federal-te: Kolster 'B' Socket Power Unit

A while back I was working on this old power supply and had corresponded with someone in Radiomuseum on this. I am back on this project and hope to get some added help. Here is what I know:

The transformer in this unit is putting out 250 (+/-) volts DC. I tested the voltages at the taps and they are all wrong, indicating a bad resistor I think. The 135 VDC tap reads a full 250, the 90 vdc tap also reads 250. The 67 and 22 vdc taps all read low.

I substituted the big Mershon liguid filled cap with three smaller electrolytics rated for 600 vdc. (10uf as indicated on the original).

The resistor resistor reads open all the way up, except there is resistance between the top two (135 & 90) but not to the common below. The posibility of finding a replacement resistor is probably nill. Likewise any schematics.

My goal was to make this a functioning unit again, not just a display.

Can anyone offer some assistance?

Thanks in advance,

Mark

 

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> transformer in this unit is putting out 250 (+/-) volts DC.

Transformers put out AC.

Rectifier and first filter cap makes the AC into DC at some different voltage.

Resistors drop the raw high DC voltage to lower voltages. Caps add filtering.

> there is resistance between the top two

It would be very useful to know what that resistance is.

The schematic _must_ be like this:

The values are my guess based on the estimated capacity of the '01A tube and likely power drains of radios without on-board wall-power systems. If my "11.5K" reads different on yours, within 5:1, then scale all other values the same ratio.

All resistances may be +/-20%, not fussy. If restoring with five new resistors, use 2W parts in nearest standard value: 12.1K, 4.7K, 2.2K etc. Alternatively a similar-to-original single tapped resistor might be made with 25K 20 Watt "tap" resistor and four tap-brackets (however it may not be possible to set taps 2.3K apart on a 25K resistor).

I have not indicated the grid connection on the '01A. It may be open or may be strapped to another pin. Assume the un-tampered original is correct. I'd like to know how they did it, for my curiosity. 

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> a similar-to-original single tapped resistor might be made with 25K 20 Watt "tap" resistor and four tap-brackets

This is what I am thinking:

Digi-Key Part Number D50K25KE-ND

It is supplied with one tap-bracket which can be set at any point along the resistor. In the old days you could buy additional taps for a few cents. I can not find those taps in DigiKey's listings. A clever metal-bender could fabricate taps, perhaps slim enough to get the close-space taps.

As a make-it-work plan, individual resistors will do the job.

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Mr. Reid,

Thank you for an awesome drawing and information. I left the unit at work but will post added information next week. In the meantime, however:

* there is an additional cap, .6 uf if I remember, that needs to be inserted in the drawing. It is hidden in the photo of the model.

* I don't recall what the resistance was but will get that information.

* There seems to be two (2) transformers on this unit. Actually, I think the top one is really a choke?

* I do not understand where the 5 volts for the filiment is coming from? From another tap off the main transformer maybe?

I will take photos of the underside so you can see how it is wired.

Thanks again!

 

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> where the 5 volts for the filiment is coming from?

As it should have been drawn: from another winding on the same core as the 110V input and the 180V secondary. (I didn't draw the core-lines long enough, sorry!)

In that connection, the 5V must be a separate winding.

Last night I realized that, with just one diode (the '01A is a triode which can be diode-strapped, but not a dual-diode), they could have reversed the diode, put it in the negative leg, and now the filament could be a 5V tap on the hi-volt winding. One less lead. While the cost difference is small, the simplicity may be compelling.

> there is an additional cap
> two (2) transformers  ... really a choke?

Yeah, there may be other details. However you say you have 250V DC. I assume this is at the external screw-clamp terminals. So the main power is working. The smaller voltages are not. They are derived from that multi-tapped power resistor next to the terminals.

Your readings do seem mostly consistent with an open-circuit inside this resistor. I'm not clear how several sections could be open, but age and tarnish can cause odd failures.

The make-it-work answer is new resistor(s). While a bunch of shiny metal-film resistors would be visually incorrect, maybe visual authenticity is less important. Or a string of 2W carbon-composition resistors would be very approriate as a "1940s repair". (We are fixing a 1941 Plymouth. There is a large welded patch on the frame from a long-ago accident: after cleaning and checking, we will leave it be as an authentic repair, rather than try to make this car factory-new again.)

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