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PAS-3 hum (I know, I know)

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Posted on January 17, 2017 at 08:34:08
CaseyTheLibrarian
Audiophile

Posts: 13
Location: Maine
Joined: January 17, 2017
Good morning everyone :) I am hoping to solve the pesky hum in the right channel problem people seem to have with the PAS-3.

I recently picked up a ST-70 and PAS-3, both completely gone over, recapped, some mods added, etc. I also picked up a Parks Audio Budgie phono pre so my system looks like this:

Dual TT > Budgie > PAS3 >ST-70 >Boston Acoustic A70s.

Now, onto the issue.

There is a very low and nearly inaudible hum in the right channel. Once the music begins, it's a non issue but it is definitely there.

The ST-70 has 2 individual gain/volume controls. When the ST-70 and PAS-3 are fired up, that is when the hum begins. If I reduce the power in the right channel on the ST-70, I can effectively get rid of the hum. But I have to reduce the gain by about 75%. It becomes audible right around when I hit half-way on the dial. When the PAS is off, the ST-70 is dead quiet.

Things I've tried: switching ICs, reversing plugs, moving the units.

I brought the unit to a guy in town who works on things and is an electrical engineer and had him take a look. He verified that there was a hum (which made me happy that I wasn't crazy and dreaming it up) but was unable to find a solution. I asked him to email me the things he did and this is what he sent:

1. Swapped inputs to the second stage amp. Hum stayed in right channel. This proves the problem is in the second stage amp.
2. Temporarily installed very large value filter caps in all three power supplies - High voltage, and the Plus/Minus heater supply. Hum stayed in right channel.
3. Hum goes away if I unplug, and let the amp run on the residual voltage in the power supply. This leads me to believe it's a power supply issue. Took aluminum foil, and shielded the entire power supply. Hum stayed in right channel.
4. Followed the advice from Dynaco, concerning the grounding issue using temporary jumpers. No change, hum stayed in right channel.
5. Swapped ALL capacitors between the two channels in the second stage amp. Hum stayed in right channel.
6. Swapped the tubes...no change
7. Checked ALL components...all are good.

So that is where I am right now. I've read some old posts on this issue here but was hoping someone could chime in with a possible solution.

I am an absolute electronics dummy, so any additional information provided will be processed through that filter :) I can attach photos of things if that is of help.

 

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RE: PAS-3 right channel hum, posted on January 17, 2017 at 13:53:29
petercapo
Audiophile

Posts: 476
Joined: December 29, 2012
Kind of short on time, but I'd like to take a stab at this.

I'd like to know how many wires, and where they come from, are connected to PC5 eyelets 1 and 8 whether from above or below the board?

 

RE: PAS-3 right channel hum, posted on January 17, 2017 at 14:52:02
CaseyTheLibrarian
Audiophile

Posts: 13
Location: Maine
Joined: January 17, 2017



So it looks like the a black wire from Audio Out to eyelet 1 and a black wire from the red power (?) board to eyelet 8. Those are on the top. Underneath, there is one black wire to 8 and 3 to black wires to 1.

 

RE: PAS-3 right channel hum, posted on January 17, 2017 at 14:52:43
CaseyTheLibrarian
Audiophile

Posts: 13
Location: Maine
Joined: January 17, 2017




Underneath...

 

RE: PAS-3 right channel hum, posted on January 17, 2017 at 14:53:09
CaseyTheLibrarian
Audiophile

Posts: 13
Location: Maine
Joined: January 17, 2017



Underneath...

 

RE: PAS-3 right channel hum, posted on January 17, 2017 at 16:22:37
petercapo
Audiophile

Posts: 476
Joined: December 29, 2012
Okay, so it is not in the original, stock configuration, meaning, you have the cap board replacing the quad section capacitor and the filament supply. Perhaps there have been other changes, I don't know. Hum problems are notoriously difficult, and your PAS having undocumented changes could make a solution more elusive.

It could be helpful to know where the other ends of the wires at eyelets 1 and 8 go. However, if the goal here is to get the hum equal in both channels, here is something you can try. Any rework I describe here should be done with reversal in mind, in other words, don't do anything that cannot be put back the way it was.

You know your black wire that goes to eyelet 8 underneath the board? I can't quite tell from the image, but if the other end of this wire is soldered to the ground lug on the chassis, then I suggest removing this wire from the underside of eyelet 8. Insulate the bare end of the wire so it cannot short to anything.

On the top side of the PC board at eyelet 8, you have a black wire that goes to the cap board (the red, tubenirvana power supply board). I suggest moving this wire from the top side of eyelet 8 to the top side of eyelet 1. That is, desolder the wire from the top of eyelet 8 and then solder it to the top side of eyelet 1. This might be tricky, because you already have several wires at eyelet 1. You also might have to get a longer piece of wire. Be sure that none of the wires can short to the chassis or anywhere else other than where they are supposed to go.

 

RE: PAS-3 right channel hum, posted on January 17, 2017 at 16:50:13
CaseyTheLibrarian
Audiophile

Posts: 13
Location: Maine
Joined: January 17, 2017
So would the goal of these changes be to even the hum out to both channels or eliminate it?

