Heathkit SB-310 Dial Cord Replacement

Discussion in 'Radio Circuits, Repair & Performance' started by KU4BY, Nov 26, 2019.

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  1. KU4BY

    KU4BY Ham Member QRZ Page

    Good Morning Group,

    I recently picked up an old Heathkit SB-310 in need of some serious work. I was wondering if anyone in here has restored one of these before and if so, where I might find the manual that shows how to string the dial? The unit I picked up doesn't have a string in it at all.

    I found a partial manual online but there isn't anything in it pertaining to a dial string. Any help would be greatly appreciated.

    Thanks!

    Steve
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    There shouldn't be any dial string for the main tuning in a Heathkit SB-310.

    The Heathkit SB series used a pinch roller design and an associated lever indicator for main tuning and there was no dial string for the main tuning dial.

    Consult any Heathkit SB series transmitter or receiver manual for assembly instructions for the tuning dial mechanism. Mike has this SB-101 assembly manual on his website that includes information on the main tuning dial assembly: https://w5rkl.com/wp-content/uploads/2014/04/SB101-complete-assembly-manual.pdf (description of dial assembly starts around page 50)

    Basically on the Heath SB series transmitters, receivers and transceivers the main tuning knob turns a brass pinch roller that's basically two very thin, tapered brass washers with a small gap between them. The main numbered plastic dial (the circular dial numbered 0 to 100) is pinched between those brass washers and you get an effective planetary reduction gear so that it takes many turns of the front panel knob to completely tune across the band. There's also a small lever assembly that has a pin riding in a spiral groove in the main plastic indicator dial that moves the top pointer as you tune across complete 500 kHz bands.

    The biggest problem with this design is adjustment of the brass pinch roller assembly so that the dial doesn't slip or bind. If your dials don't turn at all when you turn the main tuning dial then the pinch roller has probably slipped and is much too loose, that's an easy adjustment after you remove the front panel knob and perhaps the plastic bezel.
     
  3. KU4BY

    KU4BY Ham Member QRZ Page

    Thanks! I saw the pinch roller and it looked like it was the perfect size for a dial cord which is why I assumed it was just missing. Perhaps I should look for a copy of the build guide as there are a few things going on here that don't mesh up with your explanation.

    When I turned the knob, nothing was spinning. Your explanation of the pinch roller design make sense here.

    When manually I turned the main tuning dial, the log dial up top does not slide. I see the grooves in the back of the main dial and the lever that is supposed to ride in the grooves is there but I guess I am missing the pin that goes in the plastic sleeve as well.

    The plastic piece with the tuning indicator on it that sits between the main tuning dial and the front panel is loose as in it stays downward towards the bottom of the radio and at first glance makes it look like there is no indicator at all. Should that be held in place by the pinch roller that is mounted on the back of the plastic face for adjustment of the indicator?

    Thanks!

    Steve
     
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Yes, it's been a while since I've been inside my SB-102 but as I recall there's a large brass nut on the pinch roller assembly that helps hold the plastic bezel in place. There's also the graticule adjuster knob (small tapered silver knob in the upper right hand corner) that passes through the bezel that helps keep it in place.

    If the plastic bezel is slipping down then the brass pinch roller assembly is loose and likely not making a good friction contact with the main tuning dial. From your description it sounds like something happened to the pin that rides in the spiral track and a previous owner took this apart and didn't put it all back together again.

    FWIW, parts of this assembly show up on eBay all the time, try searching for Heathkit SB dial. Luckily they're all the same across the line of SB series rigs.
     
  5. KU4BY

    KU4BY Ham Member QRZ Page

    Yeah so the graticule adjuster barely makes contact and does slip. I had to take the escutcheon off and put it back on to get it to somewhat work but then it slipped off again. There's also a lot of play in it as I can push it in and pull it out about 1/16th of an inch. I'll probably need to tighten the pinch rollers a little to help with the looseness. THanks for your assistance!

    Steve
     
  6. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    You don't have to remove the escutcheon to get the main dial drive correctly positioned.

    If you look closely, there is a slot where the drive mechanism goes through the escutcheon. Remover the main tuning knob. You loosen the nut on the sleeve through which the shaft, with the "pinch" drive, goes through. Then, lift the mechanism and slide it backwards, and forwards, until you "feel" the inside of the main dial. You might have to move the mechanism slightly downward to make the contact. If you feel this contact when pulling the mechanism outward, then lift it very slightly and pull it outward like 1/32nd an inch and then push the mechanism downward. When correctly positioned, the "pinch" drive will slip over the inner portion of the main dial. Finger tighten the nut that holds the shaft in place. Then turn the shaft. If it still slips, then move the shaft very slightly downward until you get the shaft to turn the dial without slipping. If the shaft is hard to turn, then move it very slightly upward and try again.

    If the shaft makes contact when first pushing it in, then move it slightly upward and then inward about 1/32nd of an inch. Then follow the instructions above.

    After doing this a couple of times, it does get really easy to do! After positioning the shaft correctly, tighten the nut holding the shaft bushing in place and then put on the main tuning knob.

    I have adjusted, literally, well over 100 of these drives and, unless the "pinch" drive is badly damaged, have never had to remove the escutcheon.

    This "pinch" drive is used on the SB-100, SB-101, SB-102, SB-110, SB-110A, SB-300, SB-301, SB-303, SB-310. SB-313, SB-400, and SB-401.

    The SB-310 is the SWL equivalent of the SB-300. There was an optional conversion kit that removed the 11-meter "CB" band and replaced that band with the 13-meter and part of the 15-meter band. When this kit is installed, there is an additional "plate" over the band-switch silk-screening. My particular SB-310 has this modification kit installed.

    Glen, K9STH
     

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