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Author Topic: Heathkit SB-102 Tuning problem  (Read 3975 times)
KD0ILM
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Posts: 36




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« on: March 17, 2013, 08:32:29 PM »

I have a new to me SB-102. It has good output  of 100 watts on all bands although it is only about 70 watts on 15 and 10. In receive it seems to have good ears but..... The best way I can describe the tuning is scratchy. When I tune the dial a lot of scratchy noise gets kicked out of the speakers. It is all but impossible to zero beat an SSB conversation. It can be done but requires a lot of time, patience and a bit of Vodoo. Any ideas?
Kevin
KD0ILM/XV2KM
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AB1MN
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Posts: 42




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« Reply #1 on: March 17, 2013, 09:05:37 PM »

Hi Kevin,

From your description, can I assume that when you are in receive mode and adjusting the frequency dial (the big knob, the LMO) to tune in a station, you hear a scratchy sound or static from the speaker?

Let's eliminate an easier one first. If you turn the volume all the way down so that you don't hear anything from the speaker and tune the dial do you hear the sound, or anything unusual, like maybe a rubbing or squeaking sound? The reason I ask is that I have seen multiple cases where the circular dial is rubbing on the cable bundle that is very nearby at the bottom of the dial area. This usually makes a bit of noise and may also cause wear on the outside edge of the dial. In addition, it may make the tuning stiff and perhaps somewhat erratic. This can usually be cured by moving the cable clamp for the offending cable to the other side of the chassis to pick up a bit more clearance (the plastic cable clamp deforms over time allowing the cable to eventually rub against the dial).

If the above test is negative, it may be that the wiper on the LMO tuning capacitor is making erratic contact causing the frequency to jump rather than change smoothly. Unfortunately, the cure for this involves removing the LMO and opening it up to clean the contacts (and clean the old grease off the gears and apply new grease). Here are a couple of things you can try (the first one is easy, the second is more involved).

Set the bandswitch to 80 meters, turn on the Calibrator and slowly tune through the calibrator signal (every 100 KHz on the dial). Listen to the tone is the speaker as you tune dial. It should change smoothly with no burbles or jumps. If it burbles or jumps, the LMO needs to be fixed.

If you have a frequency counter, you can check the LMO using the following procedure (it is not necessary to remove the rig from the cabinet). Sometimes the LMO is erratic at specific areas of the dial due to wear on the wiper and it may be fine at other areas.

1. Power off and allow a few minutes for the capacitors to completely discharge
2. Remove the small coax cable with the RCA connector from the back of the LMO (the rectangular aluminum box immediately behind the frequency dial)
3. Connect your frequency counter to the RCA jack on the back of the LMO (note: if you have a short 2:1 RCA Phono splitter cable (1 male, 2 female), you can plug the LMO cable into one leg and your counter into the other - this way the rig will still receive while performing this test.). If you don't have a splitter, just leave the original coax cable disconnected but make sure it isn't touching anything.
4. Turn the rig on. You should see the LMO frequency displayed on your counter. If the original LMO cable was left disconnected, you won't hear any signals as you adjust the tuning dial.
5. As you turn the main tuning, the LMO frequency should change smoothly without any sudden jumps as you tune from one end of the dial to the other.  Low end of the band = 5.500 MHz, high end of the band = 5.000 MHz (or, was it the other way around?).
6. After you have completed this test, power off the rig, remove the cable to the frequency counter and reconnect the original cable.

Good luck and let us know what you find out,

Bob,  AB1MN
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K1ZJH
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« Reply #2 on: March 18, 2013, 06:49:59 AM »

I'd also second the wipers on the tuning capacitor inside of the LMO as being a good candidate for the problem.

Pete
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KB4QAA
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« Reply #3 on: March 18, 2013, 08:18:23 AM »

I agree sounds like the LMO wiper.  Also note that power drop off at higher bands is not unusual, with aging driver or final tubes.  Remember the difference between 70w and 100w on the receiving end is not distinguishable by ear.  If it gets worse or you must have closer to 100w, then try changing tubes  starting with the driver.

Glen K9STH has a procedure for servicing LMO:
http://www.k9sth.com/uploads/LMO_rebuild-1.pdf
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K8AXW
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« Reply #4 on: March 18, 2013, 08:40:23 AM »

You can forget the power drop off on the higher bands.  This is normal. 

I agree with the others that your LMO tuning cap wipers are probably crudded up.  After all, this transceiver is over 30 years old!

The LMO is a very complex and delicate piece of engineering.  This is why if comes from Heath pre-assembled and sealed.  In all the years I owned my 102 it was never necessary to open the LMO.

As I recall the LMO will have to be removed from the unit to work on it.

If and when you open the LMO don't bother anything except the main tuning cap wipers.  This will probably require a good light and a judicious application of contact cleaner while the cap is rotated.  While you're in there you might try to get a drop of light oil (like gun oil) on the capacitor bearings. I syringe might help here. Don't adjust or move any components. 

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AD4U
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« Reply #5 on: March 18, 2013, 09:14:46 AM »

If everything posted here fails, there are often Heathkit LMO's offered for sale on ebay or elsewhere.  I bought one that was said to be NOS for less than $10.  Nobody placed a bid but me.  Even if it was NOS it was still 35+ years old.  Just remember that. 

I installed it in a homebrew SB-640 external LMO I built for my SB-102 and it works as new.  Your results may vary.

