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I've had my 707 for a while. I bought it used but completely rebuilt and it has until now calibrated just fine and worked perfectly. Lately I've noticed the meter needle being very unstable when doing the sensitivity readings, and tubes which had been checked in the past and are unused are now reading much lower - and again somewhat unstabl-ey. I went through the calibration procedure and was concerned when I got to the Balance adjustment. I put the Sensitivity dial up to 100 and it looked right from the start like it was okay (meter needle riding right at zero), but when I went to move the Sensitivity dial just to see what would happen the needle hopped around a bit but kept settling at zero, even when I had the dial set to 80. I don't remember this happening the last time it was calibrated. Doesn't seem right.
The actual sensitivity adjustment was fine - 1.504 VAC dead on and that was stable. Where should I start looking for the problem?
Follow Ups:
Checkin' in late... glad to hear things much better.
If it still acting a little off, there are a handful of caps and resistors that can be replaced. When I did mine, I think shipping cost more than the parts.
You might still want to pick up a 6BN8 or 2. I think I bought a pair or so. Not expensive if I recall. Also might want to pick up a couple of sets of #55 bulbs. Check eBay. Can often get a sleeve of 5 bulbs. The old ones are much better than the new issues. Don't know why, but I do remember reading this and indeed I found this to be true when I purchased NOS versus new ones sold at auto part stores.
Don't mean for you to go overkill on back up stuff. I think I am a bit of a hoarder, but it sure is nice to have extras on hand when things require troubleshooting. Whether it is a component tube or for the tester itself.
And of course listening and troubleshooting usually end up happening on a weekend. When parts cannot be bought.
Cheers!
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
Did you check the 6BN8?
jsyrm
No, but I thought the 6BN8 was pretty much just for the grid emissions part of the tester. That (along with everything else) calibrates out fine.
When my B&K 700 needle was wonky during the sensitivity calibration, it turned out to be capacitors. I just replaced them all as there aren't that many.
As yours has recently been rebuilt, you might first try cleaning all the control and switch contacts. It might be as simple as that.(Make sure unit turned off and capacitors have had time to discharge!)
I used a zero residue contact cleaner on mine. (Of course debates run wild on which cleaners best.)
Also clean your tube pins, sockets.
Don't forget to check for any loose wires and cold solder joints.
I am not a technician, but forum folks helped me with mine. You may want to search threads of mine. Also "Big Harry" threads. Either with B & K tube testers. He was a great help.
Also, I eventually bought spare tubes for the tester. Good to get them when it is working well and you can actually test them! Sometimes it is a tube that needs replacing in the tester. (Otherwise, my goodness, we may have to buy another tester to check our tubes in our tube tester!)
Sorry couldn't be more specific. Maybe someone else can chime in?
Cheers!
Jonesy,
"I know just enough to get into trouble. But not enough to get out of it."
Also clean the contacts on the two bulbs (#55) in the grid signal circuit - I have heard some recommend SOLDERING these into the circuit.
Well, I cleaned the #55 bulbs and sockets and also did a SS replaceent for the #83. That, and liberal cleaning of the switch contacts, and a recalibration and things seem a lot better. And it's nice the tester no longer heats up like a frying pan with the #83 cooking away.
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