On Tue, March 10, 2009 8:58 am, Vic K2VCO wrote:
> Tom Mandera wrote:
>
>> I switch to OPR and key the exciter. I'm supposed to see 50-80ma of
>> plate, but I see 100ma immediately.
>
> This refers to the plate current with ZERO drive, the STBY/OPR switch in
> the OPR position
> and the RELAY connector on the back shorted to ground. You can check this
> without an exciter connected.
Got it. I'll check the current by just shorting the relay connector
without the exciter/radio connected or on, but with a load affixed to the
RF out.
This does help explain why 100ma would be the minimum with ~5w drive, vs
0w drive. I also found several others remarking on 100ma at minimum input
and it seems relatively normal.
>> I heard the (tube?) sizzle (sounds like a good weld, not a good thing in
>> a radio) while attempting to increase the exciter drive to hit 125ma of
>> grid, and never reached it.
>
> Probably an arc in the tank circuit or (hopefully not) the bandswitch,
> caused by underloading.
> Do NOT allow this to continue, it will do real damage!
I suspected it was a bad noise and immediately quit. After two such
incidents, I called it a night and sought further guidance - and
appreciate the responses I've received.
The amp has been sitting there for an entire week and I'd finally wired it
up, rearranged my operating station around it, etc. How could I not give
it a go?
> Make sure that you adjust the load control high enough. When my SB-1000 is
> Too much is an indication of underloading.
Noted.
> The simplest way to learn to tune up an amplifier is to get an experienced
> ham to show
> you. It's easier than it sounds in the SB-1000 manual, which is very
> confusing. Just
> remember to keep the grid current down (and the plate current to 400 ma).
That's my next course of action. I'll contact a few local elmers for
assistance - including the one that strongly encouraged me to pick up this
amp.
Thanks, Vic.
On Tue, Scott KBØNLY wrote:
> ...If you don't have a 80mA at this point (zero drive) then don't bother
> connecting it to a radio and applying
> drive until you fix the bias circuit. The Zener D2 is part of the bias
> supply. I have worked on this model before,
> D2 is a common failure, but its commonly related to a bad
> tube also. It can also be that someone overdrove the amp and arced it
> causing the cathode current to shoot up and take out the diode. I can't
> tell you without a doubt what caused it to blow, but you should check the
> diode first and then see if the Plate Idle Current comes back in line
> after replacement.
I forgot to mention last night that (probably quite obvious at this point)
I haven't pulled the cover on the SB-1000 yet. Given all of the "High
Voltage" warnings and the positive input about the builder, I thought I'd
give it a shot without ever peeking inside.
If a few more tests don't garner better results, and another ham can't
help tune it up (or in the meantime, I get impatient) I'll have a look-see
inside for obvious signs of arcing, in the band switch in particular, and
elsewhere in general, being cautious about the HV supply.
Is there a preferred method to ensure the capacitors are drained? Pull
the plug and wait a day?
It's been a long time since I've gotten elbows deep into any electronics
projects, and I think just shorting each cap might be problematic, though
that's what I've done in the past (seems to me one or two might've
exploded doing that though).
Thank you all.
-Tom Mandera
KE7VUX
Helena, MT
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