Adding Opt 001 and Opt 004 to the 5342A - Part 1

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. 

Fixing an HP 5342A microwave counter. The anticlimactic last part.

In Part 3 I stopped while waiting for parts to make the adapter board for the ROM. Well, I'm still waiting for these, looks like the Covid situation doesn't make it simple.

However, while casually browsing eBay, I found a seller in Israel selling a CPU board for an insanely high price. However there was a "Make an Offer" opportunity, so I sent in an similarly insane, but low offer. I was astonished when a couple of hours later eBay popped up the "Offer accepted, please pay now" message. Sometimes it pays to be aggressive in bargaining.

So two weeks later this came in the mail.

This is actually a newer version of the board, part no.: 05342-60072, but the only difference seems to be the use of one larger ROM instead of the 3 2k parts.

I plugged the board in, and magically everything came to life. The counter was working fine, option 002 measured the dBm's etc. So after much fun with debugging and researching I was able to take the easy way and a simple board swap fixed it. Isn't it cheating?