How linear are Heathkit SB series LMOs?

[david@drumheller.org]

I just finished repairing an SB-301 receiver. It works well, however,
I can’t reliably calibrate the dial. For example, if I zero the dial
at, say, 3.5 Mhz using the internal 100 Khz crystal oscillator, the
needle is about 4 Khz too high at 3.6, 3.7, 3.8 and 3.8 Mhz, but is
again zeroed at 4.0 Mhz. Is this normal behavior for linear master
oscillators (LMOs)?

NOTE: This LMO needed to be repaired. Two of its tabbed slip rings
on the tuning shaft were “glued” together (hardened grease) and its
capacitor plates moved beyond the fully meshed position on the low end
of the tuning range. Anyhow, I removed and cleaned the slip rings and
adjusted the rotation offset of the tuning capacitor. I’m now left
wondering if, out of necessity, I fouled up the LMO’s calibration.

This is a manageable shortcoming, as I can calibrate to the nearest
100 Khz mark. Nevertheless, I’m curious if such variability is
normal.

-Dave Drumheller, K3WQ

[Antonio Vernucci]

------------------------------------------------------
> ha scritto nel messaggio
...
I just finished repairing an SB-301 receiver. It works well, however,
I can’t reliably calibrate the dial. For example, if I zero the dial
at, say, 3.5 Mhz using the internal 100 Khz crystal oscillator, the
needle is about 4 Khz too high at 3.6, 3.7, 3.8 and 3.8 Mhz, but is
again zeroed at 4.0 Mhz. Is this normal behavior for linear master
oscillators (LMOs)?

NOTE: This LMO needed to be repaired. Two of its tabbed slip rings
on the tuning shaft were “glued” together (hardened grease) and its
capacitor plates moved beyond the fully meshed position on the low end
of the tuning range. Anyhow, I removed and cleaned the slip rings and
adjusted the rotation offset of the tuning capacitor. I’m now left
wondering if, out of necessity, I fouled up the LMO’s calibration.

This is a manageable shortcoming, as I can calibrate to the nearest
100 Khz mark. Nevertheless, I’m curious if such variability is
normal.

-Dave Drumheller, K3WQ

LMOs (or PTOs) are never perfect and a +/- 1kHz error, or even a little more, is
normal.

+/- 2kHz, as in your case, is probably on the high end.

But hard to tell how you can improve linearity any further.

Obviously it would be better to tune the oscillator so that it is at -2kHz on
3.5 and 4 MHz, and at +2KHz on 3.6, 3.7 and 3.8 MHz. Or even better at -3kHz on
3.5 and 4 MHz, and at +1KHz on 3.6, 3.7 and 3.8 MHz, so that you are closer to
true frequency on the major part of the scale without recalibration.

73

Tony I0JX

[COLIN LAMB]

Hi Dave:

First, let me say I have not played with the Heath LMO alignment for decades
and
I do not remember specifics. But, here are things I would do - not
necessarily in order.

1. Draw a graph of the LMO error so that you have a visual representation
of the error. It may be that the error is simply occurring at one end,
which could mean that a mechanical shift of the plates could accomplish your
goal.

2. Most capacitors in receivers generally have tabs on the outside plate
divided into sections that can be bent to facilitate adjustment. You
sometimes can make minor physical admustments even without the plates that
are divided with slots.

3. A trick to reduce the end error is to align for a point in from the end,
such as 3.6 and 3.9 MHz. Track the error. Then, try 3.55 and 3.95 and
track the error. You might be able to find a compromise you can live with.

4. You can physically take a Dremel tool and custom shape the outside rotor
or stator to give you the curve you desire. This requires careful
documentation of the shift. A computer graph made by a spreadsheet works
great.

With patience and care, you should be able to get it right on the button.

73, Colin K7FM

[Chuck Harris]

wrote:
> I just finished repairing an SB-301 receiver. It works well, however,
> I can’t reliably calibrate the dial. For example, if I zero the dial
> at, say, 3.5 Mhz using the internal 100 Khz crystal oscillator, the
> needle is about 4 Khz too high at 3.6, 3.7, 3.8 and 3.8 Mhz, but is
> again zeroed at 4.0 Mhz. Is this normal behavior for linear master
> oscillators (LMOs)?
>
> NOTE: This LMO needed to be repaired. Two of its tabbed slip rings
> on the tuning shaft were “glued” together (hardened grease) and its
> capacitor plates moved beyond the fully meshed position on the low end
> of the tuning range. Anyhow, I removed and cleaned the slip rings and
> adjusted the rotation offset of the tuning capacitor. I’m now left
> wondering if, out of necessity, I fouled up the LMO’s calibration.
>
> This is a manageable shortcoming, as I can calibrate to the nearest
> 100 Khz mark. Nevertheless, I’m curious if such variability is
> normal.

