FM Alignment & Restoration 75 Yr Old Zenith AM/FM Vacuum Tube Radio - 1949 Part 3 Model 7H921 [4K]
ฝัง
- เผยแพร่เมื่อ 5 ก.พ. 2025
- Finale 1949 ZENITH AM/FM Tube Radio I picked up a few years ago finally on the bench. In Part 2 the AM alignment was completed and now we move on to the FM alignment - and uh-oh! Did this radio project just brick? Includes FM band alignment, FM detection, FotoPhalse, design problems, history of FM radio, etc. Enjoy Part 3.
See the checklist which appears when status changes, for tracking what is discovered that needs addressing in Red, followed by Green when resolved.
NOTE: This is not instruction, it is for entertainment for people like me who enjoy watching projects and the experiences gained. If you attempt to do any of the things in this video you are at your own risk. I am using special tech-bench safety equipment not detailed here so don't take for granted you can just plug these old items in safely without them being checked and repaired correctly. Check out the link below to Mr Carlson's Lab video below.
ACKNOWLEDGEMENTS AND REFERENCES:
[note: am in no way representing how these folks have shown doing these things - I don't even think they would approve.]
Schematics: Sams Publishing , Riders
Thanks to shango066 for championing the approach of "fix it first", before mass re-capping. See shango066 for many examples of that approach.
@shango066
Awesome discussion of bench safety including variacs and isolation transformers by Mr Carlson's Lab (highly recommended):
• Tech Tips Tuesday, Iso...
@MrCarlsonsLab
Modification method for a commercial Tripp Lite isolation transformer for tech-bench use by Todd Harrison, his channel is called ToddFun:
• ToddFun.com: Isolation...
@ToddFun
Camera: Canon G7X Mark III
Mic: Rode Micro
Video Editing Software: Power Director
Video Editor: just me
People interested in the following topics may be interested in the subject matter of this video: 1940s radios, Zenith Radio, Bakelite Radio, 1950s radios, tube radio repair, tube radio restoration, vacuum tube radio recapping, silver mica disease, SMD, silver ion migration, IF can repair, IF transformer repair, Radio, Zenith tube radio, radio recapping, valve radio, Vintage Radio, antique radio, old radio repair, vintage radio, shango066, mr carlson, radio repair and restoration, 1950s radio restoration, tube radio restoration, radio restoration videos, all American five radio, aa5, aa4, aa6, buy it fix it, repair cafe
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That's a Bakelite radio 📻. I think Radio 📻 Rescue worked 💪 on one exactly 💯 alike that one. Best 2025!! Your friend, Jeff!!
Thank you , still waiting on the sams, I will check it out.
I know I made a great comment but it was zapped out maybe because where on the web where one can get replacement
set screw type knobs. The replacement dial cover was sold by a great guy on the web with lots of dial covers. As for the
rest well you did a great job with a very thorough alignment. Exceptional well receiving radio PF. Might have been a long
video but was worth it! Steve from IL
Thanks Steve. Yes it was long but I believed the problems I dealt with at the end were worth covering and I didn't want to make a 4th part. People can always pause and come back later, I do it all the time out of necessity.
Where to find this knob with set screw: I don't know, and yes they are still cr@ppy for that application, in the meantime I hope it works as well as it ever did. I think the shaft choice for that band switch was the core error. That type shaft should have been a round shaft with a flat or a D shape as an internal engineering application standard for band switches. We are left with how to deal with that design mistake.
Dial cover: yep, I gave Mark's url for Dialcover dot com on screen at 54:47 and said how much I liked it. He provides a great service.
[grump on] Am still grumpy about the YT copyright hassle. Wouldn't anyone want to be able to show the way the FM receives music, esp compared to talk which is about all we get on AM where I live? Who cares if Sade, or some CW person I have never heard of, could be heard for about 6 seconds? Worst hassle I ever experienced and it ate up even more time on editing and uploading, the least fun part of this. [grump now turned off] Thanks again.
@@PracticallyFixed There is a TH-cam music library that you can peruse which would allieviate those music copyright problems for the most part. A guy named Kevin MacLeod is one of those who I give high recommendations to put on a video but I'm not sure if you want to superimpose YT creator music over that which you are really listening to from a radio! LOL.
Zowie! The sensitivity difference was unreal! But man you had to work for it. I’ve never attempted FM alignment. I don’t believe I can do it with my DSO. Or can I? I know I can’t sweep with my fluke 6061 but my tiny sa is supposed to be able to. Really over the top with the new dial glass. My example here’s dial glass has shrunk nearly 1/4 inch all the way around and foggier than my early years. Fantastic work and great lesson! Thanks much
Hey man, thanks! Yes it was a lot of work. Concerning the sweep, I have a TinySA my son gave me for Christmas, but I don't think it can do it without some work which I saw a guy do using a mixer and figuring out a signal to feed from a signal generator. The problem is the TinySA can sweep and can give you a bode response plot, but it doesn't do both and doesn't have a trigger so it can't do it itself apparently. Hopefully that will be remedied very soon with a tracking generator function add-on.
Have a couple of ideas in the meantime..
1) your Fluke and DSO *might* be able to do it. I have an HP signal synthesizer above my bench which does sweeps. It also will put out a (single) marker which goes into the Z-drive of a scope, if it has one. I don't know if your Fluke does that, or if your DSO has a Z-drive. The way the HP would do it is it draws the sweep on the screen via X-Y on the scope, and at the marker frequency on the X axis, the intensity of the trace changes from bright to dim, or visa versa. I haven't tried it as I use the SG165 instead. BUT...
