Heater to Cathode Short - UK Vintage Radio Repair and Restoration Discussion Forum

GW3OQK Andrew · 29th Nov 2016, 7:32 pm

I had a heater-cathode short in an N37, the output valve of an Atalanta. The cathode resistor overheated of course, as the heater is about 80v from ground. (Series heater chain.) I replaced the valve and resistor and all was well until next switch-on and the same thing happened.

The valveholder pin 7, screen, is intermittent, which previously interrupted the audio when waggled but I would not expect it to cause damage. Any theories out there?

I've sent for a "new" valve and B7G.
73, Andrew

PS, in my days as a Marconi R/O I never recall such a failure.

29th Nov 2016, 7:34 pm

Sheer bad luck I guess.

Keith · 29th Nov 2016, 11:09 pm

Is it possible that there's another break in the heater chain causing the Vhk max of 150V to be exceeded?

GW3OQK Andrew · 30th Nov 2016, 10:55 am

The heater chain and HT is 115v DC max.

The failed N37 replacement cost me £15. It's box was marked 13/6 so I suppose it is a reasonable rate of inflation. Hoping for good luck with the next one.
73, Andrew

SteveCG · 30th Nov 2016, 3:57 pm

Quite a few years ago I looked at an Atlanta which had demised a stick thermistor in the heater chain. I noticed that an attempt had been made to alter a B7G base.

Reading the issue raised by Andrew makes me I wonder if the bases were not up to the voltage strain ??

Craig Sawyers · 30th Nov 2016, 4:36 pm

Replace with a ceramic one?

AC/HL · 30th Nov 2016, 9:46 pm

https://www.vintage-radio.net/forum/...67&postcount=2

GW3OQK Andrew · 1st Dec 2016, 7:08 pm

Here's what I found fellers. First, I was wrong about the voltage. The DC from my supply would rise to 166 off load. I've fitted some 10K load resistors to keep it down off load.

I did resistance tests and found there was an open circuit in the heater chain wiring. I fitted a new piece of wire to by pass it. I could see by a previous solder blob joint someone had looked for this before.

Besides the N37 audio valve the next valve down in the chain, an EF91, had a heater to cathode short. Both replaced and the set is running again. The nature of the EF91 short would draw very high current through the N37 heater. I've never liked series heaters, or live-chassis sets.

All the best
Andrew

Keith · 1st Dec 2016, 7:21 pm

Looks like my guess was right, Andrew. Another disadvantage of series heater chains which, I must admit, hadn't occured to me before!

GW3OQK Andrew · 2nd Dec 2016, 10:00 am

Yes Keith, you were spot on.

Now for my next job. My Rees Mace 619 receiver is dead. Its broken dial cord looks a horrendous job to tackle.
73
Andrew

Keith · 2nd Dec 2016, 11:14 am

Well done - glad to know you sorted it.

Good luck with the 619 (CAT) receiver. I assume you've downloaded the "Preliminary Information" from the VMARS archive (no dial stringing info unfortunately). I was surprised how little I needed to do to get mine running ( http://www.tibblestone.com/oldradios...0and%20CAT.htm).

GW3OQK Andrew · 2nd Dec 2016, 5:27 pm

My CAT has been working for 8 years. I'm a mod king and modified it to allow RF gain to be available all the time, and to provide muting for CW break-in. I use CW 99.9% of time and after careful adjustment the crystal filter works well with single-signal effect. The whole front panel is going to have to come off to replace the cord, but meanwhile I have logging scale figures noted. Perhaps I'll never do it. Well done having the CAT companion transmitter. I have a mixture with the Murphy 618 tx pictured on my qrz page.

The Atalanta has shown another fault, the audio going silent then returning of its own accord. The filaments were all glowing steadily. It might be the dodgy output valve screen contact and I have a B7G base now so that's the next job.

Andrew

29th Nov 2016, 7:32 pm	#1
GW3OQK Andrew Pentode Join Date: Apr 2011 Location: Swansea, Wales, UK. Posts: 144	Heater to Cathode Short I had a heater-cathode short in an N37, the output valve of an Atalanta. The cathode resistor overheated of course, as the heater is about 80v from ground. (Series heater chain.) I replaced the valve and resistor and all was well until next switch-on and the same thing happened. The valveholder pin 7, screen, is intermittent, which previously interrupted the audio when waggled but I would not expect it to cause damage. Any theories out there? I've sent for a "new" valve and B7G. 73, Andrew PS, in my days as a Marconi R/O I never recall such a failure.

29th Nov 2016, 7:34 pm	#2
Guest Guest Posts: n/a	Re: Heater to Cathode Short Sheer bad luck I guess.

