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Old 19th Feb 2017, 5:33 am   #1
carlosmattos
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Default Decca Decola amplifier restoration

Split from this thread: https://www.vintage-radio.net/forum/...d.php?t=133868

Starting to restore the Decola PSU and amplifier.
Strange at it seems,with exception of two 600 Ohms 3w resistors,all the rest of the resistors are or precise or in the 10% of mesurment.All the litycs are out as also the coupling PIOs.
As i don't have the schems of it,(and it would be very welcome if any of you have),i'm having some trouble in identifying what's seems to be 6 tinny ceramic caps of 4pf,that i would like for you, if know what it is,try to help me identify them.
Please check the pics
Thanks
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Last edited by AC/HL; 19th Feb 2017 at 2:21 pm. Reason: Thread split
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Old 19th Feb 2017, 3:41 pm   #2
Cobaltblue
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Default Re: Decca Decola amplifier restoration

The 4pF capacitors are for neutralisation

They reduce the effective input capacitance of the Triodes.( from around 90pF to 4pF)

The closest modern equivalent would be 3.9pF

Cheers

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Old 19th Feb 2017, 7:19 pm   #3
carlosmattos
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Default Re: Decca Decola amplifier restoration

Hi Mike,
Thanks!
And what voltage should I get?
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Old 19th Feb 2017, 7:38 pm   #4
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Default Re: Decca Decola amplifier restoration

During warming up these capacitors could be subjected to full HT for a short period so I go for 500V parts which I think is what the originals are.

Cheers

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Old 19th Feb 2017, 7:49 pm   #5
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Default Re: Decca Decola amplifier restoration

Mike,Thank you so much!����
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Old 20th Feb 2017, 3:26 am   #6
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Default Re: Decca Decola amplifier restoration

Mike,
Perhaps you can help me on this! there are two strange resitors they have more strips colors then usual,they are in both PX25 output plates before output trasformer primary,it reads 45ohms but colors are:magenta,yelow,black,yelow,silver
please check the pics.Where can i find the electronic diagram?
Cheers
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Old 20th Feb 2017, 6:07 am   #7
Ed_Dinning
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Default Re: Decca Decola amplifier restoration

Hi Carlos, the ceramic caps probably do not need changing as they seldom go leaky. They could also have a neg temperature coefficient to reduce tuning drift. Paper caps, certainly replace them.

Ed
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Old 20th Feb 2017, 6:50 am   #8
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Default Re: Decca Decola amplifier restoration

Hi Carlos

The circuit can be obtained here: http://www.service-data.com/product....05/1623/a11205

If they are in the Anode of the PX25's then they are 47 Ohms

Ed at least one of the Capacitors has physically failed please see the pictures in post #1, they are in the power amp not the RF stages

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Old 20th Feb 2017, 9:10 am   #9
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Default Re: Decca Decola amplifier restoration

Mike Once again ����THANK YOU!!!
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Old 20th Feb 2017, 9:16 am   #10
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Default Re: Decca Decola amplifier restoration

Quote:
Originally Posted by Ed_Dinning View Post
Hi Carlos, the ceramic caps probably do not need changing as they seldom go leaky. They could also have a neg temperature coefficient to reduce tuning drift. Paper caps, certainly replace them.
Hi Ed,thanks for reply
Unfortunately those caps have to be replaced as soon as you touch them they tend to loose there contact wires, i have attached a pic of it...
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Old 20th Feb 2017, 2:54 pm   #11
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Default Re: Decca Decola amplifier restoration

Mine were the same and I replaced them all. They are used to reduce the miller effect by 'cancelling out' the grid/anode capacitance using the opposing valve in each pair. It's a true differential design all the way from the pickup matching transformer. Might be interesting to see if they really make a difference to the HF response. There is no overall -ve feedback so the design is reliant on the valve performance.

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Old 28th Feb 2017, 2:04 am   #12
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Default Re: Decca Decola amplifier restoration

For thouse who never had the opportunity of seeing a Decca Decola insides...
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Old 28th Feb 2017, 2:07 am   #13
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Default Re: Decca Decola amplifier restoration

a few more...
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Old 1st Mar 2017, 1:29 pm   #14
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Default Re: Decca Decola amplifier restoration

Quote:
Originally Posted by carlosmattos View Post
Perhaps you can help me on this! there are two strange resitors they have more strips colors then usual,they are in both PX25 output plates before output trasformer primary,it reads 45ohms but colors are:magenta,yelow,black,yelow,silver
please check the pics.
Those resistors are as said, 47 ohms. The marking is in the old body, tip spot (or ring) system where the base body colour (yellow) is the first digit, the tip (purple) is the second digit and the ring(black) is the multiplier (number of zeroes). The other tip (silver) is the tolerance, 10%. At 45 ohms they're still well within spec.

This system included some interesting monochromatic resistors in 20% (no tolerance marking) types like 2.2k (all red) and 33k (all orange). Other possibilities exist but would have been non standard values like 110R, 440k etc.
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Old 2nd Mar 2017, 3:13 am   #15
carlosmattos
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Default Re: Decca Decola amplifier restoration



Quote:
Those resistors are as said, 47 ohms. The marking is in the old body, tip spot (or ring) system where the base body colour (yellow) is the first digit, the tip (purple) is the second digit and the ring(black) is the multiplier (number of zeroes). The other tip (silver) is the tolerance, 10%. At 45 ohms they're still well within spec.

This system included some interesting monochromatic resistors in 20% (no tolerance marking) types like 2.2k (all red) and 33k (all orange). Other possibilities exist but would have been non standard values like 110R, 440k etc.
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Old 13th Mar 2017, 4:55 pm   #16
carlosmattos
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Default Re: Decca Decola amplifier restoration

Hi there,
After everything desmonted,re-glued,wood compensated etc,I'm stucked with a gear that holds the cover from flick back. It must be easy to desmont but I just can't see how...any tip about to do it would be super!

Much easier to polish it with separated parts...
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