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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Wed 10, 2018 4:22 pm 
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Chris, thanks for the info as it somewhat confirms the range of AVC voltage to be expected in this family of receivers even if not exact.

Rodger, thanks for the confirmation. I'll forge ahead assuming the initial -9.6v is a reasonable starting point. Initial evidence is pointing towards the tubes, as a quick check of resistors near the source showed them to be within tolerance but I want to check the EL-Menco micas there later today before moving further down the bus.

My grandson, now 16, has played soccer for many years even after a knee injury requiring screws! However, a skiing accident just after Christmas causing a dislocated shoulder may finally bring his career to an end. We'll see.

BTW, Rodger, it's hard for me to imagine you ever being ready to throw in the towel because of a technical problem. I'm still waiting for publication of your Encyclopedia of Vintage Communications Gear that will have pictures, complete technical specifications, feature reviews, common problems found, and repair hints. Hopefully each copy will come with a discount coupon for admission to Rodger's Radio Ranch. :D

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Wed 10, 2018 9:37 pm 
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Chuck,

I came pretty close to throwing in the towel with a NC-183D which was the first major restoration I had ever done. It had one remaining nagging problem that I finally traced to a kinked winding in a HFO coil that was killing the Q causing reduced oscillator output and poor sensitivity on the highest range. I have the equipment now to measure parameters like that but that doesn't mean some new challenge won't come along to bite me. The HRO-500 I finished a few weeks ago had an instability issue that was because of a defective new Panasonic brand cap I had installed that was open and I am careful not to get them hot while soldering so I am pretty sure it was defective from the start. There is always something new to learn with these vintage beasts.

I have grown used to seeing Anna sliding on the grass or turf at least once per game so I don't cringe anymore. She almost tied the game in the last minute but the keeper on the other side made an incredible save and Anna congratulated her on the incredible move she made. It will be a fun season.

My SX-42 (and SX-62) notes don't have anything on the AVC voltage but if you measure at the control grids you will be able to find where things are going less negative. It isn't likely with the sockets used in the SX-42 but with the printed circuit boards used near the end of the tube era it was common to find problems like this due to conductive residue building up on the board. The Heathkit SB-100/HW-100 series are very prone to this issue and it is one of the potential causes of the drifting S meter zero issue often seen with them.

I hope the AVC problem is quickly resolved. I spent the morning running errands and getting a camshaft sensor replaced (happily under warranty ) so now I can finally get to the bench.

Rodger WQ9E


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Thu 11, 2018 2:22 pm 
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rsingl wrote:
It isn't likely with the sockets used in the SX-42 but with the printed circuit boards used near the end of the tube era it was common to find problems like this due to conductive residue building up on the board.


Good point. I was thinking that a long shot was either residue, or a tube socket that had somehow become conductive, causing enough leakage resistance to drop the AVC voltage.


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Thu 11, 2018 2:34 pm 
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Tom Herman wrote:

Good point. I was thinking that a long shot was either residue, or a tube socket that had somehow become conductive, causing enough leakage resistance to drop the AVC voltage.


With printed circuit boards the problem usually gets worse as the set continues to heat up. I guess the "residue stew" becomes more conductive as it grows warm and in this way it closely mimics the behavior of a tube with secondary emission from the control grid and tries to lead you the wrong direction.

One of the more hilarious (to those who didn't have to initially troubleshoot it) was an old Motorola television service note for a ticking noise that appeared when the set was operated in low humidity conditions. The sound was being misdiagnosed as breakdown in the CRT high voltage circuit but applying anti-corona dope to all suspected areas wasn't curing it. The factory finally traced the sound to the safety warning tag for high voltage. The warning tag was a card suspended by a string close to the CRT and it was building up a charge from its proximity to the high voltage circuit and when it built up sufficient charge it was attracted to and would swing over and discharge with a "tick" sound to the grounded foil shield on the inside of the set back cover. Since the cover was a necessary part of the sequence the poor service tech couldn't identify the issue when set was operated using a "cheater" cord with the back cover removed. Stuff like that must have driven the poor local service tech nuts.

