor.

12th Nov 2009, 9:17 pm	#9
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,422	Re: Murphy 310 - EHT fault? Hi DC doesn't always crackle it can also hiss in damp surroundings its called corona discharge often called "spraying" by engineers. As David says you should get a spark from the EHT cap to deck. A new U26 really discounts the line stage though, try tapping the CRT neck when the faults on to see if there are any flashes, If the CRT is duff these 17 inch 70 degree tubes are common. Cheers Trevor __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

12th Nov 2009, 9:38 pm	#10
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Murphy 310 - EHT fault? Hello Trevor, You may find the EHT rectifier heater winding is breaking down to the core as the voltage builds up. This will cause all the symptoms you describe. Worth a check. Regards, John.

13th Nov 2009, 11:12 am	#12
HamishBoxer Dekatron Join Date: Aug 2007 Location: W.Butterwick, near Doncaster UK. Posts: 8,935	Re: Murphy 310 - EHT fault? U26 glowing blue if a good one is normally excess current not AC David

13th Nov 2009, 7:32 pm	#13
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Murphy 310 - EHT fault? Hello again Trevor, I am a bit confused here. I presume you are still using a U26 eht rectifier. If you are it must be that heater winding breaking down. Have you tried a stick with just the + end going to the crt with the heater winding isolated? J.

13th Nov 2009, 8:12 pm	#14
TNC Hexode Join Date: Apr 2003 Location: Westcliff-on-Sea, Essex, UK. Posts: 386	Re: Murphy 310 - EHT fault? Hi John, Yes, I am still using the U26 - I have been busy today so no time to try disconnecting the EHT supply from the CRT...I will try and check it out later tonight and report back! atb Trevor PS - I don't have a TV20 stick so will have to post a request soon!

14th Nov 2009, 1:22 am	#15
TNC Hexode Join Date: Apr 2003 Location: Westcliff-on-Sea, Essex, UK. Posts: 386	Re: Murphy 310 - EHT fault? Well a little late night experimentation yielded results. Disconnect the EHT cap from the CRT - no arcing/noise - usual line o/p frequency sound. Remove heater feed to the CRT and substitute a bench p/s - link out heater leads with a 35ohm 5w, reconnect the EHT/CRT cap and switch on...result - good looking raster! The question it leaves is - is it an EHT heater short...or is it a CRT heater/cathode short?...and does it matter? In other words for originality, it would be nice to keep the original EHT rectification which can be done by fitting a separate CRT heater tranny - alternatively, the CRT heater feed could be restored and a TV20 stick fitted to the lopty. I would welcome thoughts on that. I guess there is another consideration...will an H/C crt short also affect picture performance - I didn't have time to feed a signal so that will have to wait until Sunday. Anyway, thanks for the comments so far - most helpful. Trevor.

14th Nov 2009, 12:46 pm	#16
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Murphy 310 - EHT fault? This is a bit of an odd one Trevor. The Mazda CRM172 tube used in the Murphy V310 did not suffer from heater cathode shorts as did the earlier CRM121 series. It is very rare indeed. A H/C short will result in a flooded white screen with no picture and a very heavy load on the EHT system resulting in arcing around the dag spring. It was only designed to pass around 100 microamps and with uncontrollable peak white would probably exceed this greatly. The capacity to earth of the cathode connection must be very low indeed and the use of an ordinary 12.6v transformer or power supply will result in a very smeary picture due to loss of H.F. picture content. Special transformers were wound for this purpose with a separate secondary winding well spaced from the primary. You will probably get a picture with the heater powered from your power supply but it might be a poor one. Regards, John.

14th Nov 2009, 7:03 pm	#17
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,422	Re: Murphy 310 - EHT fault? Hi Yes I agree here with John the CRM172 never really suffered with heater cathode shorts but from what you say it does sound like thats the problem. I have a V320A the 21inch fringe version it uses a CRM212 and it has a HK short, with mine I fitted a 12v transformer from a wall wart it give 13v which I am happy with but was rather smeary, fortunately the murphy's have cutouts for other valve bases so I fitted another valve base and fitted a 6/30L2 with both triodes strapped together and wired it as a cathode follower this cured the poor LF responce and restored the poor sync. Its easy to do Anodes to HT, I used the the HT point where the video amp takes it HT from, Grid to the connection where the crt cathode goes to the video amp circuit (its thro a choke) cathode thro a 12k 1 watt to chassis cathode of the crt to the cathodes of the new 6/30L2, wire both sections in parallel and fit the heater in the chain somewhere where there is about 60-70vac. Cheers Trevor Mine had been OK for about a year or so although I would still like to get a better tube I'll draw the cct and post it here if it helps __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

15th Nov 2009, 4:28 pm	#18
TNC Hexode Join Date: Apr 2003 Location: Westcliff-on-Sea, Essex, UK. Posts: 386	Re: Murphy 310 - EHT fault? Thanks again for v. helpful comments. Before making any mods. I plan to carry out one last test - to restore the original CRT heater feed and then route the EHT AC away from the U26 to a high-voltage diode and thence to the CRT. If that is OK then I would surmise that the short is somewhere in the EHT heater feed - if not then definitely it's an HK short in the CRT...and before I do that I will just check performance with a signal...and look for "smearing". Am away for a few days from tomorrow so may not be able to update until next weekend! Thanks all, Trevor.

15th Nov 2009, 6:24 pm	#19
TNC Hexode Join Date: Apr 2003 Location: Westcliff-on-Sea, Essex, UK. Posts: 386	Re: Murphy 310 - EHT fault? Last update for a few days... Just applied a signal to the Murphy in its current "set-up" i.e. separate, isolated 12v DC feed to the CRT heater and regular EHT arrangement with a U26 rec. It gives an excellent response - strong contrast, no smearing and it's a good tube. (apart from pre "cap-change" cramping and cogging) So I am guessing the problem must lie in the EHT heater circuit - some form of leakage there?? Trevor.

2nd Dec 2009, 10:34 am	#20
TNC Hexode Join Date: Apr 2003 Location: Westcliff-on-Sea, Essex, UK. Posts: 386	Re: Murphy 310 - EHT fault? Forum member Norman Raeburn has been kind enough to let me have a couple of TV20 EHT sticks so I have been able to move this restoration on... (thanks Norman!) I therefore reverted the CRT heater supply back to the heater chain and fitted the TV20 EHT stick...result - success...no dag spring arcing! Which, I think, goes to prove that there is some form of short/leakage between the EHT heater-coil/choke assembly and the LOPT case. Anyway I have now removed (cut) the EHT rec. heater supply lead from the LOPT. By the way, I was pleased to find that the centre hole in the EHT rec. valve base required only minimal drilling to enlarge it sufficiently to allow the rec. stick to push through and be held tight - which also made the soldering connection at that end much easier to achieve - in all, a neat solution. So it's back to the recapping - so far the time-bases have been done but the IF/RF remain. The CRT is excellent but I am getting cogging - or maybe it's better described as tearing - when the contrast/brighness is lowered. Most pronounced on Test Card but also occurs on programme. I will take a photo and post asap. Thanks for help/input thus far! Trevor.