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Old 4th Jan 2017, 2:41 pm   #1
DonaldStott
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Default Bush A.C.91

Thanks to Santa and Past Times Radio I've just acquired a Bush A.C.91 - my next restoration project!

Bakelite cabinet in good condition - some minor scrapes that can easily be polished up.

Before I start I thought I'd post asking for any general advice and known issues/ challenges with these sets??

The pictures below do however show two elephants in the room:-

1. Although this is a Bush AC91 it has a DAC90A back cover - same period, but many of the holes are in the wrong place. I'll post in the Wanted section for an AC91 back cover.

2. At some point someone has replaced the original two pin socket with a modern IEC style connector! A bit worrying as there is also evidence of modern capacitors behind the IEC socket - recent replacements. The Service Manual notes "When servicing this receiver remember that one side of the electricity supply is connected directly to the chassis and may, under certain conditions, be "live." Do not connect any earthed equipment or a direct earth to the chassis without first isolating it by a fixed capacitor of approximately .005 MFD. Care should be taken to avoid handling the chassis"

Hopefully everything will be much clearer when I get the chassis out the cabinet?
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Old 4th Jan 2017, 2:51 pm   #2
ms660
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Default Re: Bush A.C.91

Yes, although fitted with a mains transformer it's of the auto transformer type and the chassis can become live if the mains live and neutral are not connected the correct way round, if those capacitors are across the mains then they should be X Class types, there should be no mains earth connection.

Lawrence.
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Old 4th Jan 2017, 3:13 pm   #3
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Default Re: Bush A.C.91

The EF39 seems to be starting to lose its metallic coating. That's the first thing to look at if there is any instability.
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Old 4th Jan 2017, 4:51 pm   #4
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Default Re: Bush A.C.91

I've never in person seen a Bush AC91 before with it's white grill, they must be very rare indeed. on the other hand I must have seen hundreds of Dac 90's and Dac 90A's over the years!
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Old 4th Jan 2017, 5:35 pm   #5
DonaldStott
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Default Re: Bush A.C.91

Thanks guys for your quick responses - as you will appreciate I'm still at the "What Have We Got" stage, so no rush!

Lawrence - one I get the chassis out the cabinet I'll be able to assess what work has been done behind that IEC connector! Thanks for the reminder about X Class capacitors.

Paul - what's the current thinking and best practice relating to the EF39 and redoing the metallic coating or using aluminium foil?

Clive - I'm pleased you like the set and agree that the white grill is a bit special!

The Service Manual lists mica, electrolytic and tubular capacitors - I'm assuming all the electrolytics and tubulars should be replaced??
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Old 4th Jan 2017, 5:52 pm   #6
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Default Re: Bush A.C.91

The EF39 may be ok, try it out.

Kitchen foil is a good short term fix, but cosmetically good repairs are difficult.
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Old 4th Jan 2017, 6:00 pm   #7
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Default Re: Bush A.C.91

Donald , i did one of these last year , same valve line up as the dac 90 but with shortwave and no internal aerial and as has been said an autotransformer instead of the back burning dropper. nice set ,her indoors even lets me keep it in the living room . regards, Tim
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Old 4th Jan 2017, 6:05 pm   #8
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Default Re: Bush A.C.91

Quote:
Originally Posted by MurphyNut View Post
I've never in person seen a Bush AC91 before with it's white grill, they must be very rare indeed. on the other hand I must have seen hundreds of Dac 90's and Dac 90A's over the years!
Yes the AC91 is much less common than the DAC90 and they do look quite striking with the Bakelite grill.
It was one of the first of the series I obtained.
I have at least one AC91 with a cloth speaker grill the same as a DAC90 I need to ask Mike Izycky if it was built that way or its a DAC90 case with an AC91 chassis.
If I can lay my hands on it (not sure where it is) I will post a picky.

Cheers

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Old 4th Jan 2017, 6:25 pm   #9
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Default Re: Bush A.C.91

Quote:
Originally Posted by Cobaltblue View Post
I have at least one AC91 with a cloth speaker grill the same as a DAC90 I need to ask Mike Izycky if it was built that way or its a DAC90 case with an AC91 chassis.
One of my AC91's has a cloth grill too. The pattern is slightly different to the DAC90 so I've always believed it to be an early version.

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Old 4th Jan 2017, 6:44 pm   #10
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Default Re: Bush A.C.91

Quote:
Originally Posted by tim.norris View Post
Donald , i did one of these last year , same valve line up as the dac 90 but with shortwave and no internal aerial and as has been said an autotransformer instead of the back burning dropper. nice set ,her indoors even lets me keep it in the living room . regards, Tim
Wow Tim, very impressive!

How did you get everything do shiny??

You may have set the bar too high for me but at least I know who to go to for advice on this set? Where should I start??
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Old 4th Jan 2017, 7:09 pm   #11
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Default Re: Bush A.C.91

Shame Bush persisted with the AC/DC arrangement for so long!
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Old 4th Jan 2017, 7:11 pm   #12
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Default Re: Bush A.C.91

"Where should I start??"

A good visual of the chassis, top and bottom to see if any components have been replaced with modern types, check the HT reservoir and filter capacitor can for any signs of bulging, check for any shorts across the HT rail and across the heater chain etc, if all's well then make sure the mains plug is fitted with a fuse of the correct rating (3 Amps max) Make sure that the mains neutral pin on the mains plug reads very close to 0 (zero) ohms to chassis with the on/off switch set to on.

