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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Tue 10, 2020 6:26 am 
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Joined: Mar Tue 10, 2020 5:11 am
Posts: 91
Location: Mission Viejo, CA
I am presently restoring a NC98, myself. When I drove to pick it up, the seller had it plugged into a variac. Before I realized what he intended to do and could stop him, he turned up the variac, slowly, but turned it up... Got it up to 115V before I had the presence of mind to tell him to shut it off... I would prefer to bring the old radio up myself, under my own test conditions 8-/ Anyway, I paid him, took it home, and last weekend finally got around to working on it. I found the internal-tooth lockwasher from the volume control loose inside the chassis, along with the metal link used to connect one antenna terminal to the ground screw. Obviously, someone's been inside this NC98 before, and somehow, didn't get all the hardware back in place.

A couple months ago, I bought a replacement capacitor kit from Hayseed, so that was the first thing I intended to install after checking the tubes. Also a few months back, I bought a Superior TV-12 tube tester fairly-cheap, I thought. It works just fine, although it doesn't directly show Gm. I carefully put removable labels on all the NC98 tubes then proceeded to check them. All were fine, until I got to V8, the 6AQ5 audio output, and found it somewhat questionable... it was barely into the green on the TV-12 meter. Then I noticed the meter was still slowly increasing, even after 3 or 4 minutes. I sat back to wait a few minutes more. After maybe 5 or 7 minutes, pressing the button on the TV-12 moved the meter solidly into the middle of the green. After 10 minutes, the needle stopped rising. Perhaps it's been so long since this radio was powered up that a very small amount of residual gas has seeped into the tube, and then my warming it up on the TV-12 has slowly "gettered" the tube, similar to what hams must do with very old glass transmitting tubes that haven't been powered up for decades. This might be what's wrong with Scott's radio: the 6AQ5 is still slightly gassy and it takes a few minutes for the cathode and getter to burn it off. More likely, however, I suspect a leaky capacitor somewhere that "warms up" after a few minutes after B+ is applied and becomes less-leaky.

I carefully replaced both the 0.1 uF, both the 0.047 uF, and the three 0.01 uF original caps in my NC-98 (as well as the 3-section electrolytic). In my receiver, one of the 0.047 caps was a "Black Beauty", whereas the 0.1 and 0.01 caps were all ceramic-cased (I presume there are paper caps inside the ceramic tubes). Hayseed provided four 0.01 caps, however, and it took me awhile to figure out that in my radio, the fourth one was actually a 0.01 uF disc ceramic, of which there are a bunch in my radio.

Then I undertook a concerted resistor-measurement exercise. To my horror, I discovered that a number of resistors measured way out-of tolerance and needed replacement before my radio could be powered-up. There are 43- or 44-some carbon composition resistors in my radio, plus a 500-ohm 10W sand resistor in the power supply. The remaining four resistors are all potentiometers, which I haven't checked yet. Of the 44-odd composition resistors, one is supposed to be a 27K, 1W but in my radio, is actually a 33K-1W type; I'll replace it, as I suspect my radio was built before the manual that I have was published and the 27K is likely a factory modification. Only 13 resistors are within their 10% tolerance. At least 16 other resistors are all reading way high, at least 15% high, with some as much as 50% high. R36, a 47K 1/2W, is reading 470K. About 13 of the 44-odd can't be easily measured in-circuit, like the pair of 1M R19 and R20, and the pair of 270K R17 and R18 resistors (but those are in the ANL circuit, so may not be critical). The two 15K-1W resistors, R5 and R21, are both around 18K or so and need replacement.

Bottom line: I need a whole slew of carbon composition replacement resistors. I just did some shopping on epay tonight for the values I need for this receiver, as well as a Kaar KE-23A receiver (that I don't have yet).


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Tue 10, 2020 7:08 am 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
Steve,

Very informative post and I am glad there are two of us refurbishing these beautiful receivers. Yes-I have been concerned with a gassy audio amp 6AQ5, so I substituted an NOS and it made no difference in the warm up time or the audio distortion.

