[Central-Electronics] 20-A Rebuild

[George Maier]

Well Ron...
I think Jim is the resident guru of 20A's, but I'd like to add a few more
thoughts.
Make sure that you check the resistor values all along the RF chain, and
especially around the 12BH7 driver stage and PA grid circuit.  Also, test
the mod diodes for front to back ratio, and replace them if they're out of
balance, or course if they're defective.  
Finally, go to Ken Grimm's great web site, BAMA, and download the 20A
changes PDF from the CE section.  There is some good info there on factory
updates, that you may want to consider.
Good luck & hope to hear you on this winter.
73, George - K1GXT

-----Original Message-----
From: central-electronics-bounces at mailman.qth.net
[mailto:central-electronics-bounces at mailman.qth.net] On Behalf Of Jim
candela
Sent: Monday, October 10, 2005 8:34 PM
To: K3PID; central-electronics at mailman.qth.net
Subject: RE: [Central-Electronics] 20-A Rebuild


Ron,


    In a yes / no answer scenario, NO!

Some 20a's use silver mica bypass capacitors. If you replace these, be
careful! I had one short out on me, so I replaced them all on a particular
20a I was going through. I was using equal value ceramic disc capacitors
(Z5U or Z5V dielectric). Then I lost gain in the mixer circuit as a result.
Replace mica with mica! All those big green bypass capacitors (like .22 uf
400 v)need to go since they all either leak, or soon will. The power supply
electrolytics all must go too. The pi filter 'CAN' capacitor can be replaced
with a equal capacitance 'CAN', but preferably one rated at 500 volts
instead of 450. Today's line voltage is higher than it was in 1954, so the
extra voltage rating adds margin where there is none.

http://www.angela.com/
50uF+50uF/500VDC, 1&3/8" diameter, 2" tall, clamp mount type like LCR (use
mounting clamp #6 described above). EACH $8.

For the large bypass, I like to use Metalized Polypropylene.




In case you feel like following one of my QRO 20a projects, here is a link
to my last converted 20a project:

http://pages.prodigy.net/jcandela/CE20AQRO/

open 20aqro3.doc

My 1st two QRO projects used 7591 output tubes without neutralization for
160-40 meter operation.

Warning: Don't QRO your 20a unless your 20a is working fine already at stock
power, and don't QRO your 20a unless you have about 1000 expendable hours.
:-)

Regards,
Jim Candela
WD5JKO


-----Original Message-----
From: central-electronics-bounces at mailman.qth.net
[mailto:central-electronics-bounces at mailman.qth.net]On Behalf Of K3PID
Sent: Monday, October 10, 2005 6:48 PM
To: central-electronics at mailman.qth.net
Subject: [Central-Electronics] 20-A Rebuild


I have my 20-A on the bench in preparation for this winters rebuild. I know
that I've seen about as many answers to this as there are members but here
goes again. For the record: Should I just carte blanche replace the paper
caps? I hate the look of orange drops but they do work. Any other
suggestions on replacements?

K3PID
Ron H.

Old Friends, Old Radios, Old Music, life is good!


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