I've had the radio for around 20 years, just as a display, but I want to get it working. What I need right now is how to get the main tuning mechanism to work. I did a quick youtube check, and saw nothing. I do need three tubes for it, and filter caps, but want to get the tuning working first. EDIT: The mechanism is slipping.

RW

http://tubularelectronics.com/Heath_Man ... /sb300.pdf Doesn't look good. Maybe this is better.

https://www.manualslib.com/manual/96596 ... b-300.html

Neither are any good. They leave out all the construction details...both have only 37 pages. It's like 6 or 7 pages, then it jumps to page 59 to the end. I downloaded them last spring, thinking they were complete.
RW

I found instructions online, but boy, is it complicated. I would have to tear down almost the whole LMO...I might just stick it back in the cabinet. I have no patience for that kind of detailed work. Thing is, I was given it years ago with the stipulation I don't sell it until the guy who gave it to me passes. And he's still around. Bollocks.
RW

The SB series LMO are driven by a pulley that engages with the drive ring. Slippage is usually caused by this being out of adjustment, NOT an internal LMO issue. It is adjusted by removing the tuning knob, loosening the lock nut that secures the drive shaft assembly, and pushing it upward to achieve greater engagement. Smoothest operation may require loosening but NOT removing the mounting hardware for the LMO "box", use the engagement adjustment and slight changes in orientation of the LMO assembly for smoothest operation. The photo below is from a SB-101 manual but the LMO drive mechanism is the same style for all of the SB series except for the SB-104.

Rodger WQ9E

Ok. Will look into that. Thanks, Roger
RW

Actually DOWNWARD rather than upward. The "pinching disks" of the tuning shaft engage the inner ring of the dial from the "inside" and you have to push downward to engage the dial inner ring. Also, you want to "just engage" it where it is pinching but not forced all the way down. If you force it too much you risk breaking one of the "pinch disks" (yes I found out the hard way) or having the mechanism bind badly. If you get it right the mechnism is quite smooth, tunes easily, and has no backlash or other odd behavior.So care must be taken but it really isn't hard. It takes a little practice to make sure the "pinch disks" are actually "pinching" the inner edge of the dial ring rather than being in front of it or behind it. You kind of have to do that by "feel" because you can't see in there too well. Once you get the turning feel the way you like it you tighten the shaft nut and reinstall the knob.

Curtis Eickerman

Thank you for the correction Curtis, it has been some time since I had to adjust one of these.

The outer disks have become more fragile with age and are also prone to breaking so definitely don't force anything.

It is very common to find that you need to SLIGHTLY adjust the placement of the LMO to provide smooth tuning if you find it intermittently slipping. Again, slight adjustment without forcing anything is key.

Rodger WQ9E

Ok. I used a small screw driver through the hole above the tuning shaft. It turns stiffly. Now, most of the of the other controls were stiff. Those I loosened with a little light oil. I suspect the LMO shaft is gummed up as well. Probably why the tuning knob is slipping. So how do I take care of that problem?
RW

Did you put some light oil on the tuning shaft where it passes through the bushing, this is called for in the assembly instructions and must be renewed with age/use.

Rodger WQ9E

Did about 15 minutes ago.

My SB 310, which I assembled from a kit in about 1968, had problems with its tuning mechanism when I went to restore it a few years ago. I played around with it and then just gave up and took the full assembly apart and cleaned and re-assembled it all. It then worked like new. I have restored a few other SB units with the same mechanism and found that most of them needed to be torn down and re-assembled. It isn't hard and once you get into it, can be rewarding, as you know the tuning will function properly for years to come.

Norm

Just noticed the zero set dial is not aligned with the zero set drive pulley.
RW

I got the zero thing working...and now it seems the main tuning is working too. Just a little slippage. Maybe the zero dial operating correctly had summat to do with it all.
RW

And it's working. Picked up some 80 meter AM, and a SSB net. The S meter is pegged past 0...a little voltage discrepency there. Most of the controls have loosened up. Will need to dig up a proper power cable for it. And order the tubes. Thanks for the suggestions, all. Cheerio.
RW

Ok...still more headaches...the AGC switch is jammed. I can move it if I use a Vice Grip pliers. I've put cleaner around the shaft, and then put some light oil in. No good. I just sprayed it down with WD-40 to see if that will loosen it. I removed the switch from the radio. Now, I noticed that all the rotary switches have some kind of goo along the detent mechanism. Whatever it is, is hardened. I suspect that is interfering with the proper operation of the switch. Would that stuff be something that Heath would have put on their switches, like maybe a grease? If so, I will pick it off.
RW

Usually, the switches and pots seize up between the shaft and the bushing. I use a syringe filled with 5W-20 synthetic motor oil to free these up. Heat the bushing with a soldering iron and then remove the iron and apply a bit of oil to where the shaft enters the bushing. As it cools, it will draw the oil in. Repeat this one or two times and it should free up and work properly.

Rodger WQ9E

Ok...will give that a shot.
RW

Got it loosened up. Not like new, but at least it will switch. Reinstalled the switch; just need to rewire it.
RW

The S-meter is not functioning correctly. It goes "negative" after the radio is powered up and warmed up. The meter zero control does nothing. I've used DeOxit Fader Lube on it. I just replaced the 6AS11 audio, 6HF8 and 6BZ6. Same old. Not sure what to check in that area. I do have a schematic, but it's in four parts...probably should print them off and then cut and tape it together so it makes more sense.
RW