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Agilent E3632A +/- 17.4V rails out 0.6 - 1.2 volts, this a problem?

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I have an Agilent E3632A power supply. I'm troubleshooting an intermittent problem where after turn on, the display turns on, all segments light up then goes black with a single beep. I hit this about 1/10 times turning the unit on. When the unit starts successfully, it seems to function normally. It can drive loads within its specified voltage and current range.

To troubleshoot I was measuring the bias supply voltages. For the +17.4V rail I get a reading +18 volts. For the -17.4 volt rail I get a reading of -18.6V.

Using the Bias Supply schematic as a reference, I tested the capacitors, bridge rectifier and diodes in this part of the circuit and they seemed OK. I suspected perhaps an issue with the resistors feeding into the ADJ pins of the LM317T and LM337T but the matched the values in the datasheet. Might both regulators be going bad or is the voltage being high by up to a volt really not a problem?

Service guide with schematic: https://xdevs.com/doc/HP_Agilent_Keysight/E3632A/pdf/E3632_service_sch.pdf

The -+/- 17.4 V supplies are only for the display. I doubt they are that critical. It's telling that the troubleshooting steps don't tell you to measure those rails, but the +/- 15V that are referenced from them. So I would compare the +/- 15V rails with the values in table 5-1 in the manual.

I would also look at the ripple of the rails. Dry electrolytics are a common issue in older equipment, and excessive power supply noise could explain the intermittent operation of the digital circuits.

I get about 15 mV RMS ripple on the 4 bias supplies, which is within the input noise of my DP10007 probe in 10X mode. So I think the noise is OK.

I checked the bias supply voltages. All but one were within spec and very close to the center of their range. The +15 V supply measured 14.04 V below the bottom end of the valid range from 14.25 V-15.75 V.

The Zener diode CR9 which feeds the drooping +15 V supply has a temperature around 65-70 degrees C (according to a thermal cam). CR10 which feeds the -15V supply shows around 50 degrees C.

This CR9 temperature seems high to me like something is drawing too much current. My next steps was going to be to try to figure out what that may be.

Does this make sense? Any pointers on what to look out for?

Sounds indeed like excessive current is drawing that one rail down. My first guesses would be either a shorted tantalum cap (I don't remember if these supplies have any) or an opamp that's oscillating for some reason. Is there anything else that appears excessively hot on the thermal cam that you wouldn't expect and is connected to the rail?

I tested all the tantalum caps on the board and they all seemed OK with no short circuit and they were not hot on the thermal cam.

There are 3 op amps that are some of the warmest things on the board. They are all around 45 degrees C. So not overly hot. I think and since they are all around the same temp, perhaps it's normal. (Op amps are U38, U41 pp 121 Power Circuit and Protection Circuit and U31 pp 123 DAC system).

There was also resistor R112 at 85 degrees C. It is a 200 ohm 2 watt resistor part of the Power Circuit and Power Protection Circuit on page 121. It's fed by the +15V and then goes into Q11 a p-channel MOSFET. I took the MOSFET out of circuit to test it and it checks out OK.

With the MOSFET out of circuit (no current through R112) I retested the +15V supply. It reads +14.44 volts (up from 14.04V). So perhaps the problem is near here. It's not clear to me what the purpose of this part of the circuit is for. Why use the MOSFET to connect the +15V to OUT- in this circuit?


Full circuit in page 121 of https://xdevs.com/doc/HP_Agilent_Keysight/E3632A/pdf/E3632_service_sch.pdf


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