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Old 11-11-2018, 04:57 PM
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Zenith 7H918 FM Radio Restoration.

Hi all,

While I'm waiting on a tube to come in for my RCA television, I decided to get to work on a project I've been itching to do for awhile. This is an oddball radio, an FM only set from 1949 or 1950, if I've understood the schematic correctly.

I'm in the midst of replacing some crumbled wiring, and planning to start on the paper and electrolytic capacitor replacement soon. I had to order a new resistor and 1N4004 diode to replace the sand resistor (I think that's what it's called? It's powdery) and the silicone rectifier (I had already ordered the dropping resistor for that, but apparently ran out of diodes, go figure.)

Anyway, I had some questions about the tuner. I was in the midst of replacing the broken dial string, which is quite possibly the worst part of radio restoration I've ever experienced, when I noticed that the rubber mounting grommets under the tuner are deceased. I actually believe I have rubber grommets that will fit in those slots, but I can't wrap my head around this tuner. Apparently it's not a variable capacitor - the schematic lists it as two distinct variable coils. They have these long posts that go above the tuner, and what appears to be a masonite board glued onto the posts connecting them to each other. The glue has failed, and while I can see where the board is supposed to go, it's basically free floating, though I can't slide it off without removing the glue further. The coils seem to move together just fine without that linkage, and I'm not seeing where this is an electrical connection, but I'm at a loss as to what exactly I'm looking at. That's not to mention the bizarre roller setup to move the coils - it has two cylinders, but only one seems to rotate while the other just helps to keep the posts in place.

Is anyone familiar with this kind of tuner design? Am I going to find myself in adjustment hell if I mark where this board is supposed to sit, pull it off, replace the grommets, and glue it back in place? Also, if anyone is familiar with it, are there any recommended points to grease/oil/contact clean?
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Old 11-11-2018, 10:08 PM
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Unless the coils are loose I'd ignore the gromets...There should not be any significant force applied to those voils when tuning.
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Old 11-11-2018, 11:06 PM
old_coot88 old_coot88 is offline
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Quote:
Originally Posted by Electronic M View Post
Unless the coils are loose I'd ignore the gromets...
Ditto that. Maybe consider re-impregnating them later on with some kind of silicone product.

Looks like those pinch rollers should both turn, just like the old time washing wringers. First time I ever seen that type of setup in a tuner.

https://www.youtube.com/watch?v=SvJW4Nx3IDA
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Old 11-11-2018, 11:28 PM
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The board may help the two slugs track together. The tuner is very much like a ganged variable capacitor tuner only on this one the coils are the variable type and the caps are fixed. I know Zenith used variable inductor tuning on most of its early FM radios although I've never seen this mechanical set up before. I've seen ones where a cam on the shaft pushes against a yoke that the rods on the slugs are attached to.
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Old 11-12-2018, 11:05 AM
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Hmmm - wonder if the grommets are mainly to reduce microphonics?
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Old 11-12-2018, 09:01 PM
zeno zeno is offline
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The FM uses a PTO ( permeability tuned oscillator). Zenith used it
up till the mid 60's. The coils tune instead of the caps. Very stable, precise
& expensive ! Only found on car radios & high end communication receivers
to my knowledge.

I would at least change the grommets, they may effect the dial cord if
nothing else. Other parts can be stripped from a junker. I would guess
Zenith made the minimum changes over the years & they built millions
of these in almost 20 yrs.

Also think twice about changing caps in the RF amp, osc & mixer. The
front end is tricky to align & its best to avoid it. New caps will almost
have to throw things off.

BTW the dial cord IS the hardest thing to deal with on radios. I never
got the hang of it. We had one cat that had the special talent & all of
us passed dial cords to him.

73 Zeno
LFOD !
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Old 11-18-2018, 05:58 PM
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Alright, well, I'm back. Project isn't done, but I just wanted to update y'all.

I did go ahead and replace the grommets. It was a job in itself. The entire tuner machanism had to come apart, and I ended up having to completely desolder and remove both coils from the underside to access the holes. I took the opportunity to measure and reglue the plate, clean the inside of the coil forms with rubbing alcohol, and clean and regrease the mechanism. In fact, one of the rollers was seized - both are now rotating as expected.

So then I went ahead and replaced the filter capacitors which were giving off a hum. Had an opportunity to replace one of the paper caps while I was at it. Gave it a test, and everything works just fine. Pulled in stations where I expected them, no filter hum, so things are moving in the right direction. I've still got three more paper caps to replace. It also looks like someone replaced the silicone rectifier with a diode in the past. It's a professional job, done in the 60s by the look of the part, but there's no dropping resistor, so I'm planning on removing it and using a 1N4004. Then I've just got that dial string. Honestly, I may be willing to admit defeat and just pay someone local to do that one. There's a few shops within driving distance of where I live.

Does anyone have any recommendations for a good bakelite adhesive? One of my knobs has a crack that has caused some splitting on the face. The shaft is fine, but I'd like to glue it and clamp it to get it looking better. Eventually I'll try to replace it, but I've had no luck finding a replacement.
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Old 11-18-2018, 07:35 PM
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The original re rectifier was SELENIUM, and your 1n4004 is silicon. Silicon rectifiers date back to the 60s and unless subjected to over voltage, over current, or something else that exceeds specs will last pretty much indefinitely.
selenium dates back to the 30s was phased out in the 60s typically looks like a square plate tuning capacitor and let's out a stinky toxic smoke when it eventually burns out.
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Old 11-18-2018, 09:35 PM
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Quote:
Originally Posted by Electronic M View Post
The original re rectifier was SELENIUM, and your 1n4004 is silicon. Silicon rectifiers date back to the 60s and unless subjected to over voltage, over current, or something else that exceeds specs will last pretty much indefinitely.
selenium dates back to the 30s was phased out in the 60s typically looks like a square plate tuning capacitor and let's out a stinky toxic smoke when it eventually burns out.
My mistake. I meant to type selenium, but must've made a typo