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Author Topic: HQ-180A - Strange S-Meter Behavior  (Read 1525 times)
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KW4H
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« Reply #25 on: January 12, 2022, 05:39:51 PM »

I'm in urgent need of more ways to isolate this problem.  The good news is that I'm learning more about the HQ-180A's AVC design than I ever thought necessary.  The bad news is that I'm continuing to get nowhere.

Extensive troubleshooting today, but no fix:  
- Finished the resistance and voltage checks on V-13, A and B sections.  All good.  There were somewhat elevated voltages on pins 6, 7, and 8; however there are no voltages on those pins when AM is selected, therefore the higher voltages there are not really significant as the S-meter issue occurs in all modes.
- Bypassed the harness wiring to the AVC switch in the event of a degraded wire somewhere.
- Cleaned the tube socket again, and checked the tube socket ground, and clipped in additional grounding as a test.
- Replaced C-41 for good measure.
- Checked & bypassed the ground to the AVC switch to ensure good grounding there.
- Swapped out the AVC switch from a donor HQ-180A.
- Swapped out the meter sensitivity pot.
- Looked for wiring & component replacement errors by comparing against the donor HQ-180A.

There is a reason the S-Meter is misbehaving like this.  Once the meter is zeroed, the needle should stay at zero when you turn the AVC off.  It doesn't -- it pins negative.  And if you zero the meter in the AVC "Off" position and then flip over to Slow, Med, or Fast the needle swings positive with the RF gain at zero.  

You can see it happening in the numbers.  The meter amplifier tube is half of a 12AU7.  Here's the data:
Plate (Pin 1)- Off: 81v; Slow: 84v; Med: 83v; Fast: 83v.
Grid (Pin 2)- Off: 0v; Slow: -1v; Med: -.8v; Fast: -.6v
Cathode (Pin 3)- Off: 3.3v; Slow: 2.9v; Med: 3.0v; Fast: 3.1v

As a further test, I pulled V16 -- that eliminated any signal coming to pin 7 of the AVC detector.  In fact, I pulled all the wiring off of pin 7 of V-8 and it had no effect on this problem. 

Steve, KW4H


* S Meter.png (346.53 KB, 1942x1440 - viewed 35 times.)
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KW4H
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« Reply #26 on: January 12, 2022, 10:32:44 PM »

A new development.  I was looking for a change in the symptoms and finally found it -- I swapped out V-8 a couple of times and the S-Meter readings varied and dropped a little.  Despite my efforts to isolate V-8 away from this, it might not have been enough.  Tomorrow I'm going to clean the V-8 socket and make sure that any relevant components around it have been fully evaluated. 

Steve, KW4H
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K8DI
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« Reply #27 on: January 13, 2022, 08:32:48 AM »

There is a reason the S-Meter is misbehaving like this.  Once the meter is zeroed, the needle should stay at zero when you turn the AVC off.  It doesn't -- it pins negative.  And if you zero the meter in the AVC "Off" position and then flip over to Slow, Med, or Fast the needle swings positive with the RF gain at zero. 

You can see it happening in the numbers.  The meter amplifier tube is half of a 12AU7.  Here's the data:
Plate (Pin 1)- Off: 81v; Slow: 84v; Med: 83v; Fast: 83v.
Grid (Pin 2)- Off: 0v; Slow: -1v; Med: -.8v; Fast: -.6v
Cathode (Pin 3)- Off: 3.3v; Slow: 2.9v; Med: 3.0v; Fast: 3.1v

This is exactly the behavior I would expect. Look at the simplified sketch attached. The circuit is a pair of voltage dividers. The variable resistor is the tube. The sensitivity is shown as fixed. The zeroing pot is shown as two resistors for clarity.

The meter reads zero when the two voltage divider center points (red dots) are the same. The zeroing pot trims the one to match the other.

The lower part of the fixed/right hand divider has four settings by the switch. This is to compensate for the four different values that occur on the upper part of the left hand divider as the tube's DC operating point changes due to the bias changing with the switch on the grid circuit. The two sets of changes are supposed to track, so that the potential between the cathode and the wiper of the zeroing pot stays zero. What's happening is that the two changes are not tracking. Something is off with the bias part, the divider switching part, or the element the bias part controls (tube). 

HOWEVER:
it may never have been perfect. And, if in your troubleshooting you've changed the meter sensitivity to a significantly higher than original point, the problem may be magnified.

