7th Jan 2017, 6:41 pm | #21 |
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Re: Bush A.C.91
A £10.00 multi-meter is fine.
With the meter set on the Ohms range and the set disconnected from the mains, with one test lead to the chassis (doesn't meter which one) and the other at the 250V tapping on the transformer, it should measure 210 Ohms. You'll need to have the valves removed to carry out that test, otherwise the series valve heaters would be in parallel with the transformer winding across points B & C. You'll see what I mean from the attached sketch. If by any chance it reads open circuit, the individual resistances of each tap are on the circuit data so you'd need to check to see where the break is. Fingers crossed! As far as you can, on any equipment it's always good practice (and safer) to do as much testing and fault diagnosis as you can using an Ohm-meter with the set disconnected from the mains. You can check the continuity of valve heaters, coils & transformers, speech coils of speakers, switches, the value of resistors and so forth. Hence, while your doing these checks, it would be worthwhile checking the output transformer for continuity, the primary winding of which should read 500 Ohms, the secondary, 0.75 Ohms. Good luck with the restoration Donald. It's always an added challenge working on a set where someone has been there before you. Hope that helps a bit.
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David. BVWS Member. G-QRP Club member 1339. |
7th Jan 2017, 7:15 pm | #22 | |
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Re: Bush A.C.91
Quote:
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9th Jan 2017, 2:56 pm | #23 |
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Re: Bush A.C.91
Looking to delve deeper I noticed that on V1 (Mullard CCH 35) the top cap is loose and I'm wary of pulling too hard on the metal connector.
Any advice on how to safely remove the connector and what I need to do with the loose top cap?
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9th Jan 2017, 3:06 pm | #24 |
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Re: Bush A.C.91
WD40 applied to the join between cap and pin and allowed to soak should help to separate them. Just use it sparingly and make sure not to get it anywhere but the top cap. Then clean off afterwards.
Others have recommended epoxy to refix top caps; preferably cheap epoxy which will not set fully and allow some flex when the glass expands in the heat. I'm currently looking for some myself, but the 'cheap' stuff I bought to date seems to set too hard for me to risk it. You might already be aware, but I don't think superglue is recommended as I've heard this can crack the glass. I also have an AC91 with white grille. They're nice little sets. I haven't got round to restoring mine as the output transformer needs rewinding (open primary). Hopefully yours is ok! Liam Last edited by Voxophone; 9th Jan 2017 at 3:12 pm. |
9th Jan 2017, 3:32 pm | #25 | |
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Re: Bush A.C.91
Quote:
I'm assuming the Pin is underneath the Top Cap (in my picture) but I'm trying to separate the Connector from the Top Cap? Although the Top Cap is loose I think that it must still be electrically connected to the Pin??
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9th Jan 2017, 3:37 pm | #26 |
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Re: Bush A.C.91
If it remains stuck fast then a pointed tool inserted between the clip and valve cap should ease things up, make sure you hold the valves cap firmly if doing that, once the clip is removed it can be straitened up/re-tensioned if needs be.
Lawrence. |
9th Jan 2017, 3:38 pm | #27 |
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Re: Bush A.C.91
You should be able to hold the top cap with a pair of small fine nosed pliers while removing the clip. Once the clip is removed you can glue the top cap in place.
Superglue isn't usually recommended for valves because differential rates of heat expansion can cause cracking, but in this case the valve runs quite cool so it will probably be OK. If you use epoxy, apply a very small quantity with a cocktail stick. You don't need to glue the top cap in place if you are careful whenever you remove the clip - it's a matter of personal preference. |
9th Jan 2017, 4:11 pm | #28 | |
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Re: Bush A.C.91
Quote:
I don't know the proper name for the removable part with the rubber coated wire that clips over the top cap (the 'connector' in your picture). The last two posts called it a clip. Someone can confirm I'm sure. |
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9th Jan 2017, 4:18 pm | #29 |
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Re: Bush A.C.91
You've got two choices - leave well alone and hope that you don't disturb the loose top cap risking breaking the wire, or carefully removing the cap and re-fixing it securely.
