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Old 1st Aug 2020, 2:28 pm   #1
Ken Taylor
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Default Bush AC.34 repair

Hello all,

I have just collected a Bush AC.34 from a dog walking associate and he would like to have it going again, he says. It seems complete and I am told it works but make a horrible noise, so that's either an electronic fault or he is tuned to a pop music station (showing my age here of course). I have downloaded the service data but would appreciate any hints and tips on the peculiarities of this set before I start, although I shall obviously change THAT capacitor as a matter of course.

Thanks
Ken
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Old 1st Aug 2020, 3:37 pm   #2
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Default Re: Bush AC34 repair

Hi Ken

The horrible noise might not be due to a fault in the radio (although I am sure it will have a few faults).

Modern devices use SMPS and these can play havoc on AM radio's reception.

So once "done" it may still make a racket once returned to the owner.

Just worth bearing in mind

Other than that the reservoir and smoothing capacitor seem to reform well on most Bush sets of this age but the UL41 can be a bit notorious for hum when aged.

For what ever reason they seem to fare better (longer quieter life) in sets using a transformer for the heaters than the AC/DC sets.

You may need to change more than just the grid coupling capacitor, many change all the wax paper capacitors on sight and if its for someone else this may be the best approach.

Nice sets they sound pretty good.

Cheers

Mike T
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Old 1st Aug 2020, 9:40 pm   #3
Ken Taylor
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Default Re: Bush AC.34 repair

Thanks Mike. Yes, it could have been interference of course. I have found that the mains switch is jammed and a previous repairer has just shorted it out! OK on the waxy capacitors, I have some replacements left over from my DAC90a refurb earlier this year and I'll change them all eventually. I checked all the resistors in situ and none seem too far out of tolerance. The cabinet needs some attention but I'm not great at woodwork. It's intact but in need of some TLC.

I'll try it on the lamp limiter tomorrow.

All the best
Ken


I would like to replace the volume/off-on switch, any idea where I could get one? 2M ohm, Log with DPST switch and fairly long shaft.
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Old 1st Aug 2020, 9:51 pm   #4
David G4EBT
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Default Re: Bush AC.34 repair

Bowood Electronics have them:

https://www.bowood-electronics.co.uk...-log-switched/

Hope that helps.

Good luck with the restoration Ken.
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Old 2nd Aug 2020, 8:19 am   #5
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Default Re: Bush AC.34 repair

I recently restored a Bush AC34, itís a nice set to work on, the main problem I had was IF problem, this turned out to be a fractured wire on the capacitor inside the IF can, I was able to bridge the break and up came the gain, made a huge difference. Also the cabinet was in poor condition. I stripped the cabinet and applied Danish Oil and obtained a Bush Transfer from David G4EBT, the final result was very pleasing. Hope you enjoy your project.
John
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Old 2nd Aug 2020, 3:05 pm   #6
Ken Taylor
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Default Re: Bush AC.34 repair

David, thanks for the Bowood link - I have put together an order.

And thank you John for your input. My cabinet needs some work but I will talk to the owner first. I was a bit concerned as to where I could get a new transfer but you have answered that one too.

All the best
Ken
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Old 2nd Aug 2020, 4:27 pm   #7
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Default Re: Bush AC.34 repair

If you do decide to tackle the cabinet, donít over do it with the sandpaper, I was a bit too vigorous In one spot.
John
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Old 2nd Aug 2020, 9:39 pm   #8
Ken Taylor
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Default Re: Bush AC.34 repair

Well I have now put some power on it via the lamp limiter. It works although the bulbs don't light but it tunes and find stations. An there is hum. It does get a little louder with volume, but when a station is tuned in the audio is very distorted. I suspect this is the "horrible noise" the owner reported. Does this sound like the UL41? I'll swap the one out of the DAC90a to test but I have to pack everything away for a week now as we have visitors next weekend and the "workshop" has to be returned to a bedroom.
Thanks again for the advice.
Ken
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Old 2nd Aug 2020, 10:06 pm   #9
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Default Re: Bush AC.34 repair

Start with the previously mentioned coupling capacitor before swapping valves. The hum is likely to be the main smoother capacitor.
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Old 5th Aug 2020, 6:10 pm   #10
Ken Taylor
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Default Re: Bush AC.34 repair

Thanks for the advice. The pot and capacitors have arrived from Bowoods so I'll make a start on Monday.

