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Old 13th Jun 2017, 10:08 pm   #1
bobskie
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Default Bush AC 34 50hz Hum

Hi everyone I have a problem with my radio, it has been fully recapped except c7 + 8 and c18 and c19 on the trader sheet. These are the ones in the cuboid boxes on top of the chassis. I'm not sure if I need to replace these or not but I couldn't quite figure out how to wire them up, could these be causing the hum? Now I'm not certain if the hum is 50hz but by a comparison from YouTube it certainly seems like it. I have checked all my wiring but I think it's ok. I It is quite a loud buzz but is not the same as the motorboating issue I had before which I think was caused by a dirty potentiometer. It also happens with the volume turned right down but gets slightly louder as it's increased.. Im not sure what to check next so hopefully someone can help. Thanks!

Last edited by bobskie; 13th Jun 2017 at 10:36 pm. Reason: Added info
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Old 13th Jun 2017, 11:18 pm   #2
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Default Re: Bush AC 34 50hz Hum

The capacitors you are referring to do not need changing and are inside the IF transformer cans.

This sounds like a typical UL41 problem. Has C26 been replaced? Clip your meter to the output valve (V4) grid and chassis, switch on, and watch the voltage. Switch off if it rises above 2V.
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Old 14th Jun 2017, 11:33 am   #3
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Default Re: Bush AC 34 50hz Hum

The only caps which should be replaced in a general re-cap are the tubular wax paper types and potentially the electrolytics (although the ones in these sets are often ok). The flat mica types in the r.f. and i.f. circuits are generally reliable and if you replace them it will affect the alignment. If you have replaced any of these they set may need realigning in order to work properly as a radio. The caps you listed inside the cans would fall into this category.

The hum problem is likely to be a separate issue and may well be caused by the UL41. Alternatively it may be the smoothing electrolytic capacitors C32/33 if these have not been replaced, or it could be that the metal body of the volume control is not connected to chassis.

Last edited by Voxophone; 14th Jun 2017 at 11:42 am.
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Old 14th Jun 2017, 2:28 pm   #4
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Default Re: Bush AC 34 50hz Hum

Quote:
Originally Posted by PJL View Post
The capacitors you are referring to do not need changing and are inside the IF transformer cans.

This sounds like a typical UL41 problem. Has C26 been replaced? Clip your meter to the output valve (V4) grid and chassis, switch on, and watch the voltage. Switch off if it rises above 2V.
Ok I have a known good valve in another bush radio which I will try, hopefully I can avoid removing the chassis again. I'll just give that a go and see what happens first. Thanks for your help!
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Old 14th Jun 2017, 2:45 pm   #5
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Default Re: Bush AC 34 50hz Hum

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Originally Posted by Voxophone View Post
The only caps which should be replaced in a general re-cap are the tubular wax paper types and potentially the electrolytics (although the ones in these sets are often ok). The flat mica types in the r.f. and i.f. circuits are generally reliable and if you replace them it will affect the alignment. If you have replaced any of these they set may need realigning in order to work properly as a radio. The caps you listed inside the cans would fall into this category..
The hum problem is likely to be a separate issue and may well be caused by the UL41. Alternatively it may be the smoothing electrolytic capacitors C32/33 if these have not been replaced, or it could be that the metal body of the volume control is not connected to chassis.
I didn't replace any micas, I forgot to mention that! I wasn't sure if the ones in the boxes would be micas but I had a feeling they would be. I will try out another ul41 from another bush radio I have and see if that improves anything. That's interesting about the volume control grounding, why would that cause a hum? I'll check that after I have tried the other valve. Thanks for the help.
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Old 14th Jun 2017, 3:22 pm   #6
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Default Re: Bush AC 34 50hz Hum

As a general rule, only caps between 0.001uF and 0.1uF need changing. Any smaller than this are almost certainly mica or ceramic and don't fail. It's very unusual to need to change the caps in an IFT, though oddly enough the IFT caps used in American sets often fail.

Lots of different things can cause hum, which makes it a difficult fault to fix. UL41s are notorious for internal leaks which can cause the heater voltage to modulate the signal. A volume control fault can cause hum pickup in the associated wiring. Diagnosis isn't helped by the fact that the public were much more tolerant of background hum in the past, and many radio designs left the factory with some hum present, though the AC34 isn't a notorious hummer.
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Old 18th Jun 2017, 2:11 pm   #7
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Default Re: Bush AC 34 50hz Hum

Well I finally had time to swap the valves over and it works fine no buzz at all! I stole one from a small Pye set which needs a lot of work for just now and I'll keep my eyes open for another. Thanks for the help
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Old 18th Jun 2017, 2:37 pm   #8
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Default Re: Bush AC 34 50hz Hum

There are several threads discussing curing and working around hummy UL41s if you search the forum. It's also possible to replace a UL41 with a UL84 or UCL82 without too much butchery if you are finding them difficult to obtain at a reasonable price.
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Old 18th Jun 2017, 2:51 pm   #9
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Default Re: Bush AC 34 50hz Hum

That's how it would have been done back in the day. Mullard published a substitution guide, probably others did too.
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Old 18th Jun 2017, 4:33 pm   #10
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Default Re: Bush AC 34 50hz Hum

10P13 is also usable without changing the valve socket, fit the correct cathode resistor though.
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Old 18th Jun 2017, 5:34 pm   #11
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Default Re: Bush AC 34 50hz Hum

Mazda also publised a valve substitution guide in their data books, one of which, dating from 1970, I still have somewhere. Not sure if the UL41 was covered, though.
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Old 19th Jun 2017, 5:13 pm   #12
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Default Re: Bush AC 34 50hz Hum

I'll keep an eye out for some of these valves around antique fares and stuff. No hurry to find a new one for the Pye as I have a few others I need to restore first. Thanks for your help I'll make a list of those valves.
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