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Marconi 2022E RF signal generator repair and refurb

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Zenith:

--- Quote from: bd139 on July 24, 2022, 12:44:09 pm ---The ex cell tower OCXOs on aliexpress etc have plenty of adjustment room yet. Plus also 5 for £10 it's probably worth just buying some to leave around as ornaments  :-DD

--- End quote ---

Thanks. I'll bag some of those. I've got enough stuff that uses OCXOs and I think it's only a matter of time before the one in the 2019 cashes its chips in.

bd139:
Non trivial update finally.

At lunch today I threw together the prototype crystal oscillator replacement module based on the original schematic. This was tested with 0-5V which gave a pull range of 9.999800 to 10.000100 MHz. The pull range is +/-12V



This was crudely tacked into the 2022E. No prizes for mounting here.



Booted straight up and the CPU runs fine. The phase lock is a bit dicky so might need some more debugging but we at least have a working frequency reference.

When you poke EXT it complains about error 11 (bad reference) and occasionally error 12 (cant lock). Which is a lie because it's clearly working as shown here:



The game is however not over. It does output a signal up to 250MHz but at 10dBm output it's about 15mV and on top of that it's well off frequency. That would possibly suggest that there's a problem with an OM345 somewhere in it as it's clearly not kicking out enough wumph to drive the output amplifier and/or synthesizer divider. Will wait until I've got the final oscillator board in for that.

Found a nice debugging feature though. 2ND+3 then enter register number and you can shift binary values. I used this to test the attenuator drivers and relays to make sure that they were operating independently which they were.

bd139:
Going to need some OM345's for this so have designed a board. This uses the MiniCircuits MAR-7SM+. Unlike other boards there will be a different mounting method which will involve inserting a pre-trimmed 5-way header pin into the edge of the castellated board. The pins can then be soldered in while still on the header plastic which is then removed. This is then inserted into the board and soldered underneath. This avoids some of the mounting issues identified with other approaches. Disclaimer: might be worse :). Also the module is the same size as the original one.



xrunner:
When I bought mine I researched that issue before it got to me, but luckily I didn't have to replace any.  :phew:

Does the board manufacturer populate your board for you? I noticed some of them will do that.

bd139:
I will order the PCBs from JLCPCB. They don’t sell the mini circuits part so will have to order those from mouser and hand solder. Will get the other two SM parts at the same time.

I need to do a trial fit of the pins first which I will do when they turn up ….  :=\

Did some debugging earlier and confirmed that OM345 IC10 is indeed dead. You can ID the dead ones because the input pin one goes to 0V and the output to 12V as the feedback network is broken.

I’m actually going to order 30 boards as that’s the cheapest JLCPCB increment band and sell the rest on eBay  :-DD

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