Murphy U604 falling HT - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

G6Tanuki · 14th Jan 2014, 5:25 pm

Sagging HT and increasing hum invariably make me think something's drawing excessive HT current so the rectifier's not able to 'keep up' with demand and the load's draining the reservoir capacitor more between cycles than the original design intended.

My instinct would be to break the circuit at the rectifier-valve's cathode and stick a milliameter in there to see what sort of HT current is actually being drawn.

ukcol · 14th Jan 2014, 6:19 pm

Quote:

Originally Posted by JoshWard

The ones fitted are 12V 2.2W.

Having the wrong lamps does indeed look like the main problem with the heater voltages. A quick google shows that there are some 19v 0.1 amp MES lamps available. It will be worth getting some to restore your voltage drop across the lamps to 38 volts instead of the current 12 volts total.

Even if this turns out to be nothing to do with your fading volume symptom at least you will have brighter lamps and a valve heater current nearer to the correct value.

JoshWard · 14th Jan 2014, 7:42 pm

I have just ordered the correct dial lamps, at the price they cost (with postage...) I have no choice but to get this set running right

Once they have arrived I will try the set out again and measure the HT current.

JoshWard · 18th Jan 2014, 6:15 pm

My lamps arrived today, the ones supplied were actually 18V 1.2W but my heater voltages are fine now

Powered the set up and it didn't work on MW, fiddling around with the wavechange switch got it going again! Think I may have to hardwire it to either MW or LW as everytime I switch it on now I have to adjust the contacts or bypass them!

HT current is 54mA at the cathode of the rectifier.

ukcol · 18th Jan 2014, 9:04 pm

I surprised the voltages are right now, 18 volts 1.2 watts equates to 0.15 amps.

I assume you still have your volume dropping problem?

JoshWard · 19th Jan 2014, 1:16 am

Hi Colin,
The voltages I am getting are as follows (sheet voltages are in brackets)
V4 Pin 5- 161.5v (164v)
V3 Pin 4- 125v (126v)
V1 Pin 4- 75v (76v)
V2 Pin 4- 56v (57v)
PL1- 36v (38v)
PL2- 18v (19v)

So whilst not perfect they are pretty close. I guess the company cannot supply the 19v ones anymore and supplied these instead (would have been nice to know what I was getting before spending a fortune on them).

As for the volume dropping problem- it sounds bizarre but I have no idea. It is now proving very difficult to get the set to work at all- let alone long enough to see if the volume drops. I can get it to work on Long Wave if I fiddle with the wavechange switch but getting it to work on Medium Wave is very difficult, sometimes even bypassing the wavechange switch doesn't work. If I do get it to work then if I switch it off and then on again it no longer works.....
When the set is 'not working' (it is all lit up, audio stages working- hum present, IF working as the wavechange switch crackles) all the DC voltages check out OK.
If I fit the dial lamps that were previously fitted (the 12v 2.2w ones) then it works as it did before!!

I haven't got a clue what is going on here! How on earth can changing the bulbs affect the operation of the wavechange switch? Or are the lower heater voltages now meaning the radio is working differently thus different faults are being revealed?

Either way I either need the set to work well with these new lamps or the 12v ones. So far all I can see is I have spent a load of money on some dial lamps that have somehow made the set even worse

ukcol · 19th Jan 2014, 10:58 am

Sorry Josh, I made a careless mistake when calculatind the lamps current rating. They are only 67mA types and will be over run and not last long.

You could get round it by connecting a single 1K wire-wound resistor in parallel with both lamps. I hope I've got the calculation right this time.

With regard to your original fault, have you tried new valves in the frequency changer and I.F. amplifier positions?

JoshWard · 19th Jan 2014, 11:04 pm

Hi Colin,
My stock of 1K resistors goes from 1 watt to 15 watts. Fitted the latter and it works well although takes up half the chassis

Powered up and it was working well on LW- with plenty of volume and no sign of it dropping. I then switched to MW and the sound dropped away as before.

Substituted V1 and V2 and all seems OK now! No sign of the volume dropping!

So what has happened here? Incorrect heater voltages damaging the original valves? Could it have caused the damage to the original output valve (which appeared to have a heater cathode short). Must check to see if that hum has gone...
It's strange because when I substituted V1 and V2 before we sorted the heater chain out it made no difference (I am using the same valves I substituted before as well).

Will put the original valves back in one at a time and see which was the cause. Hope it is the frequency changer because I have a spare

JoshWard · 20th Jan 2014, 12:49 am

Damn spoke too soon, it appears to be V2!

Hum is still there and is not affected by changing V1 or V2. Also tried another output valve but no difference.

ukcol · 20th Jan 2014, 12:52 am

Hi Josh

I am glad you have got it going OK. Not sure about exactly what was going on there except to say that over running heaters is bad news.

JoshWard · 20th Jan 2014, 1:01 am

Thanks for all your help Colin.

Will have to see how it goes. I don't like faults like this, after testing it so many times your ears can start playing tricks on you! It certainly achieves a decent volume now and seems to stay that way so that's good enough for me!

Wavechange switch is working fine again now as well.... A very strange radio!

Just need to find another valve now!

