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21
Divide and Conquer.
To test the power amp separately from the preamp just lift that orange wire (comes off TR10 to point 0 on pcb)
That is the preamp out/power amp input. So rig up a temporary input socket, make sure it's got a ground connection to amp chassis and plug your guitar into that,,,
you should get a clean undistorted signal. it won't be super loud but if it can hardly be heard then the power amp has a problem.

Or better still plug in your mp3 player or other audio player with line out/headphone out  and it should blow you head off at full volume on your player.
If only weak signal is present then you know it's a power amp problem.

If Power amp test is good then you know the preamp is at fault.
Phil.
22
Amplifier Discussion / Re: Peavey Session 400 1976 troubleshooting
« Last post by Roundtone on July 11, 2019, 12:46:23 AM »
I was measuring to the voltage rail as well as looking at mV as V, so you will see a much different situation. (I am assuming that the power amp is working correctly maybe not at peak performance, but output is strong and without hum). All voltages were taken between chassis ground and leg on my chart. The Reverb circuit is still not functioning. Is the voltage  variance on the ballast resistors something of concern?
23
Amplifier Discussion / Re: Peavey Session 400 1976 troubleshooting
« Last post by Roundtone on July 11, 2019, 12:36:03 AM »
Voltages attached
24
Amplifier Discussion / Lab Series L3
« Last post by jman on July 10, 2019, 09:33:46 PM »
Can some one tell me how to set the bias on a lab series L3 Amp.
25
Amplifier Discussion / Re: Peavey Session 400 1976 troubleshooting
« Last post by Roundtone on July 10, 2019, 01:05:12 PM »
I will double check this evening and post voltage readings. Thank you
26
Amplifier Discussion / Re: Peavey Session 400 1976 troubleshooting
« Last post by Enzo on July 10, 2019, 12:52:31 PM »
If you have 50v rails, it is about imposible to get 100v anywhere in there.  Make sure your meter is connected to ground, usually chassis, and make sure the board ground is in fact connected to chassis.
27
Amplifier Discussion / Re: Peavey Session 400 1976 troubleshooting
« Last post by Roundtone on July 10, 2019, 10:57:00 AM »
Schematic
28
Phatt,

The distorion worked when I plug the guitar in and turn the pot for it.

I didn't have a chance to check the transistor.
29
Amplifier Discussion / Re: Peavey Session 400 1976 troubleshooting
« Last post by Jazz P Bass on July 10, 2019, 12:59:47 AM »
Please post the schematic.

Those 'sandbox resistors' are most probably the output ballast resistors.
The speaker output is taken off of them.
Therefore there should be zero dc voltage on them.
30
Does the distortion work?
Phil.
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