PL700 Series 2 NO bias adjust ?? both channels

[Greg]

So I got another project a PL700 series 2 But no Bias Adjust on both Channels ?

.008vdc

90 vdc rail ok
ziener volts 15 Vdc ok

used heat gun on Bias Transistors and no change..

 

[MusicSteve]

Greg
Should check the Bis, using the other resistor on the Lt side.
I would clean the back of the board w a tooth brush (and do a Visual inspect as you go)
and also clean the back and inspect everything after cleaning.
after that I would change out both Bias Transistors
looks like a Full comp amp (which sound better)

Whats your plans?

 

[Greg]

ok thanks yes ill do a good clean and test / replace the bias transistors along with checking all the power transistors
so I should do Bias check on the the other 10 Ohm ? I was doing both 10 ohms just to compare

 

[Greg]

Just to confirm PL 700 series 2 Bias check resistor here and here ?

 

[MusicSteve]

yes
The power Transistors when bad will normally short out and blow fuses so your probaley good but it would not hurt to see what you have back their which will tell you if they mixed and match them .

 

[mlucitt]

I would also check the resistance of the 10 Ohm Bias Resistors. Yours look original and they have been known to drift out of specification.

 

[Greg]

Thanks ! will do Am I on the correct Resistors for bias check ?

 

[laatsch55]

Yep..

 

[wattsabundant]

B+/- fuses are ok, right? Oxidation on the resistors could cause no apparent bias. Shorted bias transistor (unlikelly) would result in no bias.

 

[Greg]

Ok very good ! Yes Im going to do a clean and deOX it thank you for suggestion

 

[Greg]

So Picked up a couple testers On Amazon / Ebay for testing transistors and caps etc Its pretty cool does all the work and identifies the leads on unidentifiable parts ! has anyone used these with any luck ?

 

[mlucitt]

Those Chinese chip-based testers are as good as the programming inside the chip. They are great for comparing similar parts and good vs. bad parts. As long as the battery is strong, it will give consistent results. Just don't expect full power or full current flow through any part to determine internal breakdown or leakage.

 

[Greg]

Sorting out ZERO Bias volts on BOTH Channels ...Just Got back to AZ today Ill post pics of test results in a methodical way to show what I have going on for test results .
If there are any specific requests of test points ?
Thx all !!

 

[Greg]

Bias channel A

 

[Greg]

Bias channel B

 

[Greg]

Rail voltages left to right pic set 1

 

[Greg]

Rail
Voltage pic set 2

 

[Greg]

Rail voltage pic set 3

 

[Greg]

Rail voltages pic set 4

 

[Greg]

Output volts A