Test Equipment Pr0n: The HP 3312A

A few weeks ago, I purchased an HP 3312A 12MHz function generator for the lab.  After living without a decent signal source for years, I figured that it would bepo handy to have a good general purpose function generator around.  A quick visit to eBay and a few clicks later, one was on the way.

Unfortunately, when I first powered it up, the output was clamped to one supply rail and the sync output was giving me a much-too-large, out of spec voltage swing.  D’oh!

The generator was sold as-is (like most test equipment on eBay), so I decided to take a crack at fixing it myself.  Armed with a barely intelligible, poorly scanned-faxed-photocopied copy of the 3312A service manual that I downloaded from Agilent’s website, I loosened two captive screws and slid the top and bottom covers off the unit.

What I found inside really blew me away.  What follows are some snapshots of the unit.

The 3312A is the most elegantly designed and well-preserved piece of electronic test equipment I that have ever owned.  The circuit boards, which look brand new, use entirely two-layer  through-hole construction and are laid out with generous component spacing and helpful silkscreen labels.  There is no inter-board wiring to prevent quick removal of any of the PCBs; all of the wiring harnesses use straightforward connectors.  Connections between the top and bottom PCBs are via clever gold plated removable posts that extend through the center panel of the instrument chassis.  The chassis itself, which is cast aluminum, is light but sturdy.  Every aspect of the instrument design appears to have been carefully thought out and is perfectly executed.

Here’s the 3312A on the bench, ready for some serious troubleshooting action:

HP 3312A Repairs

The aforementioned aluminum chassis.  Very nice!

HP 3312A Repairs

When I flipped the generator over, I immediately noticed a problem.  Here are the remains of four 200 ohm, 1W carbon resistors, burnt to a crisp:

HP 3312A Repairs

These resistors provide an internal 50 ohm termination for the sync output, and explain why the sync voltage swing was out of spec.  A quick trip to Jameco for some new 200 ohm power resistors and the sync problem was fixed.

The broken main generator output took some more serious troubleshooting.  One of the emitter follower transistors that drives the push-pull output driver was burning hot to the touch and a good candidate for replacement.  An hour of troubleshooting with the diode test function of my Fluke 87V identified one of the push-pull transistors had failed as well.  This is the device that had failed short and was clamping the output voltage to the -15V supply rail.

The final push-pull drivers are shown here; they are the two devices with the largest heatsinks.  The emitter followers are the two metal can transistors just to the left.

HP 3312A Repairs

I was able to find suitable replacements for the failed transistors at Jameco.  Neither of the original devices were still available but I was able to find some devices that were “close enough” by examining a few datasheets and cross reference guides.  With the faulty output devices replaced, the generator powered up and was good as new!

Here’s another couple shots of the main PCB.  Gorgeous gold-plated traces and component layout, and some pretty components too:

HP 3312A Repairs

HP 3312A Repairs

Here’s a shot of the inside of the generator with the top (modulator) PCB removed so you can see the header posts that connect the top and bottom PCBs.  The center aluminum plate that holds the pins in place is also removable.  This allows for rework of components on the bottom (main) PCB without disassembling the entire instrument.  Cool!

HP 3312A Repairs

Want to see more?  Check out the 3312A repairs album on flickr!

12 thoughts on “Test Equipment Pr0n: The HP 3312A”

  1. HP test equipment is as good as it gets. The documentation is unmatched. That makes this gear easy to fix and calibrate. All my audio test gear is HP.

  2. A bit of an old thread, but this page was top of the list when I googled hp3312a.

    Mine had a problem with square wave output : just spikes at each transition, no proper square wave. But sine and triangle were fine, and so was the sync out (and sync is just the same signal as the internal square wave).

    So I took it apart and poked around the function switches S9,10,11. Same story as the output. Tracing the square wave back, I soon found it : J2 pin 4 (square wave from mod board to output board) wasn’t connecting. Unplugging and replugging the connector fixed the problem.

    So .. those nice connections between the boards aren’t perfect ! They’re not gold-plated, either .. if you find odd things happening they’re probably worth cleaning.

    While waiting to find time to fix it, I happened to see a 33120A on ebay. It cost me a lot more than the 3312A but it was still cheap as these things go. I probably wouldn’t have bought it if the 3312A were working, but I’m happy to have both.

    1. Alan – One of the boards has no solder mask. The other has a light green semi-transparent mask. The traces are gold plated on both boards, so corrosion of the copper shouldn’t be an issue in either case!

  3. Reading this reminds me that I had exactly the same story with my Tektronix signal generator. Bought from the ‘bay, strange offset on the output, one defective output transistor… seems to be common!
    Lesson learned: always lookout if there’s a service manual available before buying some second hand equipment.

  4. I picked up an HP 34_6_8B 5.5 digit DMM not long ago. I like it too much to pop the lid (yet). “Obsolete”? Feh!

    1. Very nice! Dave Jones of the EEVBlog really likes those old HP DMM’s. My collection includes a Fluke 8442A and a Fluke 45. I really wanted an HP/Agilent 34401A but they are quite expensive, even on eBay.

  5. *DROOL*

    This reminds me of a 310A I have gathering dust on a shelf in my apartment:


    The coolest thing about that scope is that it was apparently built before the advent of the PCB, so there are all these weird ceramic interconnects and resistors hanging in the air. It’s still great to look at though, and opens up nicely in a way that shows it was intended to be serviced.

    Something tells me these days the expectations are that if something stops working you just throw it in the trash. I’ve heard about this even in the case of giant waterjet cutting machines. In China they consider many of them to be “disposable”!


    1. I used to own a Tek 491 spectrum analyzer that was also pretty interesting, but it was less beautiful and more crowded inside! There were probably a few pounds of gold in its construction if you added up every single trace, can, and connector that was gold plated.

      I think that you have a point about even test equipment and industrial machinery becoming disposable. Maybe that is why now everything has to be ROHS? So it is more compatible with the landfill… 🙁

  6. Jeff, if you want test equipment porn, the only place to look is Agilent’s 3458A multimeter. To what you saw in the 3312A, add fiber optic cables, two independent power supplies, sliding aluminum panels, and even bigger PCBs. There are a few pictures here: http://www.prc68.com/I/HP3458DVM.shtml, but they hardly do it justice.

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