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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jun Tue 27, 2017 10:48 pm 
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Location: Portland, TN, USA
Well, yeah! :oops:

I just didn't look close enough down in there. I thought it was a slug. :oops:

I must now take back half of the bad things I've said about Hallicrafters engineers.

I must also hold to all the GOOD things I've said about the good folks here who make the effort to help those like me who need it.

Norm you have brightened my outlook this afternoon. Thanks for the pictures.

Now for the surgery. Wait, I only do surgery first thing in the morning during the 15 minutes before my coffee wears off.

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73, de Chuck K4CCW

If talk is cheap, it's because the supply usually exceeds the demand.


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jun Wed 28, 2017 12:23 am 
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Chuck,

You aren't the only one to have been taken in by those mounting screws :)

As to coffee one of the people I worked with in risk management was a heavy duty cardiac surgeon and he wouldn't drink coffee (or any caffeinated beverage) for at least 48 hours prior to surgery to make sure that his hands were as steady as possible. Of course he did have a little more to worry about than burning a little insulation if his hand quivered a bit.

Rodger WQ9E


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jun Wed 28, 2017 1:39 am 
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Hi Rodger,
I think frustration at the end of a pretty long day played a big part in my missing the obvious. It's times like those that it's great to be able to put the problem out there so those watching over your shoulder can put you back on track. Sometimes when you're in the middle of the forest...

As for coffee, I limit myself to 2 cups first thing and no more caffeine during the day normally. Sometimes I think I want three but when I succumb to that urge, I very often will get minor shakes later that morning.

Tomorrow's surgery will have me mostly worried about those very fine wires on that coil as I try to repair it. I'm thinking that whoever added an SO-239 right next to that coil may have done the damage, but I'm not ruling out myself as a suspect somewhere along the line even though I try to be careful.

I tried to be especially mindful of those kind of wires when cleaning the bandswitch contacts. There are enough challenges in this radio without introducing more.

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If talk is cheap, it's because the supply usually exceeds the demand.


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jul Mon 03, 2017 9:46 pm 
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Surgery update:

Removal of T5 for examination was relatively painless. I just carefully marked each lead before removing it and moved each out of the way, leaving plenty of room to take out the T5 assembly. You can see the added SO239 connector on the left of the second photo.

Attachment:
File comment: T5 Before removal
2017-06-28 13.18.56.jpg
2017-06-28 13.18.56.jpg [ 68.47 KiB | Viewed 3650 times ]


Attachment:
File comment: Showing SO239
2017-06-28 13.19.06.jpg
2017-06-28 13.19.06.jpg [ 72.8 KiB | Viewed 3650 times ]


Two traumatic events seem apparent from the primary coil's appearance. I suspect it took a lightning hit based on the charred look of the primary coil, as well as the fact that a large part of it kind of slid down into the secondary coils on one side.

Attachment:
2017-06-29 09.26.33.jpg
2017-06-29 09.26.33.jpg [ 54.35 KiB | Viewed 3650 times ]


On top of this drooping portion was a blob of what I'm guessing is melted enamel. I was able to see one of the open windings and I suspect there were more hidden beneath the "slide".

Attachment:
2017-06-29 09.27.26.jpg
2017-06-29 09.27.26.jpg [ 55.88 KiB | Viewed 3650 times ]


Attachment:
2017-06-29 09.27.10.jpg
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2017-06-29 09.27.49.jpg
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Attachment:
2017-06-29 09.26.02.jpg
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Another lightning indicator to me was a splattering of tiny solder blobs on the side where the primary would have connected to terminal C of T5. These are visible in the first photo of the exposed coil.

The second traumatic event I suspect was caused by someone's careless work on or near T5 evidenced by bare, indented abrasions in several places on the primary coil. These shiny copper areas are visible in a couple of the pictures.

As for repairing T5, I would be little enough inclined to rewind this large of a solenoid-wound coil. This one is a pi-wound (or universal-wound if you prefer) and well beyond what I would attempt.

