Bias Problem

[rhinson]

hello there, double check and make sure your resistor is 47k and nod 470ohms or 47ohms, and also check your pot and make sure it's in working order and not shorted from end to end. your supply is either not producing any voltage or you're bleeding it all off to ground. also check and see what the ac voltage is after the 220k resistor and what the dc voltage is after the diode. these measurements will help in telling you what's going on. rh

[robkeeler]

220K -> 150VAC -> Diode -> 0VDC

Should I be getting DC after the diode?

The band on the diode is facing the 220K resistor.

[mlp-mx6]

Yes, there should be a negative DC voltage there, in the -30V to -40V range, perhaps as much as -50V. There IS a filter capacitor there too, right? (Since I can't see your amp I have to ask the dumb questions...)

You should have...

220K resistor
diode
filter cap with the + side to ground (if you're using the Kelly '90 schem/layout you will have a network of 2 resistors and 2 filters)
20K bias pot with the negative voltage on the wiper, and a direct connection to ground on the other
wire from the other end of the pot to the 220K resistors at the PI

[robkeeler]

Yes there is. I'm using the layout based on "richies_fixed_tw_184.jpg" The only thing I haven't changed in this bias section is the 2 10uf caps. I'm gonna change these and give it a go.

[mlp-mx6]

OK, so what negative voltage do you measure on the 220K resistors in the PI?

Are the filter caps mounted w/ the + side to ground?

[robkeeler]

0V at the 220K junction and the 15k/47k junction. The caps have the posistive side to ground.

[mlp-mx6]

Are you sure that diode is working properly? How about replacing just that? There should DEFINITELY be a negative voltage on the "other" side (non-AC) of the diode.

[robkeeler]

I already replaced the diode because I thought the first one might be the culprit. I might have put in a dud.

[rhinson]

hello there, check your diode with the diode check function on your mutimeter---most have that function these days. you must have either a cold solder joint there (ie--the diode/cap junction where you're getting 0vdc) or maybe you accidently put the ground wire there under the board (i can't see your board so i'm pulling reasons out of the air). you could take out the old diode, check a known good one with your test function, solder it in to make sure you have a good solder joint. if the diode is good, then somehow here you have grounded out your neg. volage supply. where is the diagram you used----i didn't see it in the files section. rh

[Richie]

yes make sure you didn't leave off the ground wire on the end of the board.

[robkeeler]

I had a friend of mine go through the amp. It was a grounding issue. The nut holding the the ground lug down was a little loose.

It's always the the most obvious thing!!!!!!!

Although I would have never realized it if he didn't go over my amp. I really owe him big time.

I want to thank all of you for your advice and help. This really is a great forum!