PAS 3X upgrade excessive hum

I hope I have the correct place to post the following question(I think my initial question on this forum was misplaced): I have recently been upgrading a PAS 3X with new TCLA and phono boards, new tone control potentiometers, volume and selector switch new tubes, except for two 12AX7 in the phono board, all from tubes4hifi, and new RCA input and output jacks(these are in, but isolated from, a 1/4" thick aluminum plate screwed to the inside back of the chassis), 3 wire AC connector and power switch on the back panel because I didn't want to hack into the front panel.

The boards are complete, installed and the sound, with the exception of a loud hum seems fine. The hum is loud regardless of the volume setting, although it diminishes slightly with the volume knob turned all the way up. The hum is about equal in volume to the sound listening at a low average level in a small room. It seems to be only in the Preamp.

I think I have all of the connections correct as well as the wiring done per the information Mr. Mottram sent me. I decided against the blend and tape switches, eliminated the power on light, and the remaining switches along the bottom front panel.

I tried cutting the F+ F- and B+ connections to the phono board, the hum continues, also disconnected the input to the line board, still the hum persists.

The only things I have not replaced are the 12X4 socket and power transformer, I'm wondering if it may be causing the hum.

I've been in touch with Mr. Mottram and so far we don't have a solution.

This is an interesting project as this is a preamp I first wired as a kit in about 1969. I did an ST 70 the same time and have also recently upgraded it with Tubes4hifi components. I don't mind these little obstacles as I'm learning things I should have learned 40+ years ago, nonetheless, I'd like to solve the problems and enjoy the improved PAS 3X and ST 70.

Any suggestions will be greatly appreciated.

tna

I just installed a similar upgrade to my PAS3. If the ground on the AC is wired to the preamp, try disconnecting it. Also, what is the purpose of the aluminum panel? My first thought though is the grounding which my be causing a ground loop.

I replaced the the original RCA jack assembly with the aluminum plate drilled for new RCA jacks. It's just a more substantial way to mount the RCA jacks. All of the jacks are isolated from the plate with the plastic washers supplied by Parts Express, and grounded as shown in Mr. Mottram's directions. 1/4" plate is heavy, but I didn't have 1/8" so I used what I have. I rewired the input and output connections this morning after increasing the drilled holes for more clearance around the RCA jacks, that made some difference, but not much really.

I've tried disconnecting the ground to the chassis, it makes no difference.

It's interesting that connected to different power amps the volume of the hum decreases, it's still way too loud though.

This morning I rewired the B+ & Ground, F+ & F- from the power supply to the phono board. I drilled two holes in the chassis just beneath the power supply board to run the wires under the upper chassis plate. It's a more direct route for the wires, and keeps both sets beneath the chassis, but made no difference in the hum.

Thanks for the suggestion, tna

This may be a silly question, but is the aluminum plate grounded to the chassis? I keep thinking that the problem is a floating ground, but don't know where.

The aluminum plate is screwed to the back panel of the chassis, it is definitely grounded. However, the RCA jacks are isolated from the aluminum by plastic washers, one of which on each jack has a centering shoulder so the metal threaded sleeve, the ground, cannot connect with the aluminum and chassis.

As I understand the wiring instructions, the RCA jacks should be isolated and connected to only one ground on the line amp board; However, I have continuity between the RCA ground and the chassis.

I can see that the nuts and washers that hold the line amp board to the chassis overlap the metal boarder around the board, perhaps this ground connection should not be made? The nuts and washers on the phono board do not connect with the boarder.

Should I have continuity? It doesn't make sense to me if I was also to isolate the RCA jacks from the chassis.

Thanks again, tna

I still haven't received any photos from you, but from your description it seems you installed the PCBs on the top side of the chassis rather than as the original boards which are installed from the bottom side, thus no ground connection, so it may just be as simple as a ground loop with the PCB ground connecting with the chassis. The boards should have been mounted the same way the originals were, from the bottom side.

I installed the boards on the bottom of the chassis.

I have very carefully re-examined the 8 screws, washers and nuts around both boards. Initially, it appeared to me that the nuts and washers around the phono board did not connect with the metalic boarder; however, looked at more closely, it appears to me that one screw, nut and washer set probably does connect with the boarder on the phono board.

