R1155 progress

Restoration of the R1155L is not progressing well at the moment, digging into some of the more inaccessible areas has uncovered a number of burnt resistors and wiring around the bias board. Most of the burnt bits appear to be part of a modification done by a previous owner, who in future I'm going to refer to as Bill the Bodger (a British term for a person who makes or repairs something badly or clumsily). His shoddy efforts have really created a lot of additional work for me.

It looks like Bill has attempted to re-wire the Meter Amplitude potentiometer into the bias circuit, although without a lot of effort I'm not going to find out why. I'm guessing that it might have been intended for variable RF Gain, but whatever it is it will have to come out.

On a more encouraging note, I was delighted to find that the two VI103 'magic eye' indicators I bought last month are both usable. The R1155 uses one as a tuning indicator, driven by the AGC voltage. Unfortunately the phosphor that gives the green glow seems to fail very quickly, resulting in a very dim display even though the valve emission is still good. Finding serviceable originals is now very difficult; New Old Stock (NOS) near-equivalents are available, but these are getting expensive.

VI103 'Magic Eye' on test, control grid grounded.

The seller said they tested OK on his AVO valve tester, but he didn't give any indication of how bright they were. They probably aren't as bright as they should be, but they are usable and at the price I paid I'm definitely well pleased with them. The seller was in Eastern Europe, so it might have been interesting to find out how two British Air Ministry valves got there.

From Eastern Europe via eBay!

The original VI103 in my set had been replaced with an EM34, a similar civilian indicator with a slightly different 'eye' shape. Fortunately I have now discovered that the pin out of the two types are slightly different, so the wiring of the socket must be changed back before I re-install a VI103!

Stuffing!

Old wax paper capacitors are notorious for going leaky. Even if they are not significantly leaky now, you can guarantee that they will be at some time in the future. My approach is usually to change all such capacitors on sight, and this receiver will be no exception to this rule.

Unusually, in the R1155, the decoupling capacitors are all housed in aluminium cans bolted to the chassis. Most of these have three 100nF capacitors inside, others have only one. I want to preserve the original appearance of the chassis, so I will be removing the contents of these cans and replacing them with modern components.

One of several capacitor cans

Removing a can from the chassis is easy. You just cut the existing wires close to where they enter the can, leaving the wires connected so you can see where you have to connect the new wires. Undoing the nut and pulling the can from the chassis completes the easy part of the task!

At this point you have a choice in the method for opening the can. Some people use a pipe cutter to separate the cylindrical part into two pieces, and then glue it back together afterwards using epoxy. Foil tape can be used to hide the join. Others may use a hacksaw to remove the end of the can, and glue that back in place afterwards. 

Fortunately I discovered an easier and neater way to do it.

Once the capacitor can has been removed, put the nut loosely back onto the threaded part. Clamp the nut in the jaws of a small bench vice with the capacitor standing vertically upwards. Now grab the end of the can and carefully rock it back and forwards, and side to side.  The cylindrical part of the can is crimped on to the threaded base part, and this rocking motion will loosen the crimping.  The trick is not to be too rough and tear the metal! Carry on this gentle loosening process until you can pull the can vertically upwards and separate it from the threaded part, which stays in the vice.

Removing the original capacitors from the can. Messy!

Carefully dig out the black pitch from the open end of the can with a small screwdriver. The now exposed capacitors inside need a bit of persuasion to come out of the can.  I use a large wood screw to do this; just screw it into the waxy yellow mass inside the can and pull it out.

In the next blog entry I'll tell you how I re-stuffed this can with new capacitors and re-assembled it.

Keep listening!