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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 11th Feb 2017, 10:56 pm   #1
vfotuner
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Default A pair of Bush AC91s

Hi All, I'm new to vintage radio repair but would line to get started on learning how. I got my hands on a pair of Bush AC91 radios and would like to get the cleaner one working properly. One radio works (sort of) with excessive AC hum. The other AC91 works as far as the heaters working, but cosmetically, the white bakelite speaker grille has a section broken and missing. The working AC91 looks great.

I checked C27/C29 and it has been leaking and bulging. Also, someone placed a secondary parallel wired 16uf 350v Capacitor across C27 doubling the value to 32UF. I propose to replace C26 with a 15UF 450v and C29 with a 33UF 450V. Ok with this?

I know one end of a soldering iron from the other, but I am new to anything over 12 volts.

Niall in Ireland.
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Old 12th Feb 2017, 4:34 pm   #2
mister valve
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Default Re: A pair of Bush AC91s

Firstly - please note that even though this set is for AC mains only, the chassis is NOT isolated from the mains. It uses an auto transformer with one end connected to the chassis, so ensure that the chassis is at Neutral potential before working on it whilst it is on.

It is good practice to change all the wax covered paper capacitors (condensers) as they will most probably be electrically leaky. The Mica, trimmers and air spaced ones are usually ok if they haven't been physically damaged. Also change the electrolytic caps, as they can deteriorate with age. Check resistor values and change any out of spec. All those capacitors should be replaced with 350VDC or higher rating (C21 the EBC33 valve cathode bypass capacitor, can be about 15VDC or above). C30 MUST be replaced with a CLASS X (or) CLASS Y and be a 0.1mfd @ 250VAC. This capacitor goes straight across the mains and therefore MUST be the type mentioned. Do this change regardless of the condition of the original capacitor for safety reasons.

Yes 15mfd can be used for C27, and 33mfd can be used for C29 instead of 16mfd & 32mfd respectively as in the circuit diagram. 450V rating should also be ok.

Good luck with the restoration.

John.
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Old 13th Feb 2017, 11:13 am   #3
Voxophone
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Default Re: A pair of Bush AC91s

Hi Niall,

It sounds like the main problem with your set is deterioration of the smoothing caps C27 and C29, leading to excessive hum. An additional capacitor added in parallel is a sure sign that someone has tried to fix the hum problem in the past. Replacing C27 and 29 as John has described should do the trick. It's best to get rid of the old ones rather than leaving them in parallel as they may leak and cause problems. The original smoothing cap on the AC91 can be hollowed out and the new capacitors installed inside if preferred.

Replacing most of the wax capacitors can probably wait until you have dealt with the hum problem. The exceptions are C30 (as John has stated) and C26, both of which should be replaced before powering the set again. C26 is the grid coupling cap for the output valve. If it leaks, it risks turning the output valve fully on and damaging the output transformer.

To replace the wax capacitors (other than C30), any modern film capacitors can be used as long as the voltage rating is sufficient (the original ratings will be printed on the capacitors). Use 630V types and you can't go wrong.

This link was helpful to me when starting out: http://www.justradios.com/captips.html

This one is also pretty good: https://antiqueradio.org/recap.htm

Good luck.

Liam

Last edited by Voxophone; 13th Feb 2017 at 11:22 am.
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Old 13th Feb 2017, 11:18 pm   #4
Mulcher
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Default Re: A pair of Bush AC91s

Hi Niall. Good luck with your project. I sent you a private message with some Dublin info that might be useful.
Martin
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Old 14th Feb 2017, 12:21 am   #5
Clydeuk
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Default Re: A pair of Bush AC91s

I would recommend after you replace C27 and C29, that you then tackle the rest one at a time, testing after each replacement, starting with the grid coupling capacitor. Doing it this way will help eliminate man made faults, which are the hardest to diagnose. If the set is worse or stops working after a replacement you will know it was the last thing you replaced.
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