UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

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.

Closed Thread
 
Thread Tools
Old 7th Dec 2018, 6:59 pm   #1
Freya
Octode
 
Freya's Avatar
 
Join Date: Jan 2009
Location: Falmouth, Cornwall, UK.
Posts: 1,987
Default Capacitor value

Just wanted to double check the value of this, I make it 330pF ?
Attached Thumbnails
Click image for larger version

Name:	20181207_175532.jpg
Views:	217
Size:	68.8 KB
ID:	174146  
__________________
Stephen
_________"It`s only an old telly" ___
Freya is offline  
Old 7th Dec 2018, 7:05 pm   #2
Al (astral highway)
Dekatron
 
Al (astral highway)'s Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
Default Re: Capacitor value

Orange 3, orange 3, brown x 10

So yes!
__________________
Al
Al (astral highway) is offline  
Old 7th Dec 2018, 7:08 pm   #3
Craig Sawyers
Dekatron
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
Default Re: Capacitor value

Acorns! Don't often see those.
Craig Sawyers is online now  
Old 7th Dec 2018, 7:10 pm   #4
Freya
Octode
 
Freya's Avatar
 
Join Date: Jan 2009
Location: Falmouth, Cornwall, UK.
Posts: 1,987
Default Re: Capacitor value

Thanks, it threw me when I tested it and it measured 34pF knowing it was likely bad.

Quote:
Originally Posted by Craig Sawyers View Post
Acorns! Don't often see those.
It's a 5 channel modulator in a video and sync generator
__________________
Stephen
_________"It`s only an old telly" ___
Freya is offline  
Old 8th Dec 2018, 6:29 am   #5
Argus25
No Longer a Member
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default 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.
Argus25 is offline  
Old 8th Dec 2018, 12:05 pm   #6
Al (astral highway)
Dekatron
 
Al (astral highway)'s Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
Default 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?
__________________
Al
Al (astral highway) is offline  
Old 8th Dec 2018, 12:33 pm   #7
Argus25
No Longer a Member
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Capacitor value

Quote:
Originally Posted by astral highway View Post
.. as the third colour- mulitplier - is brown (x 10) not black (x 1)

But that third bar looks resolutely brown.
Yes, I agree it does look brown and that could be the multiplier, and say for example it could make sense if it was an open circuit 220uH choke or 220R resistor.

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.
Argus25 is offline  
Old 8th Dec 2018, 1:18 pm   #8
Al (astral highway)
Dekatron
 
Al (astral highway)'s Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
Default 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?
__________________
Al

Last edited by Al (astral highway); 8th Dec 2018 at 1:37 pm. Reason: Clarity
Al (astral highway) is offline  
Old 8th Dec 2018, 1:34 pm   #9
Argus25
No Longer a Member
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Capacitor value

Quote:
Originally Posted by astral highway View Post
I’m still open to your strong theory but what if the component was a resistor, instead?
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.
Argus25 is offline  
Old 8th Dec 2018, 1:37 pm   #10
Al (astral highway)
Dekatron
 
Al (astral highway)'s Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
Default Re: Capacitor value

Absolutely ... I’m intrigued now!
__________________
Al
Al (astral highway) is offline  
Old 8th Dec 2018, 2:19 pm   #11
Freya
Octode
 
Freya's Avatar
 
Join Date: Jan 2009
Location: Falmouth, Cornwall, UK.
Posts: 1,987
Default 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.
__________________
Stephen
_________"It`s only an old telly" ___
Freya is offline  
Old 8th Dec 2018, 10:13 pm   #12
Argus25
No Longer a Member
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default 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.
Argus25 is offline  
Old 9th Dec 2018, 11:49 am   #13
Goldieoldie
Octode
 
Join Date: Jul 2014
Location: Petersfield, Hampshire, UK.
Posts: 1,043
Default Re: Capacitor value

Why not just sub it for a good 33 pf and then a good 330 pf if no improvement ?
Goldieoldie is offline  
Old 9th Dec 2018, 12:22 pm   #14
Freya
Octode
 
Freya's Avatar
 
Join Date: Jan 2009
Location: Falmouth, Cornwall, UK.
Posts: 1,987
Default Re: Capacitor value

Already fitted a 220pF, it's working well on all five channels now.
__________________
Stephen
_________"It`s only an old telly" ___
Freya is offline  
Old 9th Dec 2018, 12:43 pm   #15
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,801
Default 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
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline  
Old 9th Dec 2018, 12:48 pm   #16
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Capacitor value

Some Murphy TV's used capacitors that were packaged the same as those.

Lawrence.
ms660 is offline  
Old 9th Dec 2018, 1:27 pm   #17
Al (astral highway)
Dekatron
 
Al (astral highway)'s Avatar
 
Join Date: Nov 2006
Location: London, UK.
Posts: 3,496
Default Re: Capacitor value

Quote:
Originally Posted by Freya View Post
Already fitted a 220pF, ...working well on all five channels now.
Mystery solved! A great outcome, thanks for sharing!
__________________
Al
Al (astral highway) is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 10:15 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.