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.
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