Crawls Backward (When Alarmed)

IconProjects, musings about guitar builds, guitar repairs, vintage tube amplifiers, old radios, travel, home renovation, and other stuff.

Dynaco PAS-3 Finished!

With the chassis working, I can clean up the faceplate and put it into service.

Here's the faceplate and knobs waiting for cleaning.  The faceplate is brushed aluminum, and looks great, but it collects fingerprints and scratches like crazy.

I'm very wary of cleaning it with anything too caustic, because the lettering, which I'm pretty sure was silk-screened on, will come right off.

So I mixed up a weak solution of Simple Green and water.  I tried a test on a small area with lettering (aka 'inconspicuous area') and it went well.

So I carefully cleaned the whole faceplate with the mixture.  I just soaked the knobs in a bucket of it.

A side note here:  Simple Green is some amazing stuff.  It's non-toxic and biodegradable, which is a good thing for humans and the Earth both.  I buy it a the Despot in gallon containers.  You can use it straight up at "full" strength, but I find it cleans just as well mixed with water.  The label gives suggested mixing ratios.

I've used this straight to clean filthy radio chassis and it works wonders.  You can also use it on floors, furniture, you name it. Really a great all-purpose cleaner.

One of the long-term issues on the PAS that has bugged me is this little cutaway on the back of the cover.  There isn't a phono ground on a stock PAS, so folks have come up with various solutions to put one on.   Turntables have a ground lead that has to be connected to the amp or preamp they're connected to.

The previous owner of my PAS had put a banana-plug style connector on the back of the chassis, which is certainly a neat solution.  However, they sort of hacked a little divot off the case where it needs to clear the connector.  Bottom line is that it was a pain to get on and off.  So I whipped out the trusty Dremel and made a cleaner, bigger cut off the cover.  Dig those c-c-crazy sparks!

Here's the finished job.  The cutaway clears the ground better now.  It's a lot easier to get the cover on and off.  In the past, I had to sort of pry the cover over the ground lug.

My cut isn't perfect, but it will rarely be seen.  This is also known as function over form.



Now we have a fully assembled and rebuilt Dynaco PAS-3 ready to go back into service!

The faceplate came out pretty well I think.  There are some scratches I didn't want to mess with.  I mainly wanted to make it a bit more presentable.

Now let's go hook it up, yes?

Here we have a stack 'o Dynaco gear. 

The top piece is my FM-3 tuner.  I did a handful of mods to it a number of years ago and  also aligned it.  It's been in regular service since then.  This is actually one of my next projects - I'd like to change the can cap out for a PC board similar to the one that's in the PAS.

The middle component is another PAS - this one is dead stock.  I used it while I was rebuilding this one, which is on the bottom.  You can see the middle one could use a sprucing up as well - another future project.

Here's something a little crazy I have on the FM-3.  The FM-3s run notoriously hot - after an hour of use the top cage is almost too hot to touch.  So I rigged up a little exhaust fan for mine.

It's really simple - it's a 9 volt DC wall wart adapter that plugs right into the convenience outlet on the back of the FM-3.  I hardwired a computer fan (from Radio Shack...) to it.  When the FM-3 is switched on, the fan goes on.

I set this up so the hot air is pulled out of the tuner, not so outside air (and dust...) is pushed in.

One crazy thing about this arrangement is that the fan has blue LEDs on it!

This is what it looks like from the back.  I didn't set out to get one with LEDs, but that's what they had and I figured "why not?"

The fan is really effective - and it quiet too.  I keep my components in a stereo cabinet, and with a fan on the tuner and one on my ST-70 amp, they run nice and cool.


Here's the newly rebuilt PAS preamp along with my FM-3.  It's a bit dim where they are located, so the camera shutter speed was real slow.

You can see the blue LED I put in for the PAS pilot light.  If you look carefully, you can also see a blue glow from the fan on the back of the PAS.  Gives it sort of a 'Star Wars' sort of feel.

Ironically, after the mods, only three knobs and two switches do anything!  The others are just taking up space.

Since the inputs serve new functions I made a little card to track what's what.  We now have a dedicated input for the TV (a stereo audio line from the cable), cassette (I don't use it much but it's there), and a "special" line level input.  I have a lead running from the back which I can connect another input to on the fly - so far it's really useful for my Tascam DR-1 digital recorder.

The rebuild was pretty straightforward.  The most tedious part was wiring the selector switch.  I believe I mentioned the hum on the phono input that I corrected by changing the dress of a ground lead.  With all of the flying leads, the PAS has a lot of potential for noise and hum.  But this one is much quieter than my stock one.

To sum up, here's what I did:

Installed a new filter cap board with increased filter values and a line fuse.
Installed new boards for the phono and line stages (PC-5 and PC-6).
Installed a new Alps Blue Beauty volume pot.
Did the Curcio phono stage mods to improve the RIAA equalization curve.
Put 1 uF Sprague Hyrel PIO coupling caps in the phono stage and the line output stage.
Bypassed the tone controls.
Removed the loudness control from the circuit.
Removed the filter control from the circuit.
Replaced the old selector switch with a modern one and rewired all the inputs with modern teflon-insulated wire.
Rewired the inputs to convert the obsolete Tape Head and Special inputs to line level.
Replaced the incandescent pilot lamp with a blue LED.

I did a lot of research and reading before (and some during...) the project, which helped.  The final preamp now has virtually all of the upgrades and modifications I found.  There are more upgrades, but they amount to changing the circuitry dramatically.

How does it sound?  It sounds great.  Overall it seems more transparent than stock, there is less noise.  I listened to some Michael Hedges harp guitar recordings on a Windham Hill LP, and it sounded great.  The metallic ring of his wound bass strings was crystal clear.  The speakers I'm using are pretty awful (old Boston Acoustics), so I expect the improvements will be more noticeable with new speakers (in the works).

 
 
 
 

Post a Comment 0 comments:

Post a Comment