EMC 206 Tube Tester with o/c transformer winding

[Technomaniac]

Hi all. I have an EMC tube tester which I have persisted in using, though I have data for testing only tubes popular in the US, which means I am constantly having to generate new test data for Australian and European tubes. However recently when trying to use it I was unable to get the tubes to heat up. The nearest schematic I have ever been able to find is for a model 200, which is a little different from the 206. However by patient tracing with pad, pencil and ohmmeter, I have concluded that I had two problems, the first being the "GAS" switch, a two-position slider, had a contact resistance of about 62 ohms. Since the tester came with a brand-new NOS spare, that suggested that the previous owner may have had trouble with it. So I replaced it, half-expecting the problem to be solved, as it is in the heater circuit, one contact of it connected to the end of the heater winding of the power transformer. all heater current being through its contacts.
But the problem persisted, and further checking revealed that the power transformer heater winding is in fact, open-circuited. This is the winding with MANY taps, to supply a large range of heater voltages to the tube being checked.
The first tap on the winding is for 0.75 volts. The winding between this tap and the common end of the winding appears to be open.
I searched for a replacement transformer. The part number appears to be 903P1. I DID find a tranny for an EMC tube tester but it didn't look too flash in the photo, and seemed to be a lot simpler than mine.
Not too keen to start unwinding the brute, I put the brain in gear to come up with an alternative.
I have on hand a 240 to 6.3,7.5,9,10.5,12.6 and 15 volts multi-tapper. If I was to use the part of the secondary between 7.5 and 9 volts, I will have 1.5 volts available. But if I use the tranny on 110 volts, I will have 0.75 volts, which is the required voltage. The tester is 110 volt operated.
Then, if I take the end pigtail of the original transformer from the "GAS" switch and tape it up, and connect the new transformer's 0.75 volts between the original transformer's 0.75 volt tap and the GAS switch, I think we might have something that will work. I will have to connect it in the correct phase, or all other voltages, other than .75 will be out by 1.5 volts.
Does anyone have a BETTER idea?

 

[Bluejets]

Are you certain the winding is open..??
Seems for the heaters the winding would handle a couple of amps at least at a low voltage so not usual for the winding to go open.

 

[Technomaniac]

Hi, Bluejets.
I have just proven my theory wrong. I seem to be on the end of a separate and isolated bias winding.. With say, lever switch 4 down to position S to identify pin 4 of any socket as a heater pin, I reckon I should read with an ohmmeter, the winding of the tranny across the sockets between pin 4 and any other, as all other levers are set to the starting position K.. I do not, and it is for this reason that I am assuming that I still have a problem. So I need to retrace my steps. I'll report back.

 

[73's de Edd]

Also . . .are you working with the benefit of THIS supplementive information . . .?

https://www.mzentertainment.com/studio_workshop_test_equipment_emc_model_200_tube_tester.html


73's de Edd . . . . . .

 

[Technomaniac]

Yes thanks, I DO have that schematic. What do you reckon happens to the bottom end of the multitapped heater winding?
By the schematic it appears to go to one of the busbars (K) which links the lever switches. When I have lever 4 down, which you would do if you are testing a valve with heater between 4 and 5, and all of the other levers on K, I have the necessary connections between pins 4 and 5 of (any) socket and two busbars across the lever switches. One of those busbars connects to the common of the heater voltage switch, so whatever the voltage selection is, the tranny gets its appropriate tap connected to that busbar OK, but the other busbar needs to get to the bottom end of the tranny, and I can't see any path in the device, though I can see it in the model 200 schematic. I haven't identified the cotton insulated tranny lead that is the bottom of that heater winding. There are no tranny leads going to that lever switch busbar and I don't know what might be in between.

 

[Bluejets]

A couple of photos of what you can or can't see might help..

 

[Technomaniac]

This is three dimensional circuitry, you can't lift the power transformer very far to see what's underneath, and the wiring is crammed, needs haemostat to pull on the wires to see where they go. It'd be nice to have the tranny out to run a meter over the leads, but the ends are through the hole and twisted needing a lot of wiggling to remove a wire. With the large number of wires on the tranny, the odds are that something would get broken. Here are photos:

 

[Technomaniac]

I think I may have solved the problem. The NORMAL - SPECIAL switch has to be in SPECIAL position, apparently.
But If I refer to the EMC 200 schematic, there are three transformer wires shown as connecting to the K rail across the lever switches. I have in the device, three cotton covered wires from the transformer all connecting to the back of a lampholder, and a skinny wire from there to the K rail of the switch. According to me, two of the three windings are o/c. So there must be a difference between the models which is causing my confision.

MEANWHILE, it's probably all good, and for the moment at least, CASE CLOSED. Thanks to participating forum helpers.