Fortunately, Gary Brown had what I need and will be shipping it as soon as my check arrives in a couple days. He was also able to supply a replacement Reception switch and a cover for the speaker terminals. He was very prompt in communicating with me even over the holiday weekend.

I'm still looking for a side cover for the RF section - or a picture or drawing that would help in fabricating something approximating the original. I imagine it would have holes through which one could make adjustments to the oscillator and antenna coils and caps for bands 5 and 6.

In the interim I messed around a bit with some attempts to just bypass T5 and feed the Band 1 signal from the antenna on to T11, but with only marginal success. Fortunately, it looks like none of the other transformers in the path suffered damage in whatever took out the T5 primary, not even T1 as near as I can tell so far.

The only other progress to report so far is a quick alignment of the 10.7 MHz IF. The BFO adjustment proved flaky and will need further investigation.

For now I'm going to take a break from this very challenging project over the holiday. Happy 4th to all.

Thanks for reading and for your comments and suggestions.

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If talk is cheap, it's because the supply usually exceeds the demand.


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jul Sun 09, 2017 12:47 am 
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A Saturday evening update (since I don't feel like actually WORKING on the '42):

After making myself a cross-tabulation chart between the Sams alignment info and
the info in the Hallicrafters Service Bulletin #94X242 that matches my receiver
(helps me familiarize myself with physical locations, etc.), my next step while
awaiting parts from Gary Brown was to pursue the BFO flakiness - possibly the hard
way.

It looked fairly easy to do so I removed the BFO xfmr coil shield can (don't forget
the side screw) so I could hopefully discover what was loose inside that was
causing the erratic BFO behavior during both IF alignment steps involving this
xfmr, T28.

When nothing obvious jumped out at me, I took the opportunity to check the mica
caps as well as the continuity of the coils. All checked OK and, in fact, the micas
were very close to stated values and passed the VTVM leakage test.

After carefully reassembling T28's innards and starting to re-attach the can to the
inner metal frame with the side screw, only then did I discover what was likely causing the flakiness all
along.

The internal metal bracket, via the side screw and the two studs that secure the
can to the chassis, provide the ground for the circuit elements in the can. What I
found was a heavy coating of the greenish-yellow oxidation(?) of some sort that was
likely making the ground circuit erratic at best.

The symptom had been erratic adjustment of either coil, as well as the can being very microphonic.

I cleaned off the mung off of the interior bracket as well as the interior mating
surface of the can, the screw threads (male and female, side and bottom srews)
before re-assembly.

Thankfully the T28 circuit responded to the TLC and now adjusted properly for both IFs.

While re-doing some of my earlier progress testing, I then discovered that C105, the
BFO control was very noisy as well.

Getting at this air trimmer cap is another project, as it is buried inside its own little
can cover, which is in turn hidden beneath a small metal cover behind, and beneath,
the bandspread dial.

To get at the BFO and Reception controls, I had to remove all the knobs,
remove the front panel and then also remove the bandspread dial.

I went ahead and did this since I will need access to the Reception control
to possibly replace it with the one from Gary. I say possibly replace the Reception
control because if its detent isn't any stronger than the present one, there won't be any point.

Back to the BFO control, my labors were rewarded by allowing me to see a large solder
sliver resting loose between the control can bottom side and one of C105's three
terminals.

Attachment:
2017-07-06 16.30.13.jpg
2017-07-06 16.30.13.jpg [ 88.47 KiB | Viewed 3575 times ]


Since it was loose, and since C105 moves slightly as one adjusts the
BFO pitch, it would easily explain the static sound during adjustment of the BFO.

While it was disassembled, I very lightly lubed the friction points in it as well
as the same points in the identical XTAL Phasing cap, C57, located nearby under the
same metal enclosure in its own little can.

Until the parts arrive from Gary, I'll be cleaning knobs and the front panel and
its windows.

Although I'll be going back to read some of the many do's and don'ts of cleaning these
things, I'd still welcome any advice (do or don't) anyone wants to offer. I really
don't want to damage the dials, as they're in good shape now but a little dirty.
The same is true of the front panel.

