Forums :: NEW! Web Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Oct Wed 27, 2021 6:01 am


All times are UTC [ DST ]





Post New Topic Post Reply  [ 28 posts ]  Go to page Previous  1, 2
Author Message
 Post subject: Re: ** SX-17: need pix of the insides! **
PostPosted: Sep Mon 20, 2021 3:59 am 
Member
User avatar

Joined: May Wed 20, 2020 1:33 am
Posts: 400
Location: Rockford, IL
I'll check out the BFO thread. I worked on my S-20 BFO for a while and had a S-20R to compare to at the same time. I got it working rather well. I know the BFO tube I have in it is strong - but I'm going to fix the power caps and replace all of the 10pFs with some new Mylar 10pFs and go over the pots and switches before I get too deep into a full alignment. I normally like most of the changes done before alignment.

And I am thinking I need to do IFs and alignment to crystal. But its about 85% there for a nice great condition set. With my PM-23 (with "h") speaker I am thinking it is done and can join my other sets on the working radio racks.


jim rozen wrote:
See the story in the SX-11 threads about the BFO injection gimick. Often this needs to be increased to read SSB signals.


Top
 Profile  
 
 Post subject: Re: ** SX-17: need pix of the insides! **
PostPosted: Sep Mon 20, 2021 2:44 pm 
Member
User avatar

Joined: Oct Thu 04, 2018 2:11 pm
Posts: 825
Location: Suburban Chicago
I followed Jim's advice on an SX-11 I am restoring and it worked great. Originally the BFO did not seem to need any help. At some point however I discovered that I had installed a dead tube in the RF amp socket instead of the replacement that I had ordered for it! The radio worked surprisingly well with the dead tube but once it was replaced the BFO was too weak even when maxed out. There was no SSB when these were designed so they are set up to do CW. Once you have enough BFO level they work great on SSB. I have to try this some day on my S-38. It will never be as good as the SX-11 since you cannot adjust the RF sensitivity or BFO level. So far I have been unable to copy more than a word or two of SSB with it and I am hoping that if I boost the BFO coupling I will be able to get good SSB copy over at least a small range of RF input levels.


Top
 Profile  
 
 Post subject: Re: ** SX-17: need pix of the insides! **
PostPosted: Sep Wed 22, 2021 2:50 am 
Member
User avatar

Joined: May Wed 20, 2020 1:33 am
Posts: 400
Location: Rockford, IL
Sorry I just read this: I find that a signal tracer makes finding dead-ends in radio circuits easy. Basically you can hear where the radio signal drops off.

If the medium wave section is not working, give the switch a look. It multiplexes the incoming signal to the various coil bands.
Go from the mixer with a tracer to see if the signal is being brought into the set and converted to an IF you can demodulate with the signal tracer. Then go tube to tube and to the IFs so see where the chain stops. I had a S-20 I fixed this way and found the dead spot in about 2 minutes. In my case, it was a dead IF. I replaced it with another second stage IF, matched the wiring and the set came to life again.

Worse case you have a dead coil or IF transformer and that's why the band is dead. Best case is the wafer switch lost its tang or its just dirty and needs a DeOxit (Fader F5) cleaning.


yahoomarine wrote:
Hello. No help on the SX17 but I am plodding thru an SX16 with good results and need some guidance from someone with SX16 experience.

Mine is clean but had a bad primary on the first audio stage transformer. That caused a 10k resister to smoke, raised the grid V on one of the 6V6 tubes to +27 VDC and shorted it out. I tried a mod I learned from a Philco model 20 cathedral radio using a .1 cap to bypass the primary completely and it worked! Great audio and happy 6V6 tubes.

Here is the last big issue... The AM broadcast band is pretty much dead while the radio works better than expected on the shortwave bands after just a minor alignment. I connected the antenna to the grid cap on the 6L7 mixer tube and the AM band boomed in. However, as expected, with the antenna connected there the SW bands die out.

I also tried connecting the antenna lead to the grid cap of the 6K7 RF amp. No luck. That leads me to believe the issue lies in the selector switch after that section. The problem lies in the placement of that 6K7 as the socket sits beneath the really busy rack with the selector switches and coils plus a few tightly placed wax caps so I cannot get to it safely with a meter probe to check the voltages.

Since the radio works so well otherwise I fear that I may make matters worse if I really dive into that tight spot. As I am slowly replacing caps and bad resistors I likely will leave the wax caps in the bad places alone.

With that background and your experience with SX16s do you have any thoughts?

Thanks, Rich


Top
 Profile  
 
 Post subject: Re: ** SX-17: need pix of the insides! **
PostPosted: Sep Wed 22, 2021 3:08 am 
Member
User avatar

Joined: May Wed 20, 2020 1:33 am
Posts: 400
Location: Rockford, IL
I have been messing around with my SX-17 to assess how much I need to do to it before I clean it up. The BFO is pathetically weak and non-existent at best.

Has anyone had to work on the BFO of these sets?


Top
 Profile  
 
 Post subject: Re: ** SX-17: need pix of the insides! **
PostPosted: Sep Wed 22, 2021 5:33 pm 
Member

Joined: Jun Sat 18, 2011 5:14 pm
Posts: 318
Location: Washington State 98226
The BFO on my SX-17 works exceptionally well. However when I restored the rig I found that the BFO circuit did not match the schematic.

