Bush DAC34 - loud hum, nothing else - UK Vintage Radio Repair and Restoration Discussion Forum

richrussell · 15th Aug 2008, 2:34 pm

I've replaced every capacitor, including the electrolytics, and replaced the output transformer which had gone open circuit on the primary. Now I do at least get a sound out of the speaker - a very loud hum. Doesn't matter what position the waveband switch is (tried Gramophone to isolate the radio section), and the volume control has no effect on the hum either.

The HT at the UL41 output valve is only 165V - the service data suggests nearer 200V would be normal. I understand that the normal cause of output transformer primary burnout is the UL41 pulling too much current through due to 'that' capacitor passing DC. So would replacing the UL41 be an advisable thing to do, as it's probably damaged too?

Any suggestions as to where I should go from here?

I have a working Pye P75A now, after a simple wax capacitor elimination restore, and am itching to get the Bush working too as it's a much nicer looking radio.

Rich.

PJL · 15th Aug 2008, 2:44 pm

Switch It Off...

PJL · 15th Aug 2008, 2:56 pm

There are a number of ways you can reduce the risk of damaging a set when you switch on for the first time. 1) Use a lamp limiter or variac 2) Monitor the HT or output anode voltage and switch off immediately if it doesn't rise to near the trader sheet value.

Given the history, I would favour the UL41 needs replacing due to heater/cathode short or grid leak. The low HT means either the set is drawing too much current or the rectifier is tired.

richrussell · 15th Aug 2008, 6:37 pm

Don't worry - it was only on long enough for me to measure the voltage at the anodes of the output valve until it stabilised. I suspect, given the transformer's state when I got it that the UL41 is unwell.

I'll sort out a replacement - in the meantime I'll finish off building a lamp limiter.

Rich

Steve_P · 15th Aug 2008, 6:46 pm

This might sound a bit like going over old ground, but have you checked all your wiring. Also, the Smoothing Can (Like a metal thing on the chassis) - have you replaced this, and what with. Is it earthed OK.

That sounds to me like a duff smoother!

Also check R28 and R30. Then the rectifier valve. I know UL41's tend to hum. Whip V4 out and see what the anode voltage becomes.

If you don't fancy spending loads of money on a new valve, try putting a 10P14 in it's place.

Cheers,

Steve P.

richrussell · 15th Aug 2008, 7:29 pm

I replaced the 32uF/16uF smoothing can with a couple of electrolytics. It was humming before I replaced them, so as I had them to hand, I did them as well. The original can is still there, I just snipped the leads from it's terminals and soldered them to the two new electrolytics which I neatly mounted under the chassis.

I will double check the wiring that leads to/from the capacitors, as well as the other checks you mentioned. I haven't actually checked all of the resistors under the chassis are the values they should be (though I have checked all the ones around the power supply section).

I'll let you know how I get on!

Thanks

Rich.

richrussell · 18th Aug 2008, 7:04 am

I removed the UL41, bridged the heater pins and measured the voltage at the anode pin. Now it's 210V - which is in line with what the service data says. With the valve in place it's only 165V.

I've got a CV1977 on the way, which I understand is equivalent to a UL41. So we'll see what it does with that in place.

Both resistors Steve mentioned are within 10% of their stated values, so I'm happy to accept them.

Rich

richrussell · 19th Aug 2008, 6:01 pm

OK - UL41 replaced, and now the set works beautifully. Crystal clear reception of stations on MW with no hint of hum (in fact my Pye P75A hums much more).

However, the voltage at the anode of the output valve still concerns me. It now rises to 220V as it warms up, then drops to 160V and sits there after that. This seems low to me.

I will take the chassis out again and measure the resistances of all the remaining resistors, and check what the HT is doing on the other valves.

Top Cap · 19th Aug 2008, 8:13 pm

Assuming that the cathode voltages are ok, the drop in HT may be due to the rectifier being tired as suggested in an earlier reply. The rectifier may not be able to provide the full current required when the valves have warmed up and the HT voltage droops. It may have had a hard time after all with the faulty UL41.
Les

richrussell · 19th Aug 2008, 8:55 pm

Hmm, yes that's a good point. I might substitute a silicon diode (and resistor) to see what happens with the HT. I don't have a suitable rectifier valve to try.

I'll see what the measurements reveal.

Rich

15th Aug 2008, 2:34 pm	#1
richrussell Heptode Join Date: Jul 2008 Location: Selby, North Yorkshire, UK. Posts: 979	Bush DAC34 - loud hum, nothing else I've replaced every capacitor, including the electrolytics, and replaced the output transformer which had gone open circuit on the primary. Now I do at least get a sound out of the speaker - a very loud hum. Doesn't matter what position the waveband switch is (tried Gramophone to isolate the radio section), and the volume control has no effect on the hum either. The HT at the UL41 output valve is only 165V - the service data suggests nearer 200V would be normal. I understand that the normal cause of output transformer primary burnout is the UL41 pulling too much current through due to 'that' capacitor passing DC. So would replacing the UL41 be an advisable thing to do, as it's probably damaged too? Any suggestions as to where I should go from here? I have a working Pye P75A now, after a simple wax capacitor elimination restore, and am itching to get the Bush working too as it's a much nicer looking radio. Rich.

