BUSH DAC10 odd(to me) fault! - UK Vintage Radio Repair and Restoration Discussion Forum

'LIVEWIRE?' · 29th Jun 2012, 11:44 am

Having temporarily replaced the valve rectifier(left in situ with it's anode disconnected to preserve heater chain continuity)with a 1N4007 in series with a 150 ohm resistor, I now find that the HT, which initially is over 300v, settles to ca. 220v after about 30 seconds, which is some 9% high, and the decoupled HT is around 85v, a tad low(The 10k 2watt resistor has been replaced).

The main problem, however, is that the voltage at the triode anode of the UCH41 is now only about 9.5v(the 15k decoupling resistor is O.K.), when it should be 45v or thereabouts. With the rectifier valve in circuit, this was abut 35v, despite the HT being low, at 190v.

Could this indicate that the UCH41 is faulty. The radio, after I had carried out extensive repairs, worked intermittently, then stopped oscillating altogether. I have checked for intermittent connections, cleaned valve pins, sockets, the bandswitches, etc., to no avail!

Peter.N. · 29th Jun 2012, 11:48 am

I would start by removing the UCH41 while the set is on and watching what happens to the anode voltage, if it rises substantially either the valve is faulty or the grid has positive voltage on it, if it doesn't rise the problem must be leakage elswhere.

Peter

Sideband · 29th Jun 2012, 12:45 pm

Quote:

Originally Posted by 'LIVEWIRE?'

stopped oscillating altogether.

I would have thought that this would cause the anode voltage to rise unless there is positive volts on the grid coming from somewhere. Good advice to pull the valve but this will interupt the heater chain so the HT will rise anyway (with the silicon rec fitted) as the other valves will also go out. However while the other heaters are dying away, if you see a sudden rise in the anode supply, it's a good indication that the triode section is drawing heavy current. A better way may be to put a meter in series with the anode connection and monitor the current with the valve in situ. Shouldn't see more than a couple of milliamps or so, maybe even less.

Rich.

PJL · 29th Jun 2012, 3:09 pm

The HT after the 10K should be about 200V. Do you mean 185V?

Leon Crampin · 29th Jun 2012, 3:19 pm

If you start pulling valves out of an AC/DC set when live, you run a real risk of breaking down the heater cathode insulation of the valves higher up in the chain.

Intermittent cessation of oscillator operation in an "as found" set is often dirt in the tuning capacitor.

Leon.

vinrads · 29th Jun 2012, 5:43 pm

Try it with the tuning capacitor fully open, in case it has been damaged ie vane bent.

Peter.N. · 29th Jun 2012, 6:19 pm

Quote:

Originally Posted by Leon Crampin

If you start pulling valves out of an AC/DC set when live, you run a real risk of breaking down the heater cathode insulation of the valves higher up in the chain.

Intermittent cessation of oscillator operation in an "as found" set is often dirt in the tuning capacitor.

Leon.

Standard quick servicing procedure from the '50s, don't recall ever having that happen.

Peter

'LIVEWIRE?' · 29th Jun 2012, 8:15 pm

I didn't pull out any valves whilst the radio was 'on', because I knew this would stop the heaters' supply, and, thus, the HT readings would be meaningless. What I did was disconnect first the supply to the Hexode Anode, which resulted in a voltage of 45v at the15k resistor. I then reconnected this wire, and removed that to the Triode anode. The voltage remained at 45v, as it did when both leads to the UCH41 were reconnected, so that fault cleared without my knowing the cause, and the radio was working again.
As to the HT after the 10k resistor, PJL, it was 85v, not 185. I have now wired in the 1N4007 permanently, but with a 220 ohm series resistor, and will check readings again.
Apolgies for not giving component circuit references in these posts, but the radio and service manual are both in my workshop, about 3/4 mile from where this computer is!

Top Cap · 29th Jun 2012, 8:46 pm

This may be a fruitless test but check the individual valve pin sockets on the valve base. I have found these broken in the past and give intermittent connection to the valve electrodes. If you pull the UCH41, use a small spike or thin tweezers and check that if you move one of the two socket prongs then you can see the other moving as well. If one moves but the other does not then the socket has split. You may even find that one prong is held fast but the other is loose. You can, with care, replace any broken ones with a replacement taken from a spare valve holder. I find that those from a B9A will also fit into the B8 socket fitted to the DAC10. As I wrote earlier, this may not be the problem with your DAC10 but I thought it was worth mentioning just in case. Les

'LIVEWIRE?' · 12th Jul 2012, 8:14 pm

I now have the DAC10 working very well, having cleaned/tightened all the valveholder pins(sockets)replaced more of the old wiring, in particular from the frame aerial to the coilpack(although ohmmeter checks didn't reveal any problem with this) The decoupled HT is still only 90v, and I can find no cause of it's being this low, so will put it down to the UL41, which is a used, tested example from Crowthorne Tubes. Tomorrow I'll put the radio on 'soak' test, then check the readings again. Provided nothing gets too warm(the new 10k 2W resistor is dissipating about 1.5W instead of maybe 0.5W) I'll consider the job done.

Leon Crampin · 12th Jul 2012, 10:46 pm

Most of the voltage readings for the Bush DAC90A/DAC10 are wrong.