Favourite original capacitors to go electrically leaky are the tubular types, the coupling capacitor to the control grid of the audio output valve must not be leaky at all as damage may result if it is, that particular capacitor is normally the first one to be replaced.

Lot's more I could say but it's all standard stuff and has been covered on this forum many many times.

Report what you find, plenty of folk's on here to advise/help etc.

Lawrence.
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Old 4th Jan 2017, 7:52 pm   #13
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Default Re: Bush A.C.91

I have a BA91 the Battery Version of the AC91, 3 waveband and no frame aerial and this has a cloth grill as well. In fact the Bush Service Sheets shows it with one.

Regards
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Old 4th Jan 2017, 8:38 pm   #14
DonaldStott
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Default Re: Bush A.C.91

Quote:
Originally Posted by ms660 View Post
"Where should I start??"

Lot's more I could say but it's all standard stuff and has been covered on this forum many many times.

Report what you find, plenty of folk's on here to advise/help etc.
Agreed Lawrence, this Forum is a fantastic resource and I have been doing my background research in relation to the Bush AC91.

I now have a fairly good handle on all the "standard stuff", where to find it and who to ask but was wondering if there is anything specific or special about the AC91??
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Old 4th Jan 2017, 9:24 pm   #15
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Default Re: Bush A.C.91

It's a pretty standard late 40s circuit, almost identical to the DAC90.
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Old 7th Jan 2017, 2:20 pm   #16
David G4EBT
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Default Re: Bush A.C.91

Quote:
Originally Posted by DonaldStott View Post
2. At some point someone has replaced the original two pin socket with a modern IEC style connector! A bit worrying as there is also evidence of modern capacitors behind the IEC socket - recent replacements. The Service Manual notes "When servicing this receiver remember that one side of the electricity supply is connected directly to the chassis and may, under certain conditions, be "live." Do not connect any earthed equipment or a direct earth to the chassis without first isolating it by a fixed capacitor of approximately .005 MFD. Care should be taken to avoid handling the chassis"
If the IEC socket has been correctly wired, you should be reassured not worried, as it ensures that the mains lead isn't reversible. By that, I mean the mains neutral will be connected to the chassis, (via the single pole mains switch on the volume control unless the V.C has been replaced by one with a double pole switch), and the line will be to the correct main input tapping on the auto transformer. The centre (earth pin) should not be connected to anything of course, it's only role - and an important one at that - being to ensure that a correctly wired IEC mains lead when plugged in will always have the correct polarity. (It wouldn't matter if an earthed IEC lead was plugged in as the IEC earth pin won't connected to anything inside the set, or at least, it shouldn't be).

That modification suggests to me that a previous owner of the set valued safety above originality - an admirable quality in my view. Even by the much lower safety standards of the 1940s/50s, the reversible mains plugs on Bush AC/DC (DAC90A etc), and sets such as the AC91 which uses live chassis techniques, was a significant safety shortcoming. But I guess back in that pre-H&S era, the attitude was 'if you don't know what you're doing, you shouldn't play around with electrical equipment or you might kill yourself'. (It was after all, an era when - as a schoolboy - myself and many others were making little TRF sets by the likes of FG Rayer in P.W. which used only a heater transformer and took the HT direct from the mains).

As to the capacitors, looking at the 10-page Bush Service Data, there's only one RF bypass capacitor across the mains input - 'C30' - 0.1uF, which must be an X Class. The other capacitor worth noting is that from chassis to 'True Earth' - .005uF, which should be a 'Y' Class. Also worth noting, C1 (.005uF) and C4 (800pF) are connected to the aerial input socket for selectivity/sensitivity, and as they find their way to chassis via the RF input circuitry, had your set not been modified to have a non-reversible mains input, arguably, those caps should be Y Class as the chassis could have been live.

Hope these observations helps a bit.

Good luck in your quest for an original back Donald.
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Old 7th Jan 2017, 3:34 pm   #17
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Default Re: Bush A.C.91

Today I managed to locate and loosen the grub screws holding the knobs in place and extracted the chassis from the case. Allows a more thorough inspection and I can see immediately the point that David G4EBT was making about the IEC connector - it does indeed ensure correct polarity and I can confirm that nothing is connected to the earth tag. Just seems a bit out of place in a vintage radio set but I will always place safety above originality.

A big thanks to David G4EBT for all his advice and guidance - very helpful.

Some more pictures below that show the capacitors that have already been replaced.
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Old 7th Jan 2017, 3:48 pm   #18
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Default Re: Bush A.C.91

Now that the chassis is out the case I can start to draw up a plan of things to check, items to replace and chemicals to order - although I do have a fair bit bit left over from my previous restoration.

Knowing that there are differing points of view about using the term "that capacitor" I think I have located this component in the circuit diagram as C26 - Coupling to V4 control grid.
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Old 7th Jan 2017, 3:57 pm   #19
ms660
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Default Re: Bush A.C.91

Yes, that's the one, hard to tell from the photo's whether it's been previously replaced or not, plenty of wax types in there.

EDIT: Might be worth checking the continuity of the transformer windings (mains and output) before doing a pile of work.

Lawrence.
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Old 7th Jan 2017, 5:06 pm   #20
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Default Re: Bush A.C.91

Thanks Lawrence - although not sure how I test the transformers with a £10 multi-meter??

Although some work has been done, none of the wax types appear to have been replaced. I'll post some better pictures later.
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