Greetings to Dave and the Group:
?? I apologize for not getting back to y'all sooner, but other events have intervened.?? I started to post some intermediate results, but my post went off into la-la land and I didn't feel up to re-typing it, so I just waited for all the parts to get here and then did the shotgun thing.
?? I believe that I have solved the problem.?? So, I will pass along what I did; perhaps it will work for others.?? Also, I will enumerate the booby-trap that caused me to go barking up the wrong tree.
?? The first thing I did was look at the 6CB6 in the V1 position.?? I noted that the whole instrument was highly microphonic, so I chased that for quite a while before I compared the failed and good instruments and decided that the microphonics were characteristic of the design.?? I swapped V1 for another 6CB6 in the same instrument without affecting the noise or the microphonic behavior, so I went on to other things.?? After replacing all of the large red paper capacitors with modern 630 volt yellow caps from Just Radios, I discovered that the problem was still there.
?? I then decided to get out a reasonable number of 6CB6 tubes from stock, including another HP branded used tube.?? I then proceeded to try various tubes in the V1 position.?? It turns out that the 6CB6 used in the V1 position must be selected in test for proper behavior.?? The two HP branded tubes (the original and another used one from stock) exhibited the worst behavior.?? The best behaved tube was a used old-label RCA.?? However, none of them completely cured the intermittent noise spikes.
?? I then bit the bullet and completely rebuilt the range switch... not a task for the faint of heart.?? I used all new yelow film caps and replaced all of the carbon composition resistors with modern film types.?? Only the original HP precision wire-wound resistors and one Erie tubular ceramic capacitor were left in place.
?? After the range switch rebuild, I again tried tube swapping.?? The HP tubes were as usual... terrible.?? The RCA still exhibited a bit of occasional flicking of the meter needle, but after running the instrument for about 15 minutes, it settled down.?? I then went back through a couple of my other 6CB6's and found that some which I had decided earlier were still a bit too noisy actually settled down with a fair amount of "burn-in" time.
?? So... the instrument now appears to behave normally so all I have to do now is figure out how to calibrate it.?? The moral of the story is to obtain a half dozen or so 6CB6's and try them one at a time, allowing each to age for at least half an hour.?? If you can't find a tube that calms the instrument down, and you are certain all the large paper caps in the main chassis are OK, then gird up your loins and re-build the range switch.??
?? The big take-away, however, is that V1 MUST be selected in test from a reasonably sized pool of tubes.
Good Luck
--
Jim Thorusen
KB6GM
