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Agilent 66309D Power Supply Repair (schematic)

Question asked by Jester on Mar 26, 2016

First post here.

I'm repairing a 66309D that has a defective main output, perhaps someone with more experience can provide some assistance.

_The problem_ 
With no load the main output operates as it should, I can set the voltage to any value and the supply responds and meters correctly, however under load (any load greater than about 5ma) the output has short duration noise spikes every 3ms, the spikes when expanded are actually a 10 cycle burst of 400kHz.

As a test, I isolated the output regulator gate drive signal (lifted U309A-1) and used a lab supply to drive the Q303 gate, when I do this the output is clean and controllable under load. With U309A driving Q303, the noise burst seen in the output is also present in the U309A OP amp rails, so I believe the gated opamp power supply is the root cause. The block diagram refers to this supply as a gated bias +/-15V supply (it gets disabled when the output is disabled). With no load the rails are clean and the u309a op amp and hence the output is clean.

The gated supply schematic is not included in the service manual.

Only while under load the gated supply shifts up as if the common point is connected to the main output +ve supply, I'm guessing this is abnormal however without a detailed schematic I can't be sure. So for example when the output voltage is set to 5V, the positive op amp rail (u309a) shifts up to +20V and the -ve opamp rail becomes -8V. 

Output waveform when loaded

Can anyone tell me where I can get a detailed schematic of the gated bias supply, so I can determine the problem?

Edited by: Jester on Mar 27, 2016 4:02 AM

Edited by: Jester on Mar 27, 2016 4:04 AM