29th Nov 2016, 11:09 pm	#3
Keith Heptode Join Date: Jan 2003 Location: Tewkesbury, Gloucestershire, UK. Posts: 693	Re: Heater to Cathode Short Is it possible that there's another break in the heater chain causing the Vhk max of 150V to be exceeded? __________________ Keith Yates - G3XGW VMARS & BVWS member http://www.tibblestone.com/oldradios/Old_Radios.htm

30th Nov 2016, 10:55 am	#4
GW3OQK Andrew Pentode Join Date: Apr 2011 Location: Swansea, Wales, UK. Posts: 144	Re: Heater to Cathode Short The heater chain and HT is 115v DC max. The failed N37 replacement cost me £15. It's box was marked 13/6 so I suppose it is a reasonable rate of inflation. Hoping for good luck with the next one. 73, Andrew

30th Nov 2016, 3:57 pm	#5
SteveCG Nonode Join Date: Jun 2009 Location: Herefordshire, UK. Posts: 2,495	Re: Heater to Cathode Short Quite a few years ago I looked at an Atlanta which had demised a stick thermistor in the heater chain. I noticed that an attempt had been made to alter a B7G base. Reading the issue raised by Andrew makes me I wonder if the bases were not up to the voltage strain ??

30th Nov 2016, 4:36 pm	#6
Craig Sawyers Dekatron Join Date: Nov 2010 Location: Oxford, UK. Posts: 4,999	Re: Heater to Cathode Short Replace with a ceramic one?

30th Nov 2016, 9:46 pm	#7
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,643	Re: Heater to Cathode Short https://www.vintage-radio.net/forum/...67&postcount=2

1st Dec 2016, 7:08 pm	#8
GW3OQK Andrew Pentode Join Date: Apr 2011 Location: Swansea, Wales, UK. Posts: 144	Re: Heater to Cathode Short Here's what I found fellers. First, I was wrong about the voltage. The DC from my supply would rise to 166 off load. I've fitted some 10K load resistors to keep it down off load. I did resistance tests and found there was an open circuit in the heater chain wiring. I fitted a new piece of wire to by pass it. I could see by a previous solder blob joint someone had looked for this before. Besides the N37 audio valve the next valve down in the chain, an EF91, had a heater to cathode short. Both replaced and the set is running again. The nature of the EF91 short would draw very high current through the N37 heater. I've never liked series heaters, or live-chassis sets. All the best Andrew

1st Dec 2016, 7:21 pm	#9
Keith Heptode Join Date: Jan 2003 Location: Tewkesbury, Gloucestershire, UK. Posts: 693	Re: Heater to Cathode Short Looks like my guess was right, Andrew. Another disadvantage of series heater chains which, I must admit, hadn't occured to me before! __________________ Keith Yates - G3XGW VMARS & BVWS member http://www.tibblestone.com/oldradios/Old_Radios.htm

2nd Dec 2016, 10:00 am	#10
GW3OQK Andrew Pentode Join Date: Apr 2011 Location: Swansea, Wales, UK. Posts: 144	Re: Heater to Cathode Short Yes Keith, you were spot on. Now for my next job. My Rees Mace 619 receiver is dead. Its broken dial cord looks a horrendous job to tackle. 73 Andrew

2nd Dec 2016, 11:14 am	#11
Keith Heptode Join Date: Jan 2003 Location: Tewkesbury, Gloucestershire, UK. Posts: 693	Re: Heater to Cathode Short Well done - glad to know you sorted it. Good luck with the 619 (CAT) receiver. I assume you've downloaded the "Preliminary Information" from the VMARS archive (no dial stringing info unfortunately). I was surprised how little I needed to do to get mine running ( http://www.tibblestone.com/oldradios...0and%20CAT.htm). __________________ Keith Yates - G3XGW VMARS & BVWS member http://www.tibblestone.com/oldradios/Old_Radios.htm

2nd Dec 2016, 5:27 pm	#12
GW3OQK Andrew Pentode Join Date: Apr 2011 Location: Swansea, Wales, UK. Posts: 144	Re: Heater to Cathode Short My CAT has been working for 8 years. I'm a mod king and modified it to allow RF gain to be available all the time, and to provide muting for CW break-in. I use CW 99.9% of time and after careful adjustment the crystal filter works well with single-signal effect. The whole front panel is going to have to come off to replace the cord, but meanwhile I have logging scale figures noted. Perhaps I'll never do it. Well done having the CAT companion transmitter. I have a mixture with the Murphy 618 tx pictured on my qrz page. The Atalanta has shown another fault, the audio going silent then returning of its own accord. The filaments were all glowing steadily. It might be the dodgy output valve screen contact and I have a B7G base now so that's the next job. Andrew