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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Dec Sat 02, 2017 11:02 pm 
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Thanks Rodger.

Before moving on to the RF alignment, permit me to drop back and add some documentation of the crystal filter cleaning adventure for the possible edification and interest of those who haven’t been there yet.

For those unfamiliar with the SX-42, here’s what the 455khz crystal (xtal) filter looks like nestled onto the Selectivity control switch SW-2AA a couple inches behind the front panel:

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Here it is after removal for cleaning: (as always, take pictures or good notes as to orientation and connections to save later headaches)

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As you can see, it’s held together with tiny screws and nuts.
Opening mine up, here’s the sad picture I saw – the ceramic gasket was broken into 5 pieces:

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This one shows the crystal itself alongside the gasket pieces:

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I next spent waaaay too much time attempting to fabricate a new gasket out of some hard, but still slightly flexible, plastic from a chewing gum case. I’ll show the result here to demonstrate why I next elected to attempt super-glue repairs of the broken gasket:

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Unfortunately, I failed to take a picture of the repaired gasket. It wasn’t a pretty sight, but I thought it had a better chance of working than my creation.

After a final careful cleaning with 91 proof ISO and keeping my fat fingers off of things, I very gingerly reassembled the filter using the repaired gasket.

After letting it age for several days and finding the resonant frequency stable, the filter was reinstalled and appears to have survived my ministrations.

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"The question is not what you look at, but what you see." Thoreau


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Dec Sat 02, 2017 11:29 pm 
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Chuck,

Almost every one of these I have taken apart had cracks in the gasket, they must have been designed by the father of the engineer who developed the cylinder heads for the GM 6.2/6.5 diesels :)

But your gasket was definitely cracked worse than most. I have been putting these together with super glue but since the filter seems to go for years with a gasket that could have easily have been cracked for decades I think the most important function served by it is a non-yielding spacer rather than a seal so pretty and sealing integrity probably isn't that critical. I am glad your repair worked out.

For those restoring vintage receivers with these single element filters keep an eye out for FT-241 crystals which are in the category of ignored and often thrown away items at hamfests and estates. Several of these are close enough to 455 Khz. to make a good replacement for any receiver using a single element 455 Khz. IF filter and realistically with most receivers the IF can easily be realigned to anywhere in the 450-460 Khz. range at minimum to fit a different crystal without any noticeable calibration or tracking issues. Here is a good web link about the FT-241 crystals that explains the frequency/channel markings: http://people.csail.mit.edu/sw/html/NS1W/ft-241.html

Another possible source is some of the early SSB CW transceivers used 455 Khz. range crystals for carrier generation/BFO and some of these same sets are a source of 455 khz. filters. You will also find these crystals in some communications receivers although usually one good enough to use a crystal controlled BFO isn't a receiver you want to junk unless it is down to a parts carcass already.

Although a 455 Khz. ceramic resonator can sometimes be substituted it has a much lower Q than a good crystal filter so circuit performance will drop and I would use one only as a very temporary measure.

Rodger WQ9E


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Dec Sat 02, 2017 11:50 pm 
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Chuck, let me know if you need a good gasket. I have one from a holder where the crystal was not working and all attempts to get it working have failed. I did eventually find a replacement holder and crystal. As Rodger stated, a 455 KHz resonator is small enough to work and I used one while waiting for a new crystal. The resonators are dirt cheap, so buy a quantity--not that you will ever use more than one or two, but because they vary widely in their Q. I went through a batch of ten and and found one with a much better Q than the others-which was the one I used.

Norm

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Dec Mon 04, 2017 1:18 pm 
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Thanks for the kind offer, Norm.

However, so far so good with the original, not very pretty repairs I made to this one. It settled back down to very close to its original frequency, so we'll see how it does.

Today I hope to tackle the issue of insufficient HFO trimmer range by playing with the bandspread cap baseline setting as suggested by you and Rodger. It will likely be slow going, as their bi-directional bandspread scheme is interesting to say the least.

I'm assuming that the "Set" or "Zero" point is with the bandspread cap rotor completely unmeshed. The tweaking will start from there.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Dec Sat 16, 2017 2:11 am 
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Between chasing my tail and chasing rabbits, I've had a frustrating couple of weeks with this project.

After moving the bandspread dial about 13 log-scale points CCW, I was able to align and calibrate bands 1 & 2 without too much trouble.

