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WIP: HP 53131A Counter Repair

With 10 MHz signal input, channel 2 never triggers:

Display blank, not triggered

Initial inspection: wat

I found this resistor which clearly had second thoughts during soldering 20–25 years ago. Nerdy version of Where's Wally - can you spot it?

Potatocam board photo

How about now?

Resistor soldered incorrectly Resistor soldered incorrectly, angled shot

Electrically it's fine - the wonky resistor is soldered to its mate, and underneath it, there is a trace that was joining these two onto the same net anyway. The orange-brown blob looks to be a piece of glue that was probably supposed to be underneath the wonky resistor to hold it in place during the soldering process so it wouldn't wander...

But there's no surprise that it would have passed flying probes and higher level testing. Hopefully today, automated visual QA would inspect the job and reject this board. Much prodding later I concluded this unsightly mess is not the cause of CH2 failure.

Digging shallow: boring

First things first, power rails where they come from the power supply board onto the main board checked out fine. I verified the power rails along the frontend amplifiers and they checked out fine too.

Digging deeper: hmm

The next course of action was to squirt the 10 MHz ref out of the back of the unit and into its CH2 input (eerily reminiscent of the Slurm Queen in Futurama, S1E13). Following the signal path through the frontend should show up the problem.

Here is the last time the signal looks as it should. This is just after the R103-R142 divider:

Last time CH2 is intact

Here it is on an input at the next IC in the signal path, an ultra-fast complementary-output comparator U54-A:

Last time CH2 is intact

Suddenly going from a nice zero-centred 1 V pk-pk waveform to -2.7 VDC is strange. The only thing in between the points these two plots were taken is relay K7. This relay allows the microprocessor to choose whether CH2's comparator recieves CH2 or CH1's conditioned signal. Its default "off" state is to patch CH2 through, and checking its coil shows it's indeed deenergised.

My thought now is that the input to the comparator has become internally shorted/shunted due to overload, and K7 is/was soaking up the difference.

A quick resistor-to-ground confirms the opposite case. Using a mere 100 kOhm to ground, I can lift the input of the comparator from -2.7 V to around -30 mV. This indicates that the input isn't shorting or shunting, and in fact the relay has gone open. This explains why the fault is intermittent - super iffy/scratchy contacts.

Checking the solder joints on the relay without - and then with - magnification shows them to be fine. Excuse my scratches/dents from probe tips.

Square on: K7's contacts

Row of seven pins, side on:

K7's contacts, angle (one of two rows)

Two pins (extreme left and right), side on:

K7's contacts, angle (two of two rows)

Maybe one day I should photo the ridicuous setup I use to get these shots. The proportions are ridiculous to get this sort of magnification with a regular camera!