Crawls Backward (When Alarmed)

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Out With The Wacky Mod

Here's the inside of our victim...er...preamp.  If you have a PAS, the looks familar to you.  If you don't, well, let me give a quick guided tour.

The two brown rectangular things in the middle of the chassis are the printed circuit boards (aka "PC" boards).  The one to the left is "PC-6" - it's the phono section preamp.  There are three low level inputs on a stock PAS -Phono, Tape Head, and Special.  The thinking was that in addition to the phono input, there might be a need for monitoring a tape record head directly (as in a record head I would imagine), or something like a microphone or similar.  Today we don't have a need for all those low level inputs - and we'll be changing two of them to high-level as we go.

To the right of PC-6 is "PC-5" - the line level and tone control amplifier board.  The high-level inputs for FM, AM-FM, and Spare are amplified here; this is also the second stage amplifier for the low-level inputs.  The tone control circuits are also (partially) here.  I'm going to bypass the tone control circuit, so some parts are superfluous for our build.

Both the PC boards use two 12AX7A twin triodes - one for each channel - for voltage amplification.

Behind PC-5 is a selenium rectifier and two 2000 uF electrolytic filer capacitors for the tube filaments - AC filament voltage off the power transformer is converted to DC for the filaments so they'll be quieter.  To the right of the rectifier and caps is the power transformer, and in front of the power transformer is the Mallory can filter capacitor.  We'll be replacing the can, the selenium rectifier and the filament electrolytics all in one shot with our SDS power supply board.

Located on the far left of the chassis is the selector switch.  This switch sends the input signal - phono, FM, etc., to the appropriate place on the PC boards.  You can see it's an old-style open-frame bandswitch made of phenolic material.  The connectors are on the switch wafers and all the wiring goes to the appropriate tabs on the switch.

I personally dig these things because they represent the intersection of mechanical and electrical engineering.  I have quite a few old radios with similar switches.  The downside of these things is that the contacts get dirty, the wafer sections crack and get carbon deposits, and they can be generally unreliable.  In our case, this switch has to go.  I'll replace it with a modern switch.  We'll also tidy up all the flying leads in the process.

This is the underside of the chassis and a shot of what I think of as "Wacky Mod." The blue arrow in the top picture points to it as well.

When I got the preamp, it had this c-c-c-crazy modification installed in it.  I *think* it's some kind of cathode follower section for the phono amp, but I'm not sure.  It appears to be a 12A** tube of some sort (no markings on it....) and had a couple of 2 watt resistors and a diode connected to it.

I don't know what it was, but I know I'm taking it out!

Too bad somebody punched a hole in the chassis for it, but that's what it is.  When I said there were a lot of mods out there for Dyna stuff, I wasn't kidding.  Some of them are good and some are...Wacky.

Another shot of the bottom of the chassis.

You can see the underside of the circuit boards as well as the wiring to the that runs up to the various control pots and switches.  You can also see where I clipped off the wacky mod years ago and taped the ends.  Not my best work, but it has functioned for alll these years.

This is another thing we'll clean up...


...now!  I clipped off most of the Wacky Mod leads and unbolted the tube socket from the amp.  Oh, I feel so good now!

The socket's in good shape, so I can save it with my stash of tube sockets for a future project.

I believe those yellow capacitors are actually Mullard "tropical fish" (crazy audio nuts!) caps and are desirable.  Note to self:  investigate this.  Could sell on e*a* and retire off the earnings.

 
 
 
 

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