CR100/B28 Revisited.

[Murphyv310]

Hi.
I have had a CR100/B28 receiver for many years, It has no cabinet although I obtained a B28 front panel a few years back with the little plaque declaring that it was built in 1952
The set had been quite heavily modified before I got it with the original double RF stages comprising 6K7G's removed and little EF95's fitted on a new piece of alloy plate. The gain was astonishing but impossible to align with lousy tracking, images and cross mod to spoil the tremendous sensitivity.
I gave up with it around three years ago and it was put aside. It also needed rewired in some places, the socket on the back was bare with Mains and HT present which was a hazard, I'd already replaced some caps but more were in need of replacement. The set actually is around 1942.

I pulled the set out yesterday and so far I have removed the B5 socket at the rear, replaced the worst of the rubber wiring and will be fitting a Bulgin plug and socket for the mains that is a perfect fit in the recess the B5 socket was mounted on.

I really could do with a circuit for the set so I can compare the original to the mods done to the RF stages.
Watch this space.

[Murphyv310]

Hi.
Along with the above work done yesterday I substituted two 6BJ6 valves in the RF stages rather than the EF95's. This calmed down the excessive gain and other issues. These valves are just about pin compatible and in my case the extra pin had been wired up. Alignment then was possible and tracking is much better but still not quite perfect. I think that this could be due to the original valves having a screened lead to the grids and the wires being 6-7 inches long whereas the unknown modifier has just used a couple of inches of non screened PVC cable. May be an idea to use screened cable in a loop to give me an equivalent length to get the capacitance correct.

[Ed_Dinning]

Hi Trevor, there was a definitive article on the CR-- series in RB a few years ago. This had the schematics etc.

Ed

[Murphyv310]

Thanks Ed. Unfortunately I don't have that many. Any idea of what one?

[Stockden]

Hello Trevor,

Does this help?   http://www.vmarsmanuals.co.uk/archive/51...1868-1.pdf

Hugh

[Murphyv310]

Absolutely brilliant.
Thank you very much.

[Murphyv310]

                           
Hi.
As can be seen in the pictures some of the rubber wiring was past its use by date.
I have sleeved some and a couple of them have been substituted, on bad bits I have used heat shrink then sleeved the cable with the grey stuff.
I also sleeved the leads to the upper smoothing choke as the hole in the chassis has no grommet.
The radio is running nicely at the moment but there is still some work to be done, the miniature valve mod isn't neat and I am going to replace two carbon resistors that I don't like. All work in progress.

[Murphyv310]

Hi.
Well I discovered that on SSB there was something amiss. The drift was grim, even walking on the shed floor it went haywire. The issue though was simple, the KTW61 local oscillator valve had come unstuck from its base. a quick NOS replacement effected a cure.
I also found the C83 was missing, this I noticed by hand capacitance issues when trying to set up the local oscillator.
Next thing to show up was intermittent loss of gain. This took a bit of searching, I eventually took off the screening can under the AF amp and BFO to find two badly leaking 0.1uf alloy can caps. I re-stuffed the three caps that live in the can, again much improved performance.
There are about half a dozen of these caps left, all of them leak to an extent and all read high, between 0.15 to 0.2uf, this increase in value is common enough on radios of this age but leakage on AGC lines isn't good.

[Ed_Dinning]

Hi Trevor, there is an excellent article in RB about the successor to the CR100 series, the HR22. This is in issue 116, Xmas 2008.
The CR100 series is in issue 31, oct/nov1994.

Ed