B&K 700 meter specs?

[Ted Bruce]

Hi all,

I recently bought one of these tube testers in partially-working condition.  The meter is messed up.  It responds to current, but when it reaches a certain point, close to half scale, it "flops" all the way over to past full scale.  I can also manually "flop" it by hand, back and forth.  I tried adjusting the upper coil spring, but somebody had been in there before me, I could tell.

This has made it difficult to determine the meter internal resistance and full-scale reading using the series/parallel potentiometer method.  Does anyone know what the full scale current is?

This is interesting...I may have gotten lucky:  I took the meter out of my old Knight 600 and tested it.  It checks out at internal resistance of 100 ohms, series resistance for full scale about 13.2k, and dividing 12.6 V DC by 13.3k gives ~1 ma full scale.  It is the identical size as the B&K meter, and it fits the mounting holes perfectly.  It is obviously lower cost/quality, just looking at the magnets and coil assembly.  But, I tested some known good NOS and other tubes, and the meter moves to the position corresponding to the proper gm range for the tubes (2 6CB6's NOS and several used 6146's).  These tubes use the lower full test section of the tester.

Could it possibly be that the B&K meter is 1 ma full scale also?  If this can be confirmed, I'll just scan the B&K faceplate and tape it onto the Knight meter until I happen across a B&K replacement or get somebody to help me design some kind of PIC microcontroller-based LCD readout thingy.

This will be a continuing project.  The previous owner replaced the electrolytics and a few of the resistors before apparently giving up. I don't get much of any kind of reading on the upper quick-test section of the tester.  A tube inserted in Socket #1 just about falls out of it.  The switch assembly is a mess.  It looks like it must have cadmium plating, because there is white powder all over it.  Looks like I'll have to  remove it completely and give it an ultrasonic bath.  My chemical spray didn't help.

If anyone wants to join the fun, I will certainly appreciate your comments.  Especially you, Brian, since I'm one of your customers (wink-wink, nod-nod).

Thanks,

Ted  KX4OM

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