If you do wish to remove and re-fix the top cap, you need to be exceedingly careful in removing the clip from the cap or the fragile wire which emerges from the top of the valve will break. Once you've removed the clip, it's as well to de-solder the cap from the wire which emerges from the glass 'pinch' at the top of the valve, then gently remove the cap and clean away the old adhesive (which as often as not, was a mixture of shellac and wood flour). Doing that will ensure much better adhesion of the cap when you fix it back in place, rather than just trying to poke adhesive under the rim of the loose cap. Everyone has their own idea as to what to use as an adhesive - for loose valve bases and caps, I always use two-part epoxy made by Plastic Padding, known as 'Super Steel', though it isn't in fact steel. (Available from Halfords and the like). I apply it round the glass 'pinch' rather than inside the cap, then quickly slip the cap on, keeping the adhesive away from the wire. When the adhesive has set, I solder the wire in place. Well worth doing, because otherwise, if the cap is loose, there's a real risk that the wire will snap off at the pinch if the top cap is disturbed. Then the only chance you have - and a slim chance at that - is to use a diamond burr to try to gently grind away some of the pinch to reveal enough wire to solder onto to fit the cap back in place, but the risk is that you'll break the vacuum and let air in, then the valve is scrap. Hope that helps. Good luck in your endeavours.
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9th Jan 2017, 4:24 pm | #30 |
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Re: Bush A.C.91
Thanks to Lawrence, Paul, Liam and David for their help and clarification on the Posts above.
One last point (for the moment) - as the Mullard CCH 35 is a metallised valve does the top cap need to be in contact with the screening or is it only a connection to the loop of wire round the envelope at the top of the valve base which is required - I think that this wire is connected to the cathode pin of the valve??
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9th Jan 2017, 4:31 pm | #31 |
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Re: Bush A.C.91
The top cap should not be in contact with any screening, the top cap is the control grid connection (g1), shorting that out to chassis or cathode would cause loss of signal.
Lawrence. |
9th Jan 2017, 4:42 pm | #32 |
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Re: Bush A.C.91
You should be able to see that there is a gap between the top cap and the metallised coating.
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9th Jan 2017, 6:31 pm | #33 |
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Re: Bush A.C.91
I have an AC91 that I restored some years ago. I had to replace most of the waxies but from memory, the resistors were all okay. It is very sensitive, particularly on the short waveband, and sounds good. My wife also likes it!
Take great care when cleaning the scale as the paint finish can be delicate. Also try to get the correct 3.5 volt 100mA dial bulbs.
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Phil Optimist [n]: One who is not in possession of the full facts |
9th Jan 2017, 11:17 pm | #34 |
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Re: Bush A.C.91
Donald , when you do manage to remove the cap you will probably find you need to replace the wire to it anyway, most of the above chassis wiring on mine was very crumbly , even though there`s no dropper , the audio and rectifier valves put out a fair bit of heat, hence the brass heat dissipating rings fitted to both . i even had to replace the wire and grommets emerging from the if cans. I also replaced the tuning capacitor mtg grommets which had deteriorated badly. I have a couple of dac 90 chassis i use for spares inc a fair few valves , give me a shout if you need something . good luck with the restoration , regards, Tim.
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10th Jan 2017, 10:54 pm | #35 |
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Re: Bush A.C.91
Trying to understand the work that has already been done before I embark on anything too ambitious and it looks like I need some serious assistance!
Looking in detail underneath I can see that two of the large electrolytics C27 (16 μF, 275 V) and C29 (32 μF, 275 V) have been replaced by a single electrolytic rated at 47 μF, 450 V - see first picture below. This looks to me as if this may be acceptable? Behind the mains connector (remember this is now a three pin) we have C30 which should be a 0.1 μF, 500 V tubular capacitor we now have a capacitor from RS and the only identifier I can see is .1 / 20 and 1500V - anyone know what this is and if it is suitable? See second picture below. Now the mystery deepens - adjacent to C30 we have another tubular capacitor but with no markings. Contrary to my previous statement this links the earth pin on the mains connector to an earth tag on the chassis! And things get even more strange when we observe another electrolytic capacitor in there rated at 15 μF, 400 V and which appears to connect to the neutral pin on the mains connector. The Service Sheet shows no electrolytic capacitor in this location so what's this for and why has this been done? Lastly, C3 has been replaced. - a tubular capacitor that should be 0.005 μF, 500 V and is now a 0.01 μF, 1500 V - don' t think this is acceptable as the Service Sheet lists a Tolerance of + or - 20%?? Hope someone can help?
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10th Jan 2017, 11:28 pm | #36 |
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Re: Bush A.C.91
When tackling a set that has undergone previous work - either done professionally or simply 'got-at' by a dabbler - I would urge great caution. I have been led astray more times than I care to admit, simply because I took the previous work that had been done at face value and didn't check everything very carefully indeed.
