HELLLLLP. Geetar circuit got me stumped.

Denny Turner · From: Oahu, Hawaii USA

Well Hoot. I guess some sadistic design engineer genius done got me floored again. I got a 3 P'up guitar circuit that I can't figure out to save my life ....and been tryin now for almost a month. I sure would appreciate anyone being able to tell me how this darn circuit works.

A link to the schematic, and link to charts of switch & pot functions & related pickup volumes, are at the bottom of this posting.

The main 2 questions are: (1) how do the tone pots act like volume pots too in the tone control scheme, and in volume schemes of some switch positions, (ie. what in the circuit affords that capability), and; (2) What affords the elevated volume floor of those tone/volume pots when their switch is off?

I can't do any invasive testing / looking into the componants: I am restricted to making & reading the shematic and recording what all switch positions and pot ranges do, because the guitar is 42 years old and in near mint and all original condition, so other than removing the pickguard / electronics assembly to review it's componants, it can't be desoldered nor invasively harmed, scarred, etc. All the electronics are Rowe / DeArmond and mounted tighter than a drum to the underside of the pickguard; Permanantly mounted for my practical purposes of keeping it all original as the customer desires (this is the tightest / stiffest guitar harness I've ever seen in my 49 years of seeing them). The 3 rocker switches are housed in a plastic switch box that is riveted to the underside of the pickguard as are the pickups too; And there isn't a hole or slot in the switch box big enough to even get even a micro-borescope peek inside with to see if there's something in addition to switches in there. The pots and wiring harness are tight on a 3/16" spiraled steel flex tube that carries ground & shield and carries some of the wires, from the switch box, across all pots on it's way to the output jack. The diameter of the pots are not much bigger than a dime, with all the pot values covered with factory soldering, so any clues the pot values might provide are hidden. So the only tools I have to figure this circuit out for the Customer and my archives (and my granite-head curiosity), is the schematic and by auditing what the switches and knobs do to each pickup's signal in each switch and knob position.

General description of circuit: 3 On / Off rocker switches that bridge & un-bridge each pickup's "hot" lead on one side of each switch to a common bar buss on the other side of the switches which feeds a lead that sends signal to a Master Volume pot regulating signal to the output jack. There is 1 "tone" pot for each of the 3 pickups that taps from each pickup's "hot" lead before the hot lead goes to it's respective aforesaid switch. SOMEHOW each of the tone pots have strange volume functions through each pot's filter cap circuits to ground (functionally though, and not like a shorted cap). Different ones of those 3 tone/volume pots are switched into the circuit depending upon which combination of the 3 pickup switches are switched On; ie. different combinations of pickups switched on result in different knobs serving the tone-volume function(s). Switching a pickup on defaults it's volume to the master volume control; While the pickup switches that are switched Off somehow elevate the volume floor of the pickups switched off so that they have about 2/10 to 3/10 volume getting to the master volume pot; Apparantly a deliberate design to expand tone along the longitudinal strings plane. In most switch positions the tone/volume pots will lower that floor to zero, while in other switch positions the tone/volume pot(s) will regulate a respective pickup's volume between full on and an elevated volume floor. In some of both the On and Off switch positions, an unexpected pot will take on a unexpected volume function that doesn't seem to fit the symetry of the whole switching & tone/volume scheme, ...although that could be caused by (1) the inherent nature of the circuit, or (2) I might have not found all of the combinations that might show intuitive symetry, what with 3 switches and 3 tone knobs offering 49 different combinations. This paragraph coincides with the Switch & Pots Function charts linked at the bottom of this posting. I believe this paragraph would also coincide with the Electronics Schematic IF there were resistors inside the switch box, although for the life of me I can't figure out what that switchbox internal resistor(s) circuitry might be; Probably because my brain just can't digest / see the abstract possibilities of such resistors in series / parallel with the pots and their caps they would be switched / bridged to. Except for impedence load I don't see the pickups lending any role in the tone pots volume functions quandry. I understand that .047 caps on the front and middle pickups tone/volume pots will pass lower frequencies and thus more signal which will have an apprant volume reduction effect, especially with 2 pots & caps lending caps in parallel and thus an even higher farad effect; But the volume reduction is real and much more pronounced and identifiable as such than just a deep tone rolloff due to an unusually high farad effect.

I can vaguely "see" how resitors in the switch box could make the circuit act like it does, but just can't see or think of a wiring scheme for them that would make the circuit act like it does; And I darn sure don't see any way it could work like it does without additional resistors in that switch box. I suspect that someone who could envision that internal circuit would have the answer to the quandry.

Inconsequental, I think: The rear pickup tone/volume pot's cap value is .020 ...I assume to keep it's treble tone integrity deeper into the pot's resistance roll-off. I don't think the caps value is contributing to the switches and tone controls volume function quandry.

Here's the electronic schematic:
http://www.dennysguitars.com/HarmonySil3PupST1488Schematic1.html

Here's the Switches / Pots / Pickups functions charted:
http://www.dennysguitars.com/HarmonySilST1488SwitchFunctions.html

Here's pictures of the electronics:
(Scroll down to the electronics pictures; The text is just progress reports to the Customer)
http://www.dennysguitars.com/HarmonySilST1488WorkPage3.html

SURE WOULD APPRECIATE ANY HELP ANYONE MIGHT OFFER ! My brain is drained on this one.

