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Author Topic: SB-102 meter damping  (Read 914 times)
KC1BMD
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« on: December 05, 2018, 08:17:57 AM »

SB-102 meter issue (same meter as SB-101): The issue is noticed particularly when reading a signal on the upper part of the scale (e.g. S-meter, HV). When the signal is removed such that the needle travels to zero, it has momentum which is not slowed down. The needle travels past zero and hits the mechanical stop. How is the damping performed in this meter? My assumption is the meter damping is internal to the meter, so maybe it's defective?
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KD0REQ
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« Reply #1 on: December 05, 2018, 12:18:37 PM »

unless the pointer swings back and forth like a pendulum, it's probably not a broken hairspring. sounds like it's a circuit-derived fault. multiple issues in the service notes, including creepy tubes in the ALC area (including brands Heathkit and El-Menco,) and resistors way out of spec.  there is a B+ dropper of 22K 1 watt that is a common issue, replace that with a 2-watt and not have to replace it again. in some modes I understand the carrier balance screwdriver pot can affect meter actions, and they get crusty a lot. still working on a HW100 that might end up in this corner of the chassis. but little time to tickle it.
« Last Edit: December 05, 2018, 12:30:30 PM by KD0REQ » Logged
K7MEM
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« Reply #2 on: December 05, 2018, 12:30:19 PM »

You might want to try checking, changing, and/or increasing, the capacitor across the meter leads. There should be a 0.01 uF capacitor across the meter leads. This capacitor acts like a crude memory element. It charges up slowly, when there is a lot of signal, to dampen the forward movement of the meter. When the signal goes away, it should discharge slowly. Again, dampening the movement of the meter, but in the other direction.

I would get out the clip leads and try a few different values. If it bothers the other switch settings, you can try clipping the capacitor on to the meter switch itself. That way, only that setting gets the extra capacitance.
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Martin - K7MEM
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KD0REQ
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« Reply #3 on: December 05, 2018, 12:33:36 PM »

that would be an integrating capacitor, averaging out the readings. smoothing the meter.  sounds like his issue is more like a bias-off one since the reading is going negative. unless you're really monkeying around a Heathkit with no rhyme or reason to it, the meter should not hit either peg.
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W9IQ
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« Reply #4 on: December 05, 2018, 01:35:23 PM »

You might want to try checking, changing, and/or increasing, the capacitor across the meter leads. There should be a 0.01 uF capacitor across the meter leads. This capacitor acts like a crude memory element. It charges up slowly, when there is a lot of signal, to dampen the forward movement of the meter. When the signal goes away, it should discharge slowly. Again, dampening the movement of the meter, but in the other direction.

I would get out the clip leads and try a few different values. If it bothers the other switch settings, you can try clipping the capacitor on to the meter switch itself. That way, only that setting gets the extra capacitance.


A 0.01 uF capacitor has very limited integration capabilities. Even with a 100k ohm resistor in series with it (ignoring the shunting meter), tau is only 1 millisecond. This is more than likely a simple RF bypass capacitor.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
KC1BMD
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« Reply #5 on: December 05, 2018, 03:53:00 PM »

Several SB-102's (and SB-101's) I have seen do not have such a capacitor across the leads and neither does this one. I think I saw one version of a schematic that showed a capacitor but other versions did not. My sense is that the meter damping is not working and I would think any D'arsenval movement would have that. I might try another meter, before I start modifying the circuit. After that, I'll check for any out of spec components. I was just hoping someone had run into this symptom before.

KD0REQ: You commented: "since the reading is going negative". It doesn't look like the "reading" is going negative but that the needle momentum returning to zero is making it go past zero. After it hits the stop it comes right back to zero.
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K7MEM
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« Reply #6 on: December 05, 2018, 04:20:16 PM »

A 0.01 uF capacitor has very limited integration capabilities. Even with a 100k ohm resistor in series with it (ignoring the shunting meter), tau is only 1 millisecond. This is more than likely a simple RF bypass capacitor.

I'm sure it works as a RF Bypass as well. A couple of clip leads and a capacitor is all you need to find out.
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Martin - K7MEM
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KC1BMD
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« Reply #7 on: December 05, 2018, 04:25:22 PM »

Thanks. At this point, I'm thinking I don't need the bypass capacitor and that the problem is elsewhere.
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KC1BMD
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« Reply #8 on: December 07, 2018, 05:54:31 AM »

Until I'm able to substitute another meter, maybe this video will capture the issue well enough for someone to offer comments on a possible cause. The lack of damping also appears to occur on the upswing too.

SB-102 Meter Damping
« Last Edit: December 07, 2018, 05:56:47 AM by KC1BMD » Logged
KD0REQ
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« Reply #9 on: December 07, 2018, 07:24:32 AM »

actually, that looks OK. a shot hairspring will swing in a wide arc with no damping except the friction of the movement. this is a quick overshoot. meter is close enough to OK for ham radio. Heathkit did not buy $40 (at the time) meters, and that characteristic fits a lot of Asian meters I've had on radios, tape decks, broadcast equipment of the barely-pro variety, etc.
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KC1BMD
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« Reply #10 on: December 07, 2018, 07:31:01 AM »

I don't think it's normal because you can hear it when it slams into the stop (but cannot hear on the video), which I have never heard on such a rig before.
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KD0REQ
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« Reply #11 on: December 07, 2018, 07:37:47 AM »

if you were to look with a magnifier very carefully at the meter, you will almost certainly see at the sides of the pointer's bottom a couple little wires with thingies on them. those are springy wires with elastomeric (rubber) bumpers on them that the pointer bounces off of at end of travel. that's a design feature of most d'Arsonoval movements. that's what I'm seeing in the clip, that same bounce. should not need to disassemble the meter to see it, which in the case of a plastic case import, is probably destructive. a meter is a series of tradeoffs to a price point. if the meter is fast-acting, there will be enough energy as the hairspring unwinds to need bumpers with some resiliance. there is a whole 'nother set of tradeoffs in cheapo Emico iron-vane meters like on the AT-1, which ends up in wandering pointers, and a much louder click going to rest.
« Last Edit: December 07, 2018, 07:42:51 AM by KD0REQ » Logged
KD0REQ
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« Reply #12 on: December 07, 2018, 07:46:15 AM »

there might be an amplifying effect in the metering circuit that causes a brief shot of negative voltage as the voltage decays. not enough to cause damage. not enough to warrant a lifetime chase through the rig to identify. work stations, be happy.
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VE3WGO
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« Reply #13 on: December 07, 2018, 05:06:38 PM »

I have schematics of an SB-101 and 102.  The SB-101 shows a 0.01 uF cap across the meter.  That cap is missing from the SB-102 schematic.

Although is was said earlier that it might not add integration (smoothing), recall that a meter coil is an inductor. The net result is a parallel LC network that might impart some electromechanical characteristics that are not easily analyzed or understood.

Try it out anyway.

73, Ed
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KC1BMD
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« Reply #14 on: December 07, 2018, 06:00:52 PM »

Thanks for all the replies. I'll check it out.
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