Here a bit of info... Just to share some symptoms and to pass on a little trouble shooting solutions with you guys... for what it's worth.

Some many years back 1967-68, I got my ham license and a good pal let me use his National SW54 receiver.

Recently when I saw one come up on Ebay I bought it for $46.

I saw on the schematic that it uses the newer type (read lousy) silver mica slug-tuned crappy IF cans. So I was prepared to have to rebuild them.

Luckily however, for my sake, this early model uses the normal standard IF cans with trimmer caps... yeah!

The parts list shows that either one type or the other were used in this model year. ... that's nice.

Anyway after re-capping, cleaning and aligning this nice little multi-band receiver... and just before placing it back in its cabinet... I noticed an irritating ... serious degree of steady..loud, spurious static bursts.

If I didn't know better I'd swear it was the dreaded silver mica disease. Same exact loud crashing static bursts.
...but how could that BE???
Well ... it can't.
Ok ....so then what?

In tracking down the source of the noise I first grounded the grid of the 6BE6 input RF amp and the noise stayed the same.

That proved that the noise was not external RF noise entering the radio... no.. it was coming from inside!...

So then I grounded the grid of the 12BA6 1st IF amp... and then it was silent.
Ok that meant the noise is happening BEFORE this point... but where?

Alright.. so what is in between these two tubes? Only the IF can.

If I put my 10x scope probe on the grid 12BA6 grid... it seems to be capacitively filtering out most static but also loading down the signal too.
But I didn't see any obvious noise spikes.

So on a lark... I decided to just shotgun it a bit.

So I swapped both the 12BE6 and the 12BA6 with new tubes and the trouble was gone.

So as soon as I returned the old 12BE6 RF amp tube.. all the nasty crashing static returned.

Ok... but why?

So I tapped on the IF cans and the static got worse. But it was NOT the IF can... nope...
It was the 12BE6 acting micro-phonic!... Sure was.

Taping on it even slightly caused all the same static.

So I replaced it again... and now.. no amount of tapping anywhere causes any static at all.. all quiet.

Simple fix.. curious problem.
.. so I don't really know which elements in the tube were shorting.. but somehow they were intermittently interacting.

(My 1940s era Precision model 10-20 tube tester did not show any problems with that bad..(micro-phonic) 12BE6 )

Here's the print:
http://www.nostalgiaair.org/Pa ... 012433.pdf

Here's the SW54 print:

http://www.nostalgiaair.org/PagesByModel/433/M0012433.pdf

:Here's the SW54 print:
:
:http://www.nostalgiaair.org/PagesByModel/433/M0012433.pdf
:
:
:
:Hi! Peter,
Did you do a noise test, on the bad tubes using your tube tester , to see if it would show up that way?
Bob Masse

::Here's the SW54 print:
::
::http://www.nostalgiaair.org/PagesByModel/433/M0012433.pdf
::
::
::
::Hi! Peter,
:Did you do a noise test, on the bad tubes using your tube tester , to see if it would show up that way?
:Bob Masse
:
How?

:::Here's the SW54 print:
:::
:::http://www.nostalgiaair.org/PagesByModel/433/M0012433.pdf
:::
:::
:::
:::Hi! Peter,
::Did you do a noise test, on the bad tubes using your tube tester , to see if it would show up that way?
::Bob Masse
::
:
:To be honest Bob.. I didn't know there was a noise test..
:How?
:
:
:Peter,
Do you have the manual for it? If not I can email it to you.It's in there.
Bob Masse
:
:

Of course all tube testers are different, but even if you have a simple emissions tester like my EICO 625, there are ways of performing element to element sorts tests. These tests are not performed during the normal testing procedure, and so you must go through the extra efforts of testing for them. However, especially with emissions testers, element to element shorts can go unnoticed, since often many elements are tied together within the tester, and so only these extra tests will reveal these shorts.

T.

Probably the best test, as Norm and Doug would say, was just what Peter did. Test it in the radio until you're satisfied with the results.

marv

:Of course all tube testers are different, but even if you have a simple emissions tester like my EICO 625, there are ways of performing element to element sorts tests. These tests are not performed during the normal testing procedure, and so you must go through the extra efforts of testing for them. However, especially with emissions testers, element to element shorts can go unnoticed, since often many elements are tied together within the tester, and so only these extra tests will reveal these shorts.
:
:T.
:

Actually, some element-to-element shorts can cause damage to a radio. If you suspect a tube of this condition, it's best to check it with a neon shorts tester, first.

Thanks all...
Yes, I do have the manual.
.... lol.. never read it though.
I'll browse it now to try to get ed-ju-ma-kated on "noise" testing.

:Thanks all...
:Yes, I do have the manual.
:.... lol.. never read it though.
:I'll browse it now to try to get ed-ju-ma-kated on "noise" testing.
:
:
:
: Hi! Peter,
Your Paco 10-20 tube tester is from the late 1940's or early 50's.It is more than a simple emission tester.On page 15 &16, the manual tells about the principles of electronamic tube testing.Also page 8 tells why and what to do about tubes that pin the meter.I was testing used tubes with a Hickok 600a and a Paco 10-20. I found that tubes that tested weak on the 600A, even after extensive warm up and retesting, could be rejuvenated on the 10-20. What I noticed is that the reading on the 10-20 would climb during testing. So I let the heater warm for several minutes and retested the weak tube a couple more times which resulted in a reading of 80% of scale on the meter.I then retested the tube on the Hickok 600A. The original reading had been 1100 micromhos and the new test reading was 1700 micromhos. I tested that tube, the next day, and it was stable and again producing the higher readings,immediately. This happened with several tubes.So there is something special about the circuit in the Paco 10-20.I bought it just for the noise test capability, but it turned out to be a very good and well built tube tester.
:Bob Masse
:
:

:Probably the best test, as Norm and Doug would say, was just what Peter did. Test it in the radio until you're satisfied with the results.
:
:marv
:

:
:Hi! Marv,
That method of testing a tube is disputed by Hickok.It only works well if everything else is correct in the radio.A new tube can compensate for a resistor, that is out of range on the high side, without the old tube being bad at all.
:Bob Masse
:
:
::
::T.
::
: