WB4KDI Engineering Notebook

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Preventive Maintenance

1. Replace C10 and C11 (1000 µF @ 16V) -and- replace C13 and C16 (47 µF @ 16V) all on AF-GEN board.
2. Check and tighten loose mounting screws -- especially on the RF board.
3. Check to see if any mods and Service Bulletins have been applied.
4. Carefully apply contact cleaner/lubricant to all switches and pots.
5. Check and clean fan.

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Repair

1. See troubleshooting chart in Service Manual and SB-060.

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2. Final Oscillation on 20, 15, and/or 10 meters.
   1. Check for loose mounting screws.
   2. Has SB-844 been applied?
   3. Check for bad connections or loose filter components on Filter board.
   4. Oscillations can kill 2SC2290 and 2SC2509 on Final board, and 2SC2086 on RF board.

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3. Bad/leaky/shorted D2, D3, D4, and/or D5 on RF board.
   It is easier to add replacements for these diodes on the bottom of the RF board. Removal of the RF board is not necessary.

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4. Low Power Output -- 50W on 80 decreasing to 10W on 10.
   1. May be bad finals -OR-
   2. Check output at DRV pins on RF board. Should be able to get 10V P-P on 80.
   3. If one of D2, D3, D4 or D5 have been replaced, then check for damaged transmit balanced mixer Q5 and/or Q6.
      1. Signal at D5 should be nice clean sine wave.
      2. Failure of one of the balanced mixer FETs will cause signal at D5 to contain unwanted mixer products.
      3. Substitute == 3SK40 but NTE455 is direct sub and Q5 + Q6 should be reasonably matched for best operation.

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5. PLL Unlock, 15 and 10 meters only. (Dots on display)
   1. Check 20MHz signal at D24/D25 -- level should be greater than or equal to level of 10MHz seen for 20 meters.
   2. Check for and replace bad 2SC460 at Q24 and Q25.
      1. Several of these transistors have been seen to lose gain over time.
      2. Both Q24 and Q25 were good but just low gain.
      3. Substitute == 2SC945

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6. VFO Drift
   1. See SB-827.
   2. Check and clean FIX/VFO push button.
   3. Abnormal drift would be anything over 500Hz after 30 minute warm-up.
      1. Typical drift from cold start should be less than 1kHz.
      2. Later models have considerably less drift.
      3. Replacing lamps with LEDs can reduce VFO, carrier, and time-base drift.

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7. Mildly Distorted RX Audio.
   Broken trace from J3 to receive product detector (D1-D4) on AF-GEN board.

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8. Loose or broken connections at VOX controls.
   No VOX can be caused by broken trace at Delay control.

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Adjustments and Alignment

1. See ADJUSTMENTS in Service Manual
   1. AVB adjustment is VR5 of the Filter board and not VR4 as shown in manual.
   2. Carrier point adjustments can be made easier by using Spectrogram or another FFT display.
   3. Protection adjustment is VR1 of the Filter board and not VR2 as shown.
      1. It may work better to balance the SWR circuit, adjusting TC1 on the Filter board, at 15 meters.
      2. Too much protection action will prevent getting full output. Adjust VR1 on the Final board with care.
   4. S-Meter adjustments seem to be wrong in Service Manual.
      1. S-Meter can be adjusted to match standards (S9 == 50µv and 6db/S unit) from about S3 through 40 over S9.

         uv	0.2	0.4	0.8	1.6	3.3	6.3	12.6	25.1	50	160	500	1600	5000	15000	50000
         S	1	2	3	4	5	6	7	8	9	10 over	20 over	30 over	40 over	50 over	60 over

      2. On IF board, VR1 sets SPAN, VR2 sets ZERO.
   5. VR1 on RF board sets transmitter balanced mixer balance.
2. Readjustments of band pass circuits only necessary if modified for CB/11 meters.

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Mods and Improvements

1. Apply SB-816, SB-037 and SB-051.
2. DJB mods:
   1. 10 HZ Display
   2. AGC fast/slow
   3. High/low power
   4. Adjustable sidetone pitch
3. Add diodes or other surge protection at RX input to protect T1?

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