I think this stems from the change Bush made to the surge limiter in the UY41 anode - from 100 Ohms to 250 Ohms. The latter value reduces the inrush current on a hot re-start and lengthens the life of the lamps. I'm pretty sure the voltage readings quoted in the service data were taken with the earlier value.

90V for the HT line is absolutely normal for these sets - one of the reasons why they last so well.

Leon.

29th Jun 2012, 11:44 am	#1
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	BUSH DAC10 odd(to me) fault! Having temporarily replaced the valve rectifier(left in situ with it's anode disconnected to preserve heater chain continuity)with a 1N4007 in series with a 150 ohm resistor, I now find that the HT, which initially is over 300v, settles to ca. 220v after about 30 seconds, which is some 9% high, and the decoupled HT is around 85v, a tad low(The 10k 2watt resistor has been replaced). The main problem, however, is that the voltage at the triode anode of the UCH41 is now only about 9.5v(the 15k decoupling resistor is O.K.), when it should be 45v or thereabouts. With the rectifier valve in circuit, this was abut 35v, despite the HT being low, at 190v. Could this indicate that the UCH41 is faulty. The radio, after I had carried out extensive repairs, worked intermittently, then stopped oscillating altogether. I have checked for intermittent connections, cleaned valve pins, sockets, the bandswitches, etc., to no avail!

29th Jun 2012, 11:48 am	#2
Peter.N. Dekatron Join Date: Jul 2008 Location: Charmouth, Dorset, UK. Posts: 3,601	Re: BUSH DAC10 odd(to me) fault! I would start by removing the UCH41 while the set is on and watching what happens to the anode voltage, if it rises substantially either the valve is faulty or the grid has positive voltage on it, if it doesn't rise the problem must be leakage elswhere. Peter

29th Jun 2012, 3:09 pm	#4
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: BUSH DAC10 odd(to me) fault! The HT after the 10K should be about 200V. Do you mean 185V?

29th Jun 2012, 3:19 pm	#5
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,874	Re: BUSH DAC10 odd(to me) fault! If you start pulling valves out of an AC/DC set when live, you run a real risk of breaking down the heater cathode insulation of the valves higher up in the chain. Intermittent cessation of oscillator operation in an "as found" set is often dirt in the tuning capacitor. Leon.

29th Jun 2012, 5:43 pm	#6
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,767	Re: BUSH DAC10 odd(to me) fault! Try it with the tuning capacitor fully open, in case it has been damaged ie vane bent.

29th Jun 2012, 8:15 pm	#8
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: BUSH DAC10 odd(to me) fault! I didn't pull out any valves whilst the radio was 'on', because I knew this would stop the heaters' supply, and, thus, the HT readings would be meaningless. What I did was disconnect first the supply to the Hexode Anode, which resulted in a voltage of 45v at the15k resistor. I then reconnected this wire, and removed that to the Triode anode. The voltage remained at 45v, as it did when both leads to the UCH41 were reconnected, so that fault cleared without my knowing the cause, and the radio was working again. As to the HT after the 10k resistor, PJL, it was 85v, not 185. I have now wired in the 1N4007 permanently, but with a 220 ohm series resistor, and will check readings again. Apolgies for not giving component circuit references in these posts, but the radio and service manual are both in my workshop, about 3/4 mile from where this computer is!

29th Jun 2012, 8:46 pm	#9
Top Cap Octode Join Date: Mar 2006 Location: Watford, Hertfordshire, UK. Posts: 1,270	Re: BUSH DAC10 odd(to me) fault! This may be a fruitless test but check the individual valve pin sockets on the valve base. I have found these broken in the past and give intermittent connection to the valve electrodes. If you pull the UCH41, use a small spike or thin tweezers and check that if you move one of the two socket prongs then you can see the other moving as well. If one moves but the other does not then the socket has split. You may even find that one prong is held fast but the other is loose. You can, with care, replace any broken ones with a replacement taken from a spare valve holder. I find that those from a B9A will also fit into the B8 socket fitted to the DAC10. As I wrote earlier, this may not be the problem with your DAC10 but I thought it was worth mentioning just in case. Les __________________ Whether the Top Cap is Grid or Anode - touching it will give you a buzz either way!

12th Jul 2012, 8:14 pm	#10
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: BUSH DAC10 odd(to me) fault! I now have the DAC10 working very well, having cleaned/tightened all the valveholder pins(sockets)replaced more of the old wiring, in particular from the frame aerial to the coilpack(although ohmmeter checks didn't reveal any problem with this) The decoupled HT is still only 90v, and I can find no cause of it's being this low, so will put it down to the UL41, which is a used, tested example from Crowthorne Tubes. Tomorrow I'll put the radio on 'soak' test, then check the readings again. Provided nothing gets too warm(the new 10k 2W resistor is dissipating about 1.5W instead of maybe 0.5W) I'll consider the job done.

12th Jul 2012, 10:46 pm	#11
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,874	Re: BUSH DAC10 odd(to me) fault! Most of the voltage readings for the Bush DAC90A/DAC10 are wrong. I think this stems from the change Bush made to the surge limiter in the UY41 anode - from 100 Ohms to 250 Ohms. The latter value reduces the inrush current on a hot re-start and lengthens the life of the lamps. I'm pretty sure the voltage readings quoted in the service data were taken with the earlier value. 90V for the HT line is absolutely normal for these sets - one of the reasons why they last so well. Leon.