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Author Topic: SB-102 S-Meter Problem  (Read 7590 times)
KD0ILM
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Posts: 36




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« on: June 11, 2013, 12:18:22 PM »

I have an Sb-102 that I cleaned up and got on the air. You fellows helped me rebuild the LMO and it is working great except......
The S-meter does not work correctly. When I put the switch into the ALC position per the book the meter pegs to the left below zero and then does not budge with any strength signal. (This is with the RF gain fully clockwise) If I turn the RG gain counter clockwise it will rise but not indicate any up and down of the signal. Ideas?
Kevin
KD0ILM/XV2KM

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W8JX
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Posts: 5746




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« Reply #1 on: June 11, 2013, 12:43:56 PM »

It suggests a problem with AGC circuit.
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KD0REQ
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Posts: 925




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« Reply #2 on: June 11, 2013, 02:00:31 PM »

funky ALC tube (avoid ElMenco or Heathkit branded tubes here), 22K plate resistors way high, 100K resistor way high are all commonly reported faults in the HW/SB transciever ALC circuit area.  I haven't gotten that far in my HW-100, no detected audio getting to the final stages.
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AD4U
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Posts: 2164




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« Reply #3 on: June 12, 2013, 10:07:21 AM »

Had the exact same problem in the latest SB-102 I worked on.  The 100K resistor on the PCB under and just left of the S meter was open.  I am at work and cannot give you a reference or part number.  Replaced it and ..... voila.

Dick  AD4U
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AB1MN
Member

Posts: 42




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« Reply #4 on: June 13, 2013, 08:17:27 AM »

Worked on my share of HW and Sb transceivers and I can say that the S-meter circuit is problem prone. The meter is connected between a voltage divider in the cathode circuit of the 1st IF and a voltage divider derived from the B+. The cathode is biased slightly positive by the current flow through the 1st IF. The agc voltage biases the grid negative, reducing the current flow through the tube and causing the voltage at the cathode to decrease. This changes the balance between the voltage at the cathode voltage divider and B+ voltage divider causing the meter reading to increase.

The most common problems seem to be:

1. The 22k or 33k ohm resistor associated with the B+ divider leg. Many notes have commented that this should be replaced with a 1-watt resistor.
2. The S-meter zero adjust pot itself often becomes erratic (contact cleaner frequently helps)
3. The 1st IF 6au6 tube develops leakage
4. The resistive divider associated with the 6au6 cathode
5. The AGC that feeds pin 1 of the 6au6

As a starting point. when the RF gain is turned all of the way down (ccw), the S-meter should deflect to full scale as the 1st IF is biased to cutoff at this point. By adjusting the S-meter zero pot you should be able to zero the meter when the RF gain is fully clockwise and get a full scale reading when fully ccw (both in the absence of any signal - disconnect the antenna or use a dummy load).

If you have a VTVM, you can measure the voltage at pin 1 of the 1st and 2nd IF tubes as well the the Receive RF amplifier. I these normally read slightly negative with an increasing negative voltage in the presence of signal. If you see a positive voltage, it probably indicates a leaky tube (try the one that has the most positive voltage on the grid, or simply unplug the tubes one at a time).

If you cannot zero the s-meter with the rf gain fully cw, there is a probably a problem in the divider associated with the B+ (ditto for the case where the s-meter drifts excessively as the rig warms up.

Here is a link to a spreadsheet with resistance and voltage measurements that may help you troubleshoot this area (I think this is for an HW-101, but it is basically the same as the SB-102 in this area.

Good Luck,

Bob

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KD0ILM
Member

Posts: 36




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« Reply #5 on: June 14, 2013, 09:34:02 AM »

I tested the resistors on the board with a good ohmmeter (tested all in place). Surprisingly only a couple were out of spec. I have some extra "known to be good" tubes. I had one 6AU6/A tube and a 6AU6 in the radio. When I put in two 6AU6 tubes I got about 1/3 deflection in the ALC mode using the calibrator signal. Does anyone know if these are direct substitutes for each other?

Without the signal the needle still pegs to the pin on the left. Zero adjust pot has no effect to zero but can get some movement mid scale when the calibrator tone is on. Cannot get it to go all of the way to the right with the RF Gain knob turned fully CCW.

I think I am going to replace the resistors first and see what happens. I am debating whether or not to remove the board to rebuild it or try to do it in place. It looks like if I detach the various wires going to the board it will slip right out. Any advice before I give it a go?

Any other components that I should replace as a prophylactic measure while I am at it?
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KE3WD
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Posts: 5689




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« Reply #6 on: June 14, 2013, 01:24:19 PM »

I'd like to stick my 'scope or DMM probes across the existing meter and read what's happening there. 

And proceed to looking for a cause after that bit of information gathering. 


73
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AB1MN
Member

Posts: 42




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« Reply #7 on: June 15, 2013, 10:10:22 AM »

Oops,

Forgot to post the link to the spreadsheet in my earlier posting:

https://dl.dropboxusercontent.com/u/49531316/Heathkit%20Measurements.xlsx

Bob,  AB1MN
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KD0ILM
Member

Posts: 36




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« Reply #8 on: August 05, 2013, 12:33:08 PM »

I removed and rebuilt the IF board on my Sb-102 (new resistors) and am happy to report that the S-meter is really working great. Now I am having trouble understanding some of the transmitter alignment problems, but that is a another topic. I appreciate the help on this problem.
Kevin
KD0ILM/XV2KM
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