Crawls Backward (When Alarmed)

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Volume, give me more volume! (Dyna PAS Continued)

Since I'm in for a whole rebuild now, I'm going to replace the old volume potentiometer in the PAS with a new Alps RK-27 "Blue Velvet" pot - real popular in the DIY audio world.  I figure I'll try one out.

The value of the stock Dynaco pot on the schematic is 250K (it's a two section pot).  I knew mine had a 500K pot - it's is a PAS-3X with the special tone controls.  I thought for years that maybe the -3X had a 500K pot as part of the changes, even though I don't see it in the Dynaco documentation for upgrading a stock PAS-3 to a -3X.  Of course, it never occurred to me that someone might have replaced the volume pot in mine at some point.

Turns out that's exactly the case.  In fact, I was surprised to see that the pot in mine is an older Alps pot!  It was working fine - the only reason I was taking it out was due to the "wrong" value...and to upgrade to a new Alps pot.

So, I'll put this one in my stash of old pots for now.

It took me a bit of searching, but I was able to find a 250K two-section Alps pot.  Then I read that the Curcio phono mods have the best spec with a 100K pot!  Oh well...according to Joe Curcio, the difference is only .2 db...mine will follow the RIAA curve plus or minus .4 db.  I'm not going to lose any sleep over that.  I wish I had known though as it is a lot easier to find a 100K pot.  What-ever.

The new pot has one of those 'locator' tabs on the front as so many do.  Usually you can snap them off with pliers, but the one on my new pot is pretty heavy-duty - I had to resort to the 'ol X-Acto razor saw to remove it.  You need to take the tab off so the pot will lay flat against the front panel.

Here's something you'll undoubtedly face when replacing an old control like this in an amp, radio, etc.  You need to cut the shaft to make it match the old one.

I learned this trick from "Elements of Radio Servicing" by Marcus and Levy.  Just line the old and new up, mark and cut.  I usually leave a bit extra and then fine tune it with a file...easier to cut more later than to try and put it back.

Side note on the Marcus and Levy book.  There were three editions - I'm not sure of the first, but the second and third were 1947 and 1955.  The last two are the most common and the most useful.  I can't say enough about this book.  If you do old radio stuff, this is hands-down the best resource bar none.  William Marcus is a tremendous technical writer - he also wrote another classic, "Elements of Radio."  The Servicing book was written for technicians, and it's very clear and has all kinds of helpful and practical ideas.   There's even a section in the back with suggestions on how to set up your workbench. 

So now ve cut zee shaft wit de razor saw, ja?

And then we do a test fit in the chassis.  I did wind up taking a bit more off the shaft.

Looks like we have a winner.


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