[time-nuts] 9390 GPS RX

Mark C. Stephens marks at non-stop.com.au
Thu May 30 12:58:43 EDT 2013

Hi Ed, Just an update, I replaced the Triac (a 2N6343, no less) with a garden variety I had in my spares.
I checked most of the active and passive components and it all checked out.

I mean this is one simple PSU - using a 555 timer as oscillator...

Anyway, with bated breath I plugged it in. Didn't work.
Checked 500mA mains fuse, fuse open circuit.
Bugger, Put new fuse in, Display flashed briefly then went out.
Bugger, Try a 500 mA slo-blow then and crank it up slowly on the Variac.
Beauty mate, up it came.
Disconnect from Variac and Plug it back in and switch on.
Up it came. Dunno, what was up with the first 2 fuses, but I'll leave the slo-blow in there.

So, it's been sitting there for a couple of hours before the Lock LED came on.
Interestingly enough, I woke up (its 3am here) and I thought, Hmm wonder if its locked.
I walked down stairs to the workshop and looked at the front panel just as the Lock LED came on!

At the moment it is content to be swinging -1/+2 Hz.
Hopefully it will stabilise once everything has settled down in there.
I might put the lid on to in case there is some thermal shock going on, it's a pretty cool room.

How about that then :)


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On Behalf Of Ed Palmer
Sent: Friday, 31 May 2013 2:11 AM
To: time-nuts at febo.com
Subject: Re: [time-nuts] 9390 GPS RX

On 5/30/2013 1:24 AM, Mark C. Stephens wrote:
> Ed, et al,
> I was just cleaning up the workbench and this gave me time to reflect of the FRK repair.
> I'd just like to say thanks for your help and Guidance Ed, without it, I probably would have got there in the end but I would have not been as aware of why and how I fixed it.
> What I mean is, I learnt a great deal about how the Rubidium works AND I learnt that sometimes patience is the best recourse when dealing with very accurate timing instruments AND I made some very cool friends!

You're very welcome, Mark.  Glad I could help.

> With that mess out of the way, I realised, heck, I haven't fixed the PSU in the 9390 yet!
> So, Ed, what do you know about "Convertor Concepts INC." 28volt switch 
> mode PSU used in the 9390? ;)
> So far, I have found the Triac was blown, but I don't have the schematic for the PSU.
> The construction although acceptable and tidy is a bit "Ghetto" compared to modern day switch modes I have worked on.
> Mind you, there is a lot more room in there and you can actually get in there and fix things without having to remove heat sinks and separate boards.
> However, I am toying with the idea of replacing it with one of those new high efficiency / low noise supplies.

It was using a Triac as the switching device?  Are you sure?  That would mean it was switching at line frequency.  I've seen that on old high power supplies, but not on low power units.  Either that was a very unusual choice or they had some particular reason for choosing a supply like that.  Maybe they needed higher efficiency, but couldn't tolerate the noise level or noise frequency of a standard switching supply.

Fixing a switching power supply without a schematic is more luck than skill, at least for me.  If I had to fix one, the first thing I'd do is draw out the schematic.  That would help to trace from the blown device and locate other dead components.  But using a replacement power supply looks really attractive.  Keep in mind that noise from a switching supply can degrade the phase noise of a precision oscillator, but this might not matter if you're using it for time & frequency.  It could be a problem if you wanted to multiply the output frequency for RF purposes.


> I suppose I should just put the 9390 on the bench and hook it up to a bench supply and watch it for a while.
> Then again, I am also admiring the view - I can actually see the surface of the bench for a change - no mat of cables obscuring it!
> I guess it's only right I should at least attempt to fix the original PSU, if even just to keep the old 'gal just like new.
> And it is in lovely condition! I'd say it spent its working life in a nice clean room.
> BR,
> -marki
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