15th Aug 2008, 2:44 pm	#2
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Bush DAC34 - loud hum, nothing else Switch It Off...

15th Aug 2008, 2:56 pm	#3
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Bush DAC34 - loud hum, nothing else There are a number of ways you can reduce the risk of damaging a set when you switch on for the first time. 1) Use a lamp limiter or variac 2) Monitor the HT or output anode voltage and switch off immediately if it doesn't rise to near the trader sheet value. Given the history, I would favour the UL41 needs replacing due to heater/cathode short or grid leak. The low HT means either the set is drawing too much current or the rectifier is tired.

15th Aug 2008, 6:37 pm	#4
richrussell Heptode Join Date: Jul 2008 Location: Selby, North Yorkshire, UK. Posts: 979	Re: Bush DAC34 - loud hum, nothing else Don't worry - it was only on long enough for me to measure the voltage at the anodes of the output valve until it stabilised. I suspect, given the transformer's state when I got it that the UL41 is unwell. I'll sort out a replacement - in the meantime I'll finish off building a lamp limiter. Rich

15th Aug 2008, 6:46 pm	#5
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Bush DAC34 - loud hum, nothing else This might sound a bit like going over old ground, but have you checked all your wiring. Also, the Smoothing Can (Like a metal thing on the chassis) - have you replaced this, and what with. Is it earthed OK. That sounds to me like a duff smoother! Also check R28 and R30. Then the rectifier valve. I know UL41's tend to hum. Whip V4 out and see what the anode voltage becomes. If you don't fancy spending loads of money on a new valve, try putting a 10P14 in it's place. Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

15th Aug 2008, 7:29 pm	#6
richrussell Heptode Join Date: Jul 2008 Location: Selby, North Yorkshire, UK. Posts: 979	Re: Bush DAC34 - loud hum, nothing else I replaced the 32uF/16uF smoothing can with a couple of electrolytics. It was humming before I replaced them, so as I had them to hand, I did them as well. The original can is still there, I just snipped the leads from it's terminals and soldered them to the two new electrolytics which I neatly mounted under the chassis. I will double check the wiring that leads to/from the capacitors, as well as the other checks you mentioned. I haven't actually checked all of the resistors under the chassis are the values they should be (though I have checked all the ones around the power supply section). I'll let you know how I get on! Thanks Rich.

18th Aug 2008, 7:04 am	#7
richrussell Heptode Join Date: Jul 2008 Location: Selby, North Yorkshire, UK. Posts: 979	Re: Bush DAC34 - loud hum, nothing else I removed the UL41, bridged the heater pins and measured the voltage at the anode pin. Now it's 210V - which is in line with what the service data says. With the valve in place it's only 165V. I've got a CV1977 on the way, which I understand is equivalent to a UL41. So we'll see what it does with that in place. Both resistors Steve mentioned are within 10% of their stated values, so I'm happy to accept them. Rich

19th Aug 2008, 6:01 pm	#8
richrussell Heptode Join Date: Jul 2008 Location: Selby, North Yorkshire, UK. Posts: 979	Re: Bush DAC34 - loud hum, nothing else OK - UL41 replaced, and now the set works beautifully. Crystal clear reception of stations on MW with no hint of hum (in fact my Pye P75A hums much more). However, the voltage at the anode of the output valve still concerns me. It now rises to 220V as it warms up, then drops to 160V and sits there after that. This seems low to me. I will take the chassis out again and measure the resistances of all the remaining resistors, and check what the HT is doing on the other valves.

19th Aug 2008, 8:13 pm	#9
Top Cap Octode Join Date: Mar 2006 Location: Watford, Hertfordshire, UK. Posts: 1,270	Re: Bush DAC34 - loud hum, nothing else Assuming that the cathode voltages are ok, the drop in HT may be due to the rectifier being tired as suggested in an earlier reply. The rectifier may not be able to provide the full current required when the valves have warmed up and the HT voltage droops. It may have had a hard time after all with the faulty UL41. Les __________________ Whether the Top Cap is Grid or Anode - touching it will give you a buzz either way!

19th Aug 2008, 8:55 pm	#10
richrussell Heptode Join Date: Jul 2008 Location: Selby, North Yorkshire, UK. Posts: 979	Re: Bush DAC34 - loud hum, nothing else Hmm, yes that's a good point. I might substitute a silicon diode (and resistor) to see what happens with the HT. I don't have a suitable rectifier valve to try. I'll see what the measurements reveal. Rich