Welcome to Solid State Guitar Amp Forum | DIY Guitar Amplifiers. Please login or sign up.

March 28, 2024, 07:05:44 PM

Login with username, password and session length

Recent Posts

 

HELP?? Noise while turning/adjusting ANY instruments volume.

Started by Theors, August 17, 2015, 12:25:25 AM

Previous topic - Next topic

Theors

Thank you Enzo,... just completed the straighten of all bent tongs and used ohm meter to confirm proper positioning (and visual too) with and without cord plugged in. The amp is clean as a whistle (and down & dirty). Hummmm....TRS jack? Tip-Ring-Sleeve jack I'll remember that for sure!!! :)
'82 Carvin DC200 + others

Theors

Reading back......your post all makes sense now! I guess "I is just too slooooow :duh" Thanks for the "edumacation" J M Fahey.
And I did find those exact jacks online if ever needed. :dbtu:

Quote from: J M Fahey on August 17, 2015, 07:33:42 PM
1) thanks drgonz for the schematic  :dbtu:
2) 13.7V DC???  OUCH !!!!!!!!!!
3) I see you are sleeping with the enemy  :o
Don't get the full picture yet, but the schematic shows what looks like an "input in use" sensor circuit, not sure if it's some kind of mute or it enables some function, but there you have 2 x  22k resistors connected straight to jack ring pin leaf contacts and on the other end to "CA" .
Said ring pins are grounded if nothing is plugged in, you'll have current passing through the 22k resistors.
If you plug into any of both inputs, one 22k resistor gets open and you halve passing current; if you plug in both, you fully stop current there.
That can be detected for some function I mentioned.
The only "CA" I find (you might find others) is on the emitter of "Y" 2N5401 , to the right bottom of the tubes area, which is also connected to the power supply node "C+" , which happens to be .... +13.7V  :trouble

Now those +13.7V shouldn't be a problem because if:
a) nothing is plugged in, 22k resistors get shorted to jack ring legs , so 0V there.
To boot, jack tip legs are also grounded, as an extra security layer.
b) you plug into 1 or 2 inputs:
the jack tip contact gets ungrounded, and signal gets into the preamp.
c) the jack ring contact also opens, current through 22k resistors stops, some function is triggered.
You have +13.7V 2 mm away from signal contacts, but it "SHOULD" be no problem, meager 13.7V can (should)  not be able to jump 2mm of air or PCB material .
I guess either the jack is damaged, has bent leads, whatever, or resoldering shorted nearby contacts or leaf switches are worn or bent or PCB is contaminated , and now those 13.7V cxan contact the *audio* pins.
They travel backwards to your guitar and make a scratchy mess.

So I would replace those jacks, while out clean the PCB area very well, I'd use Isopropyl alcohol because it dissolves both grease and salts (Big Brother does not let you USA kids to buy 94% ethyl alcohol ;) ) and recheck that DC voltage dissapears.

This is what tip and ring means, applies both to plugs and to matching contacts in jacks:


By the way the amp schematic has tip and ring jack contacts drawn the wrong way, tip (wherev 68k resistor go)  should be drawn the furthest away from the sleeve/ground contact, and ring between them ... as it physically appears in a plug.

And the input sensing circuit is also wrong: as drawn, whether 22k resistors are grounded or not , voltage at 2N5401 transistor does not change, since it comes straight from the nominal "+15V" OpAmp supply (actually +13.7V) called "+C".
Those errors make me lose trust in that schematic.
It may be close enough as a general idea but each part should be checked/matched with what's actually on the PCB.
Of course, the amp works or they wouldn't have sold so many but the schematic is iffy.
'82 Carvin DC200 + others

g1

Inexperience does not always equal stupidity.  Not only is this a TRS jack, but has 2 internal switches (normally closed), one on the tip, one on the ring.  Thus the 5 pins.
The schematic representation for that jack would be diagram XII on the attached page.
And they get much more complicated than that.  Switchcraft has a whole other page of more complicated type jacks, this is the "simple" page.  ;)

Theors

Thank you g1, I feel a little bit better after looking at the Jack schematic page  :-\
'82 Carvin DC200 + others

g1

  In case anyone is interested, attached is pg.2, with the more "complex" jack schematics.