broken presence help please...

[Icetech]

I have an amp that i had built in a princeton reverb chassis about 20 years ago.. Somewhere along the way i stole the presence pot out of it for another project and when i put it back i have never gotten it to work again.. amp sounds fine but no presence control and its just a OCD thing making me nuts:)

Most things i am ok on figuring out.. presence has always been an issue with me sadly..

I will post a couple of pics of the wiring, and will say that impedance to ground from the NFB wire is 10k turning the pot never changes that.. but i do have a 5k sweep from lug to lug on the pot.. I am not sure exactly whos circuit the guy used that built this.

[img576]https://dl.dropboxusercontent.com/u/696 ... .43.16.jpg[/img]

[img576]https://dl.dropboxusercontent.com/u/696 ... .43.58.jpg[/img]

So.. that shows the pot itself which is a 5k pot.. and the other pic shows where it is connecting into the circuit..

The only thing i found that looks close to that pot setup is from jcm800 a guy built..

I had some ideas on fixing it, but wanted to ask before i release the magic smoke outa this amp:)

P.S. before anyone gets sad about destroying a princeton.. This was done in the late 80's and the amps were literally gotten for $50-100 bucks.. i have one that lives as a reverb unit now also This particular one has Traynor 50watt iron and a 12" speaker instead of the 10.. fun amp..

[Stevem]

That solder connection on the pot side of that Ciramic cap does not look too good, is it?
Also is the pots case making a good ground connection, and it's hard to tell by the photos, but it sure looks like the pots lugs shorting out to the brass plate?

[martin manning]

The presence lead always shows 5K to ground because the pot is grounded through the 22uF cap. You're measuring the fixed resistor soldered to the pot.

Where is the feedback from the OT secondary? You should have a resistor connecting the CW lug of the pot to one of the OT taps.

A 5k pot is used with the old style scratchy presence (which sounds better IMO). To wire it that way, get rid of the 22uF electrolytic cap, the resistor, and the ceramic cap, and put a 0.1uF cap from the pot wiper to ground, and ground the CCW lug.

If you want the newer style presence, replace the pot with a 25k, put a 0.1uF where you have the 22uF, get rid of the ceramic, and make the resistor on the pot 4k7.

Either way you need a source of feedback.

[Icetech]

thanks martin.. will switch it to just the .01uf setup..

[martin manning]

Typical presence cap value is 0.1uF, but I don't know what circuit you are working with. I see now that the feedback is connected through the brown disc cap on the board to the purple wire going to the presence pot. In typical circuits that would be a resistor.

See my edit above re grounding the pot for old style presence!