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Author Topic: HEATHKIT TUBE LMO  (Read 1579 times)
K0WRX
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Posts: 18




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« on: November 16, 2017, 07:35:33 AM »

Another local ham and myself are considering converting the old LMO from Heathkit equipment to a vfo that we could use in homebrew designs.  We would need to alter the frequency range covered by the VFO.  I have looked several places for a schematic for the LMO and have not found one.  The LMO I have uses a 6AU6 and has two slug tuner screws next to the 6AU6 tube ..outside the enclosure.  If anyone can help me find a schematic or offer suggestions for this project, I would appreciate it greatly.

Thank you ...Pete
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WA5VGO
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Posts: 38




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« Reply #1 on: November 16, 2017, 07:46:38 AM »

Heterodyne it to the frequency range you want.
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KD0REQ
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Posts: 2021




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« Reply #2 on: November 16, 2017, 08:00:22 AM »

if you have a clue which equipment it came out of, get the schematic off the web for the full transceiver.

but I'm with hetrodyning it to the band you want to cover. if it's working, why break it? silly things happen when you start modding something that was very, very carefully engineered.
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K8AXW
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Posts: 6360




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« Reply #3 on: November 16, 2017, 09:01:49 AM »

I built the SB-102 and as I recall after many decades, it was solid state.  The earlier LMOs had a tube.

In either case, I have never seen a schematic of the LMO.  I do recall the LMO was sealed and  Heath was adamant about opening it and making any adjustments.

I'm with REQ.  If it isn't broke, leave it alone. Of course hams have modded about everything but the Heath LMO.....not so much.
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K0WRX
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Posts: 18




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« Reply #4 on: November 16, 2017, 11:05:43 AM »

Thank you folks for offering input on the Heathkit LMO.  Once I find a schematic I will be in a better place to make a design decision.  All your input has been appreciated...thanks...Pete..K0WRX
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KB1GMX
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Posts: 1505




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« Reply #5 on: November 16, 2017, 11:19:21 AM »

most of the common HEathkit LMOs were a fet foran oscialltor and a 6au6 as
the buffer and gain.

This includes the SB-series and the HW series.

The manual for any of them has the schematic and as memory serves they
were near identical maybe two or three versions.

Note: shifting the range is nontrivial as it can impact the linear tuning to be
vary non linear.  Do so with care.

Allison
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N2EY
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Posts: 4455




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« Reply #6 on: November 16, 2017, 12:36:05 PM »

The LMO in the SB series tunes 5.0 to 5.5 MHz.

The LMOs supplied with the SB-300, SB-301, SB-400, SB-401, SB-100 and SB-101 are tube-type. The tube used varied, but was always a common type. They were made by subcontractors and supplied to Heath as a component.

PLEASE don't try to change the tuning range. You'd be better off building a VFO from scratch.

For example - suppose you want a VFO to cover 80 meters (3.5 to 4.0 MHz). A heterodyne unit could be easily built using a 9 MHz crystal oscillator and the LMO output. Subtractive mixing (9.0 - 5.5 = 3.5, 9.0 - 5.0 = 4.0).

The Hallicrafters HA-5 VFO used this method. The manual is here:

http://bama.edebris.com/manuals/hallicra/ha5/

73 de Jim, N2EY
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WD4HXG
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Posts: 297




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« Reply #7 on: November 16, 2017, 03:09:55 PM »

If you have a well equipped lab (stable and accurate signal
generators, spectrum analyzer, temperature controlled
soldering iron etc then you may be up to the task. The
LMO's were nominally 1/3 the cost of the kit and TRW which
was one of the subs used by Heathkit for the LMO's
did not release any schematic info as far as I know. I
doubt Heath even had a schematic for the units as they
returned them to the vendor to be repaired due too the
complexity based on what I was told by a Heath rep
in Michigan in 1978.

