The AM Forum

I'm restoring a HQ-180A.  It's clean and doesn't appear to have suffered any abuse over the years -- but the S-Meter behavior is a puzzle.  If you correctly zero-up the S-meter (with the RF Gain fully counterclockwise) and then turn the AVC "Off", the meter pegs negative.  Also, if you switch between "Fast" and "Medium" AVC, the zero point changes.  If you switch between "Slow" and "Medium" there's no change to the zero of the meter. I've checked all the resistance readings at V-13 (BFO/Meter) and all is in spec.  I've also checked all the resistors associated with the AVC/S-meter and so far haven't found anything out of whack.  Measuring the voltage at the meter, I can definitely see this behavior -- it's zero volts when the bridge is balanced, and goes slightly negative or positive when it shouldn't do so.  Still studying the schematic but maybe someone here has seen this before?

73 - Steve, KW4H

A while ago I had a problem like this in an HQ145X. The manual makes mention of swapping the 6BA6 tubes if there was a meter issue:

4. Erratic or Poor "s" Meter performance is usually due to the two 6BA6 (V4 and V5) vacuum tubes. Merely interchanging these tubes may provide sufficient improvement. Replacing one or both of these tubes may be advisable before suspecting other troubles.

I did this with no improvement but when changing out the tubes with new ones the meter problem was fixed. I left the old 6BA6 tubes in the tester long enough for them be really hot and one of them did show an intermittent problem that did not show up until about 15 minutes.

The HQ 180 has 3 of these tubes so swap them around. No cost thing to try.

Rich

Thanks very much for the reply, Rich.  That was very helpful information -- I do know that some of the tubes in this receiver are a bit "on the edge", so I have a replacement set on the way.  Think I'll take a pause until every tube is validated as in good shape.  I also have a uTracer (tube curve analyzer) and just for giggles will run some of those 6BA6's through the microscope tomorrow.

Another possible sign of some tube problems is another test today.  I injected a 10 microvolt signal on 10 MHz, and it was on the weak side -- too weak for the S-Meter sensitivity adjustment to put it S-9.

Steve, KW4H

Hammarlund's AVC circuitry in the HQ-170 and HQ-180 receivers always seem to be a work in process. .Hammarlund issued at least two service addenda on the HQ-170 concerning the AVC circuitry.

Very helpful to know, thanks!  I'm partial to Hammarlund for nostalgic reasons -- the HQ-180A was the radio that got me into hamming at a very early age, and the one I had was unfortunately destroyed in a move decades ago.  I'm now on a mission to "fix everything that's wrong in the universe" and have a restored HQ-180A at my fingertips.   8)

It sounds like we share the same nostalgia for the 180. For me it was the same story it was the first real receiver that I ever had and it also got me into the hobby. Fortunately I’m a pack rat so I still have the original box the original manual and the original radio. The 7 MHz band recently went dead I suspect due to the silver MIKA disease. I also have a 180 a carcass in the garage which has all of the IDF cans so repairing it shouldn’t be that difficult. It was a great receiver I just wish that I’m a.m. it had a bit more bandwidth

The HQ-170/180's are often maligned for not having enough bandwidth to be great AM radios.  And the reputation may be well-deserved in some respects.  Historically, however, it was an era where SSB was dawning and AM was sunsetting as the dominant mode.  The Hammarlunds did offer peak technology for the day -- but there's no way my HQ-180A will sound as good on AM as my National HRO-60.  Nevertheless -- I'm drawn to the Hammarlund over the National for a variety of technical and logistical reasons aside from nostalgia.  I'll probably end up selling my HRO-60 to make room for the HQ-180A.  I finished restoring the HRO-60 a few months ago.

