Bush AC34, dropping HT voltage. - UK Vintage Radio Repair and Restoration Discussion Forum

don200002 · 10th Jul 2013, 12:36 pm

Hi All,

I had a Bush AC34 that in the "to do" pile and having changed all the usual caps it came to life reasonably well with good sensitivity. Unfortunately there is a lot of old wax under the mains TX but it does seem to be holding up.
The problem is the HT rail drops from 270vdc down to 240vdc after about 15 mins. After some more checks I found R17 grid feed resistor had gone high (62K) but then to my suprise I found the UL41 valve base has been changed and a UL84 fitted. This is an old repair and no other components have been changed. My question is could this be the cause of the set losing HT by drawing too much current.
I have also monitored the ac volts on the rectifier it drops from 265vac to 255vac pointing to the problem being excess current being drawn after the rectifier rather than a faulty TX.
Any ideas would be welcome.
Thanks,
Don.

TrevorG3VLF · 10th Jul 2013, 1:34 pm

Bush AC34 Trader 1147

The current taken by the output valve can be obtained by measuring the voltage across the output transformer and the voltage across the cathode resistor R21.

Most of the rest of the radio current passes through R24 so the current can be measured.

The only alternative current drain is C32, the reservoir capacitor. I would think this would become warm if there is significant leakage.

Measure the grid and cathode voltage of the output valve when turned on and then after the supply voltage has decreased. A gassy valve can have some grid current, giving a positive grid voltage.

don200002 · 10th Jul 2013, 4:08 pm

Hi Trevor,

Thanks for your suggestions, the smoothing reservoir cap is running quite cool but further measurements are showing some problems. Following initial warm up the voltages on the UL84 are, Grid 0.14v rising to5.6v and Cathode11.5v rising to 14.5v. This is over a period of about 15mins. It's looking like a gassy valve.

Thanks for your help,

Don.

TrevorG3VLF · 10th Jul 2013, 5:48 pm

Before you buy a new valve, check the grid resistor R17, 470k. There is no need to disconnect.
Check the coupling capacitor, C26, 10nF. If this is disconnected, there can be no leakage but there will be no sound of course.

Good luck,
Trevor

don200002 · 10th Jul 2013, 7:38 pm

Hi Trevor,

A bit of confusion here, I was working from the manufacturers sheet.
Yes I have allready changed R17 and R18 which was reading 62K and C26 was changed prior to switch on as a matter of course. Looks like it's going to cost me a UL84.

Thanks again,

Don.

PJL · 10th Jul 2013, 8:21 pm

If you have a gas cooker with electronic ignition, wrap a wire round all the pins except the grid and connect to the cooker frame, wrap a wire round the grid and connect to the gas ignition spark pin. Press gas ignition button and watch the sparks in the valve stopping when all sparks occur inside the anode cavity. This will clear any deposits on the glass and might revive your UL84. I have tried this on UL41's and PM24A's with a very high success rate. No guarantee of results or your safety...

don200002 · 11th Jul 2013, 4:30 pm

Problem solved,
I found a UL84 hiding in a STELLA in the "to do" pile. The HT is now 275v and the grid is stable at 0.014v. Unfortunately I don't have a gas cooker to try the suggestion made by PJL but years ago we used to rejuvenate TV tubes using some very simple equipment, a 60w bulb would run the heaters and a 15w pigmy bulb would show the emission. There were only a handful of components and the whole thing was mounted on a piece of wood. I wonder if you could rejuvenate valves in the same way.
Thanks for everyone's help,
Don.