It is not a simple box to work on and for solid state work
I would be inclined to roll my own custom VFO before
messing with that unit. Building a stable vfo is fairly
straight forward. Building a miniature frequency counter
is also fairly straight forward. Building a linear scaled
VFO is a pain. Ops have done it but given the alternatives
I would suggest not signing up for it.
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KM1H
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Posts: 2598




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« Reply #8 on: November 16, 2017, 03:51:01 PM »

There are also many Collins and other brand PTO's with all sorts of frequency ranges that are usually very cheap since no hams want them. They are tube and SS.
Drake versions often show up from parted out gear and are superior to the Heath which often have poor linearity, by todays standards anyway.

Carl
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N6KP
Member

Posts: 2




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« Reply #9 on: November 16, 2017, 03:59:24 PM »

The SB series used an Linear Master Oscillator because Collins had a patent on their Plate tuned Oscillator (PTO).

These LMO units were factory built as they need the stability and precision which Heath thought amateurs could not build.

Other than the LMO/PTO parts, Heath tried to clone the Collins equipment and did quite successfully.

N6KP
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WA5VGO
Member

Posts: 38




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« Reply #10 on: November 16, 2017, 04:47:27 PM »


Other than the LMO/PTO parts, Heath tried to clone the Collins equipment and did quite successfully.

N6KP

Often misstated “fact”. Other than appearance, the Heathkits had very little in common with the Collins equipment. Different IF frequencies, different mixing schemes and mostly different tube line-up.
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KM1H
Member

Posts: 2598




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« Reply #11 on: November 16, 2017, 05:52:47 PM »

With a full 500+ kHz wide first IF "passband coupler" it was a pretty poor large signal performer. Heath took the cheap way out for double conversion.

Carl
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WD4HXG
Member

Posts: 297




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« Reply #12 on: November 17, 2017, 08:22:56 PM »

With a full 500+ kHz wide first IF "passband coupler" it was a pretty poor large signal performer. Heath took the cheap way out for double conversion.

Carl

The 40673 and 3nxx dual gate FETs in the early 70's were the
weak point. You could have had a 99th order filter in the RF
and the intermod performance would not have been anywhere
near what rigs 10 years later could deliver. I remember when
a plus 7 dBm 3rd OIP at the input was consider sierra hotel.
The simple fact is Heath designed and delivered kits which
worked if built per documents for a price no one else could
reach. I wanted a KWM-2A in 1972 until I discovered the
price Collins wanted for one. The SB-303 may not have been
the cat's meow in intermod performance but it sure provided
me a lot of fun and satisfaction for a price far lower than
any of the primo Collins gear could.

In my opinion Heath pulled off what a friend of mine called,
"Getting the Mostest for the Leastest". Pardon my abuse
of superlatives but it quite accurately describes what
Heath designers pulled off at a price many amateurs could afford.
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KM1H
Member

Posts: 2598




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« Reply #13 on: November 18, 2017, 01:49:57 PM »

That is why you never saw (or it was very rare) Any SB Line in the better contest and DXing stations. They were fine for casual users on modest (or less) antennas

Was the 40673 , just vacuum tubes, BP and a few FET's and many were quite respectable performers.

In 1970-73 I was in IL on a company transfer and guest operating at a few of the better stations for those years after having operated at winning multi-multi stations in MA and NH. Collins was poorly represented for several reasons besides price and a 200 kHz tuning range was a huge turnoff for SSB. Early S Line were dogs.

I lugged in my old CE-100V (No Tune), a highly modified 75A4, and my trusty NCL-2000 prototype (Ive had all 3 since 1964-65) which were still competitive and operated 2-3 bands (I taught many how to efficiently use 75/80M Grin and also ran a few small gatherings on how to dig deep into the noise) By 75-6 I was forced to upgrade to a pair of used Drake C Lines with transceive capability to remain competitive back in NH. Those were replaced by a pair of used TS-930's and up thru the Kenwood food chain until I tired of contesting.

Carl
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N4AST
Member

Posts: 56




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« Reply #14 on: November 19, 2017, 03:23:20 PM »

The SB series used an Linear Master Oscillator because Collins had a patent on their Plate tuned Oscillator (PTO).

I think Collins had a patent on their permeability tuned oscillator (PTO).  It used a slug moving inside a coil to change the frequency of the oscillator.

Gary N4AST
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