Author Topic: Agilent E3642A Unreg and ripple above 17.5 V  (Read 609 times)

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Offline JohnPiTopic starter

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Agilent E3642A Unreg and ripple above 17.5 V
« on: May 17, 2020, 10:00:34 pm »
My E3642A doesn't work well. When I look on the scope, at output voltages above 17.83, it shows a 120 Hz ripple, and the DC output voltage isn't very accurate. at lower voltages it is reasonably accurate. In addition, when I turn the knob to adjust the voltage, it jumps in steps of about 1.1 V in the display, not 1.0 as you would expect.

Before I tear it open and look at the transformer and caps, are there any suggestions for what to expect ? This unit was from the auction site and appears to have fallen on its front -- the front binding posts are damaged, but nothing else seems amiss. It can supply the rated currents.
 

Offline JohnPiTopic starter

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Re: Agilent E3642A Unreg and ripple above 17.5 V
« Reply #1 on: May 17, 2020, 11:21:22 pm »
I opened it up and probed some voltages. The main transformer output wires (Red, Black, White) had 26 V red-black, and 13 V black-white. Suspecting that was not providing enough headroom for the regulator, I moved the 2 slide switches beside those connection points to the '100 V' setting instead of the '115 V' setting it was at (I DO have 115 VAC). Now those voltages are ~ 30VAC and ~16VAC and the output goes to 21.8 V without any unregulated error. It also calibrates correctly for output V & I.

I don't know if the transformer is damaged (from a drop) -- it doesn't seem to overheat..

While the output voltage is correct at light and moderate loads; if I load above about 2 A, it comes unregulated again :-(. Are any (even partial) schematics of the power stage available ?
« Last Edit: May 18, 2020, 01:06:23 am by JohnPi »
 

Offline JohnPiTopic starter

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Re: Agilent E3642A Unreg and ripple above 17.5 V
« Reply #2 on: June 02, 2020, 04:55:28 am »
I was able to fix the unit.
 
The design has a transformer with 14 V and 22 V outputs. For the low V (8 V) range, the 14 V tap is used, for the 20 V output, the 22 V tap is used. This switching is via triacs that are fired via optocouplers. I'm not sure why, but there is a phase controller IC which seems to be used to fire the triacs at the most appropriate time. After replacing the triacs (which weren't bad), checking the optocouplers and their driver (which all seemed good), I eventually found the original service manual online with real schematics. Tracing the circuit through, I found the PCB trace open circuit between P106 and CR601 on the following schematic. It may have been broken if the unit fell. A new jumper wire fixed it !

The manual is at http://ftb.ko4bb.com/getsimple/index.php?id=download&file=HP_Agilent/Agilent_E3640A_E3641A_E3642A_Power_Supply_Service_Manual.pdf The relevant schematic is
« Last Edit: June 02, 2020, 04:52:15 pm by JohnPi »
 
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