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Author Topic: Can't set S-meter: HW-100 first step of alignment  (Read 3201 times)
AB5CE
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Posts: 11




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« on: June 18, 2016, 12:38:40 PM »

I am trying to get back on the air after after about 30 yrs. with two old boat anchors, an Hw-100 and an HW-101. Both have problems.
THe HW-100 at least functions well for RX, but can't get low SWR even with a 50 ohm dummy load.  So I am aligning it to see if I can then get a low SWR. The first thing I did was to replace the air variable drive o-rings. They were either broken or about to break. I started going through the alignment procedure from square 1 in the assembly manual, zeroing the S-meter.  But I could not get the S meter to zero with any setting of the zero adjust pot. It remains pegged negative. Turned off power and checked the mechanical adjustment on face of meter and the zero is correct. The directions call for setting the RF gain to max when making the adjustment. I found that by turning it back from max to the 2 o'clock position, I could then set the zero adjust pot for a zero S-meter indication....anything higher than 2 0'clock and it goes beyond the range of the zero adj pot, it stays pegged negative.   I think the S-meter has to be zeroed correctly before proceeding, it looks like some of the other steps are adjusted by reading the s-meter.  What could be knocking the signal strength circuit so far out of adjustment?
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AD4U
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Posts: 2537




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« Reply #1 on: June 18, 2016, 01:12:23 PM »

There are 4 or 5 100K 1/2 watt resistors in the S meter balance (zero) circuit. One or more almost always goes way high in value with age and heat.

Dick AD4U
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W5RKL
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Posts: 1109




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« Reply #2 on: June 18, 2016, 07:53:55 PM »

while ti is true the screen resistor has an affect on the S meter, based on the comments how the S meter reacts to warm up and rests below zero, I suspect first IF amp V3 (6AU6) is very gassy. V4 Second IF amp which is also a 6AU6 can also be gassy but the S meter is controlled by V3. I had the exact same problem with my SB-100, S meter would rise above zero when first powered on but as the tubes warmed up, the meter would drop back toward zero, slowly then eventually drop to zero and below, resting against the meter's left peg. Measuring V3's grid voltage is how I found the problem. The HW-100 and HW-101 both have the same IF amp circuits, the same S meter circuit, and the same IF tubes, both 6AU6s so this applies to both transceivers. V3's positive grid bias will affect the grid voltage of V4 second IF amp, V10 receiver RF amp, and V10 first receiver mixer. You can see this by studying the HW-100 and HW-101 main schematic.

Set the RF gain control fully CW.

If you have a DC volt meter, preferably a VTVM such as a Heathkit V7 or IM18 or similar VTVM, then connect the DC volt meter set to measure "negative" DC voltage, to pin "1" of V3 (grid) and ground (chassis). You can move V3 slightly in its socket to allow for the DC volt meter probe (RED) to be placed into the slot at pin 1.

Power up the radio and monitor the DC volt meter as well as the S meter. The S meter should rise well above zero when power is first applied. As V3 warms up and begins to conduct, the S meter will start to drop back towards zero and the DC volt meter should display a "negative" DC voltage of approximately minus 2VDC or so. As V3 continues to warm up, V3's grid voltage will drop towards zero as will the S meter. The S meter will drop to zero and continue below zero until the S meter rests at the meter's left peg. At the same time the DC volt meter's negative DC voltage will continue to drop towards zero and eventually become "positive" at an approximate +1.7VDC.

The above is caused by excessive gas in V3 (6AU6). Replace V3 with a known good 6AU6 tube that's not gassy. A simple tube tester cannot and will not tell you if a tube is gassy or not. Monitoring the grid voltage is one way to tell. If a tube has a grid resistor, measure the DC voltage on both sides of the grid resistor. If the DC voltage is more positive on the grid side of the resistor than the other side of the resistor then the tube is gassy.

73
Mike W5RKL
www.w5rkl.com
« Last Edit: June 18, 2016, 08:01:59 PM by W5RKL » Logged
W5RKL
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Posts: 1109




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« Reply #3 on: June 24, 2016, 09:40:22 AM »

I have a video on my website (see link below) that shows how my SB-100 V3 first IF amp AND V4 second IF amp grid voltage  are affected when only 1 tube, V3, is gassy.

The video is dark and grainy because I made the video using my old digital camera before I bought a much better video camera. If you look closely at the VTVM you will see the VTVM meter pointer begin to drop back towards zero and eventually drop below zero. At this point, not shown on the video, the S meter also dropped to zero and eventually rests below zero on the meter peg. Adjusting the meter zero pot had no affect on bringing the S meter back up to zero. The fix was to replace V3 with a known good non-gassy 6AU6 then readjust the meter zero pot.

http://w5rkl.com/wp-content/uploads/2015/02/V3-grid-voltage-changes-due-to-excessive-gas.avi

Again I apologize for the grainy dark video.

73
Mike W5RKL
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