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

Tue May 9 13:52:08 EDT 2017

The tubes are not irreplaceable but available on EBay, years ago when I did an R-1155 they were a lot cheaper but the other thing is that if you take a tube manual you will find that many of the tubes like the VR100 is just a 6K7 in a different package.  Think the "EYE" tube is just a 6E5
With the exception of the mica capacitors all the caps in my set were open and had the same wire that the insulation falls off when touched. Not having idea how to re-stuff the small aluminum tubes I determined that Sprague Orang drops would work and being that I do Ham radios and that was fine with me. 
Do you have the "drunken Man" meter yet? What are your plans for audio beyond the headphones? That was the other big Ham modification to add an audio power amplifier. Wonder if all British radios of that generation have capacitors that suck?
I will give you credit that so far the quality and documentation of your work has been excellent and an example for us all.

Ray F/KA3EKH

-----Original Message-----
From: Milsurplus [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of David Stinson
Sent: Tuesday, May 09, 2017 12:25 PM
To: milsurplus at mailman.qth.net; ARC-5 <arc5 at mailman.qth.net>
Subject: [Milsurplus] R1155: "Too Good to be True," Was.

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

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