PAS-3: First DIY project, looks great, loud hum

I've installed the following VTA upgrades in my Dynaco PAS 3: #1 upgraded power supply, I/O board, selector switch, LED, Aikido and Z-Mod PC-6 boards. It looks great but I've yet to get rid of a very loud hum:

•it starts when I turn it on (w/ST-70)
•it gets louder as the tubes warmup
•is unaffected by the volume pot

With only a cd player hooked up and playing, I was able to get the hum to stop completely and get a clear signal (though only through one channel/speaker) by placing my hand across the other unused RCA's, the sides of the main out RCA plugs, and the chassis. (This was done with the cover on and secured with screws)

I've since read that it is standard practice to upgrade one component at a time to simplify troubleshooting. The odd thing in my circumstance, this hum sounds exactly like the hum it had in stock form.

Any suggestions are welcome and appreciated!

" />

Check for solder bridges and move the HV center-tap

Thanks for the response Hawaii.ken!

I did as you said but sadly the hum remained. However, Monday morning I grounded the I/O board to the chassis and the hum ceased. I hooked up the PAS to my ST70 and Cornwalls and was truly amazed. The difference in sound between the solid state pre I'd been using and the PAS with Roy's boards is clear.



 

One last note:

After looking at pics of other people's projects on this forum, I'd like to amend the original title of this post from "looks great" to "looks adequate!"

allenmmusic10003 wrote:One last note:

After looking at pics of other people's projects on this forum, I'd like to amend the original title of this post from "looks great" to "looks adequate!"

Never-the-less, you did a far better job than many other projects I've seen.  Congratulations and enjoy your tunes!

Best Regards,
Charlie

allenmmusic10003 wrote:One last note:

After looking at pics of other people's projects on this forum, I'd like to amend the original title of this post from "looks great" to "looks adequate!"

Other than esthetics, what purpose is REALLY served by having all the wires at right angles and parallel to each other? Or is there actually a detriment to doing that, since it's easier for signals to be induced into parallel wires than when they cross at an angle?

After looking at pics of other people's projects on this forum, I'd like to amend the original title of this post from "looks great" to "looks adequate!"

It looks pretty good and nothing to be ashamed about.  Without adequate spacing parallel wires can induce current in adjacent conductors especially high impedance circuits like in a tube amp or preamp.  And the result is crosstalk.  So the rule of thumb if you can't avoid parallel runs is to leave adequate spacing between them.  And cross over other conductors at right angles.   But after the memory of wiring the project fades a really neat installation is often times better to debug as it is easier to trace the wires.

Although your hum problem is solved, I'd be inclined to move those wires near the AC convenience outlets farther away from the outlets.  A grommet for the hole where all the power lines come through the chassis might make things a little safer, too.  You can simply cut a proper-sized grommet with your utility knife and install it in the hole without unsoldering all the affected wires.

Just my two cents.