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selva prabhu

New Member
Dear All ,
Kindly assist me resolve my DC power source problem (Agilent E3612A spec 0Vdc to 60Vdc /0 to 500mA)
When power supply is turned on the voltage directly hits to 92 Vdc can't able to decrease and cant vary the current also.
Opened the power supply found negative terminal voltage regulator faulty (7912ct mc RFJO47G) .
replace that regulator with this part number (7912ct mc RHT830G) but still issue cant able to rectify.
Kindly assist me rectify this issue .


Well-Known Member
Ok, looking at the schematic in the manual from KISS...your 1st order of business is to verify good Bias & ref Voltages. The +/- 12V and the +5V.
Once that is ok and the problem remains the voltage control loop that throttles the series pass transistors must be investigated.

The voltage error amp should produce a max error signal for the 92V you describe and be trying to drive the 'series pass transistor block' OFF. Now if ANY of those series pass transistors have failed shorted you'll get an overvoltage and loss of control as you describe.
So check for the err signal from the opamp, if it IS there it should be a +ve voltage trying to switch off the gate drive via PNP transistor Q2 which is biased 'off' or +ve by default. If you see a -ve control voltage with excessive main VDC output then the control opamp can be faulty.
Once there is +ve control drive from pin 8 of the control opamp then it is ok.

At this stage you can verify that Q2 is off leaving its emitter +ve. If so this +ve gate drive to the P channel series pass should throttle them off to reduce the main output V. Thus a +ve Q2 emitter coupled with your 92VDC error condition points to a shorted series pass Pchannel. Pull them and test them.
If the gate driver : Q2 emitter is -ve with a +ve control drive from the opamp pin 8, then it needs to be replaced.

I hope this helps.
agilent 3610 sch.jpg


Well-Known Member
Also, check your main output/smoothing capacitors for functionality/ESR. IDK their VDC rating but if they're under 100VDC you may need to consider replacing them due to the voltage they have been sustaining.
After you've fixed the PSU.
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