And when you say move the wire from the top of 8 to the top of 1, do you mean underneath 1, where there are 3 wires already connected?

I can open it back up and see where the wires underneath go to.

 

RE: PAS-3 hum (I know, I know), posted on January 17, 2017 at 17:11:46
CaseyTheLibrarian
Audiophile

Posts: 13
Location: Maine
Joined: January 17, 2017



I opened it back up and this is what I found for connections.

8 goes to a spot right under where the rectifier (I believe is). Image provided.

1 of the black wires from 1 goes to that same spot as 8.

The other 2 go to the volume pot.

 

RE: PAS-3 hum (I know, I know), posted on January 17, 2017 at 17:12:33
CaseyTheLibrarian
Audiophile

Posts: 13
Location: Maine
Joined: January 17, 2017




More...

 

RE: PAS-3 hum (I know, I know), posted on January 17, 2017 at 17:12:54
CaseyTheLibrarian
Audiophile

Posts: 13
Location: Maine
Joined: January 17, 2017



More...

 

RE: PAS-3 hum (I know, I know), posted on January 17, 2017 at 17:37:31
CaseyTheLibrarian
Audiophile

Posts: 13
Location: Maine
Joined: January 17, 2017
Y'know, in reading the many posts on here about this issue, Bill Thomas suggests on a couple occasions that all the wires from underneath the board at eyelet 1 should be moved to the center eyelet (8).

It seems to take a little finesse to get all the wires to fit but is doable. Thoughts on that?

 

RE: PAS-3 right channel hum, posted on January 17, 2017 at 20:15:26
petercapo
Audiophile

Posts: 476
Joined: December 29, 2012
As I mentioned, you have [apparently] undocumented changes to your PAS. It is not in the original configuration, which can present additional challenges for remote troubleshooting.

The changes I suggested would be an attempt at making the hum more even in both channels, along the lines of the old post with Bill Thomas. My procedure is a little different from what Bill did, but it is also consistent with a change I see in the development of the PAS manual over the years that I think may have been implemented to deal with this very problem.

If the way I am suggesting doesn't work, then you can try to do it more closely to the way Bill Thomas described, though it might be more involved. Part of the reason why I am suggesting my particular way of doing it is because it should be relatively straightforward.

However, given the way your PAS is currently wired, all bets are off. In other words, there's no guarantee that either way will work, though one might. Hence, I recommended that you take note of the changes you make so that you can put things back to how they were, if necessary.

To achieve a more overall reduction of the hum [in both channels] would be more involved.

When I suggested moving the wire at the top of 8 to the top of 1, I meant just that. You have to be careful, because the bottom of 1 has other wires soldered to it - you don't want them to become disconnected. I understand that this is may be somewhat challenging. If you find it easier to run the wire to the underside to connect to 1, then go ahead. Just make sure that there are no shorts to adjacent circuitry.

 

RE: PAS-3 hum (I know, I know), posted on January 17, 2017 at 20:21:57
petercapo
Audiophile

Posts: 476
Joined: December 29, 2012
That would take more work. The procedure I described should be less work and is consistent with developments in the PAS manual over time that may have been to address this very problem. But, if you want to run the ground wires to 8, it's certainly worth trying.

Eyelets 1 and 8 are on the same circuit trace - they are the same point, electrically. One issue Bill was raising was the thinness of the trace and the distance the different wires and other parts are from each other on that trace (depending on whether they are connected closer to 1 or to 8). If I understood him correctly, I believe this is what he was attributing the hum imbalance to.

The old PAS manual had a ground wire from the quad section capacitor going to eyelet 8 but in the later manual this wire was moved to eyelet 1. Of course, which way ends up working better for you may need some experimenting.

 

RE: PAS-3 right channel hum, posted on January 18, 2017 at 17:34:36
CaseyTheLibrarian
Audiophile

Posts: 13
Location: Maine
Joined: January 17, 2017
Well, I dropped the unit back off at the local guy who took a crack at the PAS earlier with your recommendation as well as the one from Bob years ago. He's going to give them both a try and see if either work.

Let me ask you this: would changing the PC5 board make a difference? Like maybe upgrade/change to Tube Nirvana Z-Mod or the PAS3 Aikido?

 

RE: PAS-3 right channel hum, posted on January 18, 2017 at 19:45:47
petercapo
Audiophile

Posts: 476
Joined: December 29, 2012
If your tech is going to be diving into it, he might want to think about eliminating what appear to be a number of ground wires that might not be necessary. It might not be possible to implement Bill Thomas' fix the way he did if you have more ground wires than he did. This is another reason why I wanted to offer you a simpler approach with moving just the one wire (and also removing the wire from eyelet 8). Of course, my idea might not help depending on all the other stuff going on in your preamp.

I suggest you get your tech a copy of an original Dynaco PAS manual along with its corresponding pictorial diagrams. With regard to ground wiring, I suggest a later PAS manual, such as for the PAS-3X, though it may not be appropriate for certain other aspects of troubleshooting, especially since your PAS is not in any standard Dynaco configuration.

