Crawls Backward (When Alarmed)

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Out with the old, cont'd

I'm looking for a cliche or a line from a song to describe where we're at in the process - but I'm drawing a blank. Something about "you gotta get rid of the old before you can build anew" or the like.  For some reason all I can think of at this moment is the beginning of "Helter Skelter"

When I get to the bottom I go back to the top of the slide
And and I stop and I turn and I go for a ride
And get to the bottom and I do it again....

But that really doesn't fit, does it?  Hey, as an aside, there is (when I last looked) a video of Paul McCartney rehearsing an acoustic version of that song during the White Album sessions...crazy.

But back to our Dynaco PAS rebuild/upgrade project. 

Above you can see the front panel with the old wiring still on it.  I'm taking all of that off as part of the process.  The tone controls, the high/scratch filter switch and the loudness control will all be removed from the circuit.  I will leave all the hardware, but electronically they'll be gone.  So it's back to the old snip, snip and cutting-off-of-leads.

Here I'm not bothering to clean up the old solder joints since there won't be any new components going to the pots or switches (except the tape monitor switch).  This makes this destruction part go a lot faster.

I've decided to keep the Blend Switch in the circuit.  This is a fairly unique circuit that Dynaco incorporated into the PAS preamp.  Here's a way to visualize it: 

Think of the stereo spectrum (two speakers) in front of you.  If you had the channels separated to maximum separation, then gradually decreased the separation, you'd eventually end up with a mono signal in the 'center' of the two speakers.  This is exactly what the Blend Switch does.  

I should add that it also has a couple of extra functions for the obsolete "A" and "B" inputs - it allows one to have a mono signal (say, from a mono FM tuner) plugged into the "A" input appear in both channels, or a mono signal (say a mono AM tuner) plugged into the "B" input appear in both channels. 

I'll still have this ability, but it's not likely I'll use it.  I mainly am keeping the Blend Switch so I can "narrow" the stereo signal on certain playback material, or use a mono setting for old mono records.

Since my old Blend Switch is a bit funky, I took a page directly from Bill Thomas' PAS rebuilds.  Bill has done amazing work and documented it on Audio Circle. 

What I did was remove the switch, and take it outside and clean up the whole switch wafer with naptha.  Naptha is a good cleaning solution that won't harm the phenolic material the wafer is made of, and it evaporates quickly.  This took the grungy stuff off the switch.

Then I used an old toothbrush and the trusty Dexoit to clean up the contacts on the revolving piece of the switch itself.  Signal goes through the switch and the contacts, so it's important that it not be corroded.

You can see the end result on the right.  Nice and shiny.  Bill says the contacts were silver-plated, and mine sure looks that way.

A couple side notes:  you'll see this type of switch in tons of old American tube gear - I am most familiar with them in radio bandswitches.  The naptha and Dexoit cleaning is something I always do.  It's not always possible to get the bandswitch out of the chassis, but with patience you can get to the contacts and clean them up. 

 
 
 
 

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