One thing I'm grateful for is that this HQ-180A actually works.  It's not one of those dead-in-the-water radios that needs major surgery in order to get it to the point where it can be assessed.  That being said, I'm taking slow bites at the apple because there's bound to be underlying issues that will have to be addressed.  Generally speaking, it doesn't seem as sensitive as it should be on some bands.  Yesterday I injected a 50 uV modulated signal on 10 MHz, and it wasn't possible to adjust the meter sensitivity to S-9.  S-6 or S-7 was the best it could do.  It may all be tube-related -- I tested all the tubes and a few were "on the edge".  Replacements are on order.  Once I'm sure all the tubes are within range it will be time to check the voltages at the tube sockets.  In the meantime, resistance tests can be run.  The last thing I want to do is mess with the alignment, unless it can be shown that it's actually out of alignment.  I've heard cringe-worthy stories about alignment difficulties with the Hammarlunds.

73 - Steve, KW4H

The underlying cause of this strange behavior may have been found.  A previous owner removed the couplate Z2 (assume it went bad) and replaced it with some hodgepodge that isn't the discrete version of Z2, as well as altered a few of the parts in and around V15 and V16.  Whatever they did works -- but not very well.  The AVC behavior, as well as the audio, are both somewhat strange.  The donor HQ-180A is filthy, but has the couplate and the factory circuitry is untouched, so I'll do a transplant and arrange it all back to factory using the donor as a visual model.  If Z2 from the donor radio ends up being defective (rare, but it happens), I'll build Z2 using discrete components on a small perfboard and reinstall.

Steve, KW4H

Steve,

   I don't have any direct HQ-180 experience, but I just went through an HQ-110 and learned a tough lesson.

The audio was very distorted below 200hz, and the usual mod's helped a bunch at 1Khz, but at 200hz distortion was nasty, and going down to 100hz progressively worse.

I went down the road of audio fixing, and even replaced the audio output transformer with a nice beefy Edcor SE output XFMR. I added a 6DB NFB loop as well. Also found the 6AQ5 audio tube

being a Beam Power tube, would distort asymmetrically at the clip threshold. I found that a 6AR5 (a true Pentode) was much better. The overload was gradual, and symmetrical.  THD tests at

1 watt gave me < .5% distortion at 1 Khz. After all that, the 200hz and down distortion remained!!  :'(


What I found was that AVC-Audio Couplet had too short of an R-C time constant, and the lower audio frequencies were modulating the two AVC loops. This grid modulated the AVC

controlled stages with an increasing phase shift, and amplitude, as the audio frequency was lowered.

The solution was to lengthen each AVC loops R-C time constant to around 1 second (like 1meg - 1uf) as a test. Boom!! Distortion GONE, and I was clean down to 30hz at 1/2 watt.  ;)

YMMV, but watch out for a modulated AVC buss... My HQ-110 was not broken...it was how Hammerlund designed it.

Jim
Wd5JKO

Thanks, Jim!!  I've printed out your info and saved it with my manual.  If I run into this issue I'll have something to bite into.  What did you install to lengthen the R-C time constant?  I might very well run into this.

Steve, KW4H

Jim, your comments got me digging deeper.  And well, whaddya know -- a previous owner had in fact removed the auto-response circuit.  They then made a connection between the plate of V16 and the cathode of V15 through a .015uf capacitor and 1 megohm resistor in series.  That, in combination with a homebrew experiment to replace the couplate, is what I'm seeing.

The .015 uF capacitor and 1 megohm resistor are puzzling.  The intent of those would be to bring down distortion after disabling the auto-response circuit.  However, the writeup I've read on this modification (over on the AB2RA website) indicates that the connection is plate-to-plate, not plate-to-cathode.

That being said -- even the author of the auto-response audio mod (Larry, W9MDX) states in his article that he doesn't recommend attempting the modification on a HQ-170 or HQ-180.  As Larry points out, the additional selectivity provided by these two receivers has a trade-off, and that is audio quality.  Some things are best left alone. 

No other areas of this HQ-180A appear to be modified.  The audio/AVC needs to revert to factory.  As it stands now, the audio is unpleasant and the AVC is misbehaving.  

