The ultimate dual 500 "rising from the ashes" build

[grapplesaw]

Need some help

so I spent to day checking and double checking everything

Bias problem was easy and fast one end of 10 ohm resistor not soldered.

So so I powered up slowly but as I did I get about 1/4 rail power as B + offset on the outputs with with or without any drivers. I was starting with the first row only. I desoldered the entire right side control board without change only fuses on. I only went to 20 volts across rails. Both legs equal voltage to ground so think power supply ok

I could just just take it up and see but why the offset so great.

What damage can happen if I take it up to full power?
Do I need second row to get offset in line?

Is there excessive B+ coming from the black/white/green connected to the 47 if 100 volt cap which is connected to the B+ board supply?

 

[laatsch55]

I know the old stock boards didn't come into regulation till about 60 rail volts. Can't say for sure on the PL 36 board though...

 

[grapplesaw]

I know the old stock boards didn't come into regulation till about 60 rail volts. Can't say for sure on the PL 36 board though...

Ok then
The circuit is upgraded to be idenical to the PL36 now so will go for it

tomorrow will see.

Thanks Lee

 

[grapplesaw]

Time to check in

after many hours of checking and rechecking I have solved my offset problem

After reworking the input RCA plugs and isolating them from the chassis I was left with the the ground open for both channels thus creating a full rail voltage offset.

Anyway i I now known exactly how a Pl36/32 control board functions, but cannot tell you why the values are what they are for the components

start to to put it back together tomorrow


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[grapplesaw]

Time to get some input from everyone

need help with right channel bias fault full rail positive across 10 ohm resistor in backplane

no pre driver or driver installed in back plane


Dc offset 118 mv left 58 mv right

control board

right channel
all transistors replaced with checked good ones , output 101 and 105 new
all caps on right new
all diodes checked ,
protection circuit removed from circuit

resistance over brown and black connector correct at 1.6k

bypassed red and brown tranny trams black removed from board has same result.
Black tranny connection point on board voltage = full rail positive left is -11 volt
bias measurement over 10 ohm resistor full rail positive

left bias .55 mv
dc offset 118 mv

rail voltage 80 volts and 120 volt no reading change with either voltage

not sure what to do
one channel good enough to load drivers
want to get rid of bias over voltage in right first.

Any ideas please post

 

[laatsch55]

What's a tranny tram?

 

[grapplesaw]

What's a tranny tram?

It's the bias transistor 2n3403

 

[grapplesaw]

This is the problem area

D101 lifted from board
all parts good

excesive voltage at connection 10 circled on left of the 2n3403 transistor

 

[laatsch55]

All resistors in spec? VR good?

 

[grapplesaw]

All resistors in spec? VR good?

Both channel resistors equal and in spec

new caps with silver mica in lower pf caps. Same readings before and after changing. Double checked aluminum cap polarity

Swapped out q106 40412 - no change

VR 15 volts both + and -
I even changed them out as well just to cross that off

only one channel with problem

all wiring checked about 50 times.

Problem on on board as I unhooked blue and black/ white/ green and no reading problem on backplane after that

 

[Gepetto]

This is the problem area

D101 lifted from board
all parts good

excesive voltage at connection 10 circled on left of the 2n3403 transistor

View attachment 24057

A debug hint to let you find bias problem areas is to short C-E of the bias transistor (4 to 12 on your schematic) with a short jumper tacked in. The amp will work fine in this mode, will just have more crossover distortion than with the bias transistor not shorted.

 

[laatsch55]

What's the base voltage of the 2N3403 on the good channel?

 

[grapplesaw]

What's the base voltage of the 2N3403 on the good channel?

-11

the right is +80

 

[Northwinds]

Crossing fingers for you man, your on the home stretch!!!

 

[grapplesaw]

Will look for a hidden short

 

[laatsch55]

Joe, when you short C-E does it represent full gain of the Bias Tranny or no gain, or neither?

 

[Gepetto]

Joe, when you short C-E does it represent full gain of the Bias Tranny or no gain, or neither?

Hi Lee
It represents a transistor with huge beta (gain) and with an ability to saturate at near zero volts Vce

 

[laatsch55]

So it will eliminate the bias tranny as the source of trouble, if shorting it cures the offset problem?

 

[grapplesaw]

Hi Lee
It represents a transistor with huge beta (gain) and with an ability to saturate at near zero volts Vce

My thinking is Shorting C and E. Should create an equal output from both pre drivers and show none or very little Bias voltage

 

[grapplesaw]

I will remove the Black / white / green wire and see if it is back feeding the circuit

there is no continuity from collector and emiter with power off on backplate as well there is no offset between the positive and negative collecters