• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

PAS3X DC output Mystery . . . solved.

sabrefencer

sabrefencer

AK Subscriber
Subscriber
I'm working on refreshing a PAS3X - and yes it definitely is a 3x, and it happens to be factory wired, however there was much work done on it before I got it. My problem was that I was getting about 0.75vDC at the outputs. This is not supposed to happen on a PAS3X with the 1.0uF 25v caps at the output. These are physically located between the bass pot and pins 5(L) or 12(R) on PC5, and are there to block DC at the outputs. They were present in my amp, but probably original and they measured closer to 2uF and assumed to be leaky. I replaced them with new 1uF 63v Vishay electrolytics, but still getting DC. See blue caps in photo.

Hmmmm. Why I asked myself, and almost posted a question here.

Then I took a hard look at where these caps connect to board, which is at pin 5 or 12, which is after the bass control. But, someone put in two jumpers and connected pins 11, 12, and 13 together on the Right and 4, 5, and 6 together on the Left, thus by passing the tone controls (which were not doing anything) and also the 1.0uF caps, so they were not able to block the DC. Green and orange jumpers shown in photo, are now gone, and so is the DC at the outputs.

I will admit I've been working on this amp for several days, and did not question what those jumpers were for, or if they should be there, I just assumed they were part of the original build - this is my first PAS. Lesson learned. Never assume.

Here are some overall shots, including new DIY power supply, with fuse, and filament rectification. All voltages are good. Orange drops on the boards were there when I got it, keeping them in for now. Only other changes I made were to replace the loudness caps and tone caps, with the exception of the 0.002 Spragues, since I don't have the right value on hand.
 

Attachments

  • 0518211415_HDR.jpg
    0518211415_HDR.jpg
    102.3 KB · Views: 53
  • 0518211414c_HDR.jpg
    0518211414c_HDR.jpg
    119.2 KB · Views: 53
  • 0518211515_HDR.jpg
    0518211515_HDR.jpg
    69.5 KB · Views: 53
  • 0518211415a_HDR.jpg
    0518211415a_HDR.jpg
    110.5 KB · Views: 54
  • 0518211414a_HDR.jpg
    0518211414a_HDR.jpg
    107.4 KB · Views: 51
Register to hide this ad
Those tone control pots don't look correct for a 3x. They don't appear to have the feature that lifts the wiper when the control is centered, intended to bypass the tone control circuitry. That may be why the jumpers were added. When I had a PAS-3x I didn't like that feature anyway... there was too much "dead zone" where the controls were bypassed, and too much tone shaping on either end of the dead zone. It wasn't possible to get just a *smidge* of tone adjustment. A separate switch to bypass the tone controls would've been a better way IMO.
 
I'm not sure if these are the correct tone controls or not. They match the physical description of an article I came across (too lazy right now to find the link) that explains the difference between PAS and PAS3x. The other thing I need to get to is the volume pot. It does not track L to R well at all and has almost no output under the 9:00 position, apparently this is typical of these old Dynas. I checked output voltage for L vs R using two multimeters and checked it on a scope, and the channels are not the same (right is a bit low) across most of the rotation, but they are the same at about the 2:00 position, so i just leave it there and adjust the volume with my Oppo disc player, which is also the DAC for my FLAC library. Not a permanent solution, but it works for now. For for phono I will have to lower volume and boost the right channel with the balance pot. If anyone has a source for a 250 ohm dual gang volume pot with loudness tap, I'd love to hear about it. I searched the forums here and elsewhere, and could not find a source (without loudness tap yes, but not with it). I don't want to drop the loudness, as that opens up a can of worms with the RIAA equalization, and I don't want to mess with that. I did break down and ordered a pair of Chinese ALPS, most likely knock-offs - one to use and the other for a spare - less than $20 for both including shipping. Not my first choice, but there did not seem to be any alternatives. Who knows, maybe it will work? I will post my experience after I get it and install it.
 
I think you're right about the tone controls. They look different from how I recall, but I just looked at some other pics of 3x pots, and yours look the same. Sorry!
 
You must log in or register to reply here.
Back
Top Bottom