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Marconi 2022E RF signal generator repair and refurb

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bd139:
Thanks for the info - very interesting. I'm actually writing up all the notes people are providing and will publish a web page about it. There seem to be lots of little bits of information but nothing in one place so far.

As for the original oscillator here I can see the design trade off that was made and it makes sense. I supposed the MoD were using it to work on typical FM stuff at the time so frequency precision isn't as important as say narrow bandwidth. And yes the OCXOs take a while to warm up. The previous set up I had was a 5334B with OCXO module and you had to leave that running for at least 30 minutes for it to be usable.

So I got ahead of myself this morning and designed a PCB for the basic oscillator module.





Zenith:
This is interesting because I have a Marconi 2019 which I use a lot. When it came the frequency it produced was way below what it indicated and the adjustment was at its upper limit.

I found that someone on the WWW had opened the OCXO and traced the circuit. I recall there's was in a 2955 receiver test set. The circuit is similar to your OCXO, but with different transistors and varicap at least. I opened the OCXO and tweaked the preset cap quite a way, maybe 1/6th turn, and tried it. The adjustment multiturn preset could now bring it onto the correct frequency.

It's accurate to less than 1ppm and drifts slightly over time. I don't know what the spec is off hand, but that seems somewhat disappointing for an OCXO. It takes a fair time to settle down properly. Realistically, it's more than accurate enough for what I use it for after it's been on for five minutes. I suppose we all have Frequency Counter and GPSDOitis these days.

Since the 2019 works, I'm not inclined to start messing with it for no good reason. However it does occur that the OCXO is a dodgy number; it must have a lot of hours on it and it's likely been pulled well beyond its design limits. I wouldn't be surprised if it died or became unusable and I had to replace it. Finding one out of a scrapper wouldn't be easy and it would probably have the same problems, so making one up from new parts would make more sense.

xrunner:

--- Quote from: bd139 on July 24, 2022, 09:54:53 am ---So I got ahead of myself this morning and designed a PCB for the basic oscillator module.

--- End quote ---

That looks nice. Come to think of it (I wish I still had mine but I sold it to get my Agilent 8648A) one of the problems I had at first was the ext. ref input - when I tried to use it nothing happened. It just wouldn't act like it had any stimulus from the ext. ref. As I said reflowing solder joints fixed that issue too. I followed the ext. ref. input and found where it stopped and I re-flowed solder connections in the area. That fixed it. I got off easier than you are doing.  :phew:

bd139:

--- Quote from: Zenith on July 24, 2022, 11:29:28 am ---This is interesting because I have a Marconi 2019 which I use a lot. When it came the frequency it produced was way below what it indicated and the adjustment was at its upper limit.

I found that someone on the WWW had opened the OCXO and traced the circuit. I recall there's was in a 2955 receiver test set. The circuit is similar to your OCXO, but with different transistors and varicap at least. I opened the OCXO and tweaked the preset cap quite a way, maybe 1/6th turn, and tried it. The adjustment multiturn preset could now bring it onto the correct frequency.

It's accurate to less than 1ppm and drifts slightly over time. I don't know what the spec is off hand, but that seems somewhat disappointing for an OCXO. It takes a fair time to settle down properly. Realistically, it's more than accurate enough for what I use it for after it's been on for five minutes. I suppose we all have Frequency Counter and GPSDOitis these days.

Since the 2019 works, I'm not inclined to start messing with it for no good reason. However it does occur that the OCXO is a dodgy number; it must have a lot of hours on it and it's likely been pulled well beyond its design limits. I wouldn't be surprised if it died or became unusable and I had to replace it. Finding one out of a scrapper wouldn't be easy and it would probably have the same problems, so making one up from new parts would make more sense.

--- End quote ---

I had a 2019 for a while a few years back. Very nice bit of kit if a little large. Paid £5 for it from some ex EE who bought a sports car and was running off to the south of France with some floozie and just wanted rid of it! (slightly jealous).  Bargain of the century that was. I'm not going to get into the demise of that unit here because that's a whole thread on its own but I will say that the previous owner was a bodger and it lead to more than 5V landing on the 5V supply which basically machine gunned everything in the box connected to the rail  :palm: :palm: :palm: :palm:

All OCXOs have a lifespan unfortunately. The crystals and/or other components will drift out of specification over time. I had that with a Racal standard a few years back with a 9444/12 in a 9915. I built a replacement for it and bypassed the frequency multiplier circuitry. Details here including photos of insides of the Racal unit:

https://www.eevblog.com/forum/testgear/repairing-off-frequency-racal-9442-ocxo/

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

Actually I did a PCB for the Marconi as well and one of these OCXO's. That might be usable for your 2019 if it's the same standard. Not quite as elegant as the VCXO board and will require some pin trimming and spacers on the build but should work.



It replaces the original standard which looks like this:

bd139:

--- Quote from: xrunner on July 24, 2022, 12:18:39 pm ---
--- Quote from: bd139 on July 24, 2022, 09:54:53 am ---So I got ahead of myself this morning and designed a PCB for the basic oscillator module.

--- End quote ---

That looks nice. Come to think of it (I wish I still had mine but I sold it to get my Agilent 8648A) one of the problems I had at first was the ext. ref input - when I tried to use it nothing happened. It just wouldn't act like it had any stimulus from the ext. ref. As I said reflowing solder joints fixed that issue too. I followed the ext. ref. input and found where it stopped and I re-flowed solder connections in the area. That fixed it. I got off easier than you are doing.  :phew:

--- End quote ---

I'd rather have an 8648A but they don't turn up for £100 and some futzing here :)

I was expecting a fight with this one. It's not as bad as it could have been. At least it looks like it's recoverable  :-DD

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