The chasing my tail involved attempting to align band 3. Somehow, I set up to align the HFO for low-side injection and could not get things to adjust properly.

After several hours over several days of wrestling with it and trying several values of capacitance in parallel with C43, I was about ready to call for help again. Then one night it dawned on me that I needed to be aligning for high-side injection instead of low-side. :roll:

It still took a couple more sessions of experimentation to get the HFO to behave, because C43 still ended up needing about 3pf in parallel to bring the HFO frequency down to calibration. At the moment I'm having to use two 5pf cap's in series until I can scrounge up a single cap.

The whole process was drawn out, as usual for me, by my stopping to make up some sniffer probes for my frequency counter so I could look at the HFO frequency directly. The DX-400 digital receiver is great for monitoring the HFO when you're sitting still, but it's much handier to watch the frequency change real-time when you're having to experiment with changing values like I was.

I finally had another thought come to me, that my MFJ-259B antenna analyzer has a frequency counter built in. By the time I remembered this, I'd already built enough sniffer probes to equip both counters which really made the operation almost idiot-proof. And, if you haven't already guessed, when I'm working with me, that's needed.

Next up, band 4 RF alignment. Remember Chuck: LOW-Side injection for band 4.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Dec Tue 19, 2017 12:29 am 
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Well, some of you will probably already be wondering how I got away with moving the bandspread dial so far - of course I didn't as it turns out.

Having a few minutes late this afternoon, I fired it up to see how things looked after cleaning up the temporary two-cap padder I'd put in parallel with C43. Fortunately, the HFO was still very close to where I'd left it.

However, connecting an antenna and attempting to look at activity on the 40 meter band, I soon discovered that the bandspread dial would not go below about 7.15 mhz! At first I thought something was binding or slipping in the tuning mechanism. Closer inspection revealed that the bandspread cap was already fully meshed. :roll:

So..., it's obviously time to revisit adjustment of the bandspread dial on its shaft. It's way too far down the brain activity scale for me this late in the day, so this effort will have to take place in the morning.

My main reason for posting this now is two-fold:

1. Be warned not to follow my bad example and adjust the dial without serious forthought. I thought it was the solution to earlier issues described with the lower bands, but I'm not so sure now.

2. Please pass along any input you have after reviewing the last few posts. It should be obvious that I will not conquer this beast without your help.

Thanks for reading.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Dec Tue 19, 2017 2:40 am 
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If you look at the photo below, you see the relative number of plates for the main capacitor and the bandspread capacitor. The bandspread capacitor doesn't have a lot of capacitance, so for correcting alignment, moving the main tuning capacitor dial wheel on its shaft a little is the same as moving the bandspread dial wheel on its shaft a lot. I would start with the bandspread pretty close to where it should be from the manual and make dial wheel adjustments with the main tuning capacitor --but only if you can't achieve the alignment using the trimmers and inductance adjustment ranges. After setting the bandspread dial on its shaft, rotate it full CCW and CW a few times to see if the end stops align with where they would appear correct. Then do the same for the main tuning capacitor. That will be the starting point.

The two lowest frequency bands will tell you most of what you need to know, because they have only one inductance adjustment and band 2 shares the band 1 antenna trimmer. If you can't align the dial with the trimmers and inductance adjustment for these two bands--then you have no choice but to slightly change the main dial setting. Adjusting the bandspread dial on its shaft would be a last resort if the main dial was getting too far from its seemingly normal location. I would not get too far into the weeds on the lower bands without running through the alignment on all the bands to see if you are headed the right direction.

The real problem I think with the SX-42 is the trimmers are only 6p (if my memory serves me). That does not give one much range to correct for age and drifting component values. I ended up going through the complete RF alignment about 8 times or more before I felt I knew enough to move the dial a small amount to its best position. In the end that was a compromise. At some point, you will want to check your bandspread to see if it is accurately showing the correct frequency. That would be when you want to think about moving it on its shaft.

Norm


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Dec Tue 19, 2017 3:03 am 
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Thanks, Norm. That's the exact guidance I needed to get me back to a good starting point.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Dec Wed 20, 2017 1:21 am 
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Good news!

I put the bandspread dial back where it was before I removed it for surgery on the BFO and Phasing cap's and Reception control replacement.