I have been systematically going through the resistors, and in particular paying attention to those that are no longer shiny and appear rough and a little discolored. Most are within the +/- 10% tolerance, but a few were way off. Just a question-why are you replacing the out-of-spec resistors with carbon composition types? I am replacing mine with more modern carbon film resistors which are easy to procure.

I actually have not measured filament voltages on the tube sockets. I measured all the DC voltages but skipped the filament step. That I can easily do.

Per Paul's post today I measured how long it took to warm up the receiver to proper volume comparing headphones to a speaker. Lo and behold-headphones took about 30 seconds and the speaker took well over 2 minutes! So something must be a problem in the audio transformer primary circuit, or with the 6AQ5, or maybe it is the pesky couplate as others have suggested. At least I think I am narrowing it down to a section of the circuitry.

Steve how is your AVC? Do you get good action on the S-meter with AM broadcast stations. Mine is still pretty weak so I will have more investigation to do on the AVC buss.

73s, Scott, N6CIC


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Tue 10, 2020 9:28 pm 
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Joined: Mar Tue 10, 2020 5:11 am
Posts: 91
Location: Mission Viejo, CA
Scott, first off, there should be no difference between headphones and speaker; both are driven from the same winding on the same output transformer, with only the 22 ohm R38 across the phones jack being the difference (my 22 ohm resistor actually looks very slightly burned; but I can't measure it without disconnecting it from the transformer). BUT be certain that your phones and speaker are the same impedance, or all comparison is off!! Thus, your speaker may be misbehaving or possibly, a difference in the amount of audio power required to drive the speaker versus your phones is the culprit. That would point to the output stage. Your output transformer could be distorting the audio until some current has been flowing through it to warm it up some. (You could test that "theory" by letting the receiver get totally cold, then heating the transformer alone with a heat gun.) And don't forget that excess current through the transformer primary might be causing it to saturate, until the 6AQ5 warms up and begins drawing the proper (reduced) current. Also, make sure your Tone switch is working properly; if C56 is leaky and the switch is actually closed when it should be open, that cap might be introducing distortion at increased audio volume when using the speaker. If you can't figure it out, try putting a scope across the output terminals and look for distortion; inject a known modulated RF signal into the antenna input for consistent measurement results. (An injunear with a lab full of measurement equipment would connect a distortion analyzer across the output 8-/ )

Second: the reason I'm using carbon comp instead of carbon film is because many of the resistors in this receiver, such as R5, R33, R11, R17 through R21, and others are connected to points where any undesigned-but-introduced inductance from the metal film might cause RF or audio response changes due to resonances. (I have to admit, though, that my experience in substituting carbon film for previously-carbon comp resistors in old tube receivers is minimal, and I may be all-wet.) I know better than to use carbon film resistors in transmitters and high-power amplifiers, though =8-O (There is a reason parasitic suppressors all use carbon composition, or at least non-inductive carbon film, resistors!!) Also (although I'm giving up hope of keeping mine looking totally stock, since so many original components need replacement), original-looking carbon comps look more appropriate for radios of this vintage. Unfortunately, today's 1- and 2-watt carbon comp resistors don't look anything like the ones used in the NC-98, however 8-(. (Even the 1/2-watters look new and unlike the originals.)

I haven't fired my NC-98 up yet, so I have no idea about the AVC, because so many resistors are so far out of design spec. Phooey on National for using so many poor quality resistors; and they're ALL 10-percenters! I don't think they can even blame them on the post-war lack-of-quality-control Japanese or Chinese parts, just very cheap American-built products of the time...

SteveH


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Tue 10, 2020 9:31 pm 
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Joined: Mar Tue 10, 2020 5:11 am
Posts: 91
Location: Mission Viejo, CA
I forgot to mention: when you measure the filament voltages, don't forget there is 5-ohms in series with the 6AL5 filament; so its voltage is around 4.8VAC when warmed up (or so, according to the manual).