Rodger WQ9E


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Thu 11, 2018 2:48 pm 
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Rodger,

High impedance/resistance circuits can drive you buggy... I work with a fair number of Geiger counters, which by definition are extremely high impedance. With power supplies supplying microamps at 1000 VDC or more, you get the picture (R= E/I): ANY leakage can cause the unit to stop working!
My poor CDV-457 110 volt operated units are great humidity indicators: Last place I lived at had poor environmental control, and they were essentially in almost 100% humidity for most of the year: Plug in the unit, lights up, but no go... Won't have enough HV to count. Take out hair dryer (NEVER use a heat gun! Don't ask me how I know not to do this), warm the circuit board for a minute to drive out moisture, and Voila!, you have a working HV supply...
BTW: I love the Motorola over voltage indicator! That is HILARIOUS! Simplicity itself.. Reminds me of the Native American weather indicator: A stone suspended by a string: If the stone is wet, you know it's raining. If the stone is moving, you know it's windy. If you can't see the stone, you know it's dark...


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Thu 11, 2018 3:13 pm 
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Tom,

Great points and a hilarious story!

My worst humidity experience was with my 2006 GMC Sierra turbodiesel pickup. A few years ago the check engine light was on before I even put the key in the ignition however it started and ran normally but wouldn't shut off once started without pulling the ECM fuse. The only stored DTC was for loss of communications between the engine and transmission controllers. The problem only appeared intermittently and I later realized it was due to humidity. A mouse had urinated on the top level of the underhood bussed electrical system which has fuses and relays on the top level and bare bus bar wiring the next level down. The mouse urine created a corrosive path between the ignition switched and constant 12 volt buses and when humidity was high enough the leakage would become higher and would slowly charge capacitors on the switched bus until that bus hit the critical 10 volt level activating the ECM control relay and applying the full 12 volts to the switched bus turning on the CEL and preventing ignition shutoff since 12 volts remained present. The communications error occurred because the ECM went active while the TCM was still sleeping.

Finding the problem took several hours of studying the 5 volume set of service manuals and making measurements. Replacing the UBEC with a pull from a wrecked truck cost me $60 for the part and only took about 20 minutes. And I now have several layers of anti-mouse defense in that garage. The same year a colleague suffered $2,500 worth of mouse wiring damage to his 2 year old BMW SUV.

Rodger WQ9E


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Sun 14, 2018 4:16 pm 
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Whereas I have been STRONGLY urged by my next-door daughter to not venture out into our beautiful ice and snow, I've been going back over some of my notes. In doing so, I came across some information that I somehow missed noting here when it happened, but that may be useful to those rebuilding the SX-42.

It may also be of interest to Rodger who often warns about the micas in the '42's discriminator. It was probably his warning that had me opening mine up considerably earlier back in October tracking down distortion on the FM BCB. I tried to find a good place in the thread to go back and edit it in, but could not. Thus, I'll just insert it here.

In my case, the 47pf mica across the primary was in very bad shape, so bad that my Chinese component tester would not even recognize it as a component. Nor would my Pyramid CRA-2 show a value for it. The latter did show several MILLIamps of leakage before it even got up to 100V.

Both of the 100pf caps in the secondary checked well for both value and voltage and were left in place, as I didn't have suitable replacements anyway.

I hope my main mentor, Rodger, will overlook this minor rebellion on my part in exchange for this possible partial answer to his question of the exact failure mode inside these transformers. Perhaps the greater voltage stress seen in the primary vs the lower voltages seen in the secondary make the 47pf more susceptible to failure at this ripe old age. Or, maybe this case is just an outlier.

Maybe one of y'all who have yet to work on yours can provide more data by actually testing the discriminator caps as you go through yours.

Anyway, I'll get back to locating the cause of the loss of AVC voltage at the tube ends of the bus when the ice melts - not likely today, as the temperature was 1° this morning at 0600! :shock:

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Sun 14, 2018 5:42 pm 
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Chuck,

Thank you VERY much for testing that component! I have been shotgunning all of the caps in the discriminator when in there and the only time I tried to test them I couldn't find an issue using my old military universal bridge so I am glad you identified the bad actor. The usual failure mode for mica caps occurs when they have a high DC differential across them and silver migration occurs but that situation doesn't occur in the typical discriminator circuit. The SX-42/SX-62 setup is typical and although there is voltage applied to one side of the 22 pf coupling cap across the two sides of the discriminator transformer the other end of the 22 pf capacitor is isolated by another pair of capacitors so there isn't a direct path to create the classic setup for silver migration to occur.

