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Old 4th Apr 2012, 8:59 pm   #1
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Unhappy Bush AC11 - help needed in tracing fault.

My Bush AC11 which has been happily in use every day since I finished restoring it last November, has suddenly stopped working. :sad:

Last night I came across a spare UCH42 valve in a drawer, and not knowing if it was any good, tried it in the Bush, and got no sound.

On removing it, the valve broke in the socket. After a thorough clean up of the broken glass, and plaster on my right thumb!, I replaced the original UCH42 back in the socket.

Unfortunately on switch on, all I now get is a low level hum from the speaker. The volume control has no effect, turning the wavechange knob has no effect either. However both dial lamps light normally.

Any suggestions on how to trace the fault (to a learning novice) would be much appreciated.

Ian
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Old 4th Apr 2012, 9:49 pm   #2
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Default Re: Bush AC11 - help needed in tracing fault.

The first thing to check is whether the valves light up. The heaters of this set are in series, so failure of one valve's heater will result in all the valves going out. If all the valves are not alight check the continuity of their heaters with a meter on ohms range.

Assuming all the valves are alight check for HT by connecting a meter, set to DC volts, across C35 (Maufacturer's sheet).
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Old 4th Apr 2012, 10:17 pm   #3
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Default Re: Bush AC11 - help needed in tracing fault.

Quote:
Originally Posted by Orthicon View Post
On removing it, the valve broke in the socket. After a thorough clean up of the broken glass, and plaster on my right thumb!, I replaced the original UCH42 back in the socket.
An unfortunate habit of B8A valves- especially if the holder is the type with a wire spring retainer which can break off the pip on all glass valves. (Some B8As had a metal skirt with the pip on that).
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Old 5th Apr 2012, 9:10 am   #4
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Default Re: Bush AC11 - help needed in tracing fault.

I suppose a check to see if you have HT would be a good start do you have a circuit?
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Old 5th Apr 2012, 1:55 pm   #5
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Default Re: Bush AC11 - help needed in tracing fault.

- Yes all the valves light up. I have tested all of the valve heaters on the multimeter set to ohms, and all check OK. The meter bleeped on the continuity setting on all, apart from the UL41 which had a resistance of 68 ohms.

I have tried Capacitor C35 (the Oscillator tuning air dielectric) but the voltage is very low.

Ian.
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Old 5th Apr 2012, 2:01 pm   #6
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Default Re: Bush AC11 - help needed in tracing fault.

Can you give us a figure for HT voltage please.

EDIT. As you were I see that your C35 is a different one to mine. I'll check the Trader sheet.
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Old 5th Apr 2012, 2:07 pm   #7
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Default Re: Bush AC11 - help needed in tracing fault.

OK. It's C33 on the Trader sheet.

If the HT is low suspect C33 and C27 of being short circuit, or R21 of being open circuit.
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Old 5th Apr 2012, 3:31 pm   #8
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Default Re: Bush AC11 - help needed in tracing fault.

C33 HT 32uf smoothing measures 320V.

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Old 5th Apr 2012, 5:20 pm   #9
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Default Re: Bush AC11 - help needed in tracing fault.

That's plenty. What do you get on C27? If nothing then suspect R21 (10K) which you can test with your meter.
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Old 5th Apr 2012, 5:41 pm   #10
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Default Re: Bush AC11 - help needed in tracing fault.

C27 HT smoothing measures 89V

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Old 6th Apr 2012, 12:10 am   #11
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Default Re: Bush AC11 - help needed in tracing fault.

That should be around 120V. Makes R21 rather suspect unless something in the early stages is taking too much current.
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Old 6th Apr 2012, 3:46 pm   #12
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Default Re: Bush AC11 - help needed in tracing fault.

I have measured R21 (10k Ohms) resistor and it measures 9.87k Ohms.

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Old 6th Apr 2012, 5:31 pm   #13
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Default Re: Bush AC11 - help needed in tracing fault.

Then either C27 is leaky, or something earlier in the set is taking too much- one or all of the screen decoupling capacitors, perhaps?

Try disconnecting C27, leaving the rest of the ins and out from it in place- the HT will be a bit ripply, but it won't harm anything. See what you get then. Check the screen voltages of the earlier stages too.
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Old 8th Apr 2012, 3:35 pm   #14
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Default Re: Bush AC11 - help needed in tracing fault.

Thanks.
I have tested C27 which is OK (All caps in the set are new - the original cap was replaced with a 32uf + 32uf and C27/33 actually measures 33uf+ 34uf.
I have measured the Screen grid voltages and they are;

V1 (UCH42) 45V DC
V2 (UF41) 57V DC
V4 (UL41) 97V DC

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Old 10th Apr 2012, 5:10 pm   #15
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Default Re: Bush AC11 - help needed in tracing fault.

Quote:
Originally Posted by Orthicon
C27 HT smoothing measures 89V
Quote:
Originally Posted by Orthicon
I have measured the Screen grid voltages and they are;

V1 (UCH42) 45V DC
V2 (UF41) 57V DC
V4 (UL41) 97V DC
There's someting wrong with your readings, because C27 and V4 screen grid are the same point, so should be at the same voltage.

It would help if you could give us voltage readings for all pins on all valves, excepting the heater pins.
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Old 10th Apr 2012, 8:52 pm   #16
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Default Re: Bush AC11 - help needed in tracing fault.

Solved at Last!
I changed all the valves and by process of elimination it turns out it was a duff UY41. I tried the suspect UY41 in another radio, which produced the same results,- no sound. The radio worked again when swapped back with the original valve. The radio is now working perfectly as it should.

Ian.
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Old 16th Apr 2012, 1:11 am   #17
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Default Re: Bush AC11 - help needed in tracing fault.

Quote:
An unfortunate habit of B8A valves- especially if the holder is the type with a wire spring retainer which can break off the pip on all glass valves. (Some B8As had a metal skirt with the pip on that).
I wasn't aware one had to remove the clip until a few years ago, and out of the hundreds of these type of valves I must have removed and replaced, I only ever broke one!
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