My plans for this are to couple it to one of those miniature Lepai or Dayton Audio power amplifiers such as those sold by Parts Express, and install both in the small credenza in the living room.
The Coronet has its own volume, balance and tone controls, so a miniature audio amp would only need its volume set once, then forgotten; using the Fisher's volume control from then on.
As much as I do not like powering up a unit that is as old as I am, running on original components...I did so and found that the seller was quite truthful.
It plays very well on AM.
FM is, indeed, dead.
After some initial voltage checks I thought I had found the culprit - the 3rd IF transformer was open. I replaced it with one from a Fisher FM-50-B junker.
That brought back the plate voltage on the 2nd IF amp tube...but still no FM.
More troubleshooting is in order. I am going to try to get this tuner going again before replacing all electrolytics as well as the original selenium rectifier used to convert the filaments of the audio preamp tubes from AC to DC.
I replaced the electrolytic in the ratio detector circuit.
While I was at it, I also replaced the ratio detector transformer with one from a Fisher FM-100-C tuner, along with a number of resistors in that circuit.
Why?
Ratio detector transformers from 1962 and later tube-type Fisher tuners and receivers are "wide band" transformers which are much better suited for FM Stereo. The coupling between windings is different than in the older ratio detector transformers. This tuner (built around 1960) was not designed for our FM Stereo standard and is in fact mono, but has space on the chassis for an FM Multiplex decoder. With this ratio detector transformer it will be able to not only handle stereo (after the decoder is added), but it will perform better than would the original, narrower band ratio detector transformer.
So I tried it out on FM again and...nothing.
Then, noticing the FM band tuning eye was not moving at all, I disconnected it.
And after that...I now have FM.
The reception is very low, but it is there. It will need a complete alignment, especially with the replacement of one IF transformer and the ratio detector transformer. But FM is at least working now.
I will have to replace that tuning eye, also I suspect it is shorted internally...just enough to knock out FM reception while allowing the voltages in the FM section to appear normal.
Ron;
The FM IF transformers, as well as the AM ones, may need their mica caps replaced if the set is lacking in gain. I don't know about Fisher equipment but this is a common problem with many radios and tuners of that era.
Regards
Arran
I thought about that when I opened up the old 3rd IF transformer...and also opened up its replacement. As you can see from the photos above, these are constructed just like the common Miller 3/4" K-Tran IF transformers. I noticed both 3rd IFs have the mica "sandwich" in the base, and it did not give me that warm fuzzy feeling.
I might experiement with the old 3rd IF, try to fix or rewind the winding, and maybe try to disassemble that "sandwich". Only thing is, my service info does not give the value of that internal cap.
I will still attempt an alignment today and see if it helps.
Ron;
There is a fellow on Y.T that goes by the handle of shango066, I think his real name is Dan Yahro or something. In any event he has posted several videos on how to replace the mica caps inside those slug tuned mini cans, including finding the values. Basically you have to remove or permanently disconnect the mica cap, and then you need to connect a trimmer cap across each coil, and then peak the trimmer with an RF generator. The FM IF cans use small values, in the tens of MMF/Pf and the AM ones are usaully over 100 MMF/PF, so you need to start with an appropriate value trimmer. After you peak each can you can either leave the trimmer in place permanently, or measure the value it's set to with a capacitance meter or bridge, and replace it with a fixed silver mica or an NP0 ceramic cap of closest value. You don't really need to install the new cap inside the can afterward but you can if there is room.
On those Miller K-Trans I think that the base is held together with what looks like a brass eyelet, I think that you could probably drill it out with a hand drill though I am sure there are other methods. If I remember correctly you can disconnect the two coils from the base, and the compete coil and slug assembly will spread apart and separate from the base. I used to place with a lot of 1950s and 60s era AC/DC radios so I had the opportunity to take some of these apart, Miller K-Trans are only one style, I've even run across some that had a brass shaft attached to the slug, with NPO ceramics inside, and others that were like miniature versions of a late 1930s and 40s IF cans with trimmer caps on top. Why Fisher used the Miller style I have no idea, they weren't the crappiest style of IF can but they were far from the best.
Regards
Arran
I have noticed that the Fisher K-Trans were made by Automatic. I have a replacement Miller ratio detector transformer, and on the plastic base...it, too, has "Automatic Mfg. Co." molded in the plastic.
So I wonder if Automatic Mfg. Co. actually made all of these, then sold them to manufacturers such as Fisher, and resellers such as Miller?
