SB-101

Discussion in '"Boat Anchor" & Classic Equipment' started by WB3EIM, Nov 21, 2021.

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  1. WB3EIM

    WB3EIM Ham Member QRZ Page

    Been working on a SB-101. Got it to xmit, but it only receives strong stations (s9+) 40m and up with no receive on 80m. Has background static that increases when the AF is rotated. The meter deflects very slightly on the strongest stations. Where should I look first? I have been out of radio for a long time and have forgotten a lot so dumb it down for me.
    Thanks Stan WB3EIM
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    At the Heathkit assembly manual that has very good information including resistance checks from each tube socket and good troubleshooting advice.

    FWIW, the biggest recurring problem I've seen with Heathkit of that vintage is carbon comp resistors that have drifted out of specification especially resistors on a tube's B+ line. I'd be sure to perform the resistance checks in the manual and then get in and measure individual resistor values (sometimes you have to desolder one end of a resistor to get an accurate measurement if there are shunt paths that could alter the reading) but replace any that are more than 15% to 20% out of spec (usually on the high side).
     
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  3. W3RU

    W3RU Ham Member QRZ Page

    What is the S-meter reading on 75M with the calibrator tuned and peaked?

    Jack W3RU
     
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  4. WB3EIM

    WB3EIM Ham Member QRZ Page

    I'll be going thru the resistance chart today.
     
  5. WB3EIM

    WB3EIM Ham Member QRZ Page

    After tuning up on 75 and switching the meter to rel power it reads full scale. I have to back the cw mic control off some. Doesn't do this on other bands.
    Also what tube does the LMO 110-32 take. It has a 6BZ6 in it but the manual(not original to the radio) shows it takes a 6CB6.
     
  6. W3RU

    W3RU Ham Member QRZ Page

    I misunderstood - thought your issues were on receive.

    Looks like your RF chain is in good shape transmit-wise, so that helps a lot, especially since the Het-Osc circuit for 75M seems to be working (assumes no self oscillations are "helping"; it would be good to verify the 12.895 crystal frequency on the V6 Het-Osc injection point to be sure).

    6BZ6 tube on the -32 and original -13 LMO models is correct; the -40 LMO used the 6CB6 - the latest models. You have an "earlier" SB-101. I could never figure out which tube would be more desirable, except 6CB6 is a $3.00 tube, and 6BZ6's seem to go for $5.00 NOS NIB. 6CB6 in the LMO means one last spare to have on-hand, since V6 is a 6CB6.

    What is the grid voltage on the 75M HetOsc coil? Should be -2vdc +/-. All the coils should read around that as they are switched in with the voltage taken at the test point.

    You can verify the "internal" (non-antenna) signal strength using the calibrator when the preselector is peaked for each band - this would be between S30+ decreasing to S-6 across 75 - 10 meters. Be sure the antenna switch in the rear is set to common position. If your 75 meter receive is not peaking with IF noise, as do the other bands, I would suspect an alignment issue, or possibly 1 of the four rotary bandswitchs on the small vertical boards under the coil cover.

    The Het-Osc coil for 75 M is one of the dual-band coils and 3.5 MHz is accessible from the top of the RF board, and usually not lined up with the access hole on top of the board. Over it's life, this coil seems to be subjected to off-center mechanical forces when tuning with the hex tool. Also, this particular coil form seems to suffer breakage because it is so fragile. When you have the cover off, carefully inspect that coil. The slug is easily cracked from the mechanical forces - especially if a "dull" hex tool is used, and if the coil slug is frozen in time from 55 years of storage. These may sometimes be freed using minimal heat from a hair dryer and a NEW hex tool. The HetOsc board is the easiest to remove after the crystal board is out - no bare ground or connecting wires to contend with. You would need to find a donor coil from a beater SB/HW-10x rig to replace a broken one. Just be sure the slug(s) are adjustable and not stuck before investing too much time in a donor coil.

    I trust you have a manual to help with your restoration. Early or later does not matter much. Even an SB-100 or SB-102 would answer 95% of your questions for the SB-101.

    Best of luck. This series is the nicest Heathkit offering ever put out, lots of fun to operate and all SB models are easily repairable.

    Jack W3RU
     
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  7. WB3EIM

    WB3EIM Ham Member QRZ Page

    Sorry for the confusion, I am having trouble with weak reception. I thought you were talking about xmiter tune up. Going to check resistance, what circuit board should I start with. As I said before I'm not up on complicated troubleshooting but willing to learn.
     
  8. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I'd probably start by checking around V3 followed by V2 and V4. Those are key elements for receive sensitivity, AGC action and S metering but it could be many things like insufficient Het oscillator or VFO drive. If the resistors check out it doesn't hurt to try swapping individual tubes in the RF/IF strip as sometimes it's just a soft tube issue but I'd start with the resistance checks first.

    If you find a few carbon comps out of tolerance then you can be sure there's more as unless they've already been replaced as all those resistors are of similar vintage. So if you find some it's best practice to do a comprehensive sweep. Just going through the resistance check table based on resistances measured at the tube sockets is a very good start but often you have to test individual resistors as well.

    At first glance it may not seem like resistor values could cause RF effects like weak receivers but when something like the plate supply resistor goes very high in value on an IF amp that tube's gain can drop way below spec causing all kinds of issues depending on exactly what part of the circuit is impacted.
     
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  9. WB3EIM

    WB3EIM Ham Member QRZ Page

    Success! After replacing 8 resistors and a 6AU6 the receiver is working better. I still need to get the S meter working, it barely moves even on the strongest signals. Looks like someone has work on it before. Any thoughts appreciated, thanks.
     
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  10. W3RU

    W3RU Ham Member QRZ Page

    Stan,

    With meter switch in the ALC position, does the meter move upscale to the right hand peg limit as you rotate the RF gain control lever CCW when in receive? Does the meter zero properly close to the center of rotation of the 200 ohm adjustment pot? This pot frequently fails from intermittents and broken traces internally due to abuse and frequent diddling and forcing at the end points.

    Of the resistors you replaced, did that include the 68 ohm resistors in the s meter circuit, and the 100K screen resistor for the V3 6AU6 you replaced? These are usual culprits causing S-meter readings to be erroneous.

    Keep an eye out for a gaseous V3 which would cause problems in this circuit as the rig warms up. Do a tube switch with V2 or V4 as a quick test.

    Jack W3RU
     
    Last edited: Nov 24, 2021
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