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Old 21st Jan 2017, 7:42 pm   #81
David G4EBT
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Default Re: Bush A.C.91

If you look at the attached sketch based on the circuit, you'll see that one tag of the primary of the output transformer (which you've already identified), goes to pin 3 - the anode of V4. One end of C28 (.01 uF) is also attached to that tag. The other end of C28 goes to the other tag of the primary winding, and also to R18 (10K). With your testmeter prods across C28, it should read 500 Ohms or thereabouts. If not, the primary is open circuit and the transformer will either need to be rewound or replaced, but fingers crossed!

To check the secondary, as has been said, you need to disconnect one of the leads to the speaker - either at the speaker itself, or at the output transformer, whichever is easiest. With your testmeter prods across the secondary, it should read 0.75 Ohms, but first, with your test prods shorted together, check that the meter reads zero. It may not do on the low Ohms range due to the slight resistance of the test leads. Hence, if say the shorted leads read 0.5 Ohms, when connected across the OPT secondary, you should see about 1.25 Ohms or thereabouts.

It doesn't matter which way round the test leads are connected - black or red that is.

(The meter doesn't need to be a Fluke! )

Hope that helps.
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Old 21st Jan 2017, 7:54 pm   #82
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Default Re: Bush A.C.91

Quote:
Originally Posted by DonaldStott View Post
I have a 630v replacement in my bag of goodies from Farnell so I'll remove the existing capacitor and see what happens.

Looks like I may be placing another Post in the Wanted section - how common are Output Transformers or are there modern equivalents?
Someone on here might have one going spare, if not then most single ended valve output transformers will work reasonably well if can you pick up a second hand one.

More expensive is one that RS components do (I've not checked if other suppliers do them) it's a universal job with various impedance taps to suit most applications, the valve manufactures recommended load impedance (Ra) is given as approx. 4.5k in the valve data:

http://uk.rs-online.com/web/p/audio-...rmers/2106475/

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Old 21st Jan 2017, 10:32 pm   #83
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Default Re: Bush A.C.91

To satisfy my curiosity I removed the existing C28 to see if that would make a difference but to no avail - my DMM (not Fluke) still reading 1

I unsoldered one of the speaker tags and measured 0.8 ohms across the secondary??

Looks like the primary is open circuit and will need replaced - rewinding may be a bit beyond me at this stage. Although the new transformer from RS looks attractive that would be too easy!
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Old 22nd Jan 2017, 12:44 pm   #84
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Default Re: Bush A.C.91

Have found an output transformer from a dac 90 attached to a duff speaker , have cut the wires to the speaker. Its measuring 500 ohms primary and as David G4EBT says ,taking into account the resistance in the test leads Secondary is working out at 0.75 ohms ,with my lcr meter i am getting "inductor ,130 uh" ,micro henrys , looks good to go ,though it does need tidying and a new capacitor , whats the best value re Tone for this capacitor ? .Will send Alistair a PM .

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Old 22nd Jan 2017, 3:56 pm   #85
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Default Re: Bush A.C.91

That transformer sounds just the job for Alistair.

The stipulated cap in the Bush service data is 0.01uF 350V.

I dare say that what most likely sent the transformer to the 'workshop in the sky' was a leaky audio coupling cap (C26) causing the output valve to be overdriven and excess current through the OPT primary, turning it into a fuse. Same fate as many DAC90As, and possibly the reason they were consigned to the attic many years ago, to an uncertain fate.
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Old 22nd Jan 2017, 8:22 pm   #86
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Default Re: Bush A.C.91

Thanks guys for all your help and support.

I have a 0.01uF 630V capacitor ready and waiting...

C26 will of course be replaced as will all the waxies and the other dubious caps discussed earlier - just need to find the time.
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Old 23rd Jan 2017, 2:36 am   #87
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Default Re: Bush A.C.91

Firstly I must admit to having not read through all of this thread.

At the start of this thread it seems that you bought a radio that had been recently repaired by someone and sold in a working state.

If I gather the situation correctly it would seem that some capacitors around the mains input that should have been X and Y rated, were replaced with ordinary types, but you have now replaced them with the correct types.

As regards the other capacitors in the set - there are critical ones that have HT of varying levels of voltage on one side but on the other side perhaps just connect to the control grid of say the audio output valve and are coupling capacitors passing the AC audio signal, but block the DC voltage of the HT. While ANY slight leakage in a capacitor in this particular situation can be seriously bad news for the valve and transformers, slightly leaky capacitors in other situations in the set will cause little or no trouble whatsoever.

What you need to do is to look at the circuit and try to understand the function of the other capacitors that are original and decide whether any slight leakage that they 'may' have will have any detrimental effect on the circuit they're in. Remember, with the technology available at the time, these capacitors were probably manufactured with some form of slight leakage from new. Some would even argue that the circuits were designed taking these shortcomings into account and that your radio may not perform so well with replacement 'new' parts in these 'non-critical' situations - something to think about.