Steve, KW4H

Today I re-established the auto-response circuit, stripped out the problematic modifications around V15/16 and installed the couplate from the donor HQ-180A.  When turned on, the audio is terribly distorted and, as the set warms, becomes a nightmare of oscillations.  After re-checking all of the wiring, the problem has to be the donor couplate (totally UN-surprising).  A sign of the couplate failing is that things get worse as the radio warms.  The couplate will have to be rebuilt using discrete components, which isn't hard to do, but I don't have them all in stock.  Time for an evening adult refreshment.

Steve, KW4H

Aaaahhh...the familiar Hammarlund audio is finally back!  The problem ended up not being the couplate from the donor radio, but I had accidentally switched R77 and R80.  These two resistors attach to the AF gain control and are mounted just below the L-bracket to the front panel and connect to a terminal strip.  If you ever want to reverse the auto-response mod, you'll have to reinstall these two resistors.  Be very careful which one goes where.

Unfortunately, however, the S-meter behavior is not resolved.  The "Off" position doesn't zero the needle -- it pins it to the left.  Of course, you can re-zero the meter at that point; however, that means when you turn the AVC back on and rotate the RF control to minimum, the needle is above zero fairly significantly.

A replacement set of tubes is coming in today's mail.  I know some of the tubes are on the weak side, so I'm going to give this issue a temporary rest until I know that all the tubes are adequate.  Something is unbalancing the S-meter circuit in this HQ-180A, which is basically similar in design to a VTVM.  

73, Steve.

A correctly zeroed S-meter continues to pin backwards when you put the AVC switch in the "Off" position.  Since the last posting, I've replaced all of the resistors in the AVC circuit, including all the resistors at the AVC switch.  The circuit is pretty much entirely rebuilt.  Also swapped out V13 (meter amp), and I've confirmed that when you put the AVC switch in the "Off" position, pin 2 of V13 (the grid) is grounded.  Yet -- I can measure a slight negative voltage across the meter, which is pulling the needle to the left.  I've also swapped out the meter by jumpering over to the meter in a second HQ-180A here, and it does exactly the same thing -- so it isn't the meter.

I need some ideas.  This has become quite the head-scratcher.

73 - Steve, KW4H

I've been thinking this evening about another angle to this.  The correct procedure is to zero the meter after turning the RF gain all the way down.  This means that there should be no difference than the RF gain at zero or the AVC turned to the "Off" position.  What if the problem isn't the meter circuit, but something related to the RF gain?  In other words, what if "zeroing" the meter isn't actually a correct zero?  That could create this effect.  While there were some benefits to rebuilding the AVC circuitry (some parts were out of spec) it may not have been the actual source of the problem.  I may need to focus on the RF gain control and what happens when it's turned all the way down.  And the RF gain control on the HQ-180A is a gang of two potentiometers (and a switch).  :o

Steve, KW4H

Before proceeding further, I could really use some feedback.

Some new information, but the puzzle isn't solved yet.  With the AVC "Off" the voltage on the grid of V-13B (Meter Amp) is zero.  This happens because the grid is shunted to ground at the AVC switch.  I would expect that.  However, with the AVC switch in any other position, and the RF gain control set to minimum, the readings aren't zero, but a negative voltage that can vary slightly from Slow-to-Med-to-Fast.  This explains the behavior of the S-meter and why it's impossible to get a consistent zero point.  

Backing up to V-8B (Detector for AVC), this tube should present a reading of -4 VDC with the AVC "Off".  It doesn't.  With AVC "Off" and the grid of the meter amp shunted to ground, pin 9 is only showing a voltage of -.5 VDC.  That's way, way off.  I checked R96 and it's a carbon comp but within tolerance.  C-41 is a .01uf ceramic disc and those almost never go bad -- otherwise my inclination would be to think that C-41 might be leaking to ground and should be replaced.  

The only other voltage that's off on V-8 is pin 6 (should b e -.24 VDC, but measures -.6 VDC).  However, it doesn't appear that section of the tube is related to the AVC voltage for the S-meter.  