A quick check showed band 1, 2, and 3 HFO settings are still very close to where they need to be.

Next up: front-end alignment (again) of bands 4, 5, and 6. Stay tuned!

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Dec Wed 20, 2017 7:31 pm 
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Hi Chuck,
I am still following closely. While I have not yet pulled out my SX-42 for restoration, I have resumed work on my SX-62A, which I started about 20 years ago, and stopped, when a mouse started dinning on the BC band oscillator coil. It also dined on the calibrator RF choke. I figure much of what you are doing will apply to the SX-62A, so this is good practice, for me anyway, for working on the SX—42.
I was fortunate enough to find a good parts doner SX-62, although it too had it’s own battle with mice at some point in time. My SX-42 is in much better shape than the SX-62A, fortunately! I will report that the 62A is now working very well with the transplanted parts, new caps, and a few new resistors.
I am getting ready for the alignment stage, so will be checking the 455khz crystal to see how it is working before I decide to open it up for cleaning. Fortunately, the crystal housing looks to be intact on both the doner and the working chassis, so I may get lucky. I do not see any evidence of anyone using any tuner cleaner, as Roger has seen, anywhere on this chassis, so hoping for the best.
Good luck with your SX-42, and still following!

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Dec Wed 20, 2017 9:36 pm 
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Thanks, Chris, and good luck to you on your '62 - and on your '42 when you get to it.

Your idea of "practicing" before you tackle the '42 is a good one. I practiced on several dozen projects for over two years before tackling my '42, even though I have an electronics/communications background, and am glad I did, given the complexity and difficulty of the SX-42.

My "practice" included an SX-43 and an HBR-16 in addition to restoring a couple dozen pieces of vintage test equipment since returning to electronics as a hobby in late 2014.

Fortunately I discovered ARF right away and have received a lot of help and re-education from the great folks here.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Dec Sun 31, 2017 12:04 am 
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Happily, bands 4, 5, and 6 accepted alignment without further drama.

Several sessions of band cruising, including FM BCB, since my last posting show that everything seems to be holding steady over time and repeated on-off cycles over the days.

I want to double-check AVC/S-meter action, as I'm not sure whether what I'm observing is normal. Action seems much more like what I'd expect when set for FM BCB reception than the lower bands. Also, when set for Band 6, switching from AM to FM seems to greatly increase sensitivity. I haven't had time to mull this over since observing it this morning, but will not be insulted if you explain why this may or may not be normal.

Another area I need to look into is that I'm not convinced either detector is properly doing its job, as there is sometimes distortion noticeable in the audio on both am and fm - but not always.

I plan on at least one more careful pass through the alignment process and will then see if these concerns disappear.

One more thing for you to provide your thoughts on:
When the selectivity switch is in either the Xtal Med or XTAL Sharp positions, things get microphonic and the audio sounds kind of hollow, like I remember some low-end Hammarlunds sounding back in the day.

As always, your thoughts are appreciated.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Dec Sun 31, 2017 12:52 am 
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You will notice some apparent gain difference on bands 5 and 6 between AM and FM because the FM detector strip uses two additional limiter stages prior to detection that are designed to strip any remaining amplitude modulation from the signal. Effectively think of them as overdriven amplifier stages where the goal is to drive them into saturation but for weaker signals they continue to increase the overall signal strength and for those you will notice a difference. But you should have plenty of gain for AM on those bands although outside of 10 meters there won't be much there.

I don't understand exactly why but I have found replacing the mica caps in the discriminator transformer for these receivers has been important for the FM detector to work as it should and I have assumed leakage although they don't show anything significant and the value hasn't shifted according to comparison with modern dipped micas using my universal bridge. But the first SX-62 I did about drove me crazy trying to get the FM detector aligned as I thought it should be and replacing the two 100 pf mica caps did the trick. I ran into an issue with the next one I did and I just replaced all four mica caps in the discriminator while I had it open and then it was fine also. I would love to understand exactly what the problem is because these caps appear fine for leakage and value and they don't have the classic high DC differential that would normally cause silver migration. It is possible that the problem is just environmental buildup in the can and I am removing that in the process of replacing the caps. In this case until someone can explain what/why it is happening I will stay in the role of empiricist and accept the results.

