On Samstag, 5. Februar 2022 10:48:57 CET Julien Goodwin wrote:

The power starting high and dipping eventually means there's some regulation
for the temperature. That's a good sign.

First goal is to get the lamp to light up, once that's working, "calibration"
of the lamp exciter is only fine tuning. To see if the exciter oscillator is
starting at all you could use a makeshift RF probe with an oscilloscope and a
coil of wire connected to the probe and the get that close to the lamp. You
should see something between 70 and 90 MHz if the oscillator is working.

If it isn't, hopefully Q2 isn't damaged. Voltage across R14 should tell. There
should be a voltage drop across it, maybe 1V or so? Lower if the oscillator is
not working, but certainly not 0. If the voltage is 0V, Q2 has either failed
open or the connection to the exciter coil is broken. If the voltage is much
higher, Q2 has probably failed shorted and then R14 should also get very, very
hot.

That's quite low. Nominally, the lamp temperature should be 113°C.

The good thing about the FRK is the very detailed service manual that tells
you exact troubleshooting steps for each of the component. I guess you've
already found it?

BR,
Matthias

On 6/2/22 1:14 am, Matthias Welwarsky wrote:

I did stick a scope probe in as I mentioned, and there's certainly
oscillation now happening, although I didn't get a clear indication of
the frequency, tuning that is now second on the todo list once I sort
out the heater, and acquire a slightly safer way of probing that area
than waving a bare scope probe around (some field probes have been
ordered which should be ideal for this).

Yep, although I find myself switching between the FRK (mid-70s) and
FRK-L (late-80s) manuals as neither quite matches the device I have. I
guess that dates my device to likely early/mid-80s.

Until today I hadn't actually measured the temperature properly, and not
at all in operation, so I wasn't too worried that I was only seeing 90c.
Today I (kapton) taped a thermocouple to the outside lamp housing and
let it warm up for a while, and it ended up stabilizing still around
90c. So my earlier measurements were actually right on, sadly.

Some schematic & board review later it seems clear that someone's
reworked this at some point, the trimmer that should control the
temperature has been replaced with a fixed resistor, which also explains
the weird wires attached to the heater transistor.

Next time I do an order from my parts supplier (which, naturally I just
did the other day, oh well) I shall try and find something close enough,
as, naturally, a direct replacement is out of stock. Options that should
be close enough are there, so hopefully it'll be fine.

Sadly that means a full disassembly again, hopefully I don't lose too
many more screws this time, but at least I'll know to sort the work
surface out much better to protect against it.

With respect to the lamp oscillator the frequency is not that particular.
Not sure you need to adjust it if the lamp ignites. Think they are in the
137 MHz region.
Good luck
Paul
WB8TSL

On Fri, Feb 11, 2022 at 1:31 AM Julien Goodwin time-nuts@studio442.com.au
wrote:

On Sat, 2022-02-05 at 20:48 +1100, Julien Goodwin wrote:

I don't have any experience with this brand/model, but I have
resurrected some "dead" rubidium standards. I'm mostly going from
experience with gas lasers, but I found the same approach worked on the
rubidium units that I have here.

It's well worth being patient with the lamp, if it's been off for many
years, it may have a gas pressure/mix that's far from optimal. If you
let it sit with the heater and exciter running, it may just strike
after a while.

(You should measure the current going into the exciter, it should jump
up suddenly when the lamp strikes. Measure the unlit condition with the
lamp cold and the exciter on. On mine, the current roughly doubles when
it is lit.)

If it does strike, keep it running for a few days, don't power it off.
It can get even harder to light, before it gets easier again. You might
consider using a UPS for this procedure, as Murphy could strike just
after the lamp does!

On one of my units, I had to manually run the lamp heater to a higher
than normal temperature, before it would light. (With a thermocouple,
it's best to use some heatsink grease, otherwise the readings can be a
long way off.)

The needed temperature gradually came down, until the unit could be
reassembled and used normally. On another, I just waited for several
hours at the usual temperature, and that was enough. Although again, it
took a couple of days of operation before it would light without
hesitation.

On yet another unit, the lamp would light normally, but the photodiode
signal was too weak to achieve a lock. (The firmware said nope.) So I
just left it running overnight, and then after a power cycle, it locked
immediately!

So I wanted to point out that if the lamp has everything it needs, and
still won't light, you might just need to leave it powered for a while.
And you have not much to lose, if you need to run it at a higher
temperature for a while.

Have fun,
Darren

On Dienstag, 15. Februar 2022 08:10:12 CET Darren Freeman wrote:

On the same note, if you have a hot air rework station, you can try gently
heating the lamp assembly from a distance, it may help getting the lamp to
ignite, too.

This afternoon I swapped the fixed resistor that should have been a
trimpot back to a trimpot, and quickly dialed it up to where I was
getting ~115c on the lamp surface as expected.

As I was in there anyway I also swapped the 20 ohm resistor back to the
Vishay wirewound part it should have been from the hack I'd made up
using cheap carbon resistors from the local shop.

No obvious light, but I also still need to see if the excitation
oscillator is even vaguely in the ballpark (or possibly working, I may
potentially have messed with things while I was in there today), for
which I'm still waiting on some safer probe options.

On 15/2/22 6:10 pm, Darren Freeman wrote:

I need to replace the main filter cap in the FRT power supply before
I'll be comfortable leaving it on for an extended duration, but once I
do (and the replacement is now here, I just need to hook it up) I'll be
fine leaving it on for a week or so to see.

If it doesn't light after that, I may try just swapping in the lamp from
my house standard FRK-LN to see if it's that simple, and this has a bad
lamp (at which point I think I'd give up for now).

Fortunately Sydney power is good enough that my UPS' have caused more
outages than they've helped with.

Hi

The oscillator is a “works or it doesn’t” sort of thing. Frequency and output
level usually are close enough to do what is needed.

Bob