I've attached a schematic of the AVC and S-Meter circuits.  I'm pondering what's actually going on here, and would greatly appreciate any thoughts and suggestions.  I did swap out V-8 and it made no difference.

73, Steve - KW4H

Just joining in here, but a question…Why should there be any DC voltage on the meter amp grid in any position? There’s some resistance to ground all the time, it’s a high impedance point so that resistance will pull it to zero, UNLESS there is AC coming into the diode to become varying DC, and drive the meter…that’s how it meters the level it measures. DC going into the diode would pass through, the different settings would make different voltages and the zero would move around.

With RF gain down, what is on the IF output? Have you looked at the diodes inputs and outputs on a scope?

I’d be throwing a C39 in there, and looking at the places off your drawing that the AVC goes to to make sure one of them isn’t back-feeding DC into the meter circuit.

Ed

Ed, thanks very much for taking the time to reply.  It's great to have the help of others when trying to work a puzzle like this.

Here's what I know about how the meter circuit operates.  The circuit around it is a bridge (similar to a VTVM), with the tube and R88 on one side, and R21, R20 and R22 with the switched resistors on the other.  The meter itself is in the center of the bridge and it's set to zero in the absence of a signal (no AVC is being developed).  When a signal is received, what happens is that AVC is developed and then the tube current changes and the bridge becomes unbalanced, which pulls the needle up the scale.  The greater the signal, the greater the imbalance.  If you place a meter across the meter you can see the imbalance in the form of a voltage reading that's other than zero.  When the bridge is properly balanced and the meter is zero, there's no difference of potential across the meter connections. 

When you turn the AVC switch "Off" and ground the grid of V13B that SHOULD be identical to no signal being received (AVC zero).  There should be no difference between reducing the RF gain to zero and turning off AVC.  But that's not what's happening.  With the RF Gain all the way down, the AVC circuit somehow thinks a signal is being detected and is unbalancing the bridge.  This condition can be verified by zeroing the meter (balancing the bridge) with the AVC switch in the "Off" position and then flipping the AVC back on.  Boing!  The needle advances up above zero to close to S-2. 

You've made two great points -- what is the IF doing at C39 when the RF Gain is reduced to zero -- and is anything backfeeding DC into the circuit?  I'll be all over that today.

73 - Steve, KW4H

The entire AVC wiring has now been traced out.  Everything is going where it should.  At this point I'm thinking that the troubles with the AVC are buried in the components surrounding the tubes in the IF section that's positioned -- unfortunately -- below the wafer switch modules for the Selectivity and Sideband controls.  That's the one design feature of the Hammarlund that I do NOT appreciate, and back in the day it must have driven repair techs insane.  Anyway, I can see at least one area where someone made a repair and it looks really bad because the wafer switches were in the way.  That, at the very least, has to be fixed.

To remove this switch array you have to take lots of photos and make careful notations where certain wires go.  It's not impossible and not really all that difficult, but it's like slow-motion gymnastics.  

So that's where things stand.

And all THAT being said -- I'm going to avoid pulling that wafer switch module until I'm even more certain the problem is under there.  It's not the sort of effort you want to undertake only to find out that it didn't do any good.  

Any and all suggestions are welcome.  In the meantime I'll keep poking at the bear.

Steve, KW4H

OK, some actual progress, although it's not exactly elegant troubleshooting.  Totally fed up with this -- I circled back around again and this time, rather than just fish around and take measurements, I went hard core:  snipping things.  Looking at the schematic, here's what is now known for certain:

1.  If you totally remove the connection from the AVC circuit to the AVC line in the radio, the issue remains unchanged.  This eliminates the possibility of feedback from anything down the line.

2.  If you totally remove the connections to pin 7 of the AVC detector, the issue remains unchanged.  This eliminates anything back-stream in the IF section from creating this problem.

