ebay Phase Linear 700B rescue

[laatsch55]

Got plans on White Oaking that puppy...

 

[Billboard]

Sure, white oak is defiantly in the future, not important that it runs first?

 

[laatsch55]

Hell no!! But it's always fun to see what's wrong.

 

[Billboard]

I agree, I found a portable scope and can use my iPad with a function generator app. I will post the output waveforms here.

 

[Billboard]

Ok, under no load both channels look clean, under an 8 ohm speaker load the right channel is missing almost all of the bottom end.

I need to get some 8 ohm loads, speakers at constant loud tones is torture.



The iPad puts out about 1.4 volts peak to peak, not sure how many volts the amp outputs at this level, will attempt this measurement when I get some 8 ohm loads.

 

[laatsch55]

I'd be checking the negative pre drivers and drivers, and protection circuit trannys....

 

[laatsch55]

Is it consistent with frequency?

 

[Billboard]

The drop out occurs at all frequencies.

 

[laatsch55]

I'm not at home or i could tell you what wires to the control board to remove to cut off the base drives to the backplane.. that would somewhat isolate your problem to the board or the backwall. After you isolate the base drives, scope the output of the board then the speaker outs and see if they are the same...

 

[laatsch55]

What board do you have?

 

[Billboard]

20A

 

[Billboard]

What wire should I lift, looking at the schematic, maybe the yellow wire that connects to the Q14 collector to the driver board.

then compare the output waveform to the waveform at the base of driver Q12??

 

[laatsch55]

Base of Q12....

 

[laatsch55]

Then check the collector of Q7....

 

[laatsch55]

Detach the base drive of the positive row too otherwise it'll try to compensate for the no negative drive. Another symptom of the truncated waveform is the protection circuit. Unsolder one leg of d9, or D10 and scope it again before removing the base drives. Should have had you do that first....

 

[Billboard]

Just want to be clear, lift the wire off of the base of Q12, then compare the output to collector of Q7?

 

[Billboard]

OK, I will remove one end of the diodes, then report back.

 

[laatsch55]

And scope the collector of Q7 before we unhook any wires....make sure were in the right place...

 

[laatsch55]

Don't unhook any wires yet. I remove the base drives on the white oak boards a lot, but I better check with Joe on the original 20 board.

 

[Billboard]

With D9 and D10 removed form the circuit, the waveform looks normal, no distortion with a full sine wave at the speaker jacks.