[time-nuts] Best Rubidium Frequency Standard

Bob Camp kb8tq at n1k.org
Sat Mar 12 13:46:29 EST 2016


Hi

Another example of the “stored in magic memory” issues on some modern Rb’s:

To improve the yield of Rb’s cells, a DDS can be used to tune a bit one way or the other off of the normal resonance frequency. The reasons why they are off are a bit involved, the fact is they do get manufactured that way. To line things up with these cells, some sort of coarse tune word is dumped into the flash / eprom / whatever. That DDS now knows where to find the resonance of the cell and it has. This all works fine until you swap a cell between boards. The new cell may (or may not) line up with the old cell. Things may (or may not) lock up properly. 

Once you get past the “blown capacitor”, “no output on the regulator” or “dead power transistor” sort of fixes …. the newer Rb’s are not particularly easy to care for and feed. Indeed, the Temex units I mentioned earlier probably all could be fixed with detailed work and access to the code. I stop when the simple “replace these 4 caps” stuff does not bring them to life. As long as that gets more than 60% running, I’m happy.

Bob

> On Mar 12, 2016, at 12:14 PM, cdelect at juno.com wrote:
> 
> Mike,
> 
> If you are planning to buy "suspect units" with the intent to repair I
> would steer clear of the PRS10.
> 
> It is true you can pay for a factory repair but having the schematics and
> theory of operation only helps for particular repairs.
> 
> This is because a lot of the alignment parameters are stored in memory
> and there are no instructions on these alignments and how to store them.
> 
> An example: I had a unit come in for repair. It had no output. The
> customer, a professor, said a student applied a large DC to the output.
> The output is direct from an RF transformer. It burnt open the winding.
> 
> I thought, how hard can it be?
> 
> I popped off a good output board from a junker and installed it. I now
> had an output but the unit would not operate properly. Part of the output
> go to other circuitry that would need to be aligned to match. No way to
> do that or store the alignment! Bummer.
> 
> Luckily the transformer was a metal can type. I opened it up and rewound
> the tiny wire output coil and that restored the unit to operation.
> 
> After that I sold my pile of defunct PRS10 units and don't plan to
> purchase any more.
> 
> For ease of repair the HP 5065A is first, then the FRKL and H, these have
> bog standard thru hole circuitry and the manuals are excellent. Another
> of the worst is the M100 as its cards are conformal coated and the lamp
> oven assy. is potted! As you mentioned most of the later telecomm units
> are surface mount and most have some sort of microprocessor involved.
> 
> Cheers,
> 
> Corby
> 
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