PL700 Series 2 NO bias adjust ?? both channels

[Greg]

Output volts B

 

[Greg]

Regulator diode A

 

[Greg]

Voltage regulator diode B

 

[Greg]

Load resistor A

 

[Greg]

Load Resistor B

 

[Greg]

Last thing I did to try was spray some De ox across R107 ( bias adjust pot ) and move it around to try and clean. And nothing really changed

 

[Greg]

Both hi temp cut outs read .003 ohms across

 

[Greg]

Well Jeepers Creepers ! Looks here funny when you read the story out load to yourself …no fuses ! So embarrassing but one step forward

 

[Greg]

Omg so embarrassed no fuses. Well it came to surface

 

[Gepetto]

Why are you measuring collector rail to base rail Greg?

 

[Greg]

My mistake I was comparing left and right .................................then realized the fuses were out......... so replaced the fuses got proper Bias adjust on Left Channel ..... none on right and now removing power transistors to check for failed one... Ohhh boy what a day

 

[Greg]

Figures wife called said car wont start from shopping mall I said try the right keys................hurt is slowly going away its that kinda day

 

[MusicSteve]

Greg
before power up , you might want to power up w a Dim Bulb Tester to make sure you don't damage anything.

 

[Greg]

I used a Variac and brought it up slow and tested and let it settle in slow yes ill have to make one for sure thank you

 

[laatsch55]

Greg, A variac will limit voltage but not current, ergo the DBT...

 

[wattsabundant]

I used a Variac and brought it up slow and tested and let it settle in slow yes ill have to make one for sure thank you

I've been using the bulb for itnitial power up for more than 40 years. Variacs don't provide any current limiting. The bulb never fails. The variac will allow a catostrophic failure.

 

[Greg]

Yes Ill have to make one !! i just kinda squint at the moment lol

 

[Greg]

Is it Normal for R 1 / R2 7.5k ohm resistors to run hot even without load ?

 

[laatsch55]

Yep..

 

[Gepetto]

Is it Normal for R 1 / R2 7.5k ohm resistors to run hot even without load ? View attachment 71155

You bet, that is the nature of the beast with a shunt regulator...

It is also why you see the toasty brown marks forming around the terminals where those resistors are soldered to the board. Eventually, due to the heat, those joints oxidize and become unreliable solder joints and the trace adhesive eventually gives up around the pad. This was one of the rookie design errors that PL made with this board.