Author Topic: Agilent E3646A repair  (Read 1186 times)

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Offline STMartin

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Agilent E3646A repair
« on: November 17, 2016, 04:28:43 am »
I'm trying to re-repair an Agilent E3646A power supply.

I had bought it broken from EBay, and diagnosed the problem to a bad zener diode in the -15V bias supply. I had some confusion with the value of the zener, as they were marked 5.6 on the schematic, and to my estimation should have been 2.4V. Inaccurate schematics aren't unheard of, so I desoldered the surface mount diodes for both the +15V and -15V rails (they have the same part number), and measured the reverse bias voltage.

The bad diode was 0.6V both ways, while the good diode was about 0.7V forward, and 1.7V reversed. I measured a new 2.4V through hole zener, and found it's reverse voltage was about the same as the original good diode. I soldered in two 2.4V through hole zeners to replace the old ones, just to make sure it worked before I ordered proper surface mount replacements. The supply fired right up, passed the full self test, and everything worked great.

When checking the bias supply voltages again, I accidentally shorted the two zeners together. The magic smoke got out, and despite my attempts to stuff it back in, the only thing I accomplished was to curse and yell for thirty minutes. I replaced both diodes again, and now I'm faced with a new problem.

The display test comes on, all the segments light, but then it goes blank and beeps twice. According to the service manual, this is error 601, "Front Panel does not respond".

A link to the service manual:,%2047A,%2048A,%2049A%20User.pdf

The diodes I shorted are CR139 and CR140 on page 212

In my travels to diagnose this problem, I have:
  • Checked the bias supply voltages as listed on page 171. All are within spec.
  • Checked every voltage regulator, all are OK
  • Checked the ribbon cables and connectors for continuity and intermittent connections, all are OK
  • Ensured the PC board and front panel have the correct supply voltages, and they do
  • Prayed to any deity who might listen, none have responded

At this point, I'm not really sure what to look at next. Do I start checking op-amps? Poke around on the display board itself? That option would suck since the components are buried behind the VFD and I'd need a mile of solder wick to get it off.

Any advice from those better at this than I?

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