[TMC] SBE-2 problems. Maybe yours to.

[telegrapher at att.net]

Hello TMC list members!!

Got to working on my SBE-2 exciter the other night (again!).  Funny
things happening.  

1: Meter in MF or RF position stayed about midscale whether the
"exciter" was keyed up or not.

2: Constant carrier heard in the receiver without the exciter being
turned on.     This means the "exciter" switch not the power switch.

3: Low drive with the carrier control being turned full CW.  

4: B+ voltages out of limits, more than 10%.

5: RF output dirty and unstable.


Resolved number 1 by swapping out the meter amp 12AU7 tube.  Went thru
several new ones before it started behaving like it should.  Original
tube tested good.

Number 2 is giving me headaches trying to pin down just what is causing
this to work this way.  I've went thru all the adjustments from the 17
kc osc thru the 270 kc osc, LF balance, USB & LSB balance (null) etc. 
Appears that either i've got some leakage around some components or some
stage is not being cut off properly.  Maybe some looking with a scope at
different stages will help pin this one down.  

Number 3. I noticed this when trying to tune the unit up on 80 meters
with one of the xtals i've got, one on 3870 and the other on 3855.  The
manual says that this reading is for tuneup only and is not relevant for
operation.  My contention is that if your gonna use AM with this thing,
you need more than just a "showing" of carrier on the meter.  I hooked
the HP-8640B sig gen to the VMO input and started at -0- Dbm output. 
Had to go clear up to +14 Dbm out of it in order to get a good solid 70%
reading on the SBE meter.  No reason for the 70% reading, just happened
to go to that point on the meter.  With the Xtals i get a 30% reading. 
I would have thought that i would get a good high reading with xtal
operation providing the xtals have good activity.  Have changed out the
tubes in that chain of events with no increase in performance.  More
chasing on this one also due.  Tube socket voltages are very close.

#4.  My local line voltage at the wall is anywhere from 120 thru 125Vac,
depending on the time of day and year.  At one time a couple of months
back i actually measured 132V at the wall.  DOn't know what happened
there.  Anyway i run my SBE and GPE-1 exciters thru a variac and set it
at 105Vac.  Reason.  When running straight out of the wall all the B+
voltages are 10-20% higher than shown in the manual.  I think if your
going to trouble shoot this stuff you need to have voltages in the
ballpark.  SO lowering the input to 105 Vac now puts my B+ (4 of them)
within 10% easily and in some instances less than 5 %.  I also found
this to be true with my FRR-502 receiver.  Works much better at 105Vac
on the line than 125.

#5.  I thought maybe the unit was running good enough for me to drive
the PAL-500 with it this morning on the AM net.  HA!.  Run the output
thru the PAL-500 and watched it on the Heath SB-610.  NOT GOOD!  power
output drifting around and lot of noise  on the carrier.  Using the
GPE-1 exciter the output is nice and clean and stable. 

Looks like it's back to turning it over on it's back and working on it
some more.  

One other thing.  When i had the 2.977 Mc xtal in it and tried to tune
the MF tuning on 40 meters, the meter kept slamming against the wall
after i got below 3.3 Mc.  When i pulled the Xtal out of it's socket and
selected the VMO, most of that went away and i was able to tune it for
operation on 7293 which i could not due with the xtals in it.  I've done
the math on the frequency calculations for xtals over and over so i know
they are right. Just appears that when i get below 3.3 Mc with the MF
tuning control (and the carrier control has to be set to at maximum or i
don't see anything.  Turning it down CCW does not help the situation
any) the MF tuning tank goes into oscillation.  Any one out there see
the same situation?

Another thing.  I have a tube socket location marked 6BA6 on the
chassis.  However neither of the two SBE manuals i have show anything
other than a 6AH6 in that position.  With the 6BA6 in there the tube pin
B+ drops way down.  With the 6AH6 it reads like it should be.  Might
check your unit to see what's actually there and do you have any
information that says it should be something else.  

Otherwise i could be in the market for a better unit.  THis one might be
more than i can handle.

Larry
W0OGH