10th Jul 2013, 12:36 pm	#1
don200002 Pentode Join Date: Sep 2011 Location: Bideford, Devon, UK. Posts: 196	Bush AC34, dropping HT voltage. Hi All, I had a Bush AC34 that in the "to do" pile and having changed all the usual caps it came to life reasonably well with good sensitivity. Unfortunately there is a lot of old wax under the mains TX but it does seem to be holding up. The problem is the HT rail drops from 270vdc down to 240vdc after about 15 mins. After some more checks I found R17 grid feed resistor had gone high (62K) but then to my suprise I found the UL41 valve base has been changed and a UL84 fitted. This is an old repair and no other components have been changed. My question is could this be the cause of the set losing HT by drawing too much current. I have also monitored the ac volts on the rectifier it drops from 265vac to 255vac pointing to the problem being excess current being drawn after the rectifier rather than a faulty TX. Any ideas would be welcome. Thanks, Don.

10th Jul 2013, 1:34 pm	#2
TrevorG3VLF Rest in Peace Join Date: Apr 2011 Location: Matlock, Derbyshire, UK. Posts: 1,378	Re: Bush AC34 dropping Ht voltage Bush AC34 Trader 1147 The current taken by the output valve can be obtained by measuring the voltage across the output transformer and the voltage across the cathode resistor R21. Most of the rest of the radio current passes through R24 so the current can be measured. The only alternative current drain is C32, the reservoir capacitor. I would think this would become warm if there is significant leakage. Measure the grid and cathode voltage of the output valve when turned on and then after the supply voltage has decreased. A gassy valve can have some grid current, giving a positive grid voltage.

10th Jul 2013, 4:08 pm	#3
don200002 Pentode Join Date: Sep 2011 Location: Bideford, Devon, UK. Posts: 196	Re: Bush AC34, dropping HtT voltage. Hi Trevor, Thanks for your suggestions, the smoothing reservoir cap is running quite cool but further measurements are showing some problems. Following initial warm up the voltages on the UL84 are, Grid 0.14v rising to5.6v and Cathode11.5v rising to 14.5v. This is over a period of about 15mins. It's looking like a gassy valve. Thanks for your help, Don.

10th Jul 2013, 5:48 pm	#4
TrevorG3VLF Rest in Peace Join Date: Apr 2011 Location: Matlock, Derbyshire, UK. Posts: 1,378	Re: Bush AC34, dropping HtT voltage. Before you buy a new valve, check the grid resistor R17, 470k. There is no need to disconnect. Check the coupling capacitor, C26, 10nF. If this is disconnected, there can be no leakage but there will be no sound of course. Good luck, Trevor

10th Jul 2013, 7:38 pm	#5
don200002 Pentode Join Date: Sep 2011 Location: Bideford, Devon, UK. Posts: 196	Re: Bush AC34, dropping HtT voltage. Hi Trevor, A bit of confusion here, I was working from the manufacturers sheet. Yes I have allready changed R17 and R18 which was reading 62K and C26 was changed prior to switch on as a matter of course. Looks like it's going to cost me a UL84. Thanks again, Don.

10th Jul 2013, 8:21 pm	#6
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Bush AC34, dropping HT voltage. If you have a gas cooker with electronic ignition, wrap a wire round all the pins except the grid and connect to the cooker frame, wrap a wire round the grid and connect to the gas ignition spark pin. Press gas ignition button and watch the sparks in the valve stopping when all sparks occur inside the anode cavity. This will clear any deposits on the glass and might revive your UL84. I have tried this on UL41's and PM24A's with a very high success rate. No guarantee of results or your safety...

11th Jul 2013, 4:30 pm	#7
don200002 Pentode Join Date: Sep 2011 Location: Bideford, Devon, UK. Posts: 196	Re: Bush AC34, dropping HT voltage. Problem solved, I found a UL84 hiding in a STELLA in the "to do" pile. The HT is now 275v and the grid is stable at 0.014v. Unfortunately I don't have a gas cooker to try the suggestion made by PJL but years ago we used to rejuvenate TV tubes using some very simple equipment, a 60w bulb would run the heaters and a 15w pigmy bulb would show the emission. There were only a handful of components and the whole thing was mounted on a piece of wood. I wonder if you could rejuvenate valves in the same way. Thanks for everyone's help, Don.