Thanks for reading and commenting.

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73, de Chuck K4CCW

If talk is cheap, it's because the supply usually exceeds the demand.


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jul Sun 09, 2017 6:32 am 
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Chuck, you have inspired me to get back to my second SX 42--which I had put aside a while back because I wasn't making progress. This evening I rewound the antenna primary for my Band 1, the BC band. The BC band was a weak band. Instead of 306 turns of very fine Litz wire, I used about 200 turns of coarser Litz wire. After unwinding, I slid two plastic washers over the 1/2 form and glued them in place where the old winding had been. This I used to constrain the new winding, which I just randomly wrapped until it reached the top of my washers at 200 turns.

Because the primary is untuned, it is acting like the primary of a transformer--which it is. Having fewer turns increase the voltage on the secondary--and decreases the resistance of the primary circuit by 12 oHms--just because there is less wire. It seems to work fine now.

The mistake I have made in the past when removing dial faces is that I try and mark them so they go back where they were when I removed them. This ignores the fact that many times they are in the wrong place to start with.

Norm

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Jul Thu 27, 2017 11:16 pm 
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072417 SX-42

While awaiting the parts from Gary, I experimented a little with doing some cleaning on the chassis. At first I just used GOOP white hand cleaner by hand on the tops of the choke and the 1st IF can. Just look at that shiny top on the IF can!

Attachment:
IF Cans Before & After.jpg
IF Cans Before & After.jpg [ 149.79 KiB | Viewed 3434 times ]


The impressive results led me to bring out the Dremel tool using the cloth buffer pads, which worked even better but was more limited in its access. Well, one thing always leads to another and I was soon researching the flexible-shaft tool for the Dremel. Never being one to pass up an excuse for a new tool I bought that and a couple buffer pads (Dremel # 511E).

The parts from Gary arrived promptly and exceeded my expectations. The speaker terminal cover was the complete box and hinged cover. I was only expecting the hinged cover.

The new T5 from Gary was in great condition and must have been carefully handled in the salvage process.

The Reception switch, S3, uses a completely different type of detent mechanism from my original as you can see from the pictures.

Attachment:
OLD SW-3 with weak detent.jpg
OLD SW-3 with weak detent.jpg [ 120.44 KiB | Viewed 3434 times ]


Attachment:
SW-3 Good Detent.jpg
SW-3 Good Detent.jpg [ 114.43 KiB | Viewed 3434 times ]


Since the new one provides a much more positive detent, I went ahead with the swap out - not a trivial task, at least at this point in life.

I had to carefully mark every one of the dozen or so leads and completely disconnect each one before the old switch could be removed.

Next came the tedious chore of carefully stripping and re-tinning each lead prior to reconnecting to the new switch.

Attachment:
SW-3 Removed.jpg
SW-3 Removed.jpg [ 162.03 KiB | Viewed 3434 times ]


Also done carefully was the cleaning off the old wire remnants from the new switch connections. The switch terminals are old and fragile - like me - and should be handled carefully. I even considered using the quig/Kwikette connection method to avoid the extra stress on the terminals, but ultimately decided the switch was in good enough shape to handle gentle removal of the excess solder and old leads. Especially helpful here was my Pace PPS-100 desoldering station, which seemed to handle the large solder blobs without clogging as some have reported.

At this point I thought I was ready to install the S3 replacement but found that I had to lengthen the original lead in order to be able to solder it to the lower switch terminal prior to actually mounting the switch, as there's no accessing it afterwards. Here, I did use the quig method covered with heat shrink tubing.

For those of you not familiar with this method, here’s one of many links that illustrate the tool and the process somewhat:

[url] http://www.antiqueradios.com/forums/vie ... 8&t=303863[url]

As a bonus for those who have read this far and who may have never seen the inside of the BFO control, C-105, here's what it looks like. The Phasing control is identical to this as well.

Attachment:
BFO Control C-105.jpg
BFO Control C-105.jpg [ 110.32 KiB | Viewed 3434 times ]

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73, de Chuck K4CCW

If talk is cheap, it's because the supply usually exceeds the demand.