The LMO was spec'd to be +/- 2KHz from end to end. So, set it to be
dead on at the middle of the band, and it should be fine.

Try googling on Heath LMO recalibration... You should find some instructions
on how to clean your LMO up, and recalibrate it for best linear operation.

-Chuck Harris

[david@drumheller.org]

Thanks, everyone, for your advice.

If I spot the 100 kHz crystal oscillator at 3.7 MHz, I have no more
than 2.5 kHz of error across the band, which is a bit high, but
acceptable, so I’ll leave it alone for now. If I do attempt to reduce
the dial error, I’ll try bending the tabs on the variable cap. I’ll
first attempt to do this while the LMO is still installed in the
receiver. This will require a dental mirror and pick, but may just
work.

-Dave Drumheller, K3WQ

[Kim Herron]

Hi Chuck!!
I've fixed several of these over the years, and what I've found
is that the ground wiper on the capacitor gets dirty (see Bill Turners
response). I'll expand on what Bill said. These things tend to lose
their ground and if it's real bad, you'll get a "warble" when you tune
the radio across the band. If you partially disassemble the LMO, you
can get to the cap and clean and regrease it, and then you can get to
the adjustments inside to be able to re-linearize the thing. I've
gotten many of them to track almost as close as a Collins PTO, easily
less than 1/2 kHz off per 100 kHz. You'll find that TRW used low value
caps to pad the things and usually very little actual repair of them is
needed. FYI, you WON'T find any good info on the thing because Heath
didn't want you inside of it in the first place. Draw a schematic so
you have something to work from.
"Chuck Harris" > wrote in message
...
> wrote:
>> I just finished repairing an SB-301 receiver. It works well,
>> however,
>> I can’t reliably calibrate the dial. For example, if I zero the dial
>> at, say, 3.5 Mhz using the internal 100 Khz crystal oscillator, the
>> needle is about 4 Khz too high at 3.6, 3.7, 3.8 and 3.8 Mhz, but is
>> again zeroed at 4.0 Mhz. Is this normal behavior for linear master
>> oscillators (LMOs)?
>>
>> NOTE: This LMO needed to be repaired. Two of its tabbed slip rings
>> on the tuning shaft were “glued” together (hardened grease) and its
>> capacitor plates moved beyond the fully meshed position on the low
>> end
>> of the tuning range. Anyhow, I removed and cleaned the slip rings
>> and
>> adjusted the rotation offset of the tuning capacitor. I’m now left
>> wondering if, out of necessity, I fouled up the LMO’s calibration.
>>
>> This is a manageable shortcoming, as I can calibrate to the nearest
>> 100 Khz mark. Nevertheless, I’m curious if such variability is
>> normal.
>
> The LMO was spec'd to be +/- 2KHz from end to end. So, set it to be
> dead on at the middle of the band, and it should be fine.
>
> Try googling on Heath LMO recalibration... You should find some
> instructions
> on how to clean your LMO up, and recalibrate it for best linear
> operation.
>
> -Chuck Harris

[Chuck Harris]

Kim Herron wrote:
> Hi Chuck!!
> I've fixed several of these over the years, and what I've found
> is that the ground wiper on the capacitor gets dirty (see Bill Turners
> response). I'll expand on what Bill said. These things tend to lose
> their ground and if it's real bad, you'll get a "warble" when you tune
> the radio across the band. If you partially disassemble the LMO, you
> can get to the cap and clean and regrease it, and then you can get to
> the adjustments inside to be able to re-linearize the thing. I've
> gotten many of them to track almost as close as a Collins PTO, easily
> less than 1/2 kHz off per 100 kHz. You'll find that TRW used low value
> caps to pad the things and usually very little actual repair of them is
> needed. FYI, you WON'T find any good info on the thing because Heath
> didn't want you inside of it in the first place. Draw a schematic so
> you have something to work from.

Hi Kim,

The schematic for the solid state LMO is on bama in the Heath section.

Heath may have not wanted people inside of the LMO, but in the later years,
many an adventurous soul went in anyway.

-Chuck