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Old 13th Jul 2020, 2:02 pm   #1
Sparks
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Default CB 741 telephone - C3 capacitor

I've just acquired this phone and it appears one of the wire joints to the C3 capacitor is snapped. Assuming I can't repair the joint, will I need to replace the capacitor or can the phone manage without it ?

Thankyou.
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Old 13th Jul 2020, 3:41 pm   #2
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Default Re: CB 741 telephone - C3 capacitor

According to diagram N841, C3 (0.9μF) is in series with R2 (30Ω) between T10 and terminal 2 of the induction coil, so I would assume that it is needed - unlike C1 and C2 if you are using third-wire ringing via the capacitor in the master socket.
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Old 13th Jul 2020, 4:18 pm   #3
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Default Re: CB 741 telephone - C3 capacitor

Thanks Dave.
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Old 13th Jul 2020, 7:43 pm   #4
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Default Re: CB 741 telephone - C3 capacitor

I might be able to bodge the capacitor by dropping a blob of hot solder on the spot where the pin has snapped off and soldering a new piece of solid wire on to it. I've nothing to lose by trying but I won't be able to tell if I've been successful or not. I'll have to take the board off the base but I'd have to do that anyway to fit a replacement capacitor.

One quirk with this phone is that the board seems to be about 2 to 3mm too small for the case. When the bottom screw is tightened, one side of the board sits neatly on the lip of the case but the opposite side slips just below the lip, which can affect the switch hooks. Makes me wonder if I have a 711 case instead.
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Old 13th Jul 2020, 8:50 pm   #5
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Default Re: CB 741 telephone - C3 capacitor

Not related to your broken capacitor lead, but I notice some oddities in the wiring:
You have the blue wire (presumably from the ringing capacitor in the master) attached to T5/T6/T7, but you still have the internal capacitor in circuit (T6-T7 and T8-T9).
The bell lead at T4 doesn't appear to be connected to anything.

I was initially also puzzled by the green wire between T10 and T8/T9 until I realised it was bridging the gap in the loop left by the absence of a dial.
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Old 13th Jul 2020, 9:01 pm   #6
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Default Re: CB 741 telephone - C3 capacitor

Good spot Dave. I took that picture when I first connected a line cord (just to check for basic function). I've since moved a strap or two and put a resistor between T4 and T5 so the phone is now wired as per a consumer conversion kit (minus the dial).
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Old 13th Jul 2020, 9:07 pm   #7
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Default Re: CB 741 telephone - C3 capacitor

Yes, as you presumably know, both the T6-T7 and T8-T9 straps are redundant when using third-wire ringing, but only one needs to be removed to disconnect the internal capacitor. Removing both, however, means you have one more spare strap to use elsewhere!
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Old 14th Jul 2020, 6:07 pm   #8
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Default Re: CB 741 telephone - C3 capacitor

If I should have to replace the capacitor, can anyone suggest a likely source ? Mods, please move this to the 'wanted' thread if necessary.
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Old 14th Jul 2020, 7:22 pm   #9
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Default Re: CB 741 telephone - C3 capacitor

It's a pity that your 'phone appears to use a single 1.8μF for its ringing capacitor rather than the two 0.9μF capacitors in parallel that are shown in the N-diagram, otherwise you could have repurposed one of those.

If searching for a modern replacement, 1μF is probably the nearest available - which would probably be within the tolerance of the original component. Alternatively, you could use two 0.47μF capacitors in parallel.

N.B. you need a non-electrolytic capacitor.
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Old 14th Jul 2020, 9:47 pm   #10
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Default Re: CB 741 telephone - C3 capacitor

Thanks again Dave. I know what to look for now, although I would obviously prefer a like for like replacement. BTW, what does a capacitor do anyway and why is one needed in this instance ? I'm largely clueless in this regard.
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Old 14th Jul 2020, 10:28 pm   #11
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Default Re: CB 741 telephone - C3 capacitor

Quote:
Originally Posted by Sparks View Post
BTW, what does a capacitor do anyway and why is one needed in this instance ?
It's part of the receiver shunt-balance configuration for sidetone and transmission as far as I can make out, Sparks. There's an explanation on Bob's telephone page about the 706. It, and a resistor and a 1u8 capacitor are in parallel with the receiver and one of the induction-coil windings.

According to the blurb, capacitor tolerance of 15% is permitted so you could get away with a 1u cap, or, in reality, probably a 1u8.
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Old 14th Jul 2020, 10:51 pm   #12
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Default Re: CB 741 telephone - C3 capacitor

Thankyou Russell. I guess 1u is 1 microfarad. What does 1u8 mean ?
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Old 14th Jul 2020, 11:17 pm   #13
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Default Re: CB 741 telephone - C3 capacitor

1u8 is another way of writing 1.8 microfarads. If you're stuck for a 0.9 I might one knocking about somewhere. I'll have a look down the shed tomorrow.
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Old 14th Jul 2020, 11:22 pm   #14
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Default Re: CB 741 telephone - C3 capacitor

Most kind Russell. Thankyou.
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Old 14th Jul 2020, 11:33 pm   #15
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Default Re: CB 741 telephone - C3 capacitor

what about the cap from an old master socket?
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Old 15th Jul 2020, 8:09 am   #16
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Default Re: CB 741 telephone - C3 capacitor

I'll bear that in mind, Nick. Thankyou.
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Old 15th Jul 2020, 4:35 pm   #17
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Default Re: CB 741 telephone - C3 capacitor

If the wire has sheered off flush with the base of the capacitor, see if you can whittle a little of the resin away with a sharp knife or a file, exposing a little of the wire. 0.5mm would be more than enough.
Then take a single strand of copper wire from a mains flex and wrap a single turn around the stump, and solder.
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Old 15th Jul 2020, 6:20 pm   #18
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Default Re: CB 741 telephone - C3 capacitor

Thanks Rambo. The point in the capacitor where the wire has snapped is clearly visible so a repair should be doable.
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Old 20th Jul 2020, 6:50 pm   #19
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Default Re: CB 741 telephone - C3 capacitor

Successfully repaired this afternoon. Not too fiddly either.
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Old 20th Jul 2020, 7:03 pm   #20
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Default Re: CB 741 telephone - C3 capacitor

Glad you're sorted.
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