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Books and Resources, Electronics

Tips for finding test equipment service manuals

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2009:10:01 02:23:53

When I buy a piece of electronic test equipment, the first thing I do is turn it on and see if it works.  This is the moment of truth: was that awesome eBay find the killer deal I thought it was?  (Hint: If it’s missing case screws and came with no packing material, probably not.)

Sometimes, everything works out and I have a shiny new piece of test gear for bottom dollar.  Often, things don’t work out quite as expected.  Luckily, test equipment is often made to be fixed.

To fix it, I need a service manual.

This means that with my luck, more often than not, the second thing I do is try to track down the service manual for my new semi-functional piece of test equipment.  Even if the it’s not broken, I’ll usually try to get a service manual anyway; often the service manual doubles as a user manual and I need to figure out how to use special features, find specs, etc.

If you find yourself in a similar situation, here are some tips for finding test equipment manuals:

  1. If it’s a fairly new piece of test equipment, chances are the manufacturer will have a manual on their website, usually in pdf format.  For example, Agilent has lots of manuals online, but unfortunately, anything over 20 years old is probably not listed.  Other vendors are better about archiving old manuals.  I have had very good luck getting old Fluke manuals on their website.
  2. Google is your friend.  Are you feeling lucky?  Some manuals are easy to find, like this one for the HP 3312A Function Generator.    The first link that isn’t an ad goes right to it.  Easy!
  3. There are several free service manual repositories on the web.  These can be very hard to find when you need them (spammy links from manual vendors sometimes derail your search).   I have started keeping track of free sources for test equipment manuals on the wiki.  If you are looking for the manual for a fairly common piece of HP/Agilent or Tektronix gear, there’s a pretty good chance you’ll find it for free on one of the sites listed.
  4. As a last resort, consider paying for an electronic copy of the manual.  Beware of vendors who are simply downloading readily available manuals for free and selling them to you.  Use eBay with caution.   The wiki now includes a list of reputable service manual vendors.  The only vendor I have used is Artek Media.  They have very reasonable prices and great support.  At $5-$10 a pop, sometimes it’s easier to just buy the manual than scour the web for hours, so it’s nice to be able to trade laziness for dollars.

Usually by step 3 I have the manual I need, so I rarely have to pay for a copy, but it’s nice to know that most obscure manuals can be had for a few dollars.

I hope these tips keep more old test equipment running – remember they don’t make ’em like they used to!

If any readers have more sources for manuals to add, please leave a comment or add them to the wiki.

Electronics

Test Equipment Pr0n: The HP 3312A

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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!