I know better now. What I would suggest is that you should try to get the set back into the configuration in which it left the factory. Manufacturers did not 'gild the lily' and would only have fitted components that were essential for proper operation at the minimum cost. Hence the fact that someone, subsequently, has seen fit to replace two separate reservoir and smoothing electrolytic capacitors with a single component twice the size should start the alarm bells ringing. The reservoir and smoother do different things; most rectifier valves have a maximum recommended value of reservoir, and may be damaged or stressed if this is exceeded. The reservoir is normally smaller than or equal to the value of the smoother. Even when work has apparently been done carefully, it doesn't mean it's been done correctly. I have gone round in circles trying to trace faults, only to discover that a previous repairer had put a wire in the wrong place, or (in one case) forgotten to put a wire in place at all! So, please, do take the time to trace out the circuit very carefully, against the service sheet, and try to restore it to what it should be. Only then can you test the set, and if any faults reveal themselves, at least you know what you've got!
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Phil Optimist [n]: One who is not in possession of the full facts Last edited by Phil G4SPZ; 10th Jan 2017 at 11:32 pm. Reason: Addendum |
11th Jan 2017, 9:26 am | #37 |
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Re: Bush A.C.91
There should be two electrolytics fitted, one is the HT reservoir capacitor (C29) the other is the HT filter capacitor (C27) the +ve connection of C29 should be connected to the cathode side of R18, the +ve connection of C27 should be connected to the other side of R18, the -ve connections of those two capacitors should be connected to the chassis, as per the schematic. There should be nothing connected to the earth terminal on the IEC socket, the only thing that should be connected to the neutral connection on the IEC socket is a wire that feeds the on/off switch.
EDIT: Be aware that if the mains on/off switch is configured as original then the mains transformer/valve heaters etc will still be live when the switch is in the off position. Lawrence. Last edited by ms660; 11th Jan 2017 at 9:52 am. |
11th Jan 2017, 11:41 am | #38 |
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Re: Bush A.C.91
Thanks again to Phil and Lawrence for their helpful responses.
Phil - Although I'm a similar age to most Forum members I've only done one restoration before this - but I am very cautious and know my limits in terms of my knowledge and skill sets. Your wise words are noted and I have no intention of going anywhere near a mains plug for some time yet! Fortunately I have the Service Sheet for the Bush AC91 although my circuit tracing abilities may be a bit challenged? Lawrence - Very detailed and helpful response which gives me a head start on what needs to be done to get the set back to what it should be. I can see the connections for C27 and C29 you refer to on the circuit diagram, just need to locate these on the actual set! Does anyone have access to decent pictures of what the layout of the various components and connections should look like underneath - those in the Service Sheet are not the best. I'd be interested in any responses that could explain what has been done and why?? Seems a bit strange (to me) to introduce components not on the circuit diagram e.g. the additional 15 μF, 400 V electrolytic capacitor. To a beginner this could be construed that the person knew what they were doing, but in this case I'm just confused?
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BVWS Member Last edited by DonaldStott; 11th Jan 2017 at 11:56 am. |
11th Jan 2017, 1:00 pm | #39 |
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Re: Bush A.C.91
Donald, you are correct that many of the below-chassis images in service sheets aren't the best quality. Add the fact that the wiring layout often appears illogical when compared with the circuit diagram, and things can appear very daunting indeed!
It often helps to start by identifying the valves and their base connections. These form fixed reference points around which the rest of the circuit hangs. Octal valves have 8 pins, starting with pin 1 just to the right of the locating spigot, and counting clockwise when viewed from underneath. Pins 2 and 7 are, more often than not, the heater pins, but not always. The pin-out data normally appears on Trader service sheets but can also be found in many online sources. If a component's value has become obscured, as often happens with old sets, noting its connection to a valve pin, to chassis or to other prominent components can help. Beware situations where a valve is shown with a particular pin 'not connected' or 'N/C' but with wires going to it. Manufacturers often used non-active valveholder pins as convenient tags for unrelated purposes. What you're looking for in your set is disturbed solder joints, non-original wiring, insulation or components to show you where someone has been before. A bright light and powerful ready-readers are essential, to me at any rate! Finally, stop trying to fathom out why a previous 'repairer' has done anything. It will drive you mad, and the chances are they were wrong anyway!
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Phil Optimist [n]: One who is not in possession of the full facts Last edited by Phil G4SPZ; 11th Jan 2017 at 1:07 pm. Reason: Clarification |
11th Jan 2017, 3:03 pm | #40 |
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Re: Bush A.C.91
The 15uF that's fitted might be a replacement for the HT reservoir capacitor (C29, 16uF)
The 47uF that's fitted might be a replacement for the HT filter capacitor (C27, 32uF) If they are then the ones that are fitted should be ok value and voltage wise. Lawrence. Last edited by ms660; 11th Jan 2017 at 3:17 pm. Reason: Correction |