As you may have gathered, I'm against all this 'painting-by-numbers' approach to capacitor changing, as you learn absolutely nothing by doing it and the radio looks worse by being flooded with 'yellow' - originality, with 'honest' repairs where necessary is the way I like it and when you see a radio (or whatever) repaired in this way, then you know that it's been repaired by someone who knew what they were doing and not by a 'cap changer'.

Last edited by Techman; 23rd Jan 2017 at 2:44 am.
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Old 23rd Jan 2017, 5:49 am   #88
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Default Re: Bush A.C.91

Maybe a little harsh Techman but I do agree with you. Wholeheartedly.
Thing is, some people may not want to learn, they come to the forum just to get a radio working and they read about all these terrible leaky caps and believe that it is a universal panacea to success.
Not including the present thread, there seems to be a desire to learn here, but its not a skill that many will find easy to acquire. Today people are not used to fault finding and repair.
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Old 23rd Jan 2017, 10:48 am   #89
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Default Re: Bush A.C.91

Quote:
Originally Posted by Boater Sam View Post
Not including the present thread, there seems to be a desire to learn here, but its not a skill that many will find easy to acquire. Today people are not used to fault finding and repair.
Not sure what you mean here but I have a genuine desire to understand what has been done with this set before I swap out anything!

I would have thought that my cautious approach and detailed questions bear witness to that - if that's not obvious then I apologise for wasting everyone's time.
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Old 23rd Jan 2017, 10:59 am   #90
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Default Re: Bush A.C.91

Sorry, perhaps I am not clear.

I intended to approve of your approach which is why I said that there appeared to be a desire to learn throughout your thread.

Please don't be offended, it was a compliment!

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Old 23rd Jan 2017, 11:45 am   #91
David G4EBT
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Default Re: Bush A.C.91

Not sure how someone who admits to not having read the whole thread can be so judgemental and form a subjective view which is critical of, and detrimental to, the original poster. Alistair has sought advice at every stage, has taken that advice and acted upon it. He's demonstrated enthusiasm and a desire to learn and to go about it in a safe and competent manner. At the outset he stated this this was only his second set, and was a Christmas present. His enthusiasm and desire to go about the restoration in a safe and competent manner was evident - well at least it was to me.

As to routinely changing paper capacitors on a sixty-year old radio, many - if not most - forum members who wish to restore a radio to a safe and reliable condition into the future (not merely 'mend', 'get going', or 'do up' a radio), will routinely change all waxy paper capacitors rather than faff about testing them individually at their working voltage to form a view as to whether or not to leave them in place. There is universal acceptance that the audio coupling cap (known on the forum as 'that cap') should be replaced even before a vintage set is switched on. Likewise, RF bypass capacitors which need to be replaced with X Class capacitors. Again, that has arisen in this thread and the reasoning explained.

True, there are some tubular paper capacitors rated in the component list as being 350 VW that have very little voltage on them simply because the maker - like all makers, standardised on the maximum voltage rating called for in the set, so yes, maybe several have had an east life and could be left in place with little or no detriment to the performance of the set. As to 'flooding the set with yellow caps', they're what the BVWS sells and what most restorers seem to use as they're noted for their reliability. Other people sometimes use 'orange dip' ones. At any time during the life of a set it would have been repaired with the components available at that time, and today, that's typically yellow polyester ones.

Alistair has had the extra challenge of trying to unpick repairs by a previous owner, some of which deviate from the original circuit. Again, he's sought, been given and has acted upon advice from fellow forum members. Frankly, if some forum members don't have anything positive, constructive, supportive or relevant towards helping the original poster to reach a safe and successful conclusion then it's better not said.

Alistair has nothing to apologise for - maybe others have.

"Use what talents you possess - the woods would be very silent if no birds sang but those that sang the best".

Henry Van Dyke. (He was clever).
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Old 23rd Jan 2017, 12:00 pm   #92
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Default Re: Bush A.C.91

I normally replace all waxies with those yellow axial capacitors, for the same reasons that David has stated in the above post, and usually replace old reservoir & smoothing caps. withnew, usually F&T dual Electrolytics, but I still do check for other faults and their causes, and, in the case finding such faults, replace the faulty component(s) with the nearest available modern equivalents. Some forum members may be aware of Alan Marchant, who owns/runs The Vintage Wireless Company in Cheshire. He sells restored vintage car radios, and his approach is to completely strip down and rebuild them, replacing all faulty components, plus other components and wiring which may cause problems in future. Having seen a few radios which have passed through Alan's hands, I can confirm that they look, and sound, as if they were made yesterday, rather than 40 or 50 years ago.
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Old 23rd Jan 2017, 12:49 pm   #93
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Default Re: Bush A.C.91

As someone who is also relatively new to restoring, I don’t understand the hostility to blanket re-capping – of waxies at any rate. It’s generally accepted that this type of capacitor does not stand the test of time well, and if a set is to be in regular use it’s surely better to replace them. For a first power-up it makes sense to change as little as possible, but for long term reliability they may always be suspect. Most of my sets have sounded noticeably better after all the waxies were changed, even if they were otherwise working correctly. The same goes for Hunts ‘mouldseals’.