Last edited by WoodchuckTN on Sep Sun 17, 2017 11:30 pm, edited 1 time in total.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Aug Wed 09, 2017 12:39 am 
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Chuck, thanks for the invite to see you're workshop and collection. I thoroughly enjoyed my time there. I have enjoyed reading this thread especially as I got a close up and hands on examination of your SX-42. Looking forward to working on mine. I loved the wood holder device and will soon be making one thanks to the many pics I took of yours. Thanks for letting me get detailed measurements and pics of the locking knob so I can try and make one for my missing one. Thanks for all the advice, parts and filling my SUV with toys (supper cool vintage test equipment) Looking forward to my next visit, will bring my circle glass cutter and glass to make any you might need for meters.

Bob

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Aug Wed 09, 2017 2:55 am 
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It was my pleasure all around, Bob. Thanks for pulling the side RF deck cover on your '42 so I could make a measured drawing of it in preparation for trying to make one for mine. I'll post the drawing and whatever I end up doing to make one - unless I'm able to find one first.

I hope you'll post your process in making the brake lock knob as well as your glass cutting process, as this is a frequent need here on ARF. The latter might turn out to be a cottage industry for you.

After seeing some of your restorations in the YouTube videos you sent me links for, I can hardly wait to see what you do with the Boonton, GR, and HP stuff we ballasted your SUV with. :D Glad to find someone who appreciates the old stuff and is young enough to do something about it.

I've gotten the new Reception switch installed now as well as the new T5, but between summer chores, travel, and upcoming family and eclipse company, it's been hard to find time to get back to this project.

How did I ever get anything done when I was still working for a living!

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If talk is cheap, it's because the supply usually exceeds the demand.


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Aug Mon 28, 2017 9:20 pm 
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Next step: New brake shoes

Before reinstalling the front panel and knobs, I took closer look at the tuning gear assembly and was convinced I ought to tackle the brake shoe issue next. I had earlier visions of addressing that later, as it looked like there were only two screws holding each half of the shoe assembly to the main part of the tuning gear, and that a flex shaft screwdriver could take them loose without the need to remove the complete gear assembly.

It turns out that would have been a very bad idea because at least two of those screws required all the leverage I could muster, even with full access and a regular screwdriver, once the complete assembly was removed. Of course the better access also allowed for much easier cleaning of other parts of the assembly.

I’ll break this process down into two posts since I think there’s a 10-picture limit per post. I apologize if there are too many pictures for the tastes of some of you old-timers, but several have indicated they find them useful. I’ve even gone back to parts of my own thread for reference.

To begin this part, it should be said that the gear assembly (at least mine) does not come out without a little friendly persuasion.
This photo shows the shaft couplings which I loosened on both ends of the coupling, 4 set screws per coupling.

Attachment:
2017-08-18 10.41.37.jpg
2017-08-18 10.41.37.jpg [ 117.36 KiB | Viewed 3197 times ]


I then lifted the front of the assembly enough to allow the flywheel to clear the main chassis, allowing the assembly to move further forward until it’s stopped by the main chassis again. At this point, the back end of the shaft couplers had not cleared the capacitor shafts and it looked like the tuning and bandspread caps would have to be loosened.

Per the Hallicrafters 4-page instructions for this assembly, I tried loosening the caps’ mounting screws. This alone did not allow movement of the caps and I did not wish to risk their total removal, so I abandoned this approach.

By carefully applying pressure on the rear end of each coupling I was able to make them come forward enough to clear the caps shafts and bring the gear assembly out. I will say that the pressure required was enough that I had some concern the coupler might break. I used only my fingers to apply the pressure though, not any tool. Here's the assembly as removed.

Attachment:
2017-08-18 11.28.05.jpg
2017-08-18 11.28.05.jpg [ 113.73 KiB | Viewed 3197 times ]


This is the rear of the cam part of the drive showing the spring that applies the pressure on the bandspread lock wheel when the lock knob is turned to that position. The spring is 1/8” x ¾” overall.

