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Phase Linear 400

I found and fixed one problem. 9R (blue) had broken off the Base of pre driver Q11 and was making contact with the collector of Q11.Both sides are the same now. The bias is way to high ,they both start 280mVand quickly rise This is with a100w DLT ,pots fully CCW .I'll be checking wires
 
the offset is low and negative on start up. it was -26mV and going up towards zero. It appears its adjusting to "normal". The bias quickly went to 325mV so I shut down. Both sides are the same. The pinout is marked on the 2N5088(E-B-C) .but I have checked them and there installed correct .What would be common to both channels that would make the bias act this way ? Voltage regulators ?
 
The offset should not change that much, maybe 2-3 MV between DBT and NO DBT. Gonna have to think on this one awhile. Is the DBT staying dim or is it getting brighter?
 
The voltages are good .I'll leave the unit longer on still connected to the DBT and see if the bias will plateau .
 
after about 5 minutes the bias slowed/ stopped at 327mV on R and 323mV on L. Joe at White oak told me it takes time for the 2N5088 to work ,but I forgot to tell him I had a DBT connected . I sent you his reply to your email . Maybe I should leave it on and see if the bias starts to drop.
 
I'm back. Its been on about 10 minutes and one side is 320MV and the other is 325mV there is a little fluctuation up and down .DBT in circuit. Joe told me if the rails are up to voltage( They are + -78v) and nothing bad has happened I should put the unit on a Varriac , I have one
 
I removed the DBT and put it on a variac . At 60vAC the biaswas 250mV,this seems high to me ? I shut down at 60vAC .If I go to 120vAC the bias will be to high . What do think ?
 
I'll try it after work today .There is not going to be much if any adjustment on trim pots. What about decreasing the value of R18 ? I'm going to check that voltage point R17&R18
 
OK , with unit on full AC for 45minutes bias was 341mV on the left and 332mV on the right,pots fully CCW The amp does have a hum and when powered down 5VDC on the right speaker terminal. Would that be the speaker "thump" people talk about ?I didn't have a meter on the left speaker terminals .
 
Hi Jim
Your bias seems to be regulating properly which is good. You should tweak these when warmed up to approx. 0.350V on each side.
I asked you an earlier question:
“I am assuming you have knocked the original 2N3403 transistors out of the heat strap and glued in the 2N5088s as replacements?”
If the 5V you are measuring is a temporary bump about 10-15 seconds after turnoff, then yes this is the turn off thump. Did you install the back to back diodes on C6 as the instructions call for?
You should not be getting hum through your speakers. I am assuming you are meaning hum through the speakers rather than hum from the power transformer.
Is your rectifier bridge confirmed as good? Have you looked at your plus and minus rails with an oscilloscope?
Is the hum confirmed as there with just the amp with speakers connected and the inputs to the amp grounded? If there is no hum with this configuration then the hum is not from the amp but rather from an upstream component or a ground loop.
Did you install the new cap kit you purchased?
 
Yes to the 2N5088 ,Yes the hum seems to from the transformer (just louder than I'm use to ) , yes to the 5V after turn 10-15 sec and it quickly falls.Rectifier is new .Yes to the new cap kit .Yes to the diodes on C6's one diode with the C on the cap and one with the A on the cap (back to back ?). I haven't hooked up speakers or any other input device. I haven't look at the rails with an Oscope. The trims pots are fully CCW so I can't adjust them. Could the high bias and hum be related ?
 
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