And...all 4 screws, nuts and washer do connect with the metalic border around the TCLA board and the chassis.
I can send photos tomorrow when I have access to Broad Band, today, I have only a very slow dialup connection.

Thank you for the help, tna

If anyone has any other suggestions I'd appreciate them. Mr. Mottram looked at photos and says he sees nothing wrong, the hum persists though. It starts out loud, then diminishes as the preamp warms up, but then comes back as loud once heated. I've tried removing the screws, nuts and washers that hold the boards, that makes no difference, tried removing the screws to the transformer and insulating it from the chassis, still no change.

I've also taken it to another room using a different AC circuit, no change there either. Took it to work too and tried it there, same hum.

As I say, if anyone has ideas, I'd appreciate them.

Thank you, tna

TN Allen wrote:If anyone has any other suggestions I'd appreciate them. Mr. Mottram looked at photos and says he sees nothing wrong, the hum persists though. It starts out loud, then diminishes as the preamp warms up, but then comes back as loud once heated. I've tried removing the screws, nuts and washers that hold the boards, that makes no difference, tried removing the screws to the transformer and insulating it from the chassis, still no change.

I've also taken it to another room using a different AC circuit, no change there either. Took it to work too and tried it there, same hum.

As I say, if anyone has ideas, I'd appreciate them.

Thank you, tna

I would start with some "divide" :
- see if hum is created in pc5, test this by soldering i wire between (2) and (1) and
between (9) and (1) thus shortening the input of pc5.
If hum persists then it comes from :
- +V ( check for ripple om (16)
- see if (1) and ( are connected to psu ground
- make shure that filament midpoint is grounded as per the original psu schematic
- pull the tubes ( all tubes ) to make shure it's not a fault tube


If however amp is quiet with (2) and (9) shorted to ground you know that PC5 is ok
and you have to work towards the front, grounding the selector pins to see where
it comes.

At the other end is the intake panel you replaced. What happens if you desolder all
connections and have them floating ?

Just a few thoughts

Peter,
your suggestions don't apply because he is not using the old Dyna PC5, he is using the VTA TCLA line amp.
I think the problem is probably in the power supply, or in the ground wiring, but no one is going to know unless someone with more experience has a chance to actually look at the preamp and find the problem.

tubes4hifi wrote:Peter,
your suggestions don't apply because he is not using the old Dyna PC5, he is using the VTA TCLA line amp.
I think the problem is probably in the power supply, or in the ground wiring, but no one is going to know unless someone with more experience has a chance to actually look at the preamp and find the problem.

I know it's not a PC5 , but he can still short the inputs of the line board to divide the problem
into : within the lineboard or PSU : before the lineboard.

When dealing with "strange problems" it's always good to try to divide the problem
thus reducing the scope.By shorting the input of line ( or pc5) you get one answer.

Many times the mass noise can be suppressed by changing the position of the appliance plug.

I have left a message for a man who sold Dynaco, assembled the kits commercially, and has sold and repaired many different audio components for several decades. He has not called back yet, however, he may well find a solution when he looks at the preamp. It goes without saying that this is well beyond my meager understanding of electronics. Had I known I would encounter this problem, I probably would not have decided to do this upgrade. Nonetheless, it's an interesting problem.

I took all of the tubes out, no hum.

Unsoldered the connections between the selector switch and volume control, the hum remained.

Here is something that remains confusing to me: in the PAS-TCLA...V12-1-15-2012 instructions, regarding grounding, I read "...For example, if the RCA jacks are not isolated from the chassis, then the twisted pair of wires that connect the PCB to the jacks will each define a ground loop..."

I mentioned in an earlier post that when I test for continuity between the RCA ground rings on the output and input jacks and the chassis there is definitely a connection. If I cut the single ground wire(left) from the TCLA board, the connection between the RCA jack ground rings and the chassis is broken.

There is also continuity between the RCA phono input jack ground rings and the chassis. I did not disconnect the ground from the phono board to the jacks.

Thanks for the continued help.

Have you made any changes/mods to the High Voltage supply?

I have the power supply kit Mr. Mottram supplied. The voltages are within ~10 volts of the values Mr. Mottram wrote on the modifications information he sent. He did not include values for F+ & F-, I read these at about 20 volts.