SteveH


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Wed 11, 2020 12:39 am 
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Joined: Sep Wed 10, 2014 2:01 am
Posts: 2092
Location: Costa Mesa, California
Scott--clean the headphone speaker contacts that are disconnected when you plug in the headphone plug. These get dirty and build up carbon over time. Use some contact cleaner and wet a stiff piece of paper and slide it in between the contacts a few times. I have had radios where these prevented the speaker from working at all--let alone with reduced volume.

Norm

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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Wed 11, 2020 6:29 am 
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Joined: Jul Sat 23, 2011 9:33 pm
Posts: 978
Location: Long Beach Ms. USA 39560
My NC-98 has a characteristic that I have seen similarly on many other radios of this vintage.
The 6C4 oscillator was dying on the low end of the bands until I replaced the resistors in the circuit.
Now the three lower bands are very good, with good injection voltage to the mixer (measured with an RF demod probe) but band 4 (13 Mc to 40 Mc) finally just barely oscillates down at the low end.
The almost 30 Mc spread of band 4 makes me think it is impossible to get good performance without totally rebuilding the coils.
Since the frequency alignment comes in well, and i may not live to see another peak in sunspots (age 79) I gave up and run 20 meters off the top of band 3.
COMMENTS ? Advice? Experience?
Pat W5THT

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Unhappy tubes blush while unhappy power FETs scatter plastic


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Wed 11, 2020 6:44 am 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
Norm,
Thanks for the tip on cleaning the speaker-headphone jack contacts.

Steve,
I already replaced C-56 assuming it might have been leaky but that had no effect. To date I have only replaced one fixed resistor-R1 which was 47k and should have been 470k. I replaced it with a carbon film or metal film resistor (not sure which). I will be careful with those resistors that you mention where there could be some inductance effects at Rf. I thought you might be interested in the resistors I have found so far to have badly drifted.
R-12 measured 125k, spec 100k
R-14 measured 143 k, spec 100k
R-8 measured 174k, spec 150k
R-9 measured 265, spec 220
R-7 measured 240, spec 220
R-25 measured 93k, spec 150k ( may have to clip one end and measure again)
R-20 measured 1.5m, spec 1m
R-47 measured 2550, spec 1500

Next on my list is to measure tube filament voltages, and to continue investigation of the audio transformer primary circuit, as well as the audio amp circuit. I don't have a scope. The extent of my equipment is a DMM, Heathkit signal generator, and a Sylvania tube tester.

Scott


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Thu 12, 2020 1:08 am 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1451
Location: Champaign IL 61822
All oscillator tubes are afflicted by this, but 6C4's (and their dual, 12AU7) are notorious.
Make sure the filament voltage is 6.3 volts or higher. See how it does at 6.7 or 6.8 volts.
If it works better there, use that. If you are paranoid, you can put a dropping resistor
bigger than 500 ohms in the B+ and reduce heat by using 1N4007s and unplug the
tube.


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Thu 12, 2020 3:46 am 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
I checked the filament voltages of all tubes today, and all measured 6.8 VAC with spec at 6 VAC. However the 6AL5 detector and noise limiter had a filament voltage of 2.4 VAC as opposed to the spec of 4.8 VAC. Two 10 ohm resistors are used in parallel to drop the voltage from 6 volts. The resistors measure about 5 ohms in the circuit, but they must have drifted higher. They look dark and rough, so I will replace them.

I rechecked some key tube DC voltages. Pin 1 of V3 (6BD6) 1st IF amp measured 8.5 volts as opposed to a spec of 1.5 volts. Pin 1 is the signal grid and it is fed through Z5 (IF Crystal Unit) to R10 to the AVC buss. I think I have identified the AVC buss line correctly, but if I am wrong I welcome correction. R10, measured in place, was 246k as opposed to 270k spec. I will replace it.

Going to the audio amp circuit (V7B) R34 off the signal grid measured 11.6m as opposed to 10m spec. I will replace this resistor.

Thanks for any interest and advice.


