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Old 7th Feb 2017, 6:05 pm   #101
ms660
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Default Re: Bush A.C.91

2 watt resistors will be fine.

Lawrence.
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Old 8th Feb 2017, 10:00 pm   #102
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Default Re: Bush A.C.91

Looking under the chassis and trying to understand what is going on in the tangle of spaghetti, I noticed that C26 (Coupling to V4 control grid - "that capacitor") looked different to the other waxy caps. See picture below - it's the grey one to the left.

Suggests to me that this may have already been replaced at some point in the career of this set?

It will be replaced, just in case, but I was just wondering if anyone had any observations on this?
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Old 8th Feb 2017, 10:04 pm   #103
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Default Re: Bush A.C.91

Radiospares?

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Old 8th Feb 2017, 10:18 pm   #104
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Default Re: Bush A.C.91

Looks like one to me.
Frank
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Old 8th Feb 2017, 10:26 pm   #105
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Default Re: Bush A.C.91

Those used to leak back in the day, especially in TV's.

Lawrence.
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Old 12th Feb 2017, 1:18 am   #106
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Default Re: Bush A.C.91

One post moved to a new thread: https://www.vintage-radio.net/forum/...d.php?t=133932
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Old 12th Feb 2017, 10:33 am   #107
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Default Re: Bush A.C.91

I would also replace that 0.005uf waxie alongside the grey RS cap.
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Old 14th Feb 2017, 3:11 pm   #108
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Default Re: Bush A.C.91

Agreed - it's on my list as it will be impossible to replace C26 (Coupling to V4 Control Grid) and not disturb C25 (Fixed Tone Correction).

While I'm there I'm also going to replace R12 (V3 Triode anode load) which should be 68kΩ but is actually reading 83.3kΩ
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Old 15th Feb 2017, 11:17 am   #109
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Default Re: Bush A.C.91

Can anyone provide some background information on the electrolytic capacitor shown in the attached image - C21 on the Bush Radio Service Instructions?

This is rated at 50μF 12V DC and it's function is "V3 Cathode Decoupling" (V3 is a Mullard EBC33).

A strange looking (to me) metal enclosed capacitor located on the front of the chassis - as it's an electrolytic I'm assuming it should be replaced - but with what??
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Old 15th Feb 2017, 11:25 am   #110
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Default Re: Bush A.C.91

Replace with a 47uF, as long as the working voltage is the same or higher, most folks would use a 47uF 25 volt working.

It decouples the cathode resistor, in receivers a lot of the capacitor values aren't that critical, use an online reactance calculator to see the difference in reactance for any particular frequency, all will become apparent.

Lawrence.
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Old 15th Feb 2017, 12:06 pm   #111
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Default Re: Bush A.C.91

I routinely change output valve cathode by-pass decoupling caps as invariably they're shot.

You'll find that a modern equivalent axial lead replacement (as Lawrence says, typically 47 F/25V), will be puny in size - about as thick as a pencil and an inch or so long. With it being mounted above the chassis, depending on whether or not you wish to retain an air of originality in appearance, you might wish to consider gutting the old can and fitting the new one inside it. That's a topic which generates more heat than light, but I usually do re-stuff the can if it's on display rather than under the chassis. It's easy enough to do. Alternatively, you'll most likely find space for the new one under the chassis and just remove and discard the old can.

Well done on the effort that you're putting into this restoration Alistair, (as you did in your earlier one!).
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Old 15th Feb 2017, 1:23 pm   #112
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Default Re: Bush A.C.91

Will this one do the job: -

http://uk.farnell.com/vishay/mal2030...ial/dp/1165421

As it's on the front of the chassis I may have a go at re-stuffing!
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Old 15th Feb 2017, 1:53 pm   #113
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Default Re: Bush A.C.91

Quote:
Originally Posted by ms660 View Post
It decouples the cathode resistor, in receivers a lot of the capacitor values aren't that critical, use an online reactance calculator to see the difference in reactance for any particular frequency, all will become apparent.
Thanks Lawrence - I took your advice and now understand a bit more about Capacitive Reactance.

XC = 1 / (2 * pi * f * C), where f is the frequency and C is the capacitance in Farads.
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Old 15th Feb 2017, 3:02 pm   #114
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Default Re: Bush A.C.91

No problem, sometimes mentioning reactance to beginners can fill them with dread, it shouldn't do as you have now found out. It's a fundamental unit defined in ohms and there's nothing scary about that, you can apply the ohms law format formula to reactance as well, eg: you will no doubt be familiar with this formula...V/I*R, that applies to voltage, current and resistance (R in ohms) Here's the same formula layout but for reactance (X in ohms)...V/I*X...and again for impedance (Z in ohms)...V/I*Z...You can now see that the common theme is that although R, X and Z are different terms they are all defined in ohms.

