Philips B3G75U. Room Temperature Sensitive Hum - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Wage Slave · 22nd Mar 2012, 12:34 pm

Indeed. UL84 it is.

Wage Slave · 23rd Mar 2012, 7:07 am

Can anything about the state of a valve be determined from its external appearance? I doubt it but .....

PJC58-Hythe · 23rd Mar 2012, 9:31 am

Have the main smoothing capacitors C1/C2 been replaced?

Wage Slave · 23rd Mar 2012, 10:23 am

Yes. The old one is just there for show.

Wage Slave · 12th May 2012, 1:38 pm

Well, an update. Thanks to someone kind on here I got a NOS UL84 delivered all the way to Japan for a very reasonable cost. That improved matters noticeably. The hum went and hasn't re-appeared but I run this radio quite a lot and after a while the too loud even on minimum volume fault came back. Not nearly as severe but definitely not right. So then I thought that perhaps there was a bad joint on the resistor to earth from the bottom of the volume pot. I remade the connections and that seemed to work for a week or so but then the problem re-appeared which also coincided with a colder snap. There is/was still something a bit temperature sensitive about this fault.

So then I thought why not change the resistor and the others in that part of the circuit as new ones only cost 20p each. I did that today and the problem has gone completely and I think the tone has improved noticeably.

Now, all these resistors are original carbon composition type and all tested fine on the multimeter. Is it possible that one or more might test OK on the meter but drift high under load a bit like capacitors fail under load? Is it also possible they become sensitive to temperature after many years in service?

Anyway, I'm a happy bunny. The set is really working very well - I'm listening to Question Time on it now and it sounds terrific. It seems to do an especially good job of folk and Enka music on the high quality AM broadcasts that NHK provides. They also do some proper Jazz which sounds terrific too.

Alistair D · 12th May 2012, 1:46 pm

Old carbon compsition can show the effects you describe. It sounds like you need to invest is some more diagnostic kit namely a tin of freezer spray and a hair dryer. These really do come into their own on intermittent faults

Al

Wage Slave · 12th May 2012, 1:54 pm

Yes. I did go over it with a hair dryer but it didn't really tell me anything. Perhaps the room temperature was too high. I am on the hunt for a can of freezer spray but it doesn't seem to be a common thing to use here. It would help if I could read the squiggle covered labels as well! Not being literate is a right pain and an insight sometimes.

22nd Mar 2012, 12:34 pm	#21
Wage Slave Hexode Join Date: Sep 2010 Location: Tsu, Japan. Posts: 452	Re: Philips B3G75U. Room Temperature Sensitive Hum Indeed. UL84 it is.

23rd Mar 2012, 7:07 am	#22
Wage Slave Hexode Join Date: Sep 2010 Location: Tsu, Japan. Posts: 452	Re: Philips B3G75U. Room Temperature Sensitive Hum Can anything about the state of a valve be determined from its external appearance? I doubt it but ..... Attached Thumbnails

23rd Mar 2012, 9:31 am	#23
PJC58-Hythe Heptode Join Date: Sep 2010 Location: Hythe, Hampshire, UK. Posts: 632	Re: Philips B3G75U. Room Temperature Sensitive Hum Have the main smoothing capacitors C1/C2 been replaced? __________________ Peter - M0HYT - BVWS Member - GQRP Club http://www.retroworkshop.co.uk

23rd Mar 2012, 10:23 am	#24
Wage Slave Hexode Join Date: Sep 2010 Location: Tsu, Japan. Posts: 452	Re: Philips B3G75U. Room Temperature Sensitive Hum Yes. The old one is just there for show.

12th May 2012, 1:38 pm	#25
Wage Slave Hexode Join Date: Sep 2010 Location: Tsu, Japan. Posts: 452	Re: Philips B3G75U. Room Temperature Sensitive Hum Well, an update. Thanks to someone kind on here I got a NOS UL84 delivered all the way to Japan for a very reasonable cost. That improved matters noticeably. The hum went and hasn't re-appeared but I run this radio quite a lot and after a while the too loud even on minimum volume fault came back. Not nearly as severe but definitely not right. So then I thought that perhaps there was a bad joint on the resistor to earth from the bottom of the volume pot. I remade the connections and that seemed to work for a week or so but then the problem re-appeared which also coincided with a colder snap. There is/was still something a bit temperature sensitive about this fault. So then I thought why not change the resistor and the others in that part of the circuit as new ones only cost 20p each. I did that today and the problem has gone completely and I think the tone has improved noticeably. Now, all these resistors are original carbon composition type and all tested fine on the multimeter. Is it possible that one or more might test OK on the meter but drift high under load a bit like capacitors fail under load? Is it also possible they become sensitive to temperature after many years in service? Anyway, I'm a happy bunny. The set is really working very well - I'm listening to Question Time on it now and it sounds terrific. It seems to do an especially good job of folk and Enka music on the high quality AM broadcasts that NHK provides. They also do some proper Jazz which sounds terrific too. Last edited by Wage Slave; 12th May 2012 at 1:50 pm.

12th May 2012, 1:46 pm	#26
Alistair D Nonode Join Date: Jan 2010 Location: Edinburgh, UK. Posts: 2,008	Re: Philips B3G75U. Room Temperature Sensitive Hum Old carbon compsition can show the effects you describe. It sounds like you need to invest is some more diagnostic kit namely a tin of freezer spray and a hair dryer. These really do come into their own on intermittent faults Al __________________ I won't tell you how I discovered that.

12th May 2012, 1:54 pm	#27
Wage Slave Hexode Join Date: Sep 2010 Location: Tsu, Japan. Posts: 452	Re: Philips B3G75U. Room Temperature Sensitive Hum Yes. I did go over it with a hair dryer but it didn't really tell me anything. Perhaps the room temperature was too high. I am on the hunt for a can of freezer spray but it doesn't seem to be a common thing to use here. It would help if I could read the squiggle covered labels as well! Not being literate is a right pain and an insight sometimes.