B28/CR100 communications receivers

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One hot day while poking round in the storage shed, I uncovered my collection of old valve communications receivers and one of them in particular was begging me to clean it up and take it inside to the bench and investigate it. The one that had the loudest voice was an R1297 (s/n 241) also known as a B28 or CR100-2. As I took the outer cover off I started to regret my decision already It looked like it may have had a bit of a rough time many years ago when someone had rewired it, putting in in-approprate single ended octal valve types without a shield. I found a metal 6SJ7, a coupe of 6SK7G's and a 6Q7G. To do this they had butchered the valve shield bases.

I have two of these receivers, the one labelled R1297 and the other labelled B28. This latter one looks like it has just been taken out of service in the armed services. It even still has the strange five pin power connector that accepts a mains chord or a motor-generator power pack. The valve line-up is original and all the shields are in place.

Why do I do it to my self

On a public holiday some years ago, when I should have been putting my Wifes car back together, and with the thermometer soaring to 39C, (the good old fashioned "ton"), I gave up working on the car and started to tinker with the receiver in the shed were the temperature was wells above what it was outside.

I searched the internet for details of the B28/CR100 and I found quite a few pages devoted to restoration and I even found the original workshop manual on the Boat Anchor site. From reading the stories of peoples restorations I soon realised I would have to replace all twenty two 0.1 uF chassis mount capactors as they were always leaky twenty years ago so what is the chance of any of them being OK today. The electrolytics had already been replaced but as that was perhaps fourty years ago they all had to go also. Getting the chassis mount caps out proved to be more difficult than I thought and I soon realised whoever had replaced a lot of the wiring thirty or fourty years ago had made a mess of it.

Well in for a "penny in for a pound", take the thing apart and replace all the caps, most of the resistors and all the wiring. While ripping it apart I discovered some old pitch covered caps deap inside the coil shields.

To my relief I soon discovered the receiver was made to come apart, right down to the shields around each coil assembly, making replacing the bits around the coils quite easy. Just as well I reduced to to a big box of bits, the wiring in the coil assemblies left a lot to be desired.
In investigating the IF coils I found the BFO coil is open circuit but as the "opening" is where it terminates on a capacitor I should be able to fix it.

Just a thought, while I am rebuilding this thing perhaps I should put in a product detector. Rummaging in an old folder labelled "Comms Rx" I found some notes titled Modifications to the B28/CR100 Receivers published in Amateur Radio, October, 1967 page 5 and 6. Now that is a long time ago. Subjects covered include, Stabilising the Oscillator, Improving Selectivity, Fitting a Vernier Dial, Increasing Gain, Reducing Mixer Noise, S Meter, Improved AGC and Product Detector. The highlited link above is to a PDF of this article.

This last topic is interesting in that he has used a dual control grid pentode (6AS6) as the detector, feading the IF signal to grid one and the existing BFO circuit to grid three. The spot occupied by the Triode double diode is replace with the 6AS6 and the AGC circuit uses half a 12AT7, and a couple of solid state diodes. The other half of the 12AT7 is used for the S Meter. This circuit demodulates both SSB signals and AM signals with the BFO off. Now, if I can find a suitable dual control grid pentode with an octal base and use a 6F8 as the double triode (don't want to make the thing too modern with B7 or B9 based valves) I may just consider these modifications. An alternative is a circuit I am trying to remember that used a 6A8 or 6J8 converter as a detector and BFO. Could replace the 6J7 BFO with this tube and use the double diode triode socket for the 6F8 for the AGC and S meter. Must not get too carried away.

Progress so far as of May 2010

Photos of the second B28 stored away in the junk shed

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Written by and Copyright, Phil. Storr © Last updated 16th September 2013