I have recently fixed an HP 5342A microwave frequency counter.
The unit has options 002 (amplitude measurement which includes 003
extended dynamic range) and 011 (HPIB).
It is working fine, but this is when the Test Equipment OCD kicks in.
Still missing options 001 (OCXO) an. 004, the DAC output. How can I have an instrument without all the possible options?
Option 001 is easy. I have a 10544A OCXO waiting to be used, so out comes the standard TCXO and in goes the OCXO. Bit fiddly to have all the other cables arranged in the tight space, but simple.
Option 004 is not that straightforward. According
to the manual, it requires a new display board. However, if you
look closely at the current display board, there are some unpopulated footprints there.
The service manual actually has a photo of the Opt004 A2 board and there are schematics for it.
So I studied them and found that indeed the PCB is the same, it just needs the following parts to be installed.
U14 74ls193 - counter
U15 74ls138 - demultiplexer
U20 74ls193 - counter
U21 74ls193 - counter
U23 DAC-80-CCD-V - DAC
R23 4k7 5% - resistor
R29 10k 5% - resistor
R30 180k 5% - resistor
R31 180k 5% - resistor
R32 270k 5% - resistor
R33 270k 5% - resistor
R34 3k9 5% - resistor
R25 100k - trimmer pot
R27 100k - trimmer pot
C9 0.01uF/100V - ceramic capacitor
J2 SMB socket
Some pins and wires for connection, including a coax pigtail with SMB connector to connect the DAC out BNC at the back, which is already there.
C12, C14 and C15 are deleted according to the manual, so these will be left unpopulated
With the exception of the DAC, these are all very common and readily available parts. As luck would have it, I have one such DAC from some old instrument PCB, so of course I have to implement this option. But even If I did not have it, the DAC available on eBay for a reasonable price.
So let's go, get all the parts and see how it works. Continued in Part 2.
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