Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
HQ-180A - Strange S-Meter Behavior




 
The AM Forum
November 27, 2022, 03:05:34 PM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1] 2   Go Down
  Print  
Author Topic: HQ-180A - Strange S-Meter Behavior  (Read 4794 times)
0 Members and 1 Guest are viewing this topic.
KW4H
Member

Offline Offline

Posts: 76


« on: January 02, 2022, 06:13:57 PM »

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
Logged
WD4DMZ
Member

Offline Offline

Posts: 142


« Reply #1 on: January 02, 2022, 07:30:02 PM »

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
Logged
KW4H
Member

Offline Offline

Posts: 76


« Reply #2 on: January 02, 2022, 08:39:44 PM »

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
Logged
Pete, WA2CWA
Moderator
Member

Offline Offline

Posts: 7988


CQ CQ CONTEST


WWW
« Reply #3 on: January 02, 2022, 09:02:24 PM »

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.
Logged

Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
KW4H
Member

Offline Offline

Posts: 76


« Reply #4 on: January 02, 2022, 09:15:20 PM »

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.   Cool
Logged
WA2SQQ
Contributing
Member
*
Offline Offline

Posts: 1011


« Reply #5 on: January 03, 2022, 10:01:35 AM »

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
Logged
KW4H
Member

Offline Offline

Posts: 76


« Reply #6 on: January 03, 2022, 11:12:51 AM »

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
Logged
KW4H
Member

Offline Offline

Posts: 76


« Reply #7 on: January 03, 2022, 12:48:24 PM »

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
Logged
WD5JKO
Member

Offline Offline

Posts: 1965


WD5JKO


« Reply #8 on: January 03, 2022, 01:33:48 PM »


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!!  Cry


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.  Wink

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

Jim
Wd5JKO
Logged
KW4H
Member

Offline Offline

Posts: 76


« Reply #9 on: January 03, 2022, 01:36:21 PM »

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.  Wink

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
Logged
KW4H
Member

Offline Offline

Posts: 76


« Reply #10 on: January 03, 2022, 02:23:25 PM »

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
Logged
KW4H
Member

Offline Offline

Posts: 76


« Reply #11 on: January 04, 2022, 10:09:21 PM »

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
Logged
KW4H
Member

Offline Offline

Posts: 76


« Reply #12 on: January 05, 2022, 01:35:48 PM »

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.
Logged
KW4H
Member

Offline Offline

Posts: 76


« Reply #13 on: January 08, 2022, 05:54:49 PM »

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
Logged
KW4H
Member

Offline Offline

Posts: 76


« Reply #14 on: January 08, 2022, 09:55:24 PM »

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).  Shocked

Steve, KW4H
Logged
KW4H
Member

Offline Offline

Posts: 76


« Reply #15 on: January 09, 2022, 11:51:30 AM »

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



* S Meter.png (346.53 KB, 1942x1440 - viewed 132 times.)
Logged
K8DI
Contributing
Member
*
Offline Offline

Posts: 315


« Reply #16 on: January 09, 2022, 06:36:13 PM »

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
Logged

Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
KW4H
Member

Offline Offline

Posts: 76


« Reply #17 on: January 10, 2022, 08:28:19 AM »

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
Logged
KW4H
Member

Offline Offline

Posts: 76


« Reply #18 on: January 11, 2022, 01:59:44 PM »

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
Logged
KW4H
Member

Offline Offline

Posts: 76


« Reply #19 on: January 11, 2022, 04:44:23 PM »

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.   Tongue   Grin

Steve, KW4H


* S Meter.png (346.53 KB, 1942x1440 - viewed 119 times.)
Logged
Pete, WA2CWA
Moderator
Member

Offline Offline

Posts: 7988


CQ CQ CONTEST


WWW
« Reply #20 on: January 11, 2022, 06:42:39 PM »

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.
Logged

Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
KW4H
Member

Offline Offline

Posts: 76


« Reply #21 on: January 11, 2022, 09:22:50 PM »

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
Logged
KW4H
Member

Offline Offline

Posts: 76


« Reply #22 on: January 12, 2022, 10:11:48 AM »

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
Logged
KW4H
Member

Offline Offline

Posts: 76


« Reply #23 on: January 12, 2022, 12:03:24 PM »

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
Logged
KW4H
Member

Offline Offline

Posts: 76


« Reply #24 on: January 12, 2022, 02:14:12 PM »

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
Logged
Pages: [1] 2   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone © 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.067 seconds with 18 queries.