In some situations it may be difficult to revisit capacitors for replacement at a later date. The AC91 is a relatively easy set to work on. When working on wartime or pre-war sets however, the capacitors are often in inaccessible places with the added problem of perished wiring. Unless you have surgically steady hands it makes little sense to keep the original capacitors while replacing all the wiring around them, which may obstruct access back to the capacitors. Even newer sets like the Bush DAC10 can make it awkward to get at the capacitors multiple times – unless you fancy removing the tuning unit again and again...

As for being open to learning, unless you are lucky you will still encounter many faults which require you to delve into the theory. I knew almost nothing about valves when I started, but on successive projects have learned to identify various types of problems in all sections of a radio. Faults due to leaking capacitors aren’t terribly interesting anyway.

TV restorers seem to be more open to blanket replacements, even in the r.f. sections of the circuits which have a lot in common with radios. I’ve just started my first TV restoration and all the advice available encouraged me to replace the wax capacitors even before the first power up.
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Old 6th Feb 2017, 5:00 pm   #94
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Default Re: Bush A.C.91

I've been working away at removing the previous restoration components where required, replacing a lot of disintegrating wiring and replacing the essential X and Y class capacitors.

I've removed the open circuit Output Transformer and replaced the wiring around the loudspeaker - again, badly perished.

Had to tidy up the co-ax wiring to the On/Off switch and volume control and am forced to replace C23 (Coupling to V3 Control Grid), not "that capacitor", that is C26 (Coupling to V4 Control Grid) - hope that's right!

C23 is literally in pieces and has to come out, no choice. In series with C23 is R11 (V3 Grid return) and this is reading 2.74 MΩ when it should be 2.2MΩ - is this too high (about 20% over), how critical is this and does it need replaced??

Trying to carefully understand every change I need to make and why.
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Old 6th Feb 2017, 7:14 pm   #95
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Default Re: Bush A.C.91

It'll work ok at 2.74 meg ohm but how long for is a piece of string, resistors in the meg ohm range do tend to go high, sometimes very high, there will be other values in meg ohm range in the AGC circuit, if I was doing it I would replace them.

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Old 6th Feb 2017, 7:53 pm   #96
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Default Re: Bush A.C.91

Thanks Lawrence for the quick response and advice.

Of course, I've got to ask - " the AGC circuit " ??
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Old 6th Feb 2017, 8:17 pm   #97
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Default Re: Bush A.C.91

AGC is an abbreviation for Automatic Gain Control, also sometimes referred to as AVC (Automatic Volume Control)

The AGC voltage is developed from the signal and it's subsequent rectification, in some receivers the voltage is obtained via the normal detector circuit, in other's (including yours) a separate diode is used, it's the right hand one in the EBC33's envelope as viewed in the schematic, the signal to feed that diode is taken from the anode circuit of the IF amplifier via C24 (50pF) the rectified voltage will be -ve and will be proportional to the signal strength, that voltage is used to control the gain of the IF valve and the Mixer valve, when those valves control grids go more -ve the gain of those valves is reduced, that basically equates to less gain with a strong signal and more gain with a weaker signal, the overall effect is to keep the detected volume level reasonably constant when receiving either a weak signal or a strong signal.

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Old 6th Feb 2017, 8:20 pm   #98
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Default Re: Bush A.C.91

Quote:
Originally Posted by DonaldStott View Post
Thanks Lawrence for the quick response and advice.

Of course, I've got to ask - " the AGC circuit " ??
AGC Automatic Gain Control

It does just what it says it changes the Gain of the IF so that signals of widely differing strengths and signals that fade don't change the audio volume (within limits)

Crossed with Lawrence

Cheers

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Old 6th Feb 2017, 11:59 pm   #99
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Default Re: Bush A.C.91

Thanks guys for the helpful explanations - now I know what AGC means and what it does I can allow myself to replace the components I mentioned previously.
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Old 7th Feb 2017, 5:33 pm   #100
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Default Re: Bush A.C.91

As you all know I am more interested in restoration than authenticity up to a point but what to do about the resistors that need replacing as they are more than 20% over?

The Bush AC91 Service Sheet says that these are either 1/4 or 1/2 Watt but these would be lost under the chassis - any problem with using 2 Watt resistors, much more chunky??
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