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Vintage Telephony and Telecomms Vintage Telephones, Telephony and Telecomms Equipment |
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13th Jul 2020, 2:02 pm | #1 |
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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 ?
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13th Jul 2020, 3:41 pm | #2 |
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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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13th Jul 2020, 4:18 pm | #3 |
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Re: CB 741 telephone - C3 capacitor
Thanks Dave.
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13th Jul 2020, 7:43 pm | #4 |
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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. |
13th Jul 2020, 8:50 pm | #5 |
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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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13th Jul 2020, 9:01 pm | #6 |
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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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13th Jul 2020, 9:07 pm | #7 |
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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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14th Jul 2020, 6:07 pm | #8 |
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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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14th Jul 2020, 7:22 pm | #9 |
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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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14th Jul 2020, 9:47 pm | #10 |
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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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14th Jul 2020, 10:28 pm | #11 | |
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Re: CB 741 telephone - C3 capacitor
Quote:
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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14th Jul 2020, 10:51 pm | #12 |
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Re: CB 741 telephone - C3 capacitor
Thankyou Russell. I guess 1u is 1 microfarad. What does 1u8 mean ?
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14th Jul 2020, 11:17 pm | #13 |
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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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14th Jul 2020, 11:22 pm | #14 |
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Re: CB 741 telephone - C3 capacitor
Most kind Russell. Thankyou.
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14th Jul 2020, 11:33 pm | #15 |
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Re: CB 741 telephone - C3 capacitor
what about the cap from an old master socket?
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15th Jul 2020, 8:09 am | #16 |
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Re: CB 741 telephone - C3 capacitor
I'll bear that in mind, Nick. Thankyou.
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15th Jul 2020, 4:35 pm | #17 |
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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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15th Jul 2020, 6:20 pm | #18 |
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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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20th Jul 2020, 6:50 pm | #19 |
Heptode
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Re: CB 741 telephone - C3 capacitor
Successfully repaired this afternoon. Not too fiddly either.
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20th Jul 2020, 7:03 pm | #20 |
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Re: CB 741 telephone - C3 capacitor
Glad you're sorted.
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