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

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  1. WB9TEN

    WB9TEN Ham Member QRZ Page

    This is the modification to run the radio with an SB-640 external VFO. If you leave the SB-101 "Freq Control" in "Locked Normal", it will work just fine. The only thing you loose is the internal fixed-frequency crystal oscillator. Is the 9-pin Molex installed? It normally is on a bracket that bolts to the back of the HV cage. The Molex connected the SB-640 to the SB-101. If you want a complete copy of the SB-640 I can email one to you.
    WB3EIM and W3RU like this.
  2. W3RU

    W3RU Ham Member QRZ Page

    Good catch for Stan's info.

    If the fixed crystal operation is an important need for an operator, one actually realizes increased capability with the SB-640 because there are 2 slots for selectable fixed crystals within the confines of the unit.

    Perhaps a picture from Stan would confirm the presence of the Molex modification. This expansion capability was actually built into the SB-102 for SB-500 transverter expansion and a second Molex connector could be added for SB-640 LMO power using the same bracket. This is the reason for the rear panel knockout access on most SB-line full-size outer cases.

    Jack W3RU
    WB3EIM likes this.
  3. WB3EIM

    WB3EIM Ham Member QRZ Page

  4. WB3EIM

    WB3EIM Ham Member QRZ Page

    Yes the meter read 75-80%
  5. W3RU

    W3RU Ham Member QRZ Page

    OK - great.

    Your receiver front end is probably OK and working properly.

    Next step would be to get the stage gain chart from the manual and start from the back (6GW8 stages) and work forward using a signal generator to ID the deficiencies. Replace the 20 mfd/350 e-caps with 22/450 radial's as well. Check the product detector 6BN8 triode resistors carefully - also that the IF transformers are properly peaked. Be sure the related tube cathode voltages and resistances to ground are correct and cathode resistors don't read high.

    You will find it quickly using the 3395 KHz testing steps/process, I'll bet - maybe in a mixer stage.

    Jack W3RU
    WB3EIM likes this.
  6. WB3EIM

    WB3EIM Ham Member QRZ Page

    Changed the e-caps and checked everything else the best I could. All seemed to be in tolerance. The radio seems to be working fine now so I'm going to get it on the air and enjoy it. It's the one I wanted as a kid and couldn't afford.
    Thanks all!
    WB3EIM Stan
    KE4OH and K7TRF like this.
  7. W3RU

    W3RU Ham Member QRZ Page

    Well, great.

    And, we didn't even need to address the transmitter!

    Jack W3RU
    WB3EIM likes this.
  8. WB3EIM

    WB3EIM Ham Member QRZ Page

    Been enjoying the SB-101 but a problem just came up. After transmitting for more than a couple minutes the CB in the PS kicks. Nothing seems hot and no smells. I rebuilt the PS with all new caps, diodes and resistors. Driving the transmitter to hard?
  9. W3RU

    W3RU Ham Member QRZ Page

    Possible. What is the Plate current reading at dip point - go for 225 ma and see if it still kicks. If using a watt meter, tune for 90 watts max, assuming the 6146's are still good.

    Also, verify the resistance off the cathodes of the 6146's. Should be 1.67 ohms with the six 10 ohm parallel resistors in good condition. If higher, plate reading would be higher (especially at dip), but that would not explain the CB roblem.

    I would be betting on a weak CB in the HP-23A (fuses used in the earlier HP-23 - located in the plug.) My memory seems to recall that this is 2.3 amps for the CB. Try tacking in a 2.0 amp 3AG fuse in a holder to test the HP without the CB in use. Fuse should hold at 2.0 amps at full power output of 250 ma dip current. These are not easy to source in today's world. I would install a fuse holder in place of the breaker on the chassis apron once you identify the real culprit - those properly working CB's are really convenient in the test environment!

    Jack W3RU
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  10. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I agree it's more likely the circuit breaker itself rather than an excessive current draw problem but Jack's advice above is good, especially checking the cathode resistors/current metering shunts. I'd definitely want to double check that the finals current when dipped and loaded is pretty close to 225 mA and that the plate current metering is accurate.

    If you have a line current meter like one of the inexpensive Kill A Watt meters you could run monitor the AC supply current to see if it climbs up near the trip threshold for the circuit breaker prior to it popping. As I recall the circuit breaker should trip at 3 amps.
    WB3EIM likes this.

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