[R-390] Re capping an IF Deck

[Flowertime01 at wmconnect.com]

Dennis,

There are variations of four choices for the ACG cap.
1. Do nothing or similar more nothing.
2. The slow 2uf AGC cap can be replaced in the can like rebuilding power 
filter caps.
    Not more or less messy than power filter caps.
3. Add a new cap under the deck. Leave or remove the above deck can. C551
4. Install a socket in the deck hole and mount the cap in a crystal oven can
and plug that into the new socket. An 8 pin octal socket will mount nicely 
and the
other C548 cap will also fit in the oven can.

Along with these rebuilds it is suggested that C548 be increased from .1 uf 
to 1. uf.
Pushing C548 up into the new can fits nice. After recapping the IF deck, there
is much more space under the deck. Those brown tubes were "big" in their own 
way.

It is C553 that keeps the 1st IF B+ out of the crystal filters. Inspect this 
cap to see
if it looks like a 600 volt rated cap. You may want to replace this cap if it 
even looks
like less that a stellar quality item in your deck. Some time now for peace 
of mind over the next 50 years is a small price and some quality time with your 
receiver.

There are ranges of paranoid for your IF deck.
Real severe leaves you unwilling to even apply power to your receiver.
Not real common but known to occur.
Less sever cases accept nothing less than a real full rebuild with nothing 
less 
than orange drops. Some will not even accept a self build and require the 
work be performed by a real "professional restorer." 
Having a radio is supposed to bring you joy and not anxiety and misery.
Lesser degrees of paranoid will let you install almost any current good brand
of cap in the deck to get the brown tubes out and keep the receiver receiving.

Real caviler owners will run the receivers until the smoke escapees and then
only replace the minimum carbon needed to return the receiver to operation.

As with all preventive maintenance the objective is to put in the minimum up 
front work to avoid even more work later. So changing caps is good.

Only the brown and black tube caps in the R390/A are known to be more likely
bad than good today given there age. Also for ever bad cap found some will 
post
here that there old caps are still hanging in and working good.

As you do not have the brown tubes, your deck either never had them or has 
been
rebuilt. A look at the solder will likely provide some clue as to 
originality. Some fried
wire at the terminals is a clue someone got there with a solder iron after 
the wire had aged many years.

You will likely be OK with what is in the deck. Do inspect the cap to the 
filters,
C553, you may want a good 600 volt cap in there. Something you have some
confidence in. An orange drop is good. A Radio Shack part not so good.

Other than that you just need to reach a comfort level you can be happy with.

There are reasons to change lots of the caps. Some value changes have been 
offered over the years to improve the frequency response mostly in the audio 
deck.

Changing C553 to a break down voltage of 600VDC to protect the mechanical 
filters.

Changing C548 .1 AGC to 1. For less AGC pumping are a few.

Caps like resistors have their own popcorn noise. Parts have been hunted down 
and
changed out just to get a better receiver noise floor.

As you have no brown tubes in your deck, you have no known issue driving you
to make any changes. Now you get to make choices based on satisfaction and
enjoyment.

Enjoy

Roger KC6TRU