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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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7th Dec 2018, 6:59 pm | #1 |
Octode
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Capacitor value
Just wanted to double check the value of this, I make it 330pF ?
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Stephen _________"It`s only an old telly" ___ |
7th Dec 2018, 7:05 pm | #2 |
Dekatron
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Re: Capacitor value
Orange 3, orange 3, brown x 10
So yes!
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Al |
7th Dec 2018, 7:08 pm | #3 |
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Re: Capacitor value
Acorns! Don't often see those.
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7th Dec 2018, 7:10 pm | #4 |
Octode
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Re: Capacitor value
Thanks, it threw me when I tested it and it measured 34pF knowing it was likely bad.
It's a 5 channel modulator in a video and sync generator
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Stephen _________"It`s only an old telly" ___ |
8th Dec 2018, 6:29 am | #5 |
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Re: Capacitor value
I don't know about this. It looks red red brown on my monitor. But lets say it is in reality orange orange brown. And you measured it at 34pF right. Then it will be a 33pF. Or maybe a 22pF measuring high.
Why would I say this ? If you consider the physical construction of most capacitors, it is far more likely they will 1 go leaky, or 2 increase their capacitance with water absorption into the dielectric. Not decrease their capacitance by a factor of 10 which is what you are proposing if you think it was a 330 pF and it is now a 34 pF. Subject the circuit to study; what frequencies is it operating on ? is the capacitor more likely a 33pF or 330pF with its current circuit position. Usually Acorn tubes are chosen for working in the high VHF & UHF region, making me think 33pF or 22pF more likely. Another thing to consider perhaps..... if this component turned out to be an open circuit choke or resistor, it could very well have a capacitance around 34pF Last edited by Argus25; 8th Dec 2018 at 6:45 am. |
8th Dec 2018, 12:05 pm | #6 |
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Re: Capacitor value
Hello Hugo, I agree with your reasoning... and am prepared to imagine the first two colours could be seen as red instead of orange, for the sake of open-mindedness (it would be interesting for me to check on another monitor) But that still gives 22 pF x 10 as the third colour- mulitplier - is brown (x 10) not black (x 1)
I do also agree about the implications of the application being UHF, and your reasoning about the typical pattern of value drift over the years. But that third bar looks resolutely brown. I wonder, how did you measure it, Stephen? It looks to be connected to an inductor to picture right...so I wonder if there was an error in your measurement somehow?
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Al |
8th Dec 2018, 12:33 pm | #7 | |
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Re: Capacitor value
Quote:
That is why it is worth tracing out the circuit (if the schematic is not available) and seeing if 1) it is a capacitor in its circuit position and not a choke or resistor, should be obvious from the circuit's DC conditions and 2) what sort of value it might likely be at the operating frequency. Looking at the photo, I think its coupling from the plate circuit into a coil that is grounded, probably in the grid circuit of the other Acorn valve, so it must be a capacitor. That might stack up to be a 220pF (or 330), but if it did, it would be a bit unusual that the capacitor failed to 1/10 of its value, unless say there was some sort of physical crack in it and part of the conducting surfaces had broken away. So I agree the colours appear to be 220 or 330pF. ...also, what if the brown was a faded gold ? Then it could possibly be a 2.2 or 3.3pF that had increased in value to 34pF which is more plausible with dielectric issues. Since it look like it is wired between a plate and grid circuit, it must be at least 300V or more rated, and would be a lot bigger if it were 220 or 330pf ? Last edited by Argus25; 8th Dec 2018 at 1:03 pm. |
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8th Dec 2018, 1:18 pm | #8 |
Dekatron
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Re: Capacitor value
Hugo, your new theory is also appealing.
Only there appears to be an air-cored flattish inductor made of a couple of loops of black-sleeved wire (transformer winding?), pic right, connected to the component and grounded at the other end. This would have a very low design inductance of possibly <1uH or so, estimating from its size and my own experience of winding air-cored inductors. If so, it doesn’t quite make sense to have your proposed tiny 2.2pF capacitor but with long wire leads left intact, with their inductance of 1 nH per cm or so (effectively a series inductance) . All the precision would be annulled, with the effective reactance reduced by the stray series inductance in the capacitor leads ... only by some vague amount but possibly 2-3R at UHF+ so an undesirable amount. I can’t beleive there would be such casualness here. Note especially the left-hand lead, with black-sleeved insulation. This alone is longer than the component body. I’m still open to your strong theory but what if the component was a resistor, instead?
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Al Last edited by Al (astral highway); 8th Dec 2018 at 1:37 pm. Reason: Clarity |
8th Dec 2018, 1:34 pm | #9 |
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Re: Capacitor value
That is why its worth tracing out the schematic...as all the clues hide there. For example if the circuit was link coupling the capacitor's reactance at the operating frequency should be about equal to the inductive reactance total of the coil and wires it couples into.
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8th Dec 2018, 1:37 pm | #10 |
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Re: Capacitor value
Absolutely ... I’m intrigued now!
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Al |
8th Dec 2018, 2:19 pm | #11 |
Octode
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Re: Capacitor value
So much interest, I'm impressed.
In the light of day it actually looks more like its red red brown for sure. It was tested out of circuit initially with the multi meter on ohms which was rising indicating capacitor. Then dug out the Peak LCR45 LCR and impedance tester which identified it at 34pF tested at 15 kHz. It's on the audio side of the modulator if that assists your ponderings.
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Stephen _________"It`s only an old telly" ___ |
8th Dec 2018, 10:13 pm | #12 |
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Re: Capacitor value
So the capacitor is clearly leaky if it read anything on the Ohm meter. I guess it is possible that if it is a 220pF that the leakage confused the LCR meter. One trick, to isolate the DC leakage, would be to put a good 0.001 to 0.01uF or similar in series with it and repeat the test.
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9th Dec 2018, 11:49 am | #13 |
Octode
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Re: Capacitor value
Why not just sub it for a good 33 pf and then a good 330 pf if no improvement ?
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9th Dec 2018, 12:22 pm | #14 |
Octode
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Re: Capacitor value
Already fitted a 220pF, it's working well on all five channels now.
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Stephen _________"It`s only an old telly" ___ |
9th Dec 2018, 12:43 pm | #15 |
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Re: Capacitor value
Remember thay coloured paints age and change. Not all colours change in the same way.
If it's a coupling capacitor, perhaps both 22p or 33p would work. It looks like a tubular ceramic type and they're pretty reliable. 34p measured and orange faded to red wouldn't surprise me. David
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Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
9th Dec 2018, 12:48 pm | #16 |
Dekatron
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Re: Capacitor value
Some Murphy TV's used capacitors that were packaged the same as those.
Lawrence. |
9th Dec 2018, 1:27 pm | #17 |
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Re: Capacitor value
Mystery solved! A great outcome, thanks for sharing!
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Al |