Here are the two schematics:

Attachment:
SX-17 BFO Schematic.pdf [83.33 KiB]
Downloaded 6 times


This is the as-built schematic:

Attachment:
SX-17 BFO Schematic As-Built.pdf [127.58 KiB]
Downloaded 8 times


You have to look closely at the two schematics to see the difference. It has to due with where R21 is tied to. Originally it was tied to the ground... in the as-built it is raised above ground potential and tied to the cathode. I am not sure how much difference this makes.

When I rebuilt the BFO circuit I stayed with the as-built circuit as it appeared to be original (I did not rewire to match the original schematic).

Here is a picture of the rebuilt BFO:

Attachment:
BFO rebuilt.jpg
BFO rebuilt.jpg [ 567.94 KiB | Viewed 137 times ]


Like I say the BFO works very well... so maybe it works so well because the circuit was modified some time in the production run.

John w.


Top
 Profile  
 
 Post subject: Re: ** SX-17: need pix of the insides! **
PostPosted: Sep Thu 23, 2021 4:45 am 
Member
User avatar

Joined: May Wed 20, 2020 1:33 am
Posts: 400
Location: Rockford, IL
Now after seeing the insides of the BFO box I have to get mine off and fix it...I just don't look forward to dealing with the braided shield cabled that is soldered to the case.

I worked with the set most of today to clean it up and give it a started alignment to see what I am working with.

My XTAL is at 465.300 Khz. I was pleasantly surprised it was at the value (basically) stamped on it after 80 years. The first alignment was so-so. I can tell I have some work to do as the broad IF works good but sharp cuts the output down to nearly nothing.

The BFO is non-existent and now I am getting sucked into a rebuild. But the set is in great condition and its close.

I did have a 6K7 pop on me and cause about 30 minutes of confusion. I probably have 50 of them sitting around here so I picked another strong one and was one my way again.


Top
 Profile  
 
 Post subject: Re: ** SX-17: need pix of the insides! **
PostPosted: Sep Thu 23, 2021 6:49 am 
Member

Joined: Jun Sat 18, 2011 5:14 pm
Posts: 318
Location: Washington State 98226
Quote:
The first alignment was so-so. I can tell I have some work to do as the broad IF works good but sharp cuts the output down to nearly nothing.


That is interesting... mine behaves just the opposite. In the Broad position the signals are much more attenuated and in the Sharpe there is much more gain. I originally thought maybe I had the switch in reversed but when I looked at the IF with the spectrum analyzer I could see that the Broad was around 6Khz wide and the Sharpe was about 3khz wide.

Broad:

Attachment:
IF Broad Selectivity.jpeg
IF Broad Selectivity.jpeg [ 115.72 KiB | Viewed 121 times ]


Sharp:

Attachment:
IF Sharpe Selectivty .jpeg
IF Sharpe Selectivty .jpeg [ 119.08 KiB | Viewed 121 times ]


John w.


Top
 Profile  
 
 Post subject: Re: ** SX-17: need pix of the insides! **
PostPosted: Sep Fri 24, 2021 2:31 am 
Member
User avatar

Joined: May Wed 20, 2020 1:33 am
Posts: 400
Location: Rockford, IL
After seeing the insides of your BFO dog house, I had to get into mine. Glad I did - I removed the two wax crayon caps and replaced them with a couple of nice new ones. The resistors looked new and measured nearly right at nominal so I did not replace them.

At first it didn't work at all - the BFO had no effect. Then I realized the tube was not put back and then the BFO worked like another volume knob and no pitch at all. Then I realized the transformer might not be adjusted and a slight adjustment the BFO came in full blast and strong. I tried this adjustment before the cap change and it had no effect. So now the BFO is working strong.

I did replace a bunch of resistors that were over spec and the set works much better - so I gave the chassis a cleanup and shine to prep for the battery of alignment drills to swing the set back into a good alignment.

I had a novel way to tap into the R25 the audio detector load resistor on SX-28s for IF alignment so I could use a VOM to peak the IFs. Doing it this was with SX-28s is the only time I did an IF alignment I felt really it really worked well.


Do you have such a procedure for the SX-16 to align the IFs? I already have the frequency my XTAL runs at (465.30 Khz) and would love to get the IFs set perfect.


**** EDIT:

I tapped into the AGC line across R14 to peak my IFs using a 10M DMM and appears to have worked great. The set is snappy and sharp. That is the only indication that I have it worked - I used this same process to align SX-28s and those come out great every time (big thanks to N6PY for the nice write-up on SX-28 alignment: http://pcbunn.cithep.caltech.edu/jjb/Ha ... CEDURE.pdf).



Gasman wrote:
Quote:
The first alignment was so-so. I can tell I have some work to do as the broad IF works good but sharp cuts the output down to nearly nothing.


That is interesting... mine behaves just the opposite. In the Broad position the signals are much more attenuated and in the Sharpe there is much more gain. I originally thought maybe I had the switch in reversed but when I looked at the IF with the spectrum analyzer I could see that the Broad was around 6Khz wide and the Sharpe was about 3khz wide.

Broad:

Attachment:
IF Broad Selectivity.jpeg


Sharp:

Attachment:
IF Sharpe Selectivty .jpeg


John w.


Top
 Profile  
 
Post New Topic Post Reply  [ 28 posts ]  Moderator: Sandy Go to page Previous  1, 2

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 7 guests



Search for:
Jump to:  


































Privacy Policy :: Powered by phpBB