73s, Scott, N6CIC


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Thu 12, 2020 5:32 am 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1194
I suspect those filament resistors. Even with them not in place you'd measure a low resistance, the tube heaters (cold) and the transformer secondary with all those other tubes connected. They are probably open.


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Thu 12, 2020 1:33 pm 
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Joined: May Tue 16, 2017 3:03 pm
Posts: 287
Location: Potomac, MD 20854
Scott10k wrote:
R10, measured in place, was 246k as opposed to 270k spec. I will replace it.


Why? It is still in spec. I am no pro, but if a resistor is within 20% of what is "stated" in manual I leave well enough alone. Also reading grid voltages can be tricky, you will find many measurements are very sensitive to the signal applied (RF gain), so make sure all measurements are made using settings as stated in the manual. Also, many "manual" values can be wrong; see if you can find a Sams Photofacts ( will try to attach the Sams info here; is available on the web).

paul


Attachments:
National_NC-98_Photofact_Jan1955.pdf [2.65 MiB]
Downloaded 19 times
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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Thu 12, 2020 4:03 pm 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
Thanks Paul. I have downloaded the Sams Photofacts, and will recheck the voltages according to their readings. I did follow the settings listed in the NC-98 manual before taking my tube socket readings.

Scott, N6CIC


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Thu 12, 2020 4:27 pm 
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Joined: May Tue 16, 2017 3:03 pm
Posts: 287
Location: Potomac, MD 20854
The resistor readings table can be helpful too.

p


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Fri 13, 2020 5:52 am 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
As I stated before, the filament voltage to the 6AL5 detector/noise limiter was supposed to be 4.8 volts AC and I measured 2.4 VAC. I clipped out the two 10 ohm resistors which are installed in parallel to drop the voltage from 6.8 VAC to 4.8 VAC. One measured 34 ohms and the second over 900 ohms. On replacement with two new 10 ohm resistors the radio now warms up in less than 30 seconds whereas before it took well over 2 minutes before I got any audio.

Now my attention will go the my AVC which is anemic as judged by low S-meter readings on strong stations.

Thanks for the good advice on measuring the filament voltages, and also thanks for the link to Sams PhotoFact which has a lot of troubleshooting information.

Scott, N6CIC


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Fri 13, 2020 2:20 pm 
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Joined: May Tue 16, 2017 3:03 pm
Posts: 287
Location: Potomac, MD 20854
Is there an S meter adjustment? Maybe the AVC circuit is fine but S meter is off. If you turn RF gain to max and AVC on and then tune in a weak broadcast band signal and then a LOUD local station if the AVC is working the loud signal will not blow your ears off. Also, turn the AVC off and tune a local loud station on AM with RF gain at max (af gain not max!); with AVC non-functional it will be all garbled. If you now switch AVC ON with the same volume settings and now it sounds "normal" then AVC is working.

p


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Sat 14, 2020 5:06 am 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
Paul,

I did the test procedure you described to check AVC. On a loud AM station, there was not much difference with AVC switched on or off. I compared this with my SX-99, and with a loud station going from AVC on to off just about blasted the speaker. So I am assuming my AVC voltage is not sufficient and I need to carefully check the components on the AVC buss.
What do you think?
Thanks, Scott


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Sat 14, 2020 5:24 am 
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Posts: 4320
Location: Lincoln City, OR 97367
Greetings to Scott and the Forum:

You need to be sure the sensitivity control is set at maximum for this test. The sensitivity control overrides the AVC action except at maximum, so if it is not there, AVC action will appear to be weak or non-existent.

One of the best tests you can do is to monitor the AVC buss with a VTVM or other high-Z meter and observe the magnitude of the AVC buss voltage change when tuning onto and off a strong station. It should be at least a change of several volts. If not, look for leaky capacitors loading the buss, out of tolerance resistors or a problem with the AVC detector.

