Philips 341A Hum Problem ( EBC41 ). - UK Vintage Radio Repair and Restoration Discussion Forum

Hermit6345 · 3rd May 2007, 8:58 pm

Hi All,

I was given a Philips 341A some years ago and decided recently to refurbish it. I am working from the Trader sheet 1070, which is for the similar 310A radio.

The radio was totally dead on MW and LW but lively, but hummy, on SW. Initially I changed the decoupling/coupling caps and the main reservoir and smoothing caps. A check with my trusty old TF144H showed the set to be way off alignment on MW and thus on LW as well. Spot on on short wave. A check of the IF and re-alignment of the RF circuits on MW and LW produced a very sensitive set that receives very well on the internal foil antenna.

However, the set still has a pronounced 50Hz hum which is there all the time, even when the set is switched to gram and the grid of V3 grounded. It uses a full wave rectifier so I think I can discount hum on the HT line. In any case the EZ40 has been checked and also substituted. No difference. The EL41 was checked on my CT160 and was found to be good. However it was exchanged with a brand new one and this made no difference. I then checked the AF triode, EBC41 and that showed good on the CT160. A check of every resistor in the fairly over the top negative feedback/tone control arrangement showed R16 and R18 to be a bit on the high side but all others well within spec. C26 had been changed earlier. The only way I can get rid of the hum is to put a 25uF cap from V3 (EBC41) cathode to deck. This clears the hum completely but the tone control doesn't work.

It must have worked properly at some time. I have no spare EBC41 so can't substitute. Can I believe my CT160 in respect of cathode/heater resistance? The valve tests good in every respect. I know the reputation that both EL41s and EBC41s have for internal shorts. My CT160 shows no such shorts in either valve.

By putting a cathode bypass cap in the circuit I guess I may be bypassing the hum to ground? I am tearing my hair out (what little I have left). I have either checked or replaced every component in the AF stages. The volume pot is a device with a fixed centre tap. I guess if that is U/S I could put a couple of resistors of the same value across the pot and feed the negative feedback into the junction of the two resistors?

Anyway, anybody got any ideas regarding the hum? I have left the 25uF cap in there pro tem. Ideally I would like it to work as it was intended.

Regards,

Ian.

paulsherwin · 3rd May 2007, 9:13 pm

I have one of these. It does hum faintly, despite new HT smoothing caps.

If the radio is humming loudly I'd suspect the EBC41, despite what the valve tester says.

The volume pot is a standard 3 contact device with an additional tap on the track for loudness compensation. The radio should still work with the tap disconnected, though the sound may be a bit bright. The tone control doesn't do a lot in these sets.

Paul

Hermit6345 · 15th May 2007, 2:34 pm

Hurray! I can report that a replacement EBC41 has cured the problem completely. I suppose this just points up the fact that tests on valve testers are not always conclusive. Thanks again for all the input. This thread might as well be closed.

Regards,

Ian.

3rd May 2007, 8:58 pm	#1
Hermit6345 Rest in Peace Join Date: Oct 2006 Location: North Wiltshire, UK. Posts: 638	Philips 341A Hum Problem ( EBC41 ). Hi All, I was given a Philips 341A some years ago and decided recently to refurbish it. I am working from the Trader sheet 1070, which is for the similar 310A radio. The radio was totally dead on MW and LW but lively, but hummy, on SW. Initially I changed the decoupling/coupling caps and the main reservoir and smoothing caps. A check with my trusty old TF144H showed the set to be way off alignment on MW and thus on LW as well. Spot on on short wave. A check of the IF and re-alignment of the RF circuits on MW and LW produced a very sensitive set that receives very well on the internal foil antenna. However, the set still has a pronounced 50Hz hum which is there all the time, even when the set is switched to gram and the grid of V3 grounded. It uses a full wave rectifier so I think I can discount hum on the HT line. In any case the EZ40 has been checked and also substituted. No difference. The EL41 was checked on my CT160 and was found to be good. However it was exchanged with a brand new one and this made no difference. I then checked the AF triode, EBC41 and that showed good on the CT160. A check of every resistor in the fairly over the top negative feedback/tone control arrangement showed R16 and R18 to be a bit on the high side but all others well within spec. C26 had been changed earlier. The only way I can get rid of the hum is to put a 25uF cap from V3 (EBC41) cathode to deck. This clears the hum completely but the tone control doesn't work. It must have worked properly at some time. I have no spare EBC41 so can't substitute. Can I believe my CT160 in respect of cathode/heater resistance? The valve tests good in every respect. I know the reputation that both EL41s and EBC41s have for internal shorts. My CT160 shows no such shorts in either valve. By putting a cathode bypass cap in the circuit I guess I may be bypassing the hum to ground? I am tearing my hair out (what little I have left). I have either checked or replaced every component in the AF stages. The volume pot is a device with a fixed centre tap. I guess if that is U/S I could put a couple of resistors of the same value across the pot and feed the negative feedback into the junction of the two resistors? Anyway, anybody got any ideas regarding the hum? I have left the 25uF cap in there pro tem. Ideally I would like it to work as it was intended. Regards, Ian.

3rd May 2007, 9:13 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,943	Re: Philips 341A Hum Problem I have one of these. It does hum faintly, despite new HT smoothing caps. If the radio is humming loudly I'd suspect the EBC41, despite what the valve tester says. The volume pot is a standard 3 contact device with an additional tap on the track for loudness compensation. The radio should still work with the tap disconnected, though the sound may be a bit bright. The tone control doesn't do a lot in these sets. Paul

15th May 2007, 2:34 pm	#3
Hermit6345 Rest in Peace Join Date: Oct 2006 Location: North Wiltshire, UK. Posts: 638	Re: Philips 341A Hum Problem Hurray! I can report that a replacement EBC41 has cured the problem completely. I suppose this just points up the fact that tests on valve testers are not always conclusive. Thanks again for all the input. This thread might as well be closed. Regards, Ian.