PL400 II Input Grounding Questions

momoaz

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#1
So Hi first time poster! I bought a PL 400 Series II off CL and will be performing a WOPL Control Board, DC Caps, and Transistors upgrade. This Series II came missing all 3 fuse caps. Also all three fuse holders are different. I am planning to change out the AGX holders and replace the cap on the ACG. The input RCA jacks are wasted and someone did a grounding hack to probably clean up some hum. It has an insulated copper strip (horribly done) where the RCA jacks mount and 3 additional grounds from the control board were soldered on there (along with speaker grounds and sensitivity pots. I will attach a pic. I read through the forums found the Switchcraft RCAs to replace these. My question will be where was the original location for these 3 grounds from the control board? Two black and one white wire. I am guessing adding the newer Control Board and putting these back where they belong will take care of the grounding issue they were trying to resolve by this bad revision. I could even make another Copper buss bar there and reattach everything but I would prefer to see how it was originally done. Thanks so much!
IMG_4761.JPG
 

laatsch55

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#2
In the WOPL archives there is a schematic showing Joe's (Gepetto's) improved grounding scheme.
You're right, that was horribly done. Both speaker negatives should go back to the buss bar between the caps, as should the white ground wire from the control board. The only ground that should be on that plate is what comes to the control board input grounds..
 

Gepetto

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#3
It appears that they may have lost the insulating fiber washers on the RCA jacks based on the electrical tape on the copper tab (just a guess as I don't see any fiber washers in the picture). The RCAs need to be isolated from the chassis for proper performance.
 

momoaz

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#4
thanks for the quick replies. There is another metal plate (also wrapped in tape) on the outside of the chassis. So from looking at other posts I see the copper bar is correct just minus tape and plus insulators. How about the outer plate is it supposed to be present or did they use it to shim up smaller faced RCA connectors.
IMG_4762.JPG
 

Gepetto

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#5
Inside only plate. they used the outer plate to kludge together a way to insulate the RCAs from the chassis. The stackup from inside to outside is supposed to be RCA jack, through copper plate, through first fiber shoulder washer, through chassis, through second fiber washer, through flat washer through securing nut.

Repeat stackup for each of the 2 input jacks.
 

Gepetto

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#7
beautiful thanks
And the inside copper plate is with all the insulating electrical tape removed...the plate should be in electrical contact with the shell of each RCA jack but isolated from the chassis.
 

momoaz

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#11
Ok so without getting too confused.
1. The Ground Scheme for PL700b thread is also pertinent to the 400 got it!
2 What about the older 400 Ground scheme thread I found from 2012?
https://forums.phxaudiotape.com/threads/joes-improved-grounding-scheme.2268/
There are 6 steps there pertaining to the PL14_20 board modifications.
3. I am guessing #1 will apply and #2 is not applicable to the PL14_20RevE6 Control Board and its documentation will reflect current grounding revisions.

Incredible responses here, wow you guys are tight.
 

TravK

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#13
Not to to interject, but post #2 (thank you Lee) and post #11, comment 2. (thank you momoaz) and of course thank you Joe...y'all helped me eliminate some low-level hum today from a White Oak-ed (new cap kit plus control board) 400 I lucked into the other day. The tech I bought from had done all *except* snip the vintage 16 AWG tie between the speaker wire binding post and the 'floating' RCA copper plate. 'At-idle' hum eliminated, and best cut since my vasectomy. :)
 
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