Weird Ranger Problem

[W3UTD]

Finally got the rig on the air, and found something strange quite by accident.  When I grabbed the mike (D104) I accidentally tapped the head against a shelf, and I heard started some sort of buzz from the right side near the 1614's.  I can also hear it coming from the mic element, and the meter (in Mod position) pegs at 140

If I rotate the Audio control back to zero, then up again, it goes away and resting Modulation is back to about 60.

I can talk and whistle etc with no problem, but a tap or a loud clap and off it goes again.  And it happens in both VFO and crystal positions.  I've looked for grounds or shorts back through the mic wiring but nothing yet.

Any ideas?

[WQ9E]

Sounds like instability somewhere in the audio chain.  Is the Ranger stock or has the audio been modified?  Is this happening into a dummy load or only when connected to the antenna?

[W3UTD]

It happens on both antenna and dummy load.

To the best of my knowledge, it is stock, but most of the larger electrolytic caps had been replaced. 

The only things I can think of, audio related, that are different from a bone stock ranger are:

C52 (500pf) replaced with .02uf
C53 (200pf) was gone when I got the rig, but I didn't put it back
C59A/B (15uf ea.) replaced with 22uf each

C50B (10uf) was a straight replacement, but C50A (also 10uf, which should parallel C54) was not there, so I did put it back.

Maybe there was a mod that removed C50A at some point?

[WQ9E]

It looks like the following was done to your rig:  http://www.amwindow.org/tech/htm/ranger.htm

Somebody who has done the mod is going to need to chime in because mine are both stock so I am not sure how the modified circuit behaves.

[W3UTD]

Rodger, you're "da man" again.

I backed out my replacement of C50A, then did what the warning suggested "0.005 across the driver transformer with a 0.001"

Bingo!