I remembered that the limiter coil (3rd IF) had a mica "sandwich" that looked very clean through the plastic. So...I opened it back up again and found that one end of the coil was broken at the terminal. I removed one turn, resoldered, and bingo...continuity again.
I reinstalled the coil, touched up its alignment by ear (on that coil only) and FM then performed better (louder...and I can hear the interstation noise now where I could not previously). But it is still not up to par.
After rebuilding all of the electrolytic cans and replacing the selenium bridge rectifier in the preamp filament circuit with individual 1N4007 diodes, I tried it out again today only to notice that the FM tuning eye is responding in the reverse of how it should be responding. When I tune in an FM station, instead of the "eye" narrowing as it should, it is actually widening.
Ron;
I don't really know who made them, I seem to recall seeing "Automatic Manufacturing" marked on some, but I have an Airline (Belmont) downstairs that has a Miller sticker on the side but looks pretty much like the Automatic branded ones. I think that Automatic actually held the patents, they have them stamped into the top of the can, I suppose one could try searching for them on the U.S Patent office site to find out for sure. In any event I was thinking more of the design rather then who actually made them, the Miller/Automatic types have those cup shaped ferrite adjustment slugs that go outside the coils and move up and down inside a threaded plastic "rack", many of the others have a small ferrite slug that is threaded inside a plastic or cardboard tube, and then there are the kind that are sort of like a miniature pre war RCA IF can with a threaded rod. Anyhow I sort of treat these like they are living on borrowed time, I leave them alone if the set works well, and as replacement item if the set performs poorly after a recap.
Regards
Arran
I printed out copies of the ratio detector circuits of both the 100-T Coronet and the FM-100-C, from which the wide band ratio detctor transformer was taken.
I had my ratio detector wired as per the original 100-T circuit above, save for having the polarity of C66 reversed to accommodate the reversed diodes inside the wide band FM-100-C ratio detector transformer.
I looked over every connection...every resistor...every capacitor.
All was wired correctly.
So...
I removed the wideband ratio detector transformer and reinstalled the original 1960 transformer, also replacing the 8 uF electrolytic with another, connected as per the first diagram above, and...
I fired it up and it works great!
Reinstalling the original ratio detector transformer seems to have taken care of all of its issues. Volume seems normal now, the magic eye is responding properly, and the mysterious "double spot" tuning issue with two peaks is now gone. It tunes normally, sounds normally, and works normally. So the FM-100-C's ratio detector transformer obviously has issues, and it will find its way to the "round file" when I go back to the basement workbench shortly.
I think that I will recheck alignment on FM and AM, replace the lamps on either side of the dial, and then call this one finished. Yes, it's mono FM, but listening to my Fisher 202-R tuner on a regular basis has made me become accustomed to Living Mono FM. Nothing wrong with that.
Now to decide on which one of those miniature solid state amps to invest in, and then this Coronet will be going into the living room credenza.
Ron;
I don't know if anything is actually wrong with the FM-100-C ratio detector transformer other then it is not compatible with the surrounding circuitry, so I would not be in too big of a hurry to toss it, maybe save it as a spare for an FM-100-C. I can only go by the sections of each schematic that you posted but there appears to be a substantial difference in the ratio detector circuitry in each tuner, such in the way the cathode of the 6AU6 tube is connected, one shows the cathode connected to ground, in the other there is an RF choke, and a 5000 mmf cap, and a 1 meg resistor connected to it. Also the screen and plate voltages are different. RF circuits are like black magic to me, FM even more so, so when all else fails I go back to factory specs.
Regards
Arran
I agree with Arran, don't think I would toss that transformer. May be the simple difference of one being setup for stereo and the original mono. Also, with the detector diodes reversed, that could explain your eye tube operating in reverse. There will never be any more of those made. My 2 cents. I am glad you got it working!
If I could find the place called "Somewhere", I could find "Anything"
Tim
Jesus cried out and said, "Whoever believes in me , believes not in me but in him who sent me" John 12:44
I meant to post these schematics - one of the 100-T, the other of the FM-100-C - a few days ago. But then my "taste of retirement" (ten day holiday) ended on Tuesday morning and it was back to the salt mines, so I sort of forgot about it.