Dick  AD4U
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KG6YV
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« Reply #6 on: March 18, 2013, 09:44:56 AM »

For power output issues especially on higher frequencies check/replace the cathode resistors on the final tubes.  These
are usually very low ohm values and if they change a little the driver won't have enough "oomph" to drive the finals to full power.  On any radio I get over 15 years old with tube finals I just replace them without any further consideration.  Its normal to see some drop off in power on 15 and 10M any way...

Greg
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KD0ILM
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« Reply #7 on: March 18, 2013, 02:18:48 PM »

The frequency counter idea was a great one and simple. It showed that the LMO was dragging jumping etc. I checked the wiring harness and while it was close it was fine. I now have the LMO out with the bottom taken off. I can see the "box" inside which the shaft goes into and the 5 brass things that work with the counter. (Quite clever i might add). I am afraid to touch anything without some guidance. I also suspect that the wiper I hear about inside needs cleaning but am very unsure of what is safe to do next.

I am not at all concerned about the dropoff in the higher bands. As someone pointed out at the receiving end it is not difference. I only want to do one thing at a time before I chase screen resistors, drive tube etc. Although those are good ideas for later.

While I am in here should I replace the two plastic electrolytic capacitors? (500MFD)
Thanks
Kevin
KD0ILM/XV2KM
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KD0ILM
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« Reply #8 on: March 18, 2013, 03:32:54 PM »

I think I have found the "wipers" I assume is is a slotted wheel that rotates against a silver disc. Some of the little "arms" are "higher" than others. I was able to put a piece of notecard wetted with DeOxit under them quite easily while the others were more difficult. Should I fool with this? I also think I have made the action of the LMO smoother by re-lubing it. Once I get some advice on the wiper it will go back together and we will see.
Best
Kevin
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K8AXW
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« Reply #9 on: March 18, 2013, 04:44:16 PM »

The card and Deoxit is an OK move Kevin but I would be afraid of the card distorting the fingers.  Deoxit and rotating the cap will clean the contact(s).  You might try to get the little arms down against the opposite contact..... you call it a "silver disk."  But don't mess up the others trying this.  It's better to have some than none.

I'd leave the caps alone until you have tested the LMO.  This is an area where we all get into trouble at one time or another.  Shotgunning a fix quite often works but when it's all said and done, we have no idea what "fixed" it.

« Last Edit: March 18, 2013, 04:47:57 PM by K8AXW » Logged
KD0ILM
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Posts: 36




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« Reply #10 on: March 18, 2013, 08:30:26 PM »

Well after looking at this thing all day I think I finally got it cleaned, lubed etc. I didn't want to take it too far apart as I never get them back together and soldered as neat and clean as they were in the first place. After much experimentation I was able to clean and repack the capacitor bearing which is all but hidden by judicious use of electronic contact cleaner and then a crooked dental pick to work around the wiper and repack the bearing much like packing an old wheel bearing. Tedious but it worked. I did NOT use conductive grease because I did not have any and I could not see a reason to. One the bearing is packed with a high temp grease it appeared to me that the ground between the wiper and the inside shaft of the variable cap was the important thing. The wiper has been cleaned with Deoxit and makes good contact (at least for now). Before I button it up if someone disagrees please let me know.

I now realize that I did not pay attention to where the variable cap was in relation to the tabbed washers that limit the travel of the turns counter. I also did not pay good enough attention to how they were situated. Can anyone save me from myself?
Best Kevin
KD0ILM/XV2KM
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KB4QAA
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« Reply #11 on: March 18, 2013, 09:49:49 PM »

Did you read the service procedure and photos I posted above?
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KD0ILM
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« Reply #12 on: March 19, 2013, 10:44:21 AM »

Did you read the service procedure and photos I posted above?

Yes I did read the article and it was helpful. The article you linked me too was very good and helped me grasp the operation of the unit and the problem. Unfortunately it is for the tube type LMO and as you can see from the pictures in the article you can remove one side of it and get pretty good access. I have the later TRW model which is much much harder to get at internally. (I took a good picture but guess you cannot add a photo here). You can gain access from the bottom but you are a long way from getting to the problem. In order to get at the upper bearings, worm gear etc the counter mechanism disks have to be removed unlike the older tube version. Getting at the rest of it requires miniature tools and a good light and eyes because the "guts" of the capacitor are buried deep inside the mechanism under some boards etc.

In order to cure this problem once and for all would require the further disassembly of the essentially sealed LMO. I am not confident that I can remove and replace all of the items which are needed to get deeper into the unit so do not want to go any further.

I am open to any opinions on this and have greatly appreciated the help.
Kevin
KD0ILM/XV2KM

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KD0ILM
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« Reply #13 on: March 19, 2013, 06:35:54 PM »

Post script
I got the LMO reassembled and recalibrated and it works great...for now. At the last moment it occured to me that i had some "Butter-its-not" from the assembly of my Butternut antenna. It is basically a ground up copper grease. It is horribly messy but really does the job. I finished my job using it for better conductivity.

If you have one of the later TRW models or need to re-calibrate your LMO there is a great article written by KI4IUA on the  Yahoo Heathkit group. It is in the files section. You have to be a member to get at it (which is free and easy) but I am sure he would be happy to send you a copy. http://groups.yahoo.com/group/heathkit/
You all were very helpful and gave me the courage to do this and now that it works I am thriilled. Thanks for all of your time.
Best
Kevin
KD0ILM/XV2KM

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K8AXW
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« Reply #14 on: March 19, 2013, 06:45:58 PM »

Feels good, doesn't it?
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