3.  If you cut the connection to pin 2 (grid) of V-13B (Meter Amp) -- the problem changes.  In the AVC "Off" position, the meter is still zero.  But in the Slow, Med, or Fast positions the meter circuit unbalances and swings negative.  However, we must bear in mind that with the connection to pin 2 cut, the companion switched resistor network (part of the AVC switch) is removed.  As you switch between Off-Slow-Med-Fast and have no signal applied, the various resistances should maintain balance in the meter circuit.  That isn't happening (or maybe my understanding of the circuit operation is off-base).

The AVC switch assembly has been completely cleaned, reconstructed with new resistors, and all connections tested and checked, including ground.  In addition, I swapped out the meter by jumpering over to a spare HQ-180A and the second meter did exactly the same thing -- so it's not the meter.  The zero adjust and meter sensitivity pots seem to work fine, so i don't think the problem is either of those (or is it?).  

This may bring the focus onto V-13B, but I'm not sure what for.  I've swapped the tube out twice, replaced most every component related to this circuit at the tube.  I'll plan on re-doing the voltage and resistance checks after reconnecting my snips.

I might have to re-visit the switch for good measure -- that's another focal point.  Could also pull a switch out of the donor HQ-180A and swap it out.

This is going to make me stark raving mad.   :P   ;D

Steve, KW4H

Just a thought:
I don't recall, but if the switch wafers associated with this circuit are phenolic, the ones back then that were used were paper based. Deoxit or too much moisture, can be absorbed by the switch wafer (paper absorbs moisture) and can add almost invisible resistance to a circuit. Deoxit is especially notorious for this problem; that's why it better to add it sparingly only to the contacts and not spray the entire wafer.

You might want to replicate the entire S-meter circuit in mid-air without going through any of the switch contacts and using clip leads to mimic different switch positions.

A very helpful observation on your part.  The switch wafer is, in fact, phenolic.  I think tomorrow what I'll do is swap out the switch wholesale with the one from the spare HQ-180 and see if there are any notable differences.  At this point, it wouldn't surprise me in the least if this problem is being caused by a bizarre, esoteric issue involving something like a phenolic-based wafer switch.  Eventually we start running out of the usual suspicions and conclusions and head into the ozone. 

73 - Steve, KW4H

Been doing some remedial reading on the phenolic wafer switches.  It's really quite interesting.  There are some tried-and-true methods to cleaning and restoring them.  One of the most thorough I've found uses this process:
1.  Clean the contacts with a metal cleaner (e.g. TarnX) using a toothbrush, then give the switch a water rinse.
2.  Pat dry with a towel.
3.  Scrub the switch with 99% isopropyl alcohol using a toothbrush, then scrub with dish soap and water.
4.  Give it a hot water rinse.
5.  Put it in the oven at around 120 F for a few hours to dry everything out.

Before doing any of that, however, I'm going to transplant the AVC switch from the donor HQ-180A.  If there's any significant difference in the behavior of the S-Meter, I'll know the switch itself is the culprit and the phenolic is compromised, probably because someone sprayed it with tuner cleaner or something.  Fortunately, it's fixable -- the trick is that you can't clean a phenolic switch with anything that leaves a residue behind, and the switch needs to be completely dried after cleaning.

Steve, KW4H

AVC Switch swapped out -- the circuit is all back together again -- and NO DIFFERENCE.  Blargh!  So it's not the switch.  Dangit!  That eliminates a lot of the circuitry, though.  We know the problem still happens if you disconnect everything to pin 7 on V8B (Detector for AVC) and the entire connection to the AVC line.  Therefore this problem HAS to be contained around V-13B and the surrounding components.  R21, R88, and R22 have all been replaced.  V-13 has been swapped out with no effect.  The S-Meter itself has been eliminated.  I haven't considered R-20 (zero adj.) and R-19 (meter sens.) because those seem to be working fine.  Nevertheless, I'll check these two pots against the identical ones in the donor HQ-180A.  I'll also go back and re-measure the resistances and voltages at V-13.  The other thing I haven't tried is replacing C-41 (on Pin 9 of V-8B).  It's a bit tricky to get to and is a .01uf ceramic.  However, if it's leaking to ground I believe it could absolutely alter the operation of the circuit.   At this point I should probably replace it, even though the odds of ceramic caps going bad is pretty slim.