Ken
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Old 11th Aug 2020, 9:10 pm   #11
Ken Taylor
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Default Re: Bush AC.34 repair

Well some progress. I have changed the volume control with the seized switch for the new much smaller one from Bowood, along with the capacitors attached to it. So tracing out the source of the distortion on tuned in stations with the scope I see that the 47k "grid stabiliser" R17 on V4 hops off to the tag board via pin 4 of the valve, which being a UL41 reminded me of all the advice given when working on the DAC90a that it isn't a good idea to do that. So I lifted the resistor off pin 4 and made a temporary connection in the air above it. Bingo! Distortion gone.

There is still a prevalent 50Hz hum however. Substituting new smoothing and reservoir capacitors eliminated them as the hum stayed. It seems that there is 100mV of ripple on the grid of V3, but unaffected by the volume control, and I have replaced the screened lead.

I haven't changed all the waxy capacitors yet or cleaned valve bases, any suggestions welcome.

Ken
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Old 11th Aug 2020, 9:30 pm   #12
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Default Re: Bush AC.34 repair

Quote:
Originally Posted by Freya View Post
Start with the previously mentioned coupling capacitor before swapping valves. The hum is likely to be the main smoother capacitor.
Or a combination of the audio coupling capacitor and UL41 leaking.
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Old 11th Aug 2020, 9:51 pm   #13
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Default Re: Bush AC.34 repair

Could still be a UL41 problem. Lifting the connection to pin 4 might help but not a guaranteed cure. Caused by leakage inside the valve.
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Old 12th Aug 2020, 11:19 am   #14
Ken Taylor
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Default Re: Bush AC.34 repair

Yes, I shall continue fault-finding and replacing caps.

In the meantime, does anyone know of a source of bulbs for the scale lamps? I found them on the Electrojumble web site, but it also says they are closed. The bulbs are 6.2v 0.3A with 15mm dia. glass.

Ken
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Old 12th Aug 2020, 2:14 pm   #15
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Default Re: Bush AC.34 repair

I buy my 6.5v 300mA mes fitting bulbs from e bay in packs of 5.
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Old 12th Aug 2020, 2:25 pm   #16
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Default Re: Bush AC.34 repair

Hi Ken, I get most of my components from Chricklewood Electronics, including the bulbs.
John
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Old 12th Aug 2020, 2:26 pm   #17
John10b
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Default Re: Bush AC.34 repair

Or even Cricklewood Electronics.
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Old 13th Aug 2020, 11:13 am   #18
Ken Taylor
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Default Re: Bush AC.34 repair

Thanks for the bulb suggestions, both have blown, which I found interesting as they are in series - must have been a surge.

Anyway, the hum was reduced slightly after changing C32 (0.02uF V3 anode IF bypass) but I concur that the UL41 is the major culprit, but it's not too bad. A few more waxy caps to go.

I noticed that signal strength comes up markedly when I clip a scope ground clip to the chassis. This would suggest a 3 core mains lead might be beneficial (Yes -it's an AC only model) but are there any draw backs to this?

Ken
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Old 9th Sep 2020, 4:54 pm   #19
Ken Taylor
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Default Re: Bush AC.34 repair

Epilogue.

After replacing all the waxy caps and cleaning up the valve pins and bases hum is reduced to acceptable level. Replaced mains lead with 3 core and 1A fuse in the plug. Found I had suitable bulbs anyway. The owner is very happy with it and says he will do the cabinet work himself. I told him not to leave his coffee cup on it again. The set seems quite sensitive even without and external antenna.

This thread can be closed now.

Ken
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