DAC10 sound suddenly fades - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

westcliff · 12th Mar 2011, 2:01 am

PJL, C23 was replaced. I got 3.64V on the cathode and 0.009V on the grid of the output valve.

Gus.

frsimen · 12th Mar 2011, 9:21 am

Gus,

The cathode voltage on the UL41 is quite low (it should be 5v), suggesting that it is not drawing excessive current and isn't the source of your problem. You may have a low emission rectifier valve or the can capacitor C25/C26 is low in capacitance. One or both will need to be replaced to restore the HT to the correct level.

With increased HT, the cathode voltage on the UL41 will with any luck be nearer the published figures.

Paula

westcliff · 12th Mar 2011, 2:07 pm

Thank you Paula. I have plenty of rectifiers, so I can try another one later today. The cap can will have to wait until I order one.

Gus.

westcliff · 13th Mar 2011, 12:46 am

On the advice of a forum member who PM'd me, I measured the resistance of the volume pot, which is 500k Ohms in the manual. Mine measures 734k Ohms. Is this significant?

Gus.

frsimen · 13th Mar 2011, 11:21 am

Gus,

The volume control provides the load for the detector diode. If it were to go open circuit, then the detector wouldn't work properly, and the audio would certainly fade.

734k Ohms sounds rather higher than you would expect for a 500k Ohm control. The best check would be to try a substitute potentiometer. Anything of around that value would prove the point, but if you don't have one try connecting some resistors,to make a "fixed pot" (connect a 470k between the outer tag connected to the IF transformer and the centre tag and a 47k from the centre tag to the other outer one). The values aren't too critical.

Paula

westcliff · 17th Mar 2011, 10:38 am

Last night I found out what was wrong and fixed it. I had bought a new capacitor can to replace the replacement one currently in place. When the radio was out of its cabinet, I noticed the wire from the switching unit appeared to be badly soldered to the variable tuning capacitor. When I pulled it with my tweezers, it came off.

I soldered it back on, properly this time, and ran the radio for five hours with no unwanted loss of volume.

Still got a few things to do (replace the dial bulb assembly, switching unit and volume control), but I'm really happy to have rescued this nice old set and got it working. Thank you all for the information, advice and encouragement.

Next up - a Murphy A674.

12th Mar 2011, 2:01 am	#21
westcliff Hexode Join Date: Jan 2011 Location: Essex, UK Posts: 396	Re: DAC10 sound suddenly fades PJL, C23 was replaced. I got 3.64V on the cathode and 0.009V on the grid of the output valve. Gus.

12th Mar 2011, 9:21 am	#22
frsimen Heptode Join Date: Jan 2011 Location: Croydon, London, UK. Posts: 773	Re: DAC10 sound suddenly fades Gus, The cathode voltage on the UL41 is quite low (it should be 5v), suggesting that it is not drawing excessive current and isn't the source of your problem. You may have a low emission rectifier valve or the can capacitor C25/C26 is low in capacitance. One or both will need to be replaced to restore the HT to the correct level. With increased HT, the cathode voltage on the UL41 will with any luck be nearer the published figures. Paula

12th Mar 2011, 2:07 pm	#23
westcliff Hexode Join Date: Jan 2011 Location: Essex, UK Posts: 396	Re: DAC10 sound suddenly fades Thank you Paula. I have plenty of rectifiers, so I can try another one later today. The cap can will have to wait until I order one. Gus.

13th Mar 2011, 12:46 am	#24
westcliff Hexode Join Date: Jan 2011 Location: Essex, UK Posts: 396	Re: DAC10 sound suddenly fades On the advice of a forum member who PM'd me, I measured the resistance of the volume pot, which is 500k Ohms in the manual. Mine measures 734k Ohms. Is this significant? Gus.

13th Mar 2011, 11:21 am	#25
frsimen Heptode Join Date: Jan 2011 Location: Croydon, London, UK. Posts: 773	Re: DAC10 sound suddenly fades Gus, The volume control provides the load for the detector diode. If it were to go open circuit, then the detector wouldn't work properly, and the audio would certainly fade. 734k Ohms sounds rather higher than you would expect for a 500k Ohm control. The best check would be to try a substitute potentiometer. Anything of around that value would prove the point, but if you don't have one try connecting some resistors,to make a "fixed pot" (connect a 470k between the outer tag connected to the IF transformer and the centre tag and a 47k from the centre tag to the other outer one). The values aren't too critical. Paula

17th Mar 2011, 10:38 am	#26
westcliff Hexode Join Date: Jan 2011 Location: Essex, UK Posts: 396	Re: DAC10 sound suddenly fades Last night I found out what was wrong and fixed it. I had bought a new capacitor can to replace the replacement one currently in place. When the radio was out of its cabinet, I noticed the wire from the switching unit appeared to be badly soldered to the variable tuning capacitor. When I pulled it with my tweezers, it came off. I soldered it back on, properly this time, and ran the radio for five hours with no unwanted loss of volume. Still got a few things to do (replace the dial bulb assembly, switching unit and volume control), but I'm really happy to have rescued this nice old set and got it working. Thank you all for the information, advice and encouragement. Next up - a Murphy A674.