This morning I replaced the output wires with shielded cable. There is a marginal improvement, or perhaps wishful thinking on my part.

I am beginning to think that perhaps I should use an attenuator between the power amp and preamp. When I run the PAS 3X upgrade through a Hafler preamp I ordinarily use, I can keep the hum at more reasonable level, it's still there, but not too bad. Not as low as the Hafler alone. Perhaps the upgraded PAS 3X is a bad match for the power amps I've tried. One is an ST 70 with the tubes4hifi upgrade, the other an Updatemydynaco ST 120 upgrade. The later sems to be a good match for the Hafler, as is the ST 70. I hope it's not a problem to mention the solid state amp and preamp, but perhaps these are relevant to a solution.

Thanks for the interest and question.

tna

Try removing any wires connected to the output jacks then check for continuity between the chassis and the output ground. (both channels)

I've tried disconnecting and disassembling various parts, added an aluminum shield between the power supply area and the PCBs, no change in the hum.

I read earlier that it is advisable to do a new preamp rather than upgrade a PAS 3X, too bad I read it after this upgrade.

Today I took out the 4 screws that hold the TCLA board to the chassis, it made no difference in the hum, and there was still continuity between the RCA ground and chassis until I cut the single ground wire from the left TCLA output solder pad.

I assume therefore it is correct.

Although, it remains a puzzler as I wrote earlier because in the PAS-TCLA...V12-1-15-2012 instructions, regarding grounding, I read "...For example, if the RCA jacks are not isolated from the chassis, then the twisted pair of wires that connect the PCB to the jacks will each define a ground loop..."

There is also continuity between the RCA phono input jack ground rings and the chassis. I did not disconnect the ground from the phono board to the jacks.

I don't really know enough to go about this intelligently, it probably is best to wait and have it looked at by someone who does.

Thank you for your suggestions, tna

The original design for the output jacks make them the primary ground for the audio signal.

Try removing any wires connected to the output jacks then check for continuity between the chassis and the output ground. (both channels)

I see what you mean, but according to the instructions for the new TCLA board there should not be continuity between the RCA ground ring and the chassis. I've made the aluminum plate the jacks are mounted in so the plastic washers isolate the jacks 100% from the chassis. I looked at the old RCA jack assembly earlier today and noticed there would be a connection between the jack ground strip and the chassis, but for the new boards the connections are arranged otherwise.

As I wrote, it remains a puzzler to me, but then again, I don't know enough to know what I don't know. Consequently, I'll wait until I can take it to someone who can check it out, and perhaps offer some explanations and suggestions.

Thanks for the help, tna

I believe all the other RCA jacks should be isolated from the chassis to prevent ground loops but the output jack should be grounded to the chassis.

I've tried snipping the ground wire that connects all of the input and output ground lugs to separate the input and output jacks, no change in the hum. I also tried wiring L&R input jacks at the front of the preamp with 1" long leads grounded and not grounded to the chassis. At the same time I wired output jacks close to the front panel as well, again, with a ground to the chassis and without, still no change.

I suspect the hum is generated in the power supply and makes its way into the phono and TCLA line board through the connections to the these boards.

I wonder if the F+ to F- voltage should be the ~20 volts I measured?

Hey TN Allen. are you operating the unit with the top cover off?

Steve

I've tried it with, and without the cover; nonetheless the hum persists. Thanks for asking.

tna

I’ve been wrestling with what sounds like identical issues with a SP9 TLCA preamp connected to a VTA ST70. As an experiment I changed the ST70’s 12AT7 with 12AU7. The hum was less noticeable, but still not acceptable.

I'm still convinced you guys have ground loops (which can't be fixed via emails or forums!)
There should be a ground wire from chassis ground to the power supply.
There should be a ground wire from the power supply to ground on each circuit board.
There should be a SINGLE ground wire on all of the RCA jacks.
Every wire in the preamp should be as short as possible.
All signal wires should be twisted pairs, with one side grounded on the common RCA connection, and ONLY connected on that one end.
The volume and balance controls will of course have a ground connection.
For service, Roy on the west coast, Sal on the east coast, and Troy everywhere in between!
Also FYI, the voltage reading between F+ and F- should be about 22-25vdc. If it's lower than that, you've got a weak transformer.