B&K 650 tube tester


2013-02-24 12:48 pm
Hey folks: I am restoring a 650 tube tester and am having some issues. Namely that the schematic, and parts list, and the tester itself, don't all match up so as to enable me to restore the tester accurately. :headbash:

R10 Not my biggest problem but. In the tester unit itself I found it to be degraded and almost double the value called out on the schematic and the color code on the resistor. Then while checking I find that a service update calls for testers previous to the 6000 SN to have the value changed from 100K 10% to 120K 5%, OK, and the parts list calls it out as 120K 5%. But in the tester unit itself (SN 202934) it has a 180K 5% resistor that had drifted to 223K. I evaluated the circuitry and opted to replace it with a 180K that tested at 179,023 ohms, well within 1% value. That is the goal I am attempting. I would like to bring every component within 1% and then a 1% accuracy in calibration should yield a tester that could discern mutual conductance close to 1% for matching tubes for stereo applications and finals in ham radio gear.

The second problem I have ran into is a little more perplexing in that while reading the service bulletin on R10 it states that R11 should also be changed from 22K to 27K. Now the real problem. The parts list calls out R11 at 27K but I can not find any R11 on the schematic. I also can not find any 22K or 27K resistor on the schematic or in the tube tester unit itself - ? ? I have a feeling that this will not be the last problem I run into on this restoration since I am only about half way through it. So I decided I should go in search of an expert on the subject. So here I am, looking

Any help would be greatly appreciated.