I ran into the same issue with an older Magnavox AM/SW/Phonograph vertical console with a separate FM tuner and replacing the discriminator caps was necessary to get the FM tuner to work as it should. Next time I have the "opportunity" to work on a discriminator I will do a more in depth investigation of the caps to see if usual failure mode is the cap on the input side (from the last limiter) like Chuck found with his SX-42.

Thanks for creating such a detailed thread Chuck and I am sure this thread is going to be found and used by a bunch of SX-42 restorers long into the future.

And staying off of the ice/snow when possible is a great idea. A couple of weeks ago we had freezing rain on top of ice and it took about 5 minutes of slipping and sliding to walk the dog from the house out into the pasture and Cheyenne wasn't doing any better with it than I was but she wouldn't have as far to fall if she slipped. Coming back a barn cat was trying to follow us up the slightly inclined sidewalk and his all-wheel drive system wasn't working quite as well as it should because his rear paws were slipping quite a bit while his fronts seemed to be getting good traction. I suggested he dial in a little more front torque bias but he ignored my suggestion :)

Rodger WQ9E


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Mon 15, 2018 10:24 pm 
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Chuck, I think this might fall in the category of “for what it is worth” department. I just reported on my SX-62 thread that I solved my distortion issues. As it turned out, my problem was sticking contacts on the Noise Limiter switch. The switch is supposed to short the plate to cathode on the noise limiter 6H6, thus defeating the noise limiter. This made stations in the AM mode difficult to tune in and caused slight distortion, particularly when listening to music on BC AM. I do not think the noise limiter has any effect when using the FM detector.
I don’t know if you have checked this switch out yet, but I have had this issue before on some other Hallicrafters receivers using the toggle switches. I would assume that this may be a common issue, since I have seen it at least twice.

Good luck!

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Mon 15, 2018 10:54 pm 
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Thanks, Chris. It's a worthwhile addition to this now-lengthy saga because that switch as well as the AVC switch on mine were both not working when I began some time ago. A dose of WD-40 and many operations of each switch seemed to cure the problem at that time.

However, as I've been operating the radio during work on other areas of it, I thought once or twice that the ANL , and now AVC were not working like they should sometimes. Both switches are still on my to-do list to check out more thoroughly and the AVC switch will figure more immediately into things as I track down my AVC voltage issues.

Right now I'm trying to talk myself into going out to the shop for an hour or two before the next winter storm hits here in a couple hours. You're probably seeing it right now.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Tue 16, 2018 5:54 am 
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Progress!

I did manage to brave the elements and head back out to the shop. Actually the temperature soared to 40° here today, then back down to 14° tonight with a couple more inches of snow predicted.

Well, just as the last place I looked was where I found my missing wallet a few weeks ago, the last AVC buss branch I checked out today was where I isolated the culprit.

If I had only started at the other end of the buss, the problem would have been with the first branch off the AVC buss that I tested instead of the last of the four branches. :roll:

The good news is that I've found where the problem is. The bad news is that it's inside the T-25 IF can, apparently a leaky mica cap. At least I hope it's that easy and not a bad winding.

Supper time caught up with me before I could actually open the T-25 can and, after supper, Newton's First Law held me firmly in my recliner for the evening.

What lies inside T-25 will have to wait until tomorrow!

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Tue 16, 2018 1:34 pm 
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Chuck,

Enjoy your heatwave :) It is -2 with the wind at 20 here. I plugged the tractor block heater in this morning so I can plow later now that the drifting is pretty much gone; not much snow but the wind created a few one foot drifts in the driveway.

It sounds like you have a late production SX-42. The early production units didn't apply AGC to the second IF and the 1.2 meg resistor at the junction of the 455 and 10.7 transformer secondary windings was directly grounded. Starting with the similar SX-62, this resistor was lifted from ground and attached to the AGC bus, bypassed with a capacitor on the bus side. Apparently this change was made somewhere in the SX-42 production cycle so that it was arranged like its simpler sister.

