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

[Julien Goodwin]

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

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).

> 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?

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.