EM34 in Heathkit C3–U goes out after bridge warm–up? - UK Vintage Radio Repair and Restoration Discussion Forum

Chris55000 · 7th Mar 2020, 12:21 am

Hi!

Just a quick one!

I have a Heathkit C3–U in reasonable cosmetic condition with the red/black scale markings, which I believe was the later style used in British Heathkit front–panels, and altho' it's EM34 balance indicator is visible at switch–on, it very soon fades out completely to become completely dark!

Was the EM34 known to have a failure–mode like this or am I looking at a straightforward case of h.t. rectifier/cap failure or overload of the mains–transformer? – which would, of course, be addressed & rectified (!) when the time comes to rebuild the bridge!

Chris Williams

joebog1 · 7th Mar 2020, 12:34 am

I would be looking at the power supply, especially any high value resistors that might also be heating up with the bridge.

Joe

Techman · 7th Mar 2020, 1:13 am

I don't think this is a known EM34 fault, so I would be looking elsewhere, such as voltages disappearing etc. Of course it's possible that the EM34 has developed a fault, but that would be quite rare - the normal fault with this 'eye' type is that they just lose brightness over a long time period of usage and carry on working, but with very dim light output that can only be seen in semi darkness. These 'eyes' are normally good in test equipment as they don't see the hours of continuous usage that they would get in a radio.

Chris55000 · 7th Mar 2020, 6:13 pm

Hi!

The target–anode voltage (pin 5) of the EM34 is given as "+150V" on my circuit diagram but I have to confess I've not measured the actual voltage in this unit yet – I plan to use two silicon diodes in series with a 1k 3W resistor in the original rectifier tube to replace the old selenium elements!

Chris Williams

7th Mar 2020, 12:21 am	#1
Chris55000 Nonode Join Date: Aug 2007 Location: Walsall Wood, Aldridge, Walsall, UK. Posts: 2,873	EM34 in Heathkit C3–U goes out after bridge warm–up? Hi! Just a quick one! I have a Heathkit C3–U in reasonable cosmetic condition with the red/black scale markings, which I believe was the later style used in British Heathkit front–panels, and altho' it's EM34 balance indicator is visible at switch–on, it very soon fades out completely to become completely dark! Was the EM34 known to have a failure–mode like this or am I looking at a straightforward case of h.t. rectifier/cap failure or overload of the mains–transformer? – which would, of course, be addressed & rectified (!) when the time comes to rebuild the bridge! Chris Williams __________________ It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed! Last edited by Chris55000; 7th Mar 2020 at 12:26 am.

7th Mar 2020, 12:34 am	#2
joebog1 Banned Join Date: Jun 2015 Location: Mareeba, North Queensland, Australia Posts: 2,704	Re: EM34 in Heathkit C3–U goes out after bridge warm–up? I would be looking at the power supply, especially any high value resistors that might also be heating up with the bridge. Joe

7th Mar 2020, 1:13 am	#3
Techman Dekatron Join Date: Nov 2009 Location: Lincolnshire, UK. Posts: 5,000	Re: EM34 in Heathkit C3–U goes out after bridge warm–up? I don't think this is a known EM34 fault, so I would be looking elsewhere, such as voltages disappearing etc. Of course it's possible that the EM34 has developed a fault, but that would be quite rare - the normal fault with this 'eye' type is that they just lose brightness over a long time period of usage and carry on working, but with very dim light output that can only be seen in semi darkness. These 'eyes' are normally good in test equipment as they don't see the hours of continuous usage that they would get in a radio.

7th Mar 2020, 6:13 pm	#4
Chris55000 Nonode Join Date: Aug 2007 Location: Walsall Wood, Aldridge, Walsall, UK. Posts: 2,873	Re: EM34 in Heathkit C3–U goes out after bridge warm–up? Hi! The target–anode voltage (pin 5) of the EM34 is given as "+150V" on my circuit diagram but I have to confess I've not measured the actual voltage in this unit yet – I plan to use two silicon diodes in series with a 1k 3W resistor in the original rectifier tube to replace the old selenium elements! Chris Williams __________________ It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!