Rodger WQ9E


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Wed 17, 2018 12:06 am 
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T25 was opened up today and all components checked, even though the culprit was identified right away.

C69 was shorted down to a few megohms and showed this even with just the Fluke 73. I even cleaned off most of the crud on its case, but it was still shorted.

Of course the value, measured on the M6013, was all over the map from 4000pf down to 500or so pf. It was essentially no longer a capacitor, to the point that the EICO 950B that I threw it on for grins and giggles, to see what HV would do, would not even open up the eye when measuring value and, as expected, the eye closed as soon as the voltage started to rise a few volts during the leakage testing.

I took a number of pictures for the benefit of my fan, but won't have the time to process them until tomorrow. However, I wanted to get this much of the mystery published so he could sleep tonight. :D

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Last edited by WoodchuckTN on Jan Thu 18, 2018 5:14 am, edited 1 time in total.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Wed 17, 2018 2:40 am 
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Chuck,
I trust you received the cold weather we sent down to you by now. I tried to add a little heat to the air headed your way, but don’t think it did much good. :lol:

Chuck and Rodger,
Even though my 62 is up and running well in both AM & FM modes, I am still seeing no more than .86 meg ohms between the AGC buss and ground, so I suspect some leakage somewhere. I have been hovering around both the T24 and T25 IF transformers suspecting there might be some leakage thru the caps here. Although I am seeing -6.7 VDC or more on strong BC AM stations. Should I expect more?

I am debating removal and opening up the transformers to replace the caps as Chuck is seeing. Perhaps my 62 has not seen a whole lot of use, and is in pretty good physical condition. I am looking forward to seeing your pictures, Chuck, so I have some idea as to what to expect.

Well, there has been an interesting development since I wrote the paragraphs, above: While listening to the strongest AM station in St. Louis (50 kw, KMOX, about 20 miles away), I am getting an AGC voltage of -9 VDC or more, then the voltage will drop off to -2 or -3 VDC, with noticable static. It will usually return to -9 or I can manually “reset” it by reducing the RF gain, when it will operate normally for a short period then repeat. If I listen to a less powerful station where the AGC hovering around -6 to -7 VDC, it will operate for hours without a problem.

I got to thinking that maybe the higher AGC voltage is getting high enough to leak thru one of the T24 or T25 caps with the strong station, but it is not quite high enough to cause issues with the weaker stations. Am I all wet, or am I on to something?

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Last edited by N9whh on Jan Wed 17, 2018 3:19 am, edited 1 time in total.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Wed 17, 2018 3:10 am 
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3° here at the moment and the night is young, Chris. We also got a couple more inches of snow.

Regarding your AVC buss, are you sure you have isolated the buss from normal resistance to ground?

I ask because my version of the SX-42 has a 220K to ground at the detector end of the buss and a 2.2meg near the far end to ground.

Both of these must be lifted from ground to measure the buss properly.

I'm anxious to see what the AVC voltages will look like with the leakage fixed. Prior to repairs it was -9.6V at the junction of the two 47K resistors at the detector grid and only -5V at the tube end of R41, which was the beginning of the AVC feed.

I'm hoping to have T25 back together tomorrow and will post what I find.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Wed 17, 2018 3:23 am 
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Chuck,

I just edited my last post which must have crossed yours. I added some new information.

I am going to have another look at the schematic, now. You are right, mine is wired the same. I will dig into it tomorrow evening.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Fri 19, 2018 3:54 am 
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long thread on this one!

Just a note....I restored a 42 a couple years ago. It sat in my garage for 15 years before I finally got to it. It had the classic problem with the leakage on a band switch in the front end. It was a PITA to repair it IIRC :D . This was my second 42. It now is my back-up receiver in the ham shack (studio A). When completed they sound nice with the P-P audio. The FM doesnt quite make it all the way to 108 MHz. The cabinet design was by Raymond Loewy (sp?) the famous industrial designer. He in fact designed most of the Halli line after WW2.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Sat 20, 2018 2:07 am 
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-ps, I'm VERY glad that the notorious SW-1 issues did not rear their ugly head - so far - for me, though it was a bear to try to clean touroughly on both sides of each wafer.

