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Old 2nd Nov 2017, 6:20 pm   #1
Tonypam
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Default Plessey Electrolytic capacitor & Resistor advice

I have a very nice Radiomobile 50TM car radio with a what I believe to be a faulty Plessey 1000uF 1.5V DC cap ( Dated October 1961 ) - I cant seem to locate an equivalent with the same voltage, the nearest I have seen is a 6 or a 10 volt one. At least I assume its the cap at fault....attached to it in series is a resistor shown in the photo and I guess it could be that at fault ? Could it ? The symptoms are a very weak and distorted output. As soon as I bypass the cap with my large tweezers (!) - perfectly clear and undistorted reception !

Could anyone advise ?

Thanks as usual, Tony
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Old 2nd Nov 2017, 6:44 pm   #2
Biggles
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Default Re: Plessey Electrolytic capacitor & Resistor advice

Without looking at the circuit it's difficult to give any advice but it is perfectly in order to use a higher voltage rating capacitor as a replacement. As long as the value in microfarads is the same or near, as it probably is not that critical if you can't find a direct equivalent. Is the resistor in series or parallel with the capacitor? If the resistor is in parallel with the capacitor and you short the capacitor out you may cause damage to other components if this upsets bias conditions in the circuit. I am assuming the capacitor may be in an emitter path as it's such a low voltage value but I may be wrong.
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Old 2nd Nov 2017, 7:13 pm   #3
Tonypam
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Default Re: Plessey Electrolytic capacitor & Resistor advice

Apologies - the resistor is parallel !

Could you identify the value of the resistor from the coloured bands around it ?

I will replace that too

Thanks Tony
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Old 2nd Nov 2017, 7:21 pm   #4
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Default Re: Plessey Electrolytic capacitor & Resistor advice

The colour rendering of the resistor is unclear. What colours are the bands?
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Old 2nd Nov 2017, 7:26 pm   #5
Boater Sam
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Default Re: Plessey Electrolytic capacitor & Resistor advice

Looks like brown grey brown with a silver 10% tolerance band. If so, thats 180 Ohms.
If its not out of tolerance or otherwise faulty why change it?

Standard 3 band code. First 2 bands give the first two digits.

Black=1
Red=2
Orange=3
Yellow=4
Green=5
Blue=6
Purple=7
Gray=8
White=9

Then third band gives the number of zeros.
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Old 2nd Nov 2017, 7:32 pm   #6
Argus25
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Default Re: Plessey Electrolytic capacitor & Resistor advice

The resistor looks like a brown,grey,gold,silver, making it a 1.8 ohm, probably at least 1 or 2w rated by the size of it.With the 1000uf in parallel, most likely it is the emitter network for a class A germanium transistor audio output stage. If this is so, then the resistor is required to DC stabilise the transistor and prevent thermal runaway so don't leave it shorted out for long.The capacitor bypasses the AC signal component to maintain the gain. Probably best to replace the resistor with a 2W or 3W type. You can figure out the dissipation in the resistor by measuring the voltage across it when it's running.
Actually, assuming there is only a volt or so across the capacitor, the resistor dissipation would be that voltage squared, divided by 1.8, so probably it only needs to be a 1w rated part.
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Old 2nd Nov 2017, 7:52 pm   #7
Boater Sam
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Default Re: Plessey Electrolytic capacitor & Resistor advice

Sorry, of course if the cap is so low a voltage it can't be 180. Argus is most likely correct, 1.8 ohms.
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Old 2nd Nov 2017, 8:12 pm   #8
ukcol
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Default Re: Plessey Electrolytic capacitor & Resistor advice

The 1959-60 R&TVS book has a circuit of the Radiomobile 40T. The same source states that the "50" series sets are similar.

The set is a hybrid, the only transistor being in the output stage.

Referring to the snip below; C15 is the only 1000uF capacitor listed in the parts list and there is a 1.8 ohm resistor associate with it, R16.

C15 decouples the output transistor's base bias supply to its emitter. R16 is the emitter resistor. VR2 is the base bias adjustment pot.

EDIT:

The data for the 40T is available as an instant download here.
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Old 2nd Nov 2017, 8:36 pm   #9
Tonypam
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Smile Re: Plessey Electrolytic capacitor & Resistor advice

Thanks everyone - I have just purchased the data diagram so I will replace C15 and R16 and see what happens

Tony
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Old 2nd Nov 2017, 9:41 pm   #10
David G4EBT
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Default Re: Plessey Electrolytic capacitor & Resistor advice

Quote:
Originally Posted by Boater Sam View Post

Standard 3 band code. First 2 bands give the first two digits.

Black=1
Red=2
Orange=3
Yellow=4
Green=5
Blue=6
Purple=7
Gray=8
White=9

Then third band gives the number of zeros.
Not quite:

Black = 0
Brown = 1

Otherwise OK.

But I'm sure you knew that Sam - just a typo I guess.

More than three bands and I start to lose the plot - I grew up in the 'body/tip/spot' era!

http://www.hobby-hour.com/electronic...calculator.php
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Old 16th Nov 2017, 9:42 pm   #11
Tonypam
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Smile Re: Plessey Electrolytic capacitor & Resistor advice

Quick update - R16 changed and all is well

Many thanks, Tony
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