[time-nuts] Re: Efratom FRK with "inoperative lamp"

[Matthias Welwarsky]

On Samstag, 5. Februar 2022 10:48:57 CET Julien Goodwin wrote:
> I /think/ it's using more power than it did before (right around 20W at
> total cold, dipping to 10W after a while), although silly me didn't make
> a note of it, but even after 30 minutes it's not locking.

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

> Given that the resistor I replaced is in a key point for the lamp
> excitation oscillator I won't be surprised if it's totally off and needs
> a recalibration, but that's a task for another day. When I stick a scope
> probe in the lamp cavity as the manual suggests I do see a a signal, so
> there's certainly hope, although I don't see the expected glow.

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.

> I did confirm the heater is working, and was getting above 90c when I
> poked a thermocouple at it just after power-off.

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