Attachment:
2017-08-18 11.29.13.jpg
2017-08-18 11.29.13.jpg [ 85.48 KiB | Viewed 3197 times ]


This view shows the corrosion in the area of the brake shoes, apparently caused by either the original brake shoe material or its adhesive or both. I’ve seen pictures of much worse damage in this area so I’m glad mine wasn’t too bad.

Attachment:
2017-08-18 11.29.58.jpg
2017-08-18 11.29.58.jpg [ 81.6 KiB | Viewed 3197 times ]


Note, the corrosion even extended to the brake wheel itself where the shoes had rested:

Attachment:
2017-08-18 14.10.24.jpg
2017-08-18 14.10.24.jpg [ 87.64 KiB | Viewed 3196 times ]


Here you can see both the corrosion and the accumulated crud that 70 years brings.

Attachment:
2017-08-18 12.20.04.jpg
2017-08-18 12.20.04.jpg [ 52.21 KiB | Viewed 3197 times ]


But it’s nothing that a little elbow grease won’t cure.

Attachment:
2017-08-18 12.30.29.jpg
2017-08-18 12.30.29.jpg [ 57.12 KiB | Viewed 3197 times ]


At this point the question becomes how to re-line these brake shoes. I’ll show you one old man’s approach in my next post which should be forthcoming within the hour.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Aug Mon 28, 2017 10:33 pm 
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The final decision as to what material to use in lining the brake shoes resulted from first researching back through the many links to others’ restorations I’ve accumulated over two years of procrastination. (I’ve actually justified this as my re-training period:D )

I actually got the idea for using heavy heat-shrink tubing from Wayne Rosberg. He doesn't go into it in his excellent restoration blog,

http://ccts-inc.com/SX42/

but he told me about it during our email communication about some other parts of his restoration.

In his case, he used heat shrink that had its own adhesive. My approach, using what I had on hand (regular heavy heat shrink), was somewhat different.

Here’s the kind I used:

Attachment:
2017-08-18 14.00.27.jpg
2017-08-18 14.00.27.jpg [ 86.37 KiB | Viewed 3195 times ]


I first cut a short piece to length, leaving maybe 3/16” overlap on each end of the shoe.

Attachment:
2017-08-18 14.01.50.jpg
2017-08-18 14.01.50.jpg [ 44.54 KiB | Viewed 3195 times ]


Next, I used my flush cutters to cut out a window in the back side just very slightly larger than needed to allow room for the main arm of the shoe assembly to fit through.

Attachment:
2017-08-18 14.04.38.jpg
2017-08-18 14.04.38.jpg [ 61.09 KiB | Viewed 3195 times ]


Sliding the tubing over one ear of the shoe first, I then grabbed the other end of the tubing and stretched it gently until the window slipped over the other ear. The natural elasticity of the tubing made for a pretty good fit even before applying heat, as shown here:

Attachment:
2017-08-18 14.05.37.jpg
2017-08-18 14.05.37.jpg [ 56.63 KiB | Viewed 3195 times ]


Next came the heat application using my trusty BIC fire-starter, with this result:

Attachment:
2017-08-18 14.06.28.jpg
2017-08-18 14.06.28.jpg [ 51.07 KiB | Viewed 3195 times ]


Attachment:
2017-08-18 14.06.40.jpg
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This is the look after trimming:

Attachment:
2017-08-18 13.45.50.jpg
2017-08-18 13.45.50.jpg [ 68.65 KiB | Viewed 3195 times ]


Attachment:
2017-08-18 13.46.50.jpg
2017-08-18 13.46.50.jpg [ 52.83 KiB | Viewed 3195 times ]


Attachment:
2017-08-18 13.46.04.jpg
2017-08-18 13.46.04.jpg [ 63.28 KiB | Viewed 3195 times ]


And after re-assembly:

Attachment:
2017-08-18 14.27.27.jpg
2017-08-18 14.27.27.jpg [ 96.27 KiB | Viewed 3195 times ]


And, as Sean Connery once said, "There endeth the lesson."

