Radar Four Lead Can Capacitor Help

[atjblack]

Hello All,

In my continuing attempt to revive a pretty French radio in the absence of a positive ID (I think its very similar to an E.C.R. 752) or a schematic, I am going to replace the elderly capacitors (mostly those Valdex ones with the clear case, sealed with pitch). I thought I would start with the electrolytics in the Radar can, but I have been stymied by the labelling on the can. (See picture)

There are four leads: blue, yellow, red and black. The red lead had been cut and replaced by two 10 uF caps in parallel, so 20 uF, connecting to the grids of an ECH42 and the screen grid of the 6AQ5. The black was clearly a ground, and was connected to the chassis. The yellow was connected to the cathode of the 6X4 rectifier and the blue to the control grid of the 6AQ5.

The (nonsensical) measurements I got with my ESR tester were:
Red to black: 3000 pF
Yellow to black: 3800 pF
Blue to black: 4000 pF

If anyone has some guidance about how I could interpret these observations and replace the capacitors, I'd be very grateful.

Thanks!

Andrew

[Herald1360]

The 16 + 8 uF marking on the capacitor can only makes sense if it contained two independent capacitors. They would make sense as the reservoir capacitor between rectifier cathode and ground and smoothing capacitor fed from the rectifier cathode via a resistor or inductor with its other connection to ground. Maybe the circuit of a similar generic French set could provide clues.

https://archives.doctsf.com/document...5#lg=1&slide=0

perhaps?

[Graham G3ZVT]

I think what is happening is because the black wire wire is connected to the can, it doesn't necessarily follow that it's the common terminal for the two capacitors. That would explain why there are four wires when you would expect only three, and also those low capacitance readings, just stray capacitance.

I wouldn't expect any of the terminals to be directly feeding a control grid of any tube. A screen grid certainly, but not a control grid.

[atjblack]

Thanks Graham and Chris for your suggestions. I am confused as to what role the black wire could play as it was connected directly to the chassis and is not continuous with the can itself. I think my best bet might be to examine schematics of similar sets to determine which lead corresponds with which capacitor - I assume that two of them are the ones you mention, Chris. Then, taking into account the resistors in their immediate vicinity I might be able to come up with plausible capacitance values.

Thanks again,

Andrew

[atjblack]

Here is what I have come up with after a lot of head-scratching and eventually opening the can to see inside. If anyone is willing to vet my thought process, I would be very grateful.

As the can was sealed with pitch, it was a bit of a mess, but there were two capacitors of unequal size. The smaller seemed to have the yellow and blue wires coming from it, while the larger had the red and black wires.

I believe that the yellow/blue correspond to the 8 uF cap. The capacitance I measured between these two leads is 10 uF. The yellow wire (+ve terminal, I presume) connects directly to the cathode of the 6X4 valve at a junction which also directs a wire to the field coil of the speaker. The blue lead (-ve terminal, I presume) connects to ground through a 50 uF/50V cap (!) and also through a 200 Ohm resistor. Based on this evidence, I conclude that a 8 uF reservoir capacitor should be connected to yellow (+ve) and blue (-ve) leads.

The wire coming from the other side of the field coil connects to a terminal strip connection where the red wire was attached. (A previous owner had cut this red wire, severing the connection to the can capacitor and attached the positive end of two paralleled 10 uF capacitors at the terminal where the red wire had been attached, grounding this pair directly to a ground bus. I assume this was done as a result of the can cap failing - the capacitance measured between black and red was 1.1 nF. This terminal connection also has connections to the IF transformers, 1st audio plate, output and ECH42 screen grids, etc. I concluded that this is the smoothing capacitor that was originally 16 uF. I will replace the previous repair with a single 22 uF cap, as the current pair are registering 26 uF.

If you have remained awake to this point and are still for some reason willing to contribute to this quixotic quest, can you please let me know if my reasoning justifies my conclusion? It seems a little odd that the reservoir cap has a lower capacitance than the smoothing cap, but I think this does happen.

Thanks very much,

Andrew

[Herald1360]

The recommended reservoir cap. size for 6X4 is 10uF with a note that higher values may require attention to anode source impedance to avoid exceeding hot switching anode peak current ratings. So 8uF makes sense here.

[atjblack]

Thanks very much, Chris. That makes sense. I was planning on replacing it with a 10uF/450V cap, so that should work just fine.

Andrew