The term reactance (X) applies to inductance (Xl) as well as capacitance (Xc) If the reactance's of those two are equal and they are connected in parallel (or series) then the circuit becomes resonant, so there it is, if you didn't already know you'll now know what a resonant circuit is and what's required to achieve circuit resonance eg: L and C who's reactance values are equal.

The relationship between the voltage and the current for a reactive component (L or C) is not the same as the relationship between the voltage and the current for a resistance but that's another story.

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Old 15th Feb 2017, 4:00 pm   #115
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Default Re: Bush A.C.91

Reactance can be a strange concept but one you grasp it you’ll find it much easier to understand why certain components are used in radio and audio circuits. Since reactance depends on frequency, inductors and capacitors can be used to make a circuit behave differently at various frequencies, or to make currents at certain frequencies go where the designer needs them to go.

For example, a capacitor combined with a variable resistor can be used to make a circuit path more or less favourable to currents at certain frequencies. This is essentially what the ‘tone’ control does in some radios. It changes the frequency makeup of the audio output by changing how a circuit responds to bass, treble etc.

I’ve often found it useful when trying to understand circuits, to think of a fixed frequency (such as a radio’s i.f. frequency, or the upper/lower limits of audio) and redraw parts of the diagram with the capacitors shown as reactances. You can quickly see how the circuit operates by putting yourself in the current’s ‘shoes’ so to speak. A circuit might provide a path forward for some frequencies, while shorting others to ground and removing them.

Hope that makes sense!

Liam

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Old 15th Feb 2017, 4:07 pm   #116
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Default Re: Bush A.C.91

Quote:
Originally Posted by DonaldStott View Post
Will this one do the job: -
Yes, that's fine.

As you'll see, it's only 6mm diam (approx. 1/4") x 10mm long (3/8"), so would look a bit lost in place of the original, so it's a good candidate for re-stuffing.
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Old 15th Feb 2017, 5:39 pm   #117
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Default Re: Bush A.C.91

A huge thanks to Lawrence and Liam for the Master Class on Reactance - you work very well as a team!

As for my C21 (Cathode Decoupling) why was it located on the the front of the chassis and not underneath with most everything else??

And thanks to David for his ongoing encouragement - re-stuffing is another skill I can add to my basket!
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Last edited by DonaldStott; 15th Feb 2017 at 5:54 pm. Reason: Typos
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Old 15th Feb 2017, 5:53 pm   #118
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Default Re: Bush A.C.91

Good to know I can still talk sense occaisionally!

As far as I know, the capacitor is there purely because of space. Cathode bypass capacitors on other sets can be located pretty much anywhere in my limited experience.

Most manfuacturers seem to put things wherever they will a) fit, and b) not cause hum (due to mains wiring next to audio etc.) If your lucky, ease of access will be a distant c) though to be fair Bush were quite good at this.

One of the sets I'm currently working on has all the electrolytics chucked in the chassis corner and clamped together with long leadout wires. Note very repair-friendly...
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Old 15th Feb 2017, 7:23 pm   #119
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Default Re: Bush A.C.91

Quote:
Originally Posted by DonaldStott View Post
As for my C21 (Cathode Decoupling) why was it located on the the front of the chassis and not underneath with most everything else??
Space I guess. To digress a little, similarly, on the Bush DAC90A, (which doesn't have a cathode bypass capacitor), Bush located the 0.1uF waxy mains RF bypass cap above the chassis next to the output transformer. Always advisable to snip it out right away because if it hasn't gone bang, it surely will do. The set works fine without it, and if replaced, it must be an 'X' class as it's directly across the mains.
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Old 9th Jun 2017, 10:33 am   #120
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Default Bush AC91 - back on track

Just disassembled what I would call the Tuning Dial End Stop and am a bit intrigued why this appears to be such a complicated fixture? Why is it made of copper and what's the function of the thin strips of black tape. Just going to clean it up, give it a respray and redo the black tape with something?

See here:- http://www.vintage-radio.net/forum/p...pictureid=3786

Any inside information on this would be welcomed!
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Last edited by DonaldStott; 9th Jun 2017 at 10:36 am. Reason: Wrong URL
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