Great article - now I know that I am educated beyond my intelligence. I have an old Precision 920 that I always thought was an emmisions tester ( I bought it from Antique Electronic Supply when they still let you browse through their warehouse). Now, I'm not so sure. I checked the manual and it does say that the unit is a Mutual Conductance tester.
Can somebody help me out with this? Thanks

Many thanks, Tom, and congratulations on your 920. I'm not familiar with it, but I'm glad you were pleasantly surprised. I'm even more glad you have the manual, for they're not always easy to find for older testers.

If it needs reconditioning, all the standard procedures apply, i.e., power supply overhaul/verification, recapping (especially filter caps), resistor check and replacement of any that have drifted off tolerance, switch and socket cleaning etc. A schematic should be in your manual. If not, asking here is the best way to come up with it.

Good luck with it .

Larry

Is the Precision 920p a Mutual Conductance or an Emissions tester? Thanks

If its manual says it's a mutual conductance tester, I'd go along with it.

Larry

I wasnt able to pick up my Knight Tester until Sunday night. Here it is, looks brand new with minimal wear. Now to familiarize myself with it. More learning curves.

Congratulations! It looks fine, and I'm glad you got all the documentation. I see he even gave you one of those little resistor color-code calculator gadgets. I learned color codes on one of those, but I haven't seen one in several decades. Glad to see that some are still around!

Good luck with the tester. Stick with the manual and it should be easy to learn how to use. Let us know if you have any questions.

Larry

Precision's copywriter was prone to exaggeration. All Precisions are dynamic emission testers (as were Jacksons and the Eico 666/667). They apply proportional voltages to the tube elements and measure plate current only, but the control grid is positive, and the plate meter responds to total DC, not just AC.

Many thanks for clearing that up, Alan.

Recapped and tuned up my Weston 981 last night, all OK. Pix pending...

Larry

Inside the Weston 981 (Series 3, first edition). It might have been laid out with a service tech in mind.



I haven't done a thing to "tidy things up" here. It came from the factory as immaculate as you see it now. What kind of assembly job must it have been, with all those connections to all those sockets, terminals and switches!

The roll chart is the 1954 original. I later picked up a 1960 roll chart laid out as sheets in a booklet, along with a booklet of "obsolete" tube test settings that didn't make the original roll chart. The paper of the roll chart feels waxy to the touch (and looks a little "waxy" in the pictures).



All the caps are mounted in this terminal block where they can easily be reached. The entire job of recapping took no more than half an hour. All the original resistors in the terminal block are still present. Small transformers are mounted to either end of the block. The calibration adjustment pots are mounted on top of the block, and so is the 3A4 signal oscillator tube.

There are several silicon and selenium rectifiers in this tester. The two yellow cylindrical objects with red stripes are small selenium rectifiers. As with the two small "standard" seleniums off to the left, I chose to leave these alone. I've never been fond of the things, and I tend to update them when I consider it wise, but since these are performing up to standard, I prefer to keep them in place.



This picture gives a good view of the three calibration pots and the 3A4 tube. It doesn't look like it's completely into its socket, but it is.

The earliest 981 Series 3 tester had an overload lamp (#47 bulb paralleled with a 6.5-ohm resistor). The light glows faintly when almost any tube is under test, and glows more strongly with tubes that draw more plate current. The parallel resistor (a sandbox-style power resistor) is visible under the right-hand calibration pot. The lamp fixture is hidden beneath it.

The overload lamp was done away with in later refinements of this tester.



A view of the tester's power transformer. A few small resistors spill off it onto test socket row, but since they measured up to specs (like those on the terminal block), I let them be.

Like the two small transformers on the terminal block, this transformer appears to have been "doped" (weatherproofed?). All three transformers look as they came straight out of the box.



By comparison, the little SICO emission tester's interior is very simple, only a handful of resistors and one small cap. No rectifiers, operates purely on AC. There are no calibration adjustments either; the only way to keep this tester "honest" is to keep its fixed resistors dead on the mark.

Two more projects crossed off the list, but now it's on to worse ones...

Larry

Bump. Weston 981-3 service topic current.

L

FWIW Department: It seems as if my Weston 981-3 is an original "3." It doesn't have the ballast tube that later refinements of the "3" had. It does have a "3a" roll paper chart in it, but that must have been added by a prior owner.

Larry

Great overview for us noobs! I've noticed that I don't see stickies here on the forum. If there is the ability to make them here I would recommend this thread as a sticky.

I got lucky and picked up an EMC-211 at a local electronics shop for a song. Already found a bad tube in a radio I've been troubleshooting. Really saved me some time. It's pretty basic but will handle all my AA5 needs until I upgrade to a better one.

All thanks, Deadshort. A lot of work went into this thread, and not only my work by any means. If it could be considered for a "sticky," I'd be honored.

Thanks again and best regards,

Larry

But it is a pity that the desire is to reproduce the test conditions used to produce the tube datasheets in none of the previous mentioned testers will be met.
So results cannot be compared with the factory manuals.

Jard N.

I daresay that not many of us will lose any sleep because of that. I'm also fairly certain that not many of us would be willing to pay for the sort of equipment that could reproduce factory test conditions.

Larry

I agree,although some people do their best to make the ¨high tech¨ cheap.
http://www.dos4ever.com/uTracer3/uTracer3.html

Jard N.

Oho, another spammer with a stealth commercial...

Meanwhile, mox nix.


Larry

Bump. Still got plenty of noobs looking for guidance.

Larry

Larry, i have enjoyed this very much. I`ve been on the board several years and have yet to fix my frirst radio, but learning has been very enjoyable and one of the main tools is knowledge. Another is experience, Im having cataract surgery a little later this year and hope to gain this when I can see better.
Thanks
Donnie Mitchell

Larry,

This was a good presentation for me. I just picked up my first tube tester, as I'm just getting into this - again. I started out years ago with ham equipment; got out of it for a long while due to "life". But, I have a few tube things on hand - Philco signal generator, Eico capacitor tester, now a Hickok 51X tube tester, Philco 41-300 radio - and a bit more time now that the kids are grown, almost out on their own.

Basically, I'm having to re-learn some stuff by making my self go through certain electrical theory, tinkering on the bench, an reading anything I can. There is still a lot for me to relearn, but I'm taking my time and making sure I understand the basics again before diving in too deep on a project.

There is a lot I don't know, but presentations like the one you made is very much appreciated by someone like me who used to be active in tube radio and has been out of it for a long while.

Thanks, very much

Tim