Emailing me at the house would be icing on the cake; I'm up to my neck in alligators lately ...and often can't get to the forum !

THANK YOU,
_________________
Aloha,
Denny T~
http://www.dennysguitars.com/

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Brad Sarno · From: St. Louis, MO USA

Denny,

one visible flaw in that schematic is that the volume pot does NOT have the left terminal grounded. That terminal should be tied to ground so the volume pot will work properly. At a quick glance, it seems that if you fix that the circuit may just work.

With all the pots' shells grounded as indicated in the drawing, you could simply just bend and solder the volume pot's left (ground) lug to its own shell.

Brad

Denny Turner · From: Oahu, Hawaii USA

Thank You, Brad.

Yes, I would agree and would ground that lug were I wiring a "Y" configuration volume pot in correct alternating current phase with the amps ultimate case ground; But I don't think that's the case with this guitar for a number of reasons:

------------

I'm sorry I failed to mention that I believe the guitars circuits to be functioning properly without fault. It's just that I haven't been able to figure out how it works like it does ...unless my suspicion that a resistor(s) circuit is hidden in the switch box.

I beleive the elevated volume floor and tone control by integrating volume control of tone pots, is part of the guitar's design engineering.

There is symetry in manners the switches and pots work, illustrated in the Switch / Pots / Pickups flow charts.

My quandry is in understanding how the circuit works in the way it does.

2 discrepencies in the Switch / Pots / Pickups charts symetry are most likely errors or incomplete functions / symetry in my analysis (and probably missing some) of the 49 different combinations of controls afforded by 3 switches and 3 tone/volume pots, and infinite control of tone/volume pots ranges. But I am certain that the charts afford adequate insight into figuring out how this circuit works like it does.

------------

Regarding the ungrounded lug on the Master Volume pot's back side:

I am not arguing at all; Just presenting things as I see most of the circuit and SWAG'ing at how it works; Very open to all observations, rebuttal and ideas.

(1) The backside circuit lug of the Master Volume pot is open / empty in this original undisturbed factory wiring (see picture below).

(2) (???) The factory wiring's alternating signal current phasing on the front side of the Master Volume pot (wiring of "upstream" current circuit lug and "downstream" jack lug) is opposite of what a "Y" circuit would be for correct current flow phasing in grounding the backside lug; (???) For a designed-in reason. (???) The positive and negative sine / current flow direction would be opposite of the amp's case ground were the empty lug grounded in the factory wiring, and;

(3) (???) Were the lug grounded it would inject additional resistance to ground in parallel when the Master Volume were rolled back off of "zero" ohms and thereby effect the tone/volume pots functions / logic / scheme IF I am correct about hidden resistors being in the switch box (or some other "L padding" of resistors and caps in series / parallel that I might be missing), as it seems to me is neccessary for the whole factory-intact circuit to function as it is. ------- Folks that read the discussion chain about the 3-pickup Danelectro Dane I got stumped on for awhile last year, might remember that the final analysis found that Nate Daneiels was even using the pickups coils inductance in an "L pad" scheme of that very complicated / sophisticated guitar circuit.

(4) In any event, the Master Volume's backside lug is empty in the factory wiring, .....and again, I believe the guitar's entire circuit is working as designed. I just cannot find how it works like it does.

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Comments / rebuttal / ideas please.

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See enlarged pic of Master Volume pot backside lug here:
http://www.dennysguitars.com/files/1488MasterVolumePot2C.gif
(I posted the pic to this message but it expanded the page horizontally, requiring scrolling each sentence, so thus the link instead).

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THANK YOU,
_________________
Aloha,
Denny T~
http://www.dennysguitars.com/

Please help support humanity:
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Blake Hawkins · From: Florida

I suggest that your situation of having the tone controls from the turned off pickups interact is caused by crosstalk (leakage of the signal) between the wires going to the switch assembly and possibly by some coupling in the switch itself.

Note that while the wires are enclosed in the metal tube, they are not shielded from each other.

It is a high impedence, unbalanced circuit. Crosstalk can easily occur.

You photos and documentation are excellent.

Denny Turner · From: Oahu, Hawaii USA

Thank You, Blake.

That is certainly plausible and something I had not considered. I will study that angle and get back here with the results as soon as I can.

Tight symetry across all the controls functions is still a primary indicator imho.

And yep, the flex tube and switch might very well have enough mechanical crosstalk symetry to induce the electronic controls symetry.

Also indicative is that whatever it is, it's effect got the approval of it's original R&D (the circuit is original, intact, undisturbed); And dog-goneit, it does afford some pretty darn good tone across a wider string plane adjusted for the tone curve peak desired via the pickups selected, and control of that string plane scope and tone rolloff via the tone pots. A bit daunting to remember all the different knobs controls for a particular switch combination, but probably intuitive once a player got used to it.
_________________
Aloha,
Denny T~
http://www.dennysguitars.com/

Please help support humanity:
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