Help Needed - Heathkit SB-400 Transmitter LMO Realignment Instructions
 

Hi,

I'm restoring a Heathkit SB-400 transmitter, and need some info on how
to realign the LMO (Linear Master Oscillator). Currently, it is
running about 75 KHz high at the lower end of it's range (i.e., on the
40M band, it will not tune below 7.075 MHz), and is out of linearity
by 16 KHz at the top of its range.

There is an adjustment on the LMO sealed with red paint - adjusting
this brings the frequency range down to where it should be (not sure
if this causes any other problems, though - it was sealed for a
reason??), but the linearity problem remains.

Does anyone have any suggestions or literature on this unit? (the LMO
is Heath Part No. 110-32)

Thanks,

Pat

"Pat Richardson" wrote in message
...
Hi,

I'm restoring a Heathkit SB-400 transmitter, and need some info on how
to realign the LMO (Linear Master Oscillator). Currently, it is
running about 75 KHz high at the lower end of it's range (i.e., on the
40M band, it will not tune below 7.075 MHz), and is out of linearity
by 16 KHz at the top of its range.

There is an adjustment on the LMO sealed with red paint - adjusting
this brings the frequency range down to where it should be (not sure
if this causes any other problems, though - it was sealed for a
reason??), but the linearity problem remains.

Does anyone have any suggestions or literature on this unit? (the LMO
is Heath Part No. 110-32)


Pat -

First go to K4XL BAMA web site and download the tutorial for Heathkit LMO
Repair by Ed Kotkiewicz
ftp://bama.sbc.edu/downloads/heath/lmo/

You will need the DjVu reader -- if you do not already have it
http://bama.sbc.edu/viewers.htm

I may want to try the HeathKit reflector of owners and former engineers.

Subscription control - http://www.tempe.gov/lists/control.asp?list=HEATH
To post -
Archives - http://interactive.tempe.gov/archives/HEATH.html

Greg
w9gb

On Mon, 21 Jul 2003 13:26:12 UTC, Pat Richardson
wrote:

Hi,

I'm restoring a Heathkit SB-400 transmitter, and need some info on how
to realign the LMO (Linear Master Oscillator). Currently, it is
running about 75 KHz high at the lower end of it's range (i.e., on the
40M band, it will not tune below 7.075 MHz), and is out of linearity
by 16 KHz at the top of its range.

There is an adjustment on the LMO sealed with red paint - adjusting
this brings the frequency range down to where it should be (not sure
if this causes any other problems, though - it was sealed for a
reason??), but the linearity problem remains.

Does anyone have any suggestions or literature on this unit? (the LMO
is Heath Part No. 110-32)

Thanks,

Pat

I've been in the LMO.

SB-400? You have a tube model.

1. Tube is known good?

2. What do you mean by 75 kHz high? Do you mean the output signal
or the LMO as measured by a frequency counter is off?

3. I found a cold solder joint on one of the temp compensating caps
in a tube LMO. This was not on the wire to circuit joint. This was
the little coil of wire soldered to the end of the cap's ceramic
tube.

4. There is a known problem with dirty contacts between the LMO cap
rotor and the spring clip. Cleaning instructions are on my page
at http://www.kiyoinc.com/heathstuff.html

5. How "pure" is the LMO's signal? What does a scope show you or
how does it sound on a general coverage receiver?

6. Check the voltages too.

I'm guessing that you're close to getting it going.

de ah6gi/4

--

On Tue, 22 Jul 2003 17:03:57 UTC, Pat Richardson
wrote:


B+ is within spec - there was a short from the original construction
placing the wrong bias on the frequency shift connection (bet someone
found this thing REAL frustrating to tune on LSB!) - that was fixed
earlier in the restoration, though, and is not related to this
problem.

I'm guessing that you're close to getting it going.

Thanks - it's a fun unit to use (and would likely have been the one I
would have had if I hadn't been lazy and got my ham kicense back when
I first wanted to, in 1966....) - would be more fun if the dial
matched the actual frequency, though!

de ah6gi/4

Thanks, Pat

Then I'm guessing that there is *one* bad cap or bad solder joint in
the LMO. I don't think that adjusting the trimmer is a good idea.
I'd especially avoid even thinking about touching the plates on the
tuning capacitor.

If you can get a schematic for LMO, you might be able to "calculate"
the cap that would shift the freq by that amount.

When I had the cold solder joint on the temp compensating cap, the
symptom was that the LMO freq would jump about 20 kHz during
operation.

I found it by listening to the XTAL calibrator on the receiver and
pushing on the caps with a diddle stick. One cap made the freq jump
and jump back.

As I said, I think you are close to finding the problem.

--

de ah6gi/4