Regards,

_________________
Jim T.
KB6GM


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Sat 14, 2020 11:05 pm 
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Joined: Feb Sun 16, 2020 7:45 am
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Jim,

I first measured the resistance to ground from the buss at C-24 where it enters Z2 (2nd IF transformer). It measured 376k. Isn't that too low?

Second I tried measuring the voltage on going from a weak AM station to a very strong station. I don't have a VTVM and I think my DMM loaded down the voltage. Isn't the AVC voltage supposed to be negative? Here is what I got with the sensitivity control at max.
No AVC 0 volts
AVC on weak station 3.8 v
AVC on strong station 2.45 v

When I touched the DMM probe to pin 1 the control grid on V4 the 2nd IF amp all audio went dead. So I assume my DMM loaded it down.

Any advice would be appreciated.

Scott, N6CIC


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Sun 15, 2020 4:09 am 
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Greetings to Scott and the Forum:

I am using the Sams photofact schematic as it is clear and readable. On that schematic, C24 is in the plate circuit of the 2nd IF amp, V5. It has nothing to do with the AVC. The AVC buss has a capacitor, C22 directly connected to it and another, C17 which is isolated from the buss by a 270K resistor, R19.

Components to check would be the 2.2 meg resistor, R25 and those two capacitors. The actual AVC buss which is on the left side of the 2.2 meg resistor (R25 in the Sams schematic) should read about 1.2 megohms to ground.

The positive voltages on this buss should be normal.... remember that the AVC voltage is essentially a grid bias for the controlled stages and therefore needs to be negative with respect to their cathodes. All of the controlled tubes in the Sams schematic show a cathode voltage of plus 45 volts with respect to ground. Therefore, any voltage that is less than 45 volts positive with respect to ground is a negative voltage with respect to those tube's cathodes. How much negative (less positive) versus the received signal strength in uVolts would have to be determined by someone who owns a functioning NC-98.

I am suspicious of the voltages given on the Sams schematic, however. The voltages shown with no signal applied would represent a nearly -40 volt bias on the IF tubes, which should be more than sufficient to cut them off. This should not be the case with no signal applied.

As near as I can see, the AVC is defeated by placing a ground on the AVC buss. This then should be the most positive that the AVC buss should get, which would mean that the the AVC voltages should be negative.... an impossible condition since the tubes would already be cut off at 0 volts.

Therefore, I would suggest that you appeal for someone who owns one of these receivers in working condition to measure the cathode voltages for V1, V4 and V5 as well as the AVC buss voltage under conditions of no signal and a strong signal.

This should help narrow down the problem.

Regards,

_________________
Jim T.
KB6GM


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 Post subject: Re: National NC-98 Distorted Audio
PostPosted: Mar Sun 15, 2020 5:32 am 
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Joined: Feb Sun 16, 2020 7:45 am
Posts: 52
Thank you very much for your explanation Jim. I have a downloaded copy of the Sams PhotoFact which I got from Paul's posting earlier in this thread. The capacitor and resistor numbering is different in the PhotoFact from the NC-98 manual. So where I measured the Buss resistance to ground and the AVC voltages was actually at C22 in the Sams vs. C24 in the NC-98 manual. So the resistance to ground of 376k is too low. As you stated-it should be about 1.2 megohms.

Thank you for explaining the grid bias voltages with respect to the cathode. According to the NC-98 manual the cathode of the 2nd IF amp is 3.8 volts. I measured 3.8 volts with a weak AM signal (so no negative bias), and 2.45 volts with a very strong signal. Would that be considered enough of a bias difference?

I don't understand the 45 volts shown for the cathodes in the Sams circuit diagram. I measured the cathode of the 2nd IF amp at 3.8 volts using the settings in the manual.

The circuit layout in the Sams is much easier to understand as they show the detector circuit completely separate from the ANL circuit, and also the switch diagrams are easier to understand.

So per your recommendation I will check C22, C17, and R25 (Sams numbering) and then re-measure the resistance to ground and the AVC voltages under weak and strong signal conditions.

Thanks for you help!

Scott, N6CIC


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