With regard to changing to a different PC board... keep in mind that with the nonstandard and possibly redundant grounding scheme you have now (that could be causing ground loops), your existing line board isn't really getting a fair chance. That's not the original line board, anyway. It is a newer, fiberglass board, so it has already been changed-out. It's the ground wiring that is causing your hum problems. Switching to a different circuit board design may or may not help the hum, depending on how it integrates into your existing grounding system of wires.

I offered the suggestions that I have in the hope that it might alleviate the hum imbalance. But, what your PAS really needs is a complete reworking of the grounding by someone who is very familiar with both the original PAS and the grounding needs you now have with the tubenirvana cap board. If it were my PAS, I'd just remove every ground wire there is and start from scratch.

 

Just noticed something else..., posted on January 18, 2017 at 20:03:09
petercapo
Audiophile

Posts: 476
Joined: December 29, 2012
Are your tone control pots wired up or have the wires to them been disconnected? If they have been disconnected, then I am wondering about how the associated circuitry was managed so that the tone controls were removed properly. Typically, when the tone controls are "removed" in a PAS, I'd expect to see some wire jumpers and/or maybe some parts removed from the line board. But, in your photos, it looks like the line board is still fully populated...

 

RE: PAS-3 right channel hum, posted on January 19, 2017 at 04:03:03
CaseyTheLibrarian
Audiophile

Posts: 13
Location: Maine
Joined: January 17, 2017
I've sent all of this to my tech, so we'll see what he comes up with. He's a sharp dude, so I'm hopeful.

 

RE: PAS-3 right channel hum, posted on January 19, 2017 at 04:38:17
petercapo
Audiophile

Posts: 476
Joined: December 29, 2012
About that wire at 8... I suggested removing it because there ought to be a different wire grounding the tubenirvana cap board more directly to the chassis. But, if there isn't, then you might need to keep the wire at #8. Again, perhaps not the best way to do things, but we're trying to find something that might help short of tearing the whole grounding system apart.

 

RE: PAS-3 hum (I know, I know), posted on January 23, 2017 at 16:54:04
xaudiomanx
Audiophile

Posts: 2947
Joined: August 16, 2004
Casey mentioned his tech thought it was in the right channel only. He also mentioned the hum went away when the power cord was pulled from the wall. Does the any of the units have a three prong plug? If so use a ground lift plug and see if the hum is gone. If it is a standard two prong plug try running a wire from the center of any three prong outlet to chassis ground.

The other thing to take into consideration is the fact that both of these units are not stock and could have not been right from the mods performed.

 

RE: PAS-3 hum (I know, I know), posted on January 23, 2017 at 16:56:54
CaseyTheLibrarian
Audiophile

Posts: 13
Location: Maine
Joined: January 17, 2017
No 3-prong plugs here.

I forgot to update this. I took Peter's suggestion and also an old one from Bill Thomas and emailed my tech who wanted to try them out.

Happy to report that the hum is gone, as far as I can tell. He rerouted the ground wires to 1 instead of 8 (I think I am remembering that correctly).

Now I need to maybe reinstall the tone controls :/

 

RE: PAS-3 hum (I know, I know), posted on January 23, 2017 at 16:59:28
xaudiomanx
Audiophile

Posts: 2947
Joined: August 16, 2004
Casey,

There is a guy on another forum in this web site(DIY Tubes). His user name is Chip647. His name is Chris and he is a genius with being a tech(not to say others aren't). I have used him on various projects and he did some great things for me. I am OK at this stuff with Dynaco but he is the real deal. He could probably help you but it would be best to send it to him. You could contact him yourself if you like and I found him to be very reasonable and worth it.

Worth a try!

 

RE: PAS-3 hum (I know, I know), posted on January 23, 2017 at 17:15:15
xaudiomanx
Audiophile

Posts: 2947
Joined: August 16, 2004
If the hum is gone and the unit sounds good leave it alone. Just a suggestion. Tone controls aren't necessary but I did like them on any of the vintage equipment. I am a believer of them is a big way. You can do a lot with them for music appreciation but mainly with vinyl. CD's and the anything digital is probably as good as you can get it without the use of tone controls and tone controls when using digital can promote issues sonically.

 

RE: PAS-3 hum (I know, I know), posted on January 23, 2017 at 17:17:14
xaudiomanx
Audiophile

Posts: 2947
Joined: August 16, 2004
Casey,

It is to my very little knowledge that when you pull a power cord from the wall before the unit powers down totally and it kills the hum, it is generally a ground loop or some kind of grounding issue.

 

RE: PAS-3 hum (I know, I know), posted on January 23, 2017 at 17:24:03
CaseyTheLibrarian
Audiophile

Posts: 13
Location: Maine
Joined: January 17, 2017
So it looks like I can't send him an unsolicited message :( Could/would you put him in touch with me?

 

RE: PAS-3 hum (I know, I know), posted on January 24, 2017 at 07:33:22
xaudiomanx
Audiophile

Posts: 2947
Joined: August 16, 2004
Email me at xaudiomanx@gmail.com. I really don't want to put it out there for all. He might take exception to it.

 

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