Vintage Geiger Counter with Deketron DK24 displays

[TheFloyd]

Hi folks,
I have recently added a lovely old device to my vintage collection.   It is a Timer-Counter (Geiger Counter) made by Watson-Victor LTD, in what I assume to be the 50's/60's.
After some initial inspection and measurements, I brought it up carefully on my variac and was delighted to see that it worked perfectly.

I set to reverse engineering the circuit, taking multiple measurements along the way, with the firm assumption that all of those old electrolytic's would be dry and in need of replacement.

this is where it got interesting, and I ended up with a growing list of questions that I would love some expert guidance on.

The circuit is driven by two Toshiba 12AT7 and one 12AU7A valves.   From what I can initially tell, one of the 12AT7's is for amplifying the audio crackles and the other for amplifying the output from the detector tube.   I have not fully investigated the function of the low gain 12AU7A in the middle yet.
The front of the device includes three decade counters, that are using Okaya Musen DK24 Deketron counting tubes.    These wonderful things are the first I have ever seen in person, and their operation is fascinating to me.

Where I need help:

The power transformer, is an A&R 6069, that has a primary winding arrangement that I have not see before.    I have been unable to find any documentation on the 6069 transformer unfortunately to better explain it.
- the primary has the normal 240v black and red wires, and it also includes a single green wire.    I initially assumed that this was some kind of primary winding tap, but it is not.   The green wire is open circuit from any other wire on the transformer (both primary and secondary) and is NOT connected internally to the transformer chassis.    This had me puzzled, especially after I saw that when it connected to the PSU board, it immediatly joined to one lead of that I assume is the secondary heater filament winding.    I didnt even know how to draw this on my circuit diagram, until I found an ancient schematic for a circuit using an A&R 5776, which showed the green wire like a dashed line next to the transformer core.   I am assuming this is the same as what my 6069 has?

- - can anyone tell me what that primary wire is called, and why this power transformer specifically depicts the connection to chassis, and to one wire of the secondary, like this?


The rectification part of the circuit uses two ancient "Top Hat" style diodes that are made by Toshiba and have a part number of 1S94v.    I can't find any datasheet for these either.    They measure reasonably for silicon at 0.5v and 0.52v.
- I am not sure if it is just the way that I have laid out my schematic as i traced and measured the circuit, but I am unfamiliar with the topology of the rectification stage, and frankly, even the way to reference voltage measurements through the circuit...    for the sake of consistency, all measurements I made, are with respect to the earth/chassis. (green wire)


So far, most of the values are measuring ok, and the system operates.   Other than the poor dual-electrolytic where one of its 16uF 500V halves, has 565v across it.    I will need to get that under control before i pop it.    the Cap itself is measuring 17uF on both sides, which is far better than i would have anticipated!



I have not had the time to reverse engineer and draw the schematic of the main board yet, as I would like to get the PSU voltages all corrected first, so that I dont skew my measurements.
I would love to hear thoughts on the transformer, the diodes and the circuit design in general, as it is a bit of a mystery to me so far and I cant find any service manual or technical documentation on the device at all.   it is called "Counter-Timer" "Model-4DSPT10".

Thanks In Advance,
Floyd

[TheFloyd]

Here are some pictures of the device, and the first draft of my schematic for the PSU.

[TheFloyd]

I have just found some information that relates to the use of "electrostatic shielding" within a transformer connected to a wire that you hook to ground.   The purpose is to reduce common-mode noise of specific frequencies of 140dB+.
Looking at the schematic symbol for these transformers, and the description of their external wiring, leads me to thinking that this is what the A&R 6069 transformer is doing.

Fair assumption ?

[Analog Kid]

Don't see what else it could be for.

[gbaddeley]

Yes, it’s an electrostatic shield, a layer of foil between the primary & secondary windings that has a gap or insulation so it’s not a shorted turn. Can you see if the green wire comes out of the bobbin between the windings?  It normally connected to chassis / mains earth.

[floobydust]

An interesting mix of Australian and Japanese components. PCB marking "IU3 KEMP" but no leads.
I would add a mains fuse.

The voltages seem high, as if mains voltage is way up there +10% than design for the 1960's would have. It's a voltage doubler? Who is getting 638VDC?!
Have to look at the Rodan DK24 datasheets, it seems to require 360V. Maybe it's for an OG-3?

I could find no data on the A&R PT6069 transformer but assume the extra wire is for an electrostatic shield, usually see that for CRT scopes and sensitive instrumentation.

Those five blue electrolytics (Philips?) I would outright replace, the bung shrinks and dries out and they short or go low value.
A pic of the power board solder side and we could verify the PSU schematic.

[TheFloyd]

Yes, it’s an electrostatic shield, a layer of foil between the primary & secondary windings that has a gap or insulation so it’s not a shorted turn. Can you see if the green wire comes out of the bobbin between the windings?  It normally connected to chassis / mains earth.

Thanks for this, I suspected it was.  I don't know how i have never come across a transformer like this before.   I guess the sensitivity of the detector in the geiger counter, is reliant on reliable and clean power, to prevent false readings etc.

[TheFloyd]

Unfortunately I cant clearly make out where the green wire goes inside.  (attached image of the underside)

[TheFloyd]

Thanks for your thoughts.

An interesting mix of Australian and Japanese components. PCB marking "IU3 KEMP" but no leads.
I would add a mains fuse.

Oh Wow!   I read this, and it just dawned on me that this device doesnt have any fuse protection at all!    I will absolutely be adding one of those, well spotted and good call!

The voltages seem high, as if mains voltage is way up there +10% than design for the 1960's would have.

I suspect that voltage is so high due to some wildly out of spec components!

Have to look at the Rodan DK24 datasheets, it seems to require 360V.

I had spotted this webpage, and the image of the DK24 shows the exact device that I have, rather than the taller DK23 in the datasheet.   They are probably electrically similar for the most part, other than the differences listed in that DK23.pdf

I could find no data on the A&R PT6069 transformer but assume the extra wire is for an electrostatic shield, usually see that for CRT scopes and sensitive instrumentation.

Good to know, and it probably explains why I have not come across one of these before, as while i have pulled apart mountains of old valve gear over the years, I have never worked on an old CRO or other sensitive equipment.

Those five blue electrolytics (Philips?) I would outright replace, the bung shrinks and dries out and they short or go low value.

Totally agree!   Just need to source some appropriate replacements and get it done.

A pic of the power board solder side and we could verify the PSU schematic.

Thanks for this, I have attached some more images, including one of the underside of the PSU.    I have to say, that arrangement of rectifier diodes and electrolytics has me scratching my head...
unfortunately I might be a tricky task for you to follow the schematic from the images, because they have packed the components so closely together, that the legs on the capacitors often dont go directly through the board below them.

I cant wait to hear your thoughts on the operation of this circuit!