I've been studying this circuitry, and it seems that the older (pre-FM multiplex) Fishers took the output from pin 6 of the ratio detector transformer through a 270 ohm resistor. From late 1961 on, though, after the advent of true FM multiplex stereo, some Fishers took the output from the junction of the two 6.8K resistors while others continued to take the output from pin 6 of the ratio detector transformer through a 270 ohm resistor.
The Wide-Band ratio detector transformers (made from late 1961 on for FM multiplex) have the internal diodes reversed with respect to the internal diodes in the older FM mono ratio detector transformers. The circuitry is the same - only the 8 uF capacitor gets installed in reverse polarity with the newer transformer, because of the diodes being connected differently in the Wide-Band transformers.
It is known that the Wide-Band transformer can be substituted for an older-non-Wide Band transformer in an older Fisher where the user plans to install a multiplex decoder for FM Stereo. The only thing that needs to be done to the circuit is to switch polarity of the 8 uF electrolytic; otherwise it is a direct replacement. However, that statement does not apply to the 100-T, as I later found out.
I installed a Wide-Band transformer in my Fisher FM-100 tuner, which (like this Coronet) has space for an add-on multiplex decoder. I also reversed the leads of the 8 uF electrolytic. It works just fine, no problems. And its Magic Eye still works properly after the switch, just as it should.
This is why I suspect something is wrong with the FM-100-C transformer. The circuitry is similar enough that it should have been a direct replacement, plus switching the leads of the 8 uF electrolytic which I did.
Now, when I reinstalled the 100-T's original ratio detector transformer, I also switched the leads of the 8 uF electrolytic back to factory specs.
See the attachments for full schematics of both tuners. I apologize that the 100-T Coronet schematic is low resolution, but it should be good enough for you to see it. The FM-100-C schematic is higher resolution, easier to read.
I was just looking at the schematic of my (circa 1959) Fisher FM-100 tuner (attached, below). It takes its signal for the EM84 Magic Eye, not from the ratio detector circuit as in the Coronet, but from the 4th IF/limiter transformer, which has its own sets of diodes and an electrolytic.
I think I understand it now.
I wish I had a schematic of the FM-50-B tuner, but none are available online. It also uses an EM84 Magic Eye. I pulled a Wide-Band ratio detector transformer from an FM-50-B junker. This explains why it caused no trouble in the FM-100, since as I stated above, the FM-100's EM84 gets its signal from a different point in the circuit; hence, the Wide-Band transformer has no ill effect in the FM-100.
So, Arran and Tim...you may be right about the FM-100-C's Wide -Band transformer being OK. Yes...it does make sense to me now.
Let's compare the 100-T/Coronet C-808 tuner schematic with the Fisher 202-T tuner schematic.
In the 202-T, which a Fisher expert on Audiokarma successfully converted to FM Multiplex, the signal for the FM Magic Eye is taken from the 4th IF which has its own set of diodes internally, connected to a network consisting of a 15K resistor, 5000 pF capacitor, and 8 uF electrolytic. The signal from this is split between the EM84 Magic Eye grid and the FM muting circuitry. So adding a Wide-Band ratio detector transformer to the 202-T would not affect the operation of the FM EM84 Magic Eye tube.
In the 100-T, the signal for the FM Magic Eye is taken through R41 from the junction of R39, R40 and the negative lead of C4 (as shown in the partial Sams schematic below). Note that in the original Fisher schematic, the part numbers are R54, R50, R53 & C66, respectively. Now with a Wide-Band ratio detector transformer installed and C4 (C66) reversed, the signal polarity going to the EM84 grid would be reversed! So, in my mind anyway, to make the Wide-Band transformer conversion work in the 100-T, one would have to reverse the polarity of the Wide-Band ratio detector transformer's internal diodes. Click HERE to see how I eventually solved the problem.
I installed the Coronet in a period Fisher metal cabinet and took it to my home office, installing it in place of the Fisher 202-R tuner. The 202-R went into storage in the basement for now.
The Coronet played just fine, thank you, all weekend.
I sitll need to touch up the FM RF alignment - the high end of the FM band keeps ending up lower than it should be. I have some new plastic alignment tools on order; once they arrive, I'll repeat the FM RF alignment.
Then it will be time to start looking for one of those miniature solid state amplifiers so both the Coronet and amp can be moved to their permanent home - inside the credenza in our living room.
So that is it for this 100-T Coronet thread. Even with the FM issues, this was still probably the easiest Christmas radio rebuild I've done in the past several years.
I am glad you got it working!
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