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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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Old 26th Jul 2021, 8:37 pm   #1
John_BS
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Default Radford amp; parts queries

1. Can anyone please confirm the make-up of these grey RS capacitors? They show measurable leakage and a loss tangent much inferior to polycarbonate, which might suggest they are paper based?

2. This STA 12 amp has not been messed with too much, and it's very likely that the 'resistor' mounted on the socket of the GZ34 rectifier is original. Is this a brimistor? If, so, based on its dimensions, it's an STC CZ1?

John
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Old 26th Jul 2021, 8:47 pm   #2
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Default Re: Radford amp; parts queries

ISTR that these are Mixed Dielectric types.

See this page.

https://www.vintage-radio.com/repair...apacitors.html

Cheers

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Old 26th Jul 2021, 9:03 pm   #3
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Default Re: Radford amp; parts queries

This thread suggests it may be a CZ6? (roundabout post 10, as fitted to the MA15)
https://www.vintage-radio.net/forum/...d.php?t=164829
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Old 26th Jul 2021, 9:11 pm   #4
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Default Re: Radford amp; parts queries

I've had those grey RS capacitors fail in the past - the innards swell and crack the plastic outer-casing.

Personally I'd be installing some nice shiny new LCR or Kemet yellows in their place - then you know things will be good-to-go for another couple of decades.
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Old 26th Jul 2021, 9:57 pm   #5
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Default Re: Radford amp; parts queries

Quote:
Originally Posted by Cobaltblue View Post
ISTR that these are Mixed Dielectric types.

See this page.

https://www.vintage-radio.com/repair...apacitors.html

Cheers

Mike T

Mike: thanks for that. Having read the capacitor page above, it could perhaps do with an update!

John
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Old 26th Jul 2021, 10:04 pm   #6
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Default Re: Radford amp; parts queries

Quote:
Originally Posted by Dickie View Post
This thread suggests it may be a CZ6? (roundabout post 10, as fitted to the MA15)
https://www.vintage-radio.net/forum/...d.php?t=164829

Richard: you could well be right. Both the CZ1 and CZ6 are 1.25" long, but I was trying to guess the type from the cold resistance: this sample measures 8k2 at room temp, which is higher than either. So I susepct it's cooked at some point and changed characteristics. It measures 250 ohms at 100mA, again with normal room ambient

As this amp uses 20uF main smoothing caps, it may not be necessary to employ the brimistor at all? I may try using a fixed R to keep the HT voltage about right?

John
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Old 27th Jul 2021, 8:58 pm   #7
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Default Re: Radford amp; parts queries

Quote:
Originally Posted by Cobaltblue View Post
ISTR that these are Mixed Dielectric types.
Yes - think these are they - monochrome cat but look like they could be the grey ones.
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Old 27th Jul 2021, 9:19 pm   #8
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Default Re: Radford amp; parts queries

Quote:
Originally Posted by John_BS View Post
Quote:
Originally Posted by Cobaltblue View Post
ISTR that these are Mixed Dielectric types.

See this page.

https://www.vintage-radio.com/repair...apacitors.html

Cheers

Mike T

Mike: thanks for that. Having read the capacitor page above, it could perhaps do with an update!

John
Yes Paul has been saying that for a while now but that page was probably created almost 20 years ago and those capacitors will not have improved with age that's for sure

Cheers

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Old 27th Jul 2021, 9:47 pm   #9
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Default Re: Radford amp; parts queries

Quote:
Originally Posted by G6Tanuki View Post
I've had those grey RS capacitors fail in the past - the innards swell and crack the plastic outer-casing.

Personally I'd be installing some nice shiny new LCR or Kemet yellows in their place - then you know things will be good-to-go for another couple of decades.
Hi.

Yes, I quite agree. Those mixed dielectric capacitors often test leaky and many have swollen up cracking the case. Shades of Rifas here! I suppose the paper part of the dielectric has absorbed moisture over a long period which would cause the innards to expand and rupture the case.

I've come across various colour mixed dielectric caps. I've seen grey, white, blue and white, blue and grey, and blue and yellow cases. I don't know if there's any significance relating to the case colour. I recall the blue and yellow case versions were often used by BRC/Thorn. I think their 1400 chassis used them.

Regards,
Symon
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