The Fine Things are Always Hand Made
Philharmonic AM/FM
Ever since I first heard of EH Scott radios and joined this forum, I've been looking for a Philharmonic to "fall into my lap". Well, when Ron R. sold me his Marine SLRM I saw his AM/FM Philharmonic sitting in the corner. When he offered it for sale, I knew I would own it, I just didn't know how soon. Now, a couple of months later, it's here in my possession. Thank you Ron!
This will be a major project including some re-chroming. I want to do the radio and Ron proud! I heard Dave's (in Northern Michigan) Philharmonic (and a lot of other radios!) and decided I needed to get right on it. So, the work has begun. I decided to start with the amp/power supply as that is the easiest. Because of all the small parts around the transformers, it's also the most expensive to re-chrome (driver transformer cover, IF shields and tube shields will not be re-chromed as they can easily be destroyed, though I may give Dave's re-chroming shop in Milwaukee a try.)
This Philharmonic did not have tweeters. I would like to add them if i can find a set.
Also, since the FM is useless without a converter to the current FM frequencies, I'll be looking for a converter. i saw the schematic in the archives. Maybe i'll have to build one.
Comment
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Comment by Robert Feenstra on February 12, 2021 at 1:52pm
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Scott, the horizontal is 10 kHz/division. That's giving me about +-15k at 6db down.
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Comment by Scott Seickel on February 12, 2021 at 12:50pm
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The pictures are blurry and I can't tell what the horizontal scale is. The curves don't look too bad. I am a HP 3585A buff for SA alignments, so the Rigol screen is foreign to me.
What is the bandwidth 6db down? That will give you the bandpass.
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That seems to be as selective as I can make it. Realize that at +- 5K the signal is about 15 db down from peak.
You are absolutely correct about todays AM broadcasts. There are no HiFi stations and haven't been in many decades. HOWEVER, I usually supply my own music through an AM transmitter that is capable of 30 to 15K Hz and I can really here the difference when in the broadest position. this is where I usually listen to my "HiFi" am sets. Cymbals and snare drums really start to shimmer when I can get those frequencies through. Unfortunately, at my age, my hearing is down to about 8K so anything beyond that is pointless!
I thought of a different way to try and adjust the individual plates of the variable selectivity capacitors and may play with that today. More fun!
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Comment by Michael Lawton on February 12, 2021 at 3:06am
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Wow ! Nice to have fancy equipment. I make do with a good sig gen, a sensitive pair of ears, and more than half a century of experience. LOL ! Seriously though, given that the channels on the AM Broadcast band are 10KC wide, you might want to narrow the base in that "most selective" trace if you can. Right now it appears that the base is more than 15KC wide. That will allow some splash over from adjacent stations, which can be a problem when one is trying to separate two weak stations next to each other in frequency. Beyond that, you will be able to tell more than I can. You are the one who gets to play with the alignment and the spectrum analyzer. Roughly speaking though, given the fidelity characteristics of AM radio at its best, I would say that the ideal trace for the broadest setting would be somewhere between the top and middle traces, with flattish top, and the bottom about 20KC on each side of the middle. This circuitry, and how it functions is fascinating.
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If there are suggestion on how this should look, I'm all ears!!!!!!
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Sorry, things did not post in the proper order on the last post, but I know you can figure it out!
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I've begun a "serious" alignment. I've started with the IF. This radio has 4 IF transformers. Selectivity is controlled by varying the center frequency of all of the primary and secondary IF transformers except the primary of the first and the secondary of the forth - stagger tuning accomplished by a single shaft controlling 6 3-plate variable caps. 3 increase capacity and 3 decrease capacity as the selectivity control is turned. The IF is peaked in the most selective position of the control. Then, as the selectivity control is turned, the various stages are detuned to broaden the response curve the IF system. It seems, from my experience here, that it is very difficult to get an even transition from most selective to broadest frequency response. I'm having trouble keeping the response curve fully centered on the IF frequency of 465 kHz. This forces me to retune when I go from very selective to very broad frequency response. I'm thinking there will be compromises!
As the response curve broadens for higher fidelity, the sensitivity naturally goes down so Scott included a variable control on the selectivity that changes the bias voltage on the AGC system to compensate.. It's not perfect, but does a pretty good job of compensating.
A spectrum analyzer is very useful in this processor as I can see every very slight change in the response curve as I change the adjustments. I need to figure out how to keep the center frequency at exactly 465 kHz at all positions of the selectivity control. I may have to tweak the 6 variable caps individually. Could be hours of playing/experimenting. Fun, right!?
Following are a few pics of the Spectrum Analyzer at various positions of the selectivity control from highest selectivity to broadest frequency response. 10 kHz/division horizontal, 5 db/division vertical. You can see the curve broaden out as i turn the control to max fidelity. I just need to make everything more symmetrical. Fun Stuff!!
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I can understand, one gets so involved in the re-assembly one forgets to document everything as well as one does the disassembly. Thar IS sad, because, if you had, you could have made a video like my first one about refurbishing that NC-125, just a narrated series of photos, and followed it up with a live action video demo of the radio working. I am sure folks would have loved that. I know I would.
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Scott, Does the converter work well? I have 2 subbers that I found and was hoping the Penthode would build one for me. I've also PM'ed Bill to see what he has.
A video!? Of what, it playing? I took a LOT pictures of the disassembly, but not much of the reassembly, and no videos.
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It is Penthode. He is also pretty active on VideoKarma in the Early B&W TV section. I have one of his converters that he made from a tuner subber.
Congratulations on the good work getting that running. Now we need video or it hasn't happened ;-)
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