With the crystal filter set very sharp it will have a hollow ringing response and this is characteristic of this single element filter. It is one of its characteristics that makes it horrible for CW work with high QRN level. My experience with Hallicrafters is their filter setup usually doesn't provide quite this sharp of a response but it is pretty common with several of the National receivers when the filter is set up for sharp selectivity with a good clean (i.e. very high Q) element in place. Maybe you cleaned a little too well and you need a bit of Q killing grime back on the crystal element :)

Is your AGC issue on lower bands a lack of sufficient gain control or just a lower S meter reading than you expect?

Glad to hear that the receiver is singing for you again!

Rodger WQ9E


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Dec Sun 31, 2017 3:15 am 
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Chuck,

I have been experiencing distortion issues on my SX-62A as well which was also AGC related. My AGC voltage was drifting between -3 to -7.5 volts. To make a long story short, I traced it down to the two 6SG7, the 2nd and 3rd IF amplifiers. (The two tubes checked weak on the TT1 tube tester. So I knew they were suspect. )Fortunately, I had my NC-173 close by and borrowed a couple of good 6SG7 from it. The AGC stablized at around -7 volts and distortion went away.
I don’t know if this is of any value to you, at all. Just file as “for what ever it is worth”.

Good luck and still following closely.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Dec Sun 31, 2017 3:50 am 
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Gas in a tube, secondary emission (from contaminants on the control grid), and/or leakage between the control grid and other elements will cause the control grid to move towards positive. Distortion and associated AGC issues that show up as the set heats up are often from secondary emission but if excessive gas or grid leakage is present these faults will appear from the moment the set is turned on. Using a very high impedance voltmeter will often identify these tube faults in circuit and it is worth looking at the control grid voltage in the RF and IF chain to see if the voltage has gone the wrong direction or if it rises as the set is on for a few minutes. If you find the grid less negative on the tube side of the AGC filter resistors it is likely that tube is faulty.

A tube tester will often miss these sorts of faults so don't let the results from it mislead you. One of the issues I found in my recent HQ-180AX restoration was once it was in pretty good working shape I noticed a slight chirp on very strong CW signals. Using an external receiver to monitor the various receiver oscillators I found it was the final converter at fault. This circuit uses a 6BE6 tube operating as a converter (combined oscillator and mixer) at 395 khz. to convert the 455 Khz. IF down to the final 60 Khz. IF strip where most of the selectivity resides and sideband selection is accomplished. A new 6BE6 cleared this trouble but out of curiosity I stuck the old 6BE6 it in my B&K 700 which pronounced it fine and it probably would be for many circuits but not in that particular spot.

You will also see the same effect from external leakage often caused by careless spraying of lubricant or cleaners so that there is buildup on the bottom of the tube socket. This external buildup will create the same symptoms as grid leakage except it isn't cured by a new tube but a visual inspection will usually identify it as a culprit. I generously volunteered to help a new ham resuscitate a Heathkit SB-102 that he got for a low price and that poor thing had been soaked in some sort of cleaner/lubricant which turned it into a dust trap. I pulled the LMO, meter, relays, and AF transformer and then subjected everything else to a de-greasing bath followed by a distilled water rinse and a thorough drying. Although this sounds extreme I looked inside the unit and then pulled my DMM to make a resistance measurement from two places on top of one of the circuit boards where I wasn't touching any traces or components and with the probes an inch apart resistance was well under 1 meg and it certainly wouldn't get better with heat. It turned out to be a pretty good radio after a few minor repairs.

Rodger WQ9E


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Dec Sun 31, 2017 5:01 am 
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Chris and Rodger,

Thanks for the excellent homework assignments and explanations. All were very helpful. I'll of course let you know what I find.

I shouldn't have read your replies right before bedtime, as now I have to fight the urge to run out and check things out right away.