Steve, KW4H

Update:  replaced C-41 (also C-39 since I was working in that area), R-96, and just for the heck of it, the meter sensitivity adjustment.  No impact on the issue.  I need to focus on V-13 -- but I should probably not just look at the "B" side, but also the "A" side (BFO) -- anything that could pull that tube off track.  FYI -- the meter zero and sensitivity adjustments are panel-mount with small twist tabs that are not reversible -- you will break the tabs removing the pot.  They're nothing more than 1.5k pots -- most any panel mount will do, but you might have to just slightly widen the holes to 3/8" (a step drill bit will do nicely).  Surplus Sales has the pots.  I'd recommend replacing those two pots anyway as part of a general restoration, because the ones Hammarlund used are the cheapies where you have to insert a screwdriver and fish around for the slot.  A panel-mount with a screw adjustment visible from the outside of the chassis is an improvement.

73 - Steve, KW4H

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

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

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

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

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

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

Back at the bench today and digging deeper -- but I could really use some help understanding the functionality of V-8 as it relates to the derivation of AVC voltage.  As I understand it, pin 9 of V-8 delivers a negative voltage that varies with signal level to the grid of V-13B (the meter amp).  With the RF gain at minimum (or the AVC turned off), the voltage is considered "AVC Zero", at which point the needle on the meter is set to zero.

With no antenna connected and no signal being received, and the AVC set to "Off", the voltage on pin 9 is -.25 (negative point-two-five).  However, Hammarlund's tube socket voltage chart calls for -4 (negative four) volts on this pin under these conditions, so assuming Hammarlund's voltage chart is correct, we have a problem.  A big one.  This low voltage could very well explain the whack-a-doodle behavior of the AVC controls.  

V-8 is a 6BV8 -- a duplex-diode triode.  Pins 9 and 7 are simply the two elements of a diode, with pin 9 being the plate.  I need to see something around -4 VDC on pin 9 with the AVC in the "Off" position, but I'm only measuring a fragment (I'm using a VTVM).  Note:  when the AVC is in the "Off" position, the other side of R96 (which goes to the grid of the meter amplifier) is grounded.  The readings on pin 9 with the various AVC settings are as follows:

Off:  -.25
Slow:  -.82
Med:  -.62
Fast:  -.45

WHERE is that -4V supposed to be coming from?  

Steve, KW4H

Back at the bench today and digging deeper -- but I could really use some help understanding the functionality of V-8 as it relates to the derivation of AVC voltage.  As I understand it, pin 9 of V-8 delivers a negative voltage that varies with signal level to the grid of V-13B (the meter amp).  With the RF gain at minimum (or the AVC turned off), the voltage is considered "AVC Zero", at which point the needle on the meter is set to zero.

With no antenna connected and no signal being received, and the AVC set to "Off", the voltage on pin 9 is -.25 (negative point-two-five).  However, Hammarlund's tube socket voltage chart calls for -4 (negative four) volts on this pin under these conditions, so assuming Hammarlund's voltage chart is correct, we have a problem.  A big one.  This low voltage could very well explain the whack-a-doodle behavior of the AVC controls.  

V-8 is a 6BV8 -- a duplex-diode triode.  Pins 9 and 7 are simply the two elements of a diode, with pin 9 being the plate.  I need to see something around -4 VDC on pin 9 with the AVC in the "Off" position, but I'm only measuring a fragment (I'm using a VTVM).  Note:  when the AVC is in the "Off" position, the other side of R96 (which goes to the grid of the meter amplifier) is grounded.  The readings on pin 9 with the various AVC settings are as follows:

Off:  -.25
Slow:  -.82
Med:  -.62
Fast:  -.45

WHERE is that negative 4V supposed to be coming from?  

Steve, KW4H