[Milsurplus] R1155: "Too Good to be True," Was.

[David Stinson]

Re: The ongoing resurrection of the RAF R1155.

Tacked a 7.5V 5W zener across the Filament buss
to protect those irreplaceble Brit "valves" from 
possible Filament DC regulator failure.
https://goo.gl/photos/wTipNZJiFYcUbHLSA

Many rigs will function well with no or very few 
parts changes using "Low B+" techniques.  
My Russian US-P has run happily for years with
one capacitor changed and 60V B+, down from 
the speced 250V.  RME-45 down from the 
needlessly high 330V to 210V with no real loss
of performance.  Nationals, "ARC-5s," misc. others
have been happy and stable for years with all or 
very nearly all the original parts using Low B+.
However- there are rigs that, no matter how low
one takes the B+, some parts just won't hold-up.

Let the R1155 play for a couple of hours at
100V and noted the B-buss current began slowly 
creeping up.  Bad omen.
https://goo.gl/photos/bT1xP2LH8J9C8k7u8
Then noted that over time, it took higher and 
higher B+ to start the LO on the high band.
That's a sure sign of dying bypasses.

The long-term goal for this project, other than my
personal satisfaction, is to give the radio the best chance 
to be preserved beyond my time care-taking it. 
In my settled opinion, that requires that the radio play 
in an acceptable manner and that must be accomplished
with minimum changes to the radio.  
To meet the goals, the bad bypasses had to go.  
Here's the 4 uFd B-buss ripple filter and a 
couple of the triple-.1 (or single .5) bypass cans:
https://goo.gl/photos/iWoKsABEub3XDgFw6

In most of these sets, the "canned" bypass caps
are found open, but in this one they were all leaky.
These cans do not have terminals; they have bundles
of wires emerging from the bottom under the chassis.
The least-destructive way to address them was to 
nip a few millimeters of the lead wire out, tie the 
loose ends up out of the way and tack-solder a 
replacement cap at the wire's terminus- usually at
a tube socket or other terminal point.
https://goo.gl/photos/KhacZF6ct8Pyz3RW6
https://goo.gl/photos/P4f9fLtfkewai4Xy7

It's not "pretty," but "pretty" is not the goal.  
Attempting to remove and re-stuff these cans
will require a level of disassembly that absolutely
will cause broken terminal strips, cracked sockets,
broken switches etc.  Not acceptable.  Disconnected
the 4 mFd can cap across the B+ line and tacked-in
a new one.  Also, the cathode and AVC caps
in the big can under the "eye" tube had to go.  
One cannot remove this can for "restuffing" without
first removing the BFO, Main Tuning and maybe
the Bandswitch assemblies;  Not happening.
Unbolted the spade-lug cap leads, 
placed a square of circuit board to cover the terminals
and tacked the three replacements above the board.

The hardest cap to replace is the dual .1 can above
the band switch.  There's a "daughter board" attached
to the band switch with a couple of screws.  If you 
unsolder one wire, it can be lifted out of the way so
you can reach this cap, but it is very cramped.
Small tools will be required to do any good here.

The resistors in the set are in surprisingly good shape.
The only one that needed replacing is one that I 
broke while foozling-around in there.  Like the TCS,
10K and 100K-ranges seem to be most common
"drifters."  While about 90% of the TCS resistors
in this range required replacement, those in this
R1155 were "close enough" and was able to 
retain them in place.
All the tube grids are running with proper negative
bias, so no leaky plate couplers so far.

After the capacitor replacement and alignment 
touch-up, I put the case on the reciever, applied
200V B+ and kept a close eye on the B-buss
current for the next couple of  hours.  Current
varies with mode of operation.  In the "AVC"
position receiving around 5 MC, the buss 
current was 45 mills.  No creeping upwards
or any "mystery smoke."
https://goo.gl/photos/M2zh9orqkhEYdJDa7
https://goo.gl/photos/rVyCHPjqyxsnKXYM8

Next is the "acid test:"  Will apply 120V to 
the B-buss through a current-limited bench 
supply and leave the set running a full day.
https://goo.gl/photos/AcdcUiuYqKdri8wN6
Although 120V will significantly dim the 
"eye-tube," I think it worth the reduced stress.
If no further issues, I'll call it "good" and move
forward to a regulated power supply and 
speaker box.

GL OM DE Dave AB5S