HELP! Rescuing Dad's old PL400 S2 (pics)

lightfall said:
Sorry I did NOT adjust yet, that was the reading without any adjustment. I am still looking for a small enough piece of plastic to turn the two pots on the PCB.

I assume I make a small counter clockwise adjustment to each side respectively and when changing sides I wait >5 minutes to attach or detach leads.
Click to expand...

Is that the same reading on each channel?
 
I don't have any resistors. I am nervous about the thumbwheels! I will summon the courage and report back my readings as I go through the rows
 
OK so I tried to dial down the bias from the first reading. Looking at the PCB board face on while it is mounted the left pot only turned clockwise a small amount. I sprayed some deoxit and tried again but sure enough with the drivers in and the first row of OT the lowest the bias will read is 330mv. Turning the pot counter will ramp up the bias 3,4,500 MV etc and the DBT starts to glow brighter.

What do I make of this? I will wait 5 minutes and check the second channel.
 
Yea minimum is the same on the other channel /pot. 334mv with the first row installed. I will install the next row of OT in the morning and see what it reads next? What do you think Lee?
 
Still within spec Brad, you're lookin good...keep the bias turned as low as it will go then till you have all the outputs in.
 
We're trying to kep the bias low so we can tell if anything is bothering the DBT....the fact it dims tells us it just might work...(the amp that is)...two rows to go correct?
 
Yes sir, I plan to follow the rest of the procedure in the morning. Something about being tired and connecting those probes in a live circuit doesn't mix!
 
I can TESTIFY to that young Brad....I've done that wayyyyyyyyyyyyy to many times...
 
OK all rows are in and bias is 375 MV on both channels using the DBT. I am letting the caps bleed and then going to test on regular outlet and if that checks out then I imagine I will button the unit up, have a beer, and do a test listen if I get done before I am supposed to meet my friends for football!
 
Bias will rise on bypassing the DBT. Also check DC offset for both channels...
 
DC offset I have the unit plugged in without inputs or outputs I let it warm and then w the mm in DC MV I look for a reading of less than 10mv correct? How much does the bias rise when going to wall AC?
 
Green Bay Packers all the way! My wife's sister from Philly is coming in tomorrow so we better win today or I will get verbally assaulted tomorrow.
 
lightfall said:
DC offset I have the unit plugged in without inputs or outputs I let it warm and then w the mm in DC MV I look for a reading of less than 10mv correct? How much does the bias rise when going to wall AC?
Click to expand...



DC offset can range from a few MV to 75 mv ( acceptable range) I've seen it vary from 1.2mv to 100 VOLTS DC. In your case not much can be done to lower DC offset as that is determined by the op-amps. How far the bias will rise depends on the condition of the bias transistor and the tolerance of the relaterd RC networks.
Usually 25-30mv, but not always...
 
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