2) perhaps a better idea is: if you can find a relatively inexpensive vintage FM-TV sweep generator and use a separate signal generator, you can sweep with the TV sweep generator around 10.7MHz and use your other signal generator to place a distinct blip on the marker frequency of your choice. I have a HeathKit one (TS-4) and may try it. One of my favorite YT guys showed this recently on a nice Fisher mono FM receiver which has a bit more complicated and different alignment procedure order than Sams gave me. (It has the IF can adjusted while looking at the S curve from the ratio-detector) He usually uses a Sencore SG-80 or SG165, or a ST1000) he then uses a TH distortion meter. He even shows how to fight the sample rate on the DSO. @xraytonyb great channel.
th-cam.com/video/MryYk4Sr4lc/w-d-xo.html
Thanks again!
I have two of this same radio. After trying to align the first one, I opted to leave the second one alone. Thanks for the great video though; lots of very useful information on how it should be done. Thanks,Paul W
Thanks Paul!
Thank you for the information I have one more question. What is the brand of flexible glue you used on the speaker, mine is showing some little splits around the outside edge. Thank you
Giving credit where due, I got this from Don at @RestoreOldRadios some time back. It is Aleene's Original Tacky Glue (I get at Hobby Lobby) with a bit of water. Watch his video below to see the mixing and how he also uses to apply patches when needed. Thanks for watching my video and don't forget to subscribe and like.
th-cam.com/video/XsKWZ0Plbso/w-d-xo.html
Thank you@@PracticallyFixed
❤❤❤❤❤❤🤝🤝🤝
I do not have a Sencore SG165 how else can I do a FM alignment, I do have an RF generator thank you bob
@@robertdreitlein6613 Hi Bob. FM alignment is relatively difficult compared to the AM alignment. On the last page of the Sams it shows the procedure for doing the alignment using an AM signal generator and a VTVM. The adjustment of the discriminator will be the most difficult and I have heard some people draw a plot on graph paper to see if they are getting the even voltages on either side of "zero" and if the plot is essentially straight. You may be able to find a video of someone showing that method. Maybe try leaving it as is and see how it does! If it hasn't been touched since it left the factory and components in those sections are the same, it may not really need anything. Good luck.
Great work I am working on the same radio, Can you tell me the sams folder the diagram is in. also where did you get the plastic lens cover from, mine is shrunk. Thank you
Hi. Please check my response to your question on Part 1 of this series. Thanks
I have the same radio where the FM is not up to par. I wanted to do an FM alignment but I don't have a sweep generator. I used the AM generator method where you just peak the coils. This worked out very well for me with a nice sounding FM now. So I would like to know is the method you used really necessary and does it make the FM perform better over the AM method? Sure seems like the AM method is a lot quicker and easier than the FM method. I think I would be a little afraid to try either method on a high end FM radio but I might try the AM method over the FM method. I noticed your wave forms were not very close to the examples. Nice explanation though. BTW I have found Sams to have errors in just about everyone I have used.
Hi and thanks for watching and for the comment. I wish I could have left the music in because when you can hear it, it sounds really nice. Yes of course you can use the AM generator method as shown in Sams as well as Riders, RCA Service, etc. instead of the so-called visual method. (I also would want to use piece of dedicated equipment for multiplexed stereo FM in ~1965+ high end gear.) I believe the visual method "should" give the best result in theory, as I understand it, but sweep generators can be hard to come by and are expensive, or like mine, a little buggy. I am just working with what I have.
The part of the visual I am trying to achieve is the available FM bandwidth, or FM deviation. The 'ideal' shape would give a relatively wide and even band pass for the IF to allow the FM to go through without being cut off too fast on either side of 10.7MHz IF. What I do not know is if, in 1949, the broadcasters' signals used more compression and thus less deviation (deviation from 10.7 contains dynamic range - difference between soft and loud passages of music) and so the radios would not need to have such a flat response curve out of the IF band pass filters. That sounds possible to me. Looking at an alignment procedure of a contemporary RCA set (e.g. Model A78, 1950 RCA Service Data), it does not reference a visual method at all. I do not know if that is because it doesn't matter, or if they did not assume a repair shop would have that gear etc. Maybe Sams is just making more trouble for us by putting it in there? lol
On other, newer, FM receivers I have worked on, a previous repairman had those IF coils peaked and to my ears did not sound as nice as after I de-tuned the peak and went for the flatter or at least, more rounded shape. The discriminator/radio-detector adjustment with a sweep is easier in my view to know the shape is correct with the straightest line. I consider the shapes to be the "ideal" and I work to get as close to them as I can, but I don't expect perfection. Other, later sets seem to achieve it a bit better. See, for example, the Star-Lite radio I did a couple of years ago, from 1964.
Go to the 28 minute mark to see the "hump" better, and a bit later the "S" curve from its ratio detector.
th-cam.com/video/-MYxMiJMqxY/w-d-xo.html
Perhaps the next vintage FM (e.g. pre-1960) I do I will use the AM gen method and compare. My goal is just to show me working on these things best I can, struggles, mistakes and all, and with viewers riding along, which is the type of videos I enjoy watching the most. Thanks again.
@@PracticallyFixed Thank you for your feedback.