As with all the other re-lining methods I researched, we (or our heirs and assigns) will just have to see how well this does the job.

Now that all is reassembled, it's about time to power up again and see we have PROgressed or REgressed. Gentlemen (and ladies), place your bets!

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Aug Mon 28, 2017 11:30 pm 
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I like it!

Norm

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Sep Fri 01, 2017 2:41 pm 
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Yeah Chuck that does look good. I was going to try the Plastidip method but think I will try the shrink wrap as well after seeing this. I think I have black in several sizes. I would like the looks of black better as if anyone will really ever notice. ;)
Have you tested it yet? Looking forward to it.

Bob

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Sep Fri 01, 2017 3:34 pm 
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Hi Bob,

I would likely have used black too but all I had of the thicker type in the right size happened to be red.

Lucky for me I'm not nearly as fussy about cosmetics as some people I know. :D But, that's why your projects always look so good.

As for testing, there seems to be some sort of creative procrastination going on around here. I spent several hours in the shop yesterday but instead of working on the SX, I spent the time building and installing a new shelf above the bench but below the four already there. This was inspired by two Simpsons acquired last week, a nice RotoRanger and a 360 digital readout.

Perhaps this rainy day will see the resumption of the SX project.

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If talk is cheap, it's because the supply usually exceeds the demand.


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Sep Mon 18, 2017 12:21 am 
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You'd think an old retired guy like me could just jump on a project like this and 'git 'er done' but retired life is definitely not just composed of one long radio shop session - at least for me.

Besides the seeming need to "be in the mood" to wrestle with this beast, there's the matter of hurricanes, attacks from a swarm of bees, travel with the LOML, etc., that come between me and the finish line on this one. I'm also beginning to believe that there's never really a finish line with these old gems, just "that's good enough for now".

Enough already with the philosophical rumination and on to a sort of interim update.

Before reassembling the front panel, the main tuning and bandspread escutcheons were cleaned. The clear plastic window portions were cleaned and polished using Novus 1 & 2, being careful not to disturb the red centerline markers.

Like remodeling a room, once you clean up one part on a radio it's difficult to find a good stopping point. Consequently I went ahead and cleaned the entire front panel using Goop white hand cleaner, proceeding very gently over each of the painted lettering areas.

Very pleased with those results, I then moved on to cleaning up the knobs, moving from left to right.

Somewhere in all my reading I'd picked up the suggestion to use Mother's MAG & Aluminum Polish for aluminum items. I can report it works very well on the aluminum portions of the knobs, but is pretty labor intensive for these old hands. They only allowed me to get as far as the volume control before requiring a rest.

To allow my hands to recover, I decided to install the remaining knobs without cleaning for now so I could fire up the receiver and let it burn in while I cleaned the remaining knobs. That way I could listen to ham and short wave traffic while working.

I was pleased to see that the AM broadcast band now worked well with the new T5 in place, even before realignment.

However, when I moved to band 2 to listen for ham traffic on 80 m CW, the CW note was not clear, but sounded like there is hum modulation of the BFO. It affects both the CW note and SSB reception.

Changing the 7A4 BFO Osc/S-meter amp did not change the symptom and this tube's voltages measure close enough to the manual to be OK.

So, for the moment I'm chasing that issue, as well as a lack of a well-defined tuning peak on S-3 and S-5 during IF alignment. For the latter, I'll be checking the caps in those IF transformers.

Feel free to share your ideas.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Sep Mon 18, 2017 1:20 am 
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"My biggest problem was with the BFO. It worked for the lower bands and I was able to get a wobbly beat note, but nothing for the 10.7 MHz IF. This caused me to take several hours and check and replace parts in the panel Pitch cap area and then inside the can. I still could not get anything for the higher IF. At last the scope came out and I hooked up to the BFO side of the 7p coupling cap at the detector plate. I could see the signal and it adjusted as I adjusted the BFO slug, until I heard a faint beat note. The BFO worked as long as I kept the scope attached. I ended up adding another silver mica in parallel to get some better coupling and the BFO now works at both IF's but the upper one is pretty weak. Neither wobble, probably a result of working through so many of the solder joints and replacing some components. I have one other 7A4 BFO tube, and it is the one I bought to use as a strong replacement, but with it in the radio, the higher BFO still won't work. I think I need to get another 7A4."