Wait, it's only 17° and falling. Never mind for now. :D

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Mon 01, 2018 6:32 pm 
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Rodger,

Good call on the SB-102... You know you're in serious trouble when you can measure high resistance leakage with a DMM, or any other measuring device that supplies low voltage to the item under test...
Glad that the rig responded to a decent cleaning.
It's amazing how little leakage it takes to make a device not work right: I DeOxited the pins on the squelch tube in one of my Gonset Communicator III's, and all of a sudden, the squelch stopped working. Let it sit for 24 hours, still not working... Spray a Fluorochlorocarbon cleaner on the socket and tube base, squelch action immediately restored.
I wound up sticking test leads from my GenRad 1862-B MegOhm meter into a pool of the DeOxit D5, and even with 500 volts applied, I couldn't see any leakage, and we're talking about resistances in the teraOhm plus range... Odd as heck, but even what unmeasurable resistance was there was enough to kill that stage.
I don't even want to think about what that 102 was or wasn't doing!
Thanks for sharing the tip on the four 100 pF caps in the discriminator... When I get around to reworking my SX-42's, I'll just order a handful of new SM's.
Like the ability not to measure the leakage on the Gonset squelch stage, it's irritating when you have a situation where everything measures right, but the parts/set doesn't work until you do X... I want to understand why that's happening, but if the answer isn't short in coming, it's OK just to know that if you do X and the problem is gone, it's OK to do that and move on.
Handpicking tubes is OK as well: You use what works in a particular application. I cherrypick 5749/6BA6's for the best performance in my R-390-A's IF deck. A tube that isn't quite up to snuff there will probably work fine in less critical applications.
IIRC, some equipment manufacturers had to handpick tubes to work with certain equipment.
Happy New Year to everyone!
I have the day off, and in my cleaning up around the shop, I found a URM-25F I didn't know I had... Christmas come late...


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Wed 10, 2018 3:31 am 
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In preparation for running some of the tests suggested above, I spent some time tracing out the AVC circuit in order to make some baseline measurements before actually checking what's happening at the various IF and RF points where AVC is applied.

Measuring at the junction of the two 47K resistors coming off pin 6 of the 7H7 AM detector (control grid being used as plate of detector diode, I think), I got the most negative voltage in the circuit at -9.6V. (Using Fluke 73 DMM)

Following the AVC path from there towards the first pick-off point, the first IF, the voltage drops steadily until at the IF side of R-41 it is down to less than half at -4.6V. This doesn't seem like much AVC action, but I admit I'm not sure what to expect- even after much further study.

I just throw this out in case anyone has an idea of what level of AVC voltage I should be seeing. This was measured with a strong signal and the RF Sensitivity wide open.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Wed 10, 2018 5:08 am 
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Chuck, I hope that I am not muddying up the works here, but as I recall, on my SX-62, the AGC voltages were measuring pretty much the same on the two RF stages and the first two IF stages. As far as I can tell there is no AGC on the 3rd IF stage. (I hope I am getting this right as I am going by memory, after measuring so many voltages.) Hopefully this makes sense. I was seeing voltages of around -7.5 VDC on strong local BC stations. If I fed a strong signal in to the first IF (with the signal generator) I could see an AGC voltage of -9.5 VDC or better. I hope this helps, and that I am not miss leading you.
Because I am working in a similar area, I look forward to what the experts have to say here.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jan Wed 10, 2018 6:02 am 
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Chuck and Chris,

The voltage at the AGC detector sounds reasonable but dropping off so much as you move down the bus doesn't seem right. Normally the AGC line impedance is so high that the AGC voltage is virtually identical across the AGC bus. The 1 meg resistors and associated bypass caps are used to keep RF off the line preventing feedback and oscillation but they should not provide a significant drop in AGC voltage. I suspect you will find either a hiding leaky capacitor or a tube that is providing some positive grid bias somewhat reducing the voltage on the overall bus and more dramatically as you approach the culprit. You will probably narrow the problem down when you start making measurements on the grid side of each 1 meg resistor.

I may have notes on the AGC voltage with my SX-42 stuff and I will check that in the morning but I am beat tonight. My daughter was invited to join the high school girls soccer team in their pre-season league and we just got home from that game. The good news is Anna scored her first high school soccer goal while still in middle school but the bad news is they lost 2 to 1 because she had her only team goal. But the team they lost to won their division last year so the girls are pretty happy about their play. It is going to be tough on Anna once the regular season starts because she has to just watch the high school players while she stays with the junior high team until next year when she does start high school but in the meantime she has 4 more games in this league and the coach told her she will be starting varsity next year so all of the work and practice has paid off. I am going to try to claim a little credit since I coached her team for 3 years when they were playing in a park district league but I am MUCH better at soldering than soccer so I don't deserve much credit :) It really is her skill and hard work, something I will think about next time I am stuck on a repair problem and am ready to throw in the towel.

Rodger WQ9E


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