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Old 10th Feb 2007, 12:02 am   #1
MALC SCOTT
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Default Bush AC11; capacitor and UL41 problems.

I have restored this fine old radio. i.e replaced all the old wax caps, out of spec resistors, fitted new smoothing caps, replaced output and rectifier valve with new ones. It now plays, but not much volume for one of these. The HT volts across the smoothing block is 285v/129v. I do not have a circuit diagram, but i think that the 129v is a bit low. This voltage is across a large 15k resistor which reads OK. Any ideas what i may have overlooked. Cheers, Malc.
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Old 10th Feb 2007, 12:34 am   #2
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Default Re: Bush AC11

Hi Malc. The manufacturers cct diag. shows a 10K resistor dropping the main HT for the rest of the set, not 15K Is this original ??
This is fed among other things to the screen grid of the UL41 for which the measurement is given as 120 v so your figure of 129v is probably OK, given that the comparable anode volts is lower than your first figure, given at 260 volts.
(These are quoted against an AVO 7 on the 1kV range, so some adjustment is probably required if using a digi. ).

You really need to see if the problem is AF or RF based :
Do you get plenty of 'front end' sensitivity but low volume on all bands ? If so, given that you have changed plenty of parts, it could be that the I.F. is out of alignment or an IFT is faulty. Are all the voltages around the UF41 and the valve itself OK ? ( anode 84, screen 62 )
Just as a check, is there plenty of "buzz" if you touch the slider of the volume control when on maximum , despite the low volume of the stations ?

Interesting to see what this turns out to be. I've had a few quiet BUSH 'rogues' but usually of later vintage ( VHF62/VHFx4 variety )
ANDY
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Old 10th Feb 2007, 12:49 am   #3
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Default Re: Bush AC11

Hi Andy, many thanks for your reply. The front end is very active, plenty of stations. It looks like an output problem. The resistor across the smoothing block looks like a factory fitted item. I have the same model set i restored about 15 years ago which runs rings around this one for volume. Cheers, Malc.
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Old 10th Feb 2007, 10:45 am   #4
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Default Re: Bush AC11

Try disconnecting all the wires from pin 4 of the UL41 (with the set off of course) but keep the wires all connected together and insulated from anything else. I have one of these sets and after I did this, it belts out and lost all its distortion There are many threads on leaky UL41's
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Old 10th Feb 2007, 11:37 am   #5
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Default Re: Bush AC11

It would be worthwhile to buy a set of service info from Paul, and check all the circuit voltages. This will probably indicate where the problem lies. The first thing to check is the voltage across the UL41 cathode resistor - as others have hinted, the UL41 is the prime suspect.

Paul
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Old 10th Feb 2007, 1:52 pm   #6
MALC SCOTT
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Default Re: Bush AC11

Hi Paul, i will take your advice and obtain a circuit diagram. How do i go about buying a service sheet from Paul? Cheers, Malc.
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Old 10th Feb 2007, 2:14 pm   #7
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Default Re: Bush AC11

Quote:
Originally Posted by MALC SCOTT View Post
Hi Paul, i will take your advice and obtain a circuit diagram. How do i go about buying a service sheet from Paul? Cheers, Malc.
Click on 'Vintage Radio Service Data' at the top right of the page, click on the Service Data Index link, click on Bush, and scroll down to AC11. Paul has both the Trader sheet and the manufacturer's service data for this radio. I find the manufacturer's data more useful if you only want to buy one, but either will include a circuit diagram.

Paul
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Old 10th Feb 2007, 2:56 pm   #8
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Default Re: Bush AC11

Hi Paul, i obtained the data download from Paul. What a first class service i have been missing out on!! The culprit turned out to be the only wax cap i had not replaced, C38 0.01mfd across the primary of the sound output transformer. This fine old set is now working very well indead, plenty of output, loads of stations across all of the wavebands. Many thanks to Paul, Andy and PWH for their help. Cheers, Malc.
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Old 11th Feb 2007, 12:24 am   #9
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Default Re: Bush AC11

Hi Malc
I have found the 0.01uF capacitor across the primary of the output transformer to be unecessary. It is only a tone corrector, and these set's can be a bit bassy with it in situ. In this respect it wouldn't do any harm to change the IF filter cap(0.003, chassis/anode of the UBC41) to about 0.001uF(1000pF) either.
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Old 11th Feb 2007, 2:51 am   #10
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Default Re: Bush AC11

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Hi Malc
I have found the 0.01uF capacitor across the primary of the output transformer to be unecessary. It is only a tone corrector, and these set's can be a bit bassy with it in situ.
The 'tone correction capacitor' seems to have been an article of faith with radio designers in the 50s. Many radios will work perfectly well without it. People liked a really bassy treble free sound then.

It's often an expensive component to replace, because it needs a high working voltage. This is because the voltage across the output transformer can reach very high levels, expecially if the speaker is disconnected for some reason.

In fact, it's usually possible to replace the tone correction capacitor with a much lower voltage rating, expecially if an X2 capacitor is used. Modern film caps usually self heal after transient overvoltages, just losing a bit of capacitance. The old waxies used to fail short circuit, hence the 1kV rating.

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Old 11th Feb 2007, 12:19 pm   #11
MALC SCOTT
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Default Re: Bush AC11

Hi Tim, i have replaced the filter cap as you said. I have also fitted a brand new Mullard UL41 with the yellow print. What a difference to the output!! Cheers, Malc.
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