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Old 03-25-2015, 02:47 AM
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Tubejunke Tubejunke is offline
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Accurate Instruments Model 257 Tube Tester

This is a tester that I used for a number of years due to it's capability to test old and newer tubes as well as vintage color and other CRTs. Like my other post on the Sylvania tester, I am wondering if anyone has owned one of these, but for a more important reason than curiosity.

I noticed a long time ago that I virtually never came up with a bad tube. A member here helped me out with some needed tubes recently that he had tested on a basic emissions tester. I got them and tested them on some high grade units such as a Hickok 6000A and a B&K 747B and nearly all the tubes were bad. Recently I dug this tester out and they all tested good, but don't perform well in circuit.

I have had testers with glitches of various types, but not one that tests most tubes good when either bad or good. I assume that there is some electronic failure going on which I am unfamiliar with. I really don't know the ins' and outs' of tube tester repair. Has anyone seen this condition before?
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Old 03-25-2015, 12:27 PM
Olorin67 Olorin67 is offline
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My experience with 50plus year old used unrestored tube testers is they cant really be relied on to tell you anything more useful than if the filament lights up or not, and Ive owned over a dozen of them, including several Hickocks. Or maybe i m just unlucky enough to get the ones that were stored for years in damp basements.
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Old 03-25-2015, 06:13 PM
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Must be the damp basements I guess. I firmly believe that duration of dormancy plays a huge factor in the chances of something coming back to life and actually doing what it was meant to do. Test equipment is no different than any other electronic device.

I elaborate on duration as I have had 50s TV sets that are completely unrestored and work perfectly and a ton of radios that are the same. I can't speak for the history of many units as they are older than me. However, some I got at a young age (about 30 years ago) and they were still being used at least periodically. In those years I have always made sure that everything I have that will work is powered up at least once a year. I could go on and on about the robust nature of some of the vintage electronics, but I will spare everyone.

We all know that the BIG mistake often made is finding something and plugging it in. At least I hope so, but it does still happen. I recently acquired a 40s Hickok that by all evidence (being paperwork and such) hasn't been used since the early 60s. Brought it up on my Variac and it works great. No restoration, calibration or any of the other things we hope we find no need for. Luck of the draw perhaps???
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Old 03-28-2015, 05:24 PM
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I use one of these as my go-to tester. Never had the problems you talked about...I think your particular unit is the issue.
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Old 03-28-2015, 08:13 PM
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Quote:
Originally Posted by AUdubon5425 View Post
I use one of these as my go-to tester. Never had the problems you talked about...I think your particular unit is the issue.
I agree. I am certain that the thing wasn't made like that & frankly I can't say for sure that EVERY tube tests good, but I notice that most do. That's when I break out the 'known good' units. Both a Hickok 6000A and the B&K Dynajet 747B. I read somewhere about a problem that will make a tester do what I have described, but I can't remember if it was the tubes or what.
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Old 03-28-2015, 10:28 PM
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Most testers that I have seen have a pot for calibration in the meter circuit (either in series or as a shunt). My first guess is if the tester is reading too high or too low on known tubes, then it should be re-calibrated.

jr
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Old 03-28-2015, 10:46 PM
Olorin67 Olorin67 is offline
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Anything that old, should be checked for failed capacitors, dirty sockets, controls and switch contacts, drifted resistors.. Some testers have bridge circuits, those must be accurately balanced for accurate readings. Just normal maintenance of these old things.
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Old 03-29-2015, 02:36 AM
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Found a schematic, and simplified schematic:
http://www.mzentertainment.com/studi...be_tester.html

This is just a very simple emission checker... there is no "cailbration pot" as I mentioned above, just a selected resistor. My guess is that it changed value or the "load" pot has changed value or is dirty or has opened up. not much else in the circuit.

jr
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Old 03-29-2015, 07:50 PM
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Oddly enough most all tubes check at about the same percentage. The meter movement is not jammed disallowing further deflection. I am going to try to find the url for, or cut and paste the paragraph I found describing this very condition.

There are a few really nice pages or sites on tube testers in general. The ins' and, well; no outs concerning the nearly lost understanding of transconductance and the measurement of micromhos'. There are dynamic conductance testers out there that are mistaken and thus often misrepresented for sale as a true mutual conductance tester. Supposedly Hickok held the rights to the "real deal" for a long time and other manufacturers were forced to come up with gimmicks.
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