I would also note that old resistors with 5% tolerances were generally much closer than 5% when new. When I poke around in 50's to 60's low power circuits (i.e. no heating from current flow) I am usually astounded by how right-on the resistors are. If yours are only "within spec" it may (or may not) be good enough...

Ed


* IMG_6243.JPG (385.42 KB, 2016x1512 - viewed 29 times.)
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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
KW4H
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« Reply #28 on: January 13, 2022, 09:02:37 AM »

What's happening is that the two changes are not tracking. Something is off with the bias part, the divider switching part, or the element the bias part controls (tube). 

HOWEVER:
it may never have been perfect. And, if in your troubleshooting you've changed the meter sensitivity to a significantly higher than original point, the problem may be magnified.

I would also note that old resistors with 5% tolerances were generally much closer than 5% when new. When I poke around in 50's to 60's low power circuits (i.e. no heating from current flow) I am usually astounded by how right-on the resistors are. If yours are only "within spec" it may (or may not) be good enough...

Ed

Ed, thanks VERY much for your response.  I replaced all the resistors with 2% -- one of my initial assumptions was that the carbon comp resistors in the AVC switching and meter circuits had drifted over the years.  Happens all the time.  I found one or two that were a little drifty, but most were OK.  I replaced them anyway.  Interestingly, when I transplanted the AVC switch from the donor HQ-180A yesterday (and that one still has all the old carbon comp resistors) there was no change in the behavior.  Also, the meter sensitivity pot has minimal effect on this.

Last night a light went off -- I realized I needed to look for more conditions in the radio where a change could be induced that might tip the hand of the culprit.  So I pulled V-8, said "take that, sucker!" (not really) and powered on the radio.  And well, well, well -- the meter remained zeroed when turning the AVC switch.  Also, I swapped out V-8 multiple times and discovered variations in the severity of the S-meter misbehavior from one tube to another.  At this point, I'm going to re-focus on V-8 -- and not just V-8B, but the whole tube this time.  Basically, it appears that the AVC detector diode in V-8 is whack-a-doodle, but for reasons that might not be depicted on the simplified diagram.

Steve, KW4H
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KW4H
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« Reply #29 on: January 13, 2022, 05:32:54 PM »

Today I found some unusual resistance readings on V-8.  At least a couple are far enough of out spec that I'm chasing them down.  For example, pin 3 reads about 20k, but it should be 30k.  Pin 6 reads 22k, but it should be about 47k.  Pin 7 should read about 70 ohms, but is actually more around 55.  Interestingly, pin 6 jumps to the correct reading (47k) with the mode switch in anything but AM.

In tracking some of this down, I've found at least two or three carbon comp resistors that have drifted rather substantially DOWN in value from original.  That's a surprise -- carbon comps usually drift UP over the years.  And a resistor that drifts down is much more difficult to identify in-circuit than one that drifts up.  But, so be it.

Anyway, it seems clear that the S-meter issue, and possibly some other problems I haven't recognized yet, are buried in the set.  A number of resistors may have to be replaced in order to bring this HQ-180A back up to factory.  Hopefully I won't have to dig into the IF sections buried below the wafer switch module for the sensitivity and bandwidth settings.  Pulling that contraption out looks like a serious PITA.

As always, I appreciate any feedback and thoughts on this project.

73 - Steve, KW4H
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KW4H
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« Reply #30 on: January 14, 2022, 05:13:50 PM »

Today I spent several hours chasing down the resistance reading on pin 6 of V-8.  Tracing that out will run you from the back of the radio, all the way to the front and up the inside of the panel to the mode switch.  Then from the mode switch to the noise limiter control.  The end result is that the resistance at pin 6 is correct and Hammarlund's resistance chart is wrong.  However, in the process of studying this, I did find some modifications in the noise limiter circuit (a couple of capacitance values were changed -- and you could tell by the soldering that it wasn't factory).  Restored that back to what it should be; however, the S-meter problem remains.  

Since pulling V-8 stabilizes the meter, that circuit is going to remain in the crosshairs for now.  Pin 6 is of particular interest -- the voltage chart indicates it should be -.24v with the RF gain at maximum.  I was getting -.6 when I took the reading.  Wonder how that voltage actually varies with the RF gain control.  With the RF gain control at minimum the S-meter circuit should be at or very close to AVC zero.

My workbench is in the back of the garage and I don't have a lot of workspace, so I have to plan my work and then pull out just the tools I need, which is why my progress on this is start-and-stop and a bit sluggish. 

Steve, KW4H
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