14th Jan 2014, 5:25 pm	#21
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: Murphy U604 falling HT Sagging HT and increasing hum invariably make me think something's drawing excessive HT current so the rectifier's not able to 'keep up' with demand and the load's draining the reservoir capacitor more between cycles than the original design intended. My instinct would be to break the circuit at the rectifier-valve's cathode and stick a milliameter in there to see what sort of HT current is actually being drawn.

14th Jan 2014, 7:42 pm	#23
JoshWard Octode Join Date: Dec 2005 Location: Near Stowmarket, Suffolk, UK. Posts: 1,962	Re: Murphy U604 falling HT I have just ordered the correct dial lamps, at the price they cost (with postage...) I have no choice but to get this set running right Once they have arrived I will try the set out again and measure the HT current.

18th Jan 2014, 6:15 pm	#24
JoshWard Octode Join Date: Dec 2005 Location: Near Stowmarket, Suffolk, UK. Posts: 1,962	Re: Murphy U604 falling HT My lamps arrived today, the ones supplied were actually 18V 1.2W but my heater voltages are fine now Powered the set up and it didn't work on MW, fiddling around with the wavechange switch got it going again! Think I may have to hardwire it to either MW or LW as everytime I switch it on now I have to adjust the contacts or bypass them! HT current is 54mA at the cathode of the rectifier.

18th Jan 2014, 9:04 pm	#25
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: Murphy U604 falling HT I surprised the voltages are right now, 18 volts 1.2 watts equates to 0.15 amps. I assume you still have your volume dropping problem?

19th Jan 2014, 1:16 am	#26
JoshWard Octode Join Date: Dec 2005 Location: Near Stowmarket, Suffolk, UK. Posts: 1,962	Re: Murphy U604 falling HT Hi Colin, The voltages I am getting are as follows (sheet voltages are in brackets) V4 Pin 5- 161.5v (164v) V3 Pin 4- 125v (126v) V1 Pin 4- 75v (76v) V2 Pin 4- 56v (57v) PL1- 36v (38v) PL2- 18v (19v) So whilst not perfect they are pretty close. I guess the company cannot supply the 19v ones anymore and supplied these instead (would have been nice to know what I was getting before spending a fortune on them). As for the volume dropping problem- it sounds bizarre but I have no idea. It is now proving very difficult to get the set to work at all- let alone long enough to see if the volume drops. I can get it to work on Long Wave if I fiddle with the wavechange switch but getting it to work on Medium Wave is very difficult, sometimes even bypassing the wavechange switch doesn't work. If I do get it to work then if I switch it off and then on again it no longer works..... When the set is 'not working' (it is all lit up, audio stages working- hum present, IF working as the wavechange switch crackles) all the DC voltages check out OK. If I fit the dial lamps that were previously fitted (the 12v 2.2w ones) then it works as it did before!! I haven't got a clue what is going on here! How on earth can changing the bulbs affect the operation of the wavechange switch? Or are the lower heater voltages now meaning the radio is working differently thus different faults are being revealed? Either way I either need the set to work well with these new lamps or the 12v ones. So far all I can see is I have spent a load of money on some dial lamps that have somehow made the set even worse

19th Jan 2014, 10:58 am	#27
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: Murphy U604 falling HT Sorry Josh, I made a careless mistake when calculatind the lamps current rating. They are only 67mA types and will be over run and not last long. You could get round it by connecting a single 1K wire-wound resistor in parallel with both lamps. I hope I've got the calculation right this time. With regard to your original fault, have you tried new valves in the frequency changer and I.F. amplifier positions? Attached Thumbnails

19th Jan 2014, 11:04 pm	#28
JoshWard Octode Join Date: Dec 2005 Location: Near Stowmarket, Suffolk, UK. Posts: 1,962	Re: Murphy U604 falling HT Hi Colin, My stock of 1K resistors goes from 1 watt to 15 watts. Fitted the latter and it works well although takes up half the chassis Powered up and it was working well on LW- with plenty of volume and no sign of it dropping. I then switched to MW and the sound dropped away as before. Substituted V1 and V2 and all seems OK now! No sign of the volume dropping! So what has happened here? Incorrect heater voltages damaging the original valves? Could it have caused the damage to the original output valve (which appeared to have a heater cathode short). Must check to see if that hum has gone... It's strange because when I substituted V1 and V2 before we sorted the heater chain out it made no difference (I am using the same valves I substituted before as well). Will put the original valves back in one at a time and see which was the cause. Hope it is the frequency changer because I have a spare

20th Jan 2014, 12:49 am	#29
JoshWard Octode Join Date: Dec 2005 Location: Near Stowmarket, Suffolk, UK. Posts: 1,962	Re: Murphy U604 falling HT Damn spoke too soon, it appears to be V2! Hum is still there and is not affected by changing V1 or V2. Also tried another output valve but no difference.

20th Jan 2014, 12:52 am	#30
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: Murphy U604 falling HT Hi Josh I am glad you have got it going OK. Not sure about exactly what was going on there except to say that over running heaters is bad news.

20th Jan 2014, 1:01 am	#31
JoshWard Octode Join Date: Dec 2005 Location: Near Stowmarket, Suffolk, UK. Posts: 1,962	Re: Murphy U604 falling HT Thanks for all your help Colin. Will have to see how it goes. I don't like faults like this, after testing it so many times your ears can start playing tricks on you! It certainly achieves a decent volume now and seems to stay that way so that's good enough for me! Wavechange switch is working fine again now as well.... A very strange radio! Just need to find another valve now!