That was from my post a few months ago on the second SX 42 I restored.

I have had other radios with weak BFO's and for those, I increased the plate voltage by using a smaller plate resistor. That worked quite well. If I were you, I would at least check the resistors associated with the 7A4 tube to be sure they haven't drifted high.

Norm

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Sep Mon 18, 2017 3:11 am 
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Thanks for the insights, Norm. I'll definitely have a closer look at the BFO circuitry for starters. Thankfully, it's a lot more accessible than mucking about in the mixer/oscillator circuit.

There's something else that may be a clue, but its import escapes me so far. When I was measuring the 7A4 voltages with a Simpson 260 (20k ohms/volt as per Halli. instructions), when I touched the probe to either the plate or the grid, I'd hear signals from the speaker just as if I'd connected an antenna to the front end of the receiver.

Perhaps this is normal, it's just that I've not experienced it since I've gotten back into tube receivers.

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Sep Mon 18, 2017 1:24 pm 
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Chuck,

Listen to the HFO signal in another receiver with the test receiver's BFO on and tuned for a low pitch. Often the hum modulation originates at this stage (usually HFO but sometimes the first mixer can also cause this problem given the low signal levels at this point) but isn't readily noticeable without the BFO on. Another way to troubleshoot this area and eliminate possible sources is to temporarily disable the internal BFO and use your signal generator to provide BFO injection and see if the hum modulation is still present.

The problem with hum modulation is it often isn't readily apparent without the BFO on which can lead to focusing upon the wrong stage.

Thanks for providing such a nice detailed log of your SX-42 restoration!

Rodger WQ9E


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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Sep Tue 19, 2017 1:54 am 
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Thanks Rodger. I was hoping you would weigh in on this too.

Some days I think I may have gotten back into this hobby too late. Case in point: I'm pretty sure I did the first test you suggest yesterday before I quit. The problem is, I can't remember for sure :roll: and when I got around to having time to check my notes (which I usually keep religiously for this very reason), I could find no indication of such.

So, I'll have to do it again. That's one of the reasons I find it so frustrating to work with me sometimes. :D

I've made a list of the suggestions offered above and hope to try them in the morning when all three synapses are firing.

Thanks also, Rodger, for the encouragement on the log aspect. I always agonize over how much to include but usually err on the side of the thought that the reader is always free to skip what they deem not useful. I try to include things I've learned from others along the way that I've found useful.

Edit: Anyone care to weigh in on the antenna effect mentioned in my previous post?

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 Post subject: Re: SX-42 Begun - At Last!
PostPosted: Sep Wed 20, 2017 10:51 pm 
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I'm happy to report that the hum modulation problem has been resolved. The 7F8 HF oscillator signal (AM mode, Band 2) when monitored on an external receiver exhibited the garbled/hum modulation clearly. The BFO signal however was a nice' clear note in the external receiver. The little DX-400 is a very useful tool for troubleshooting oscillator issues in comm. receivers because of its frequency range and digital accuracy ability.

Replacing the 7F8 oscillator/mixer resolved the hum issue.

Having just a little more time before quitting for the evening and with the DX-400 still set up for monitoring , I did a quick HFO operational check on bands 1 - 5. All worked well except band 5, which was as dead as last year's birds nest.

Thus the next step will be to resolve at least three issues: no sharp adjustment peaks on S3 (T25) or S5(T26), and no oscillator activity on band 5. This latter problem rings a bell that I need to go back to a thread some time ago when Rodger and I were trying to help Aidan up in Canada with his SX-42. I seem to remember this was one of the issues he faced.

Stay tuned. Also still wondering about the antenna effect mentioned two posts ago.

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