Another problem I haven't seen so far is band 6 dropping out at the high end - so far.

Before moving on to what I hope is a final alignment pass, I wanted to post the promised pictures of the inside of T25, the 2nd IF transformer where I dound the leaky mica, C69.

Removing T25 was not too bad. The process would have gone much more quickly but I always carefully label each lead in case I get interupted for a day or two. (Actually for me a distraction of a few moments can erase the memory of where a lead goes. :roll: )

This shows one reason why you need to remove these, as the screws on the side are a challenge to reach otherwise.I'm pretty sure the BFO and Discriminator cans had these also. Rodger spoke of having worked on the discriminator without removing it, but he must have much better manual dexterity and vision than this old man.

Attachment:
T25 exterior.jpg
T25 exterior.jpg [ 154.04 KiB | Viewed 276 times ]


Here's an end view with the transformer terminals marked. The embossed numbers are sometimes hard to see under years of detritus.

Attachment:
T25 end.jpg
T25 end.jpg [ 200.9 KiB | Viewed 276 times ]


This view shows the interior and the components I replaced. C69's ailments were discussed above. R33 was over 40% high and R35 was over 34% high.

Attachment:
T25 interior annotated.jpg
T25 interior annotated.jpg [ 206.12 KiB | Viewed 276 times ]


Here's another interior view showing how the yellow "fuzz" ran all along the wire. Maybe one of you have seen this phenomenon before and have an idea what it might be???

Attachment:
Wire fuzz.jpg
Wire fuzz.jpg [ 178.47 KiB | Viewed 276 times ]


And here's the leaky culprit to take a bow.

Attachment:
the culprit.jpg
the culprit.jpg [ 46.01 KiB | Viewed 276 times ]


I'm pleased to report that T25 went back in without a hitch and the AVC seems to be working well. S-meter action also seems now to be as expected.

Chris, you may be past this already, but the AVC buss of your '62 is identical to the one in my SX-42, late version (schematic 89D257-D). To isolate the AVC buss to measure the resistance to ground, you have to lift detector end of R42 as well as either end of R88 (assuming these #s are the same).

Let me know if I can answer any questions not covered here.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Mon 22, 2018 2:11 pm 
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Hi Chuck,
Per your earlier suggestion, I did lift the resistors, you mentioned, from ground and checked the AGC buss again. I measured over 1.5 meg. Much better than previous measurements, but I still suspect some leakage somewhere. As I said before, I have been suspecting the T-24 and T-25 transformers for a while. Because I do not have the proper caps on hand, I have not removed either transformer. I have found, though, that if I reduce my RF gain, slightly, on strong signals the receiver does not overload as the AGC voltage does not reach a “threashold” voltage and drop off, as I have been seeing.

Thank you very much for posting the pictures of yours, as now I know what to expect. I decided to take the weekend off from the 62 and took advantage of the warm weather. ( My 1930 Model A Ford was feeling neglected so I gave it some attention.)

I’ll get some caps on order and go back to the 62 since colder weather will be moving in.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Mon 22, 2018 3:15 pm 
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Chris, if you have lifted those resistors and still have that low of a resistance, there is definitely leakage. Once my repairs were made, my Fluke read "OL" which meant something above 20 meg.

Just be sure you haven't fooled yourself and have lifted the wrong end of a resistor and have not really isolated the bus. Don't ask me why I say that. :roll:

After mine played beautifully for a couple hours after the above repairs, when I went back the next day and turned it on it played fine briefly and then, as it warmed up, began to fade in and out with regard to sensitivity.

Discouraged, I only played with it long enough to check the two RF amps, which tested fine. The AVC also stayed steady, but low, meaning gain should stay at max during the fades.

Briefly using signal injection around the RF and oscillator stages is pointing at the 1st RF or the HFO at this early and admittedly haphazard stage of troubleshooting. It was late afternoon which is not my best time for the motivation and mental rigor required for serious troubleshooting.

The AVC problems forced me to become intimate with AVC circuits in general and the SX-42 implementation in particular, so we'll see where this challenge leads.

And, yes, I double-checked my work and made the AVC circuit my first suspect when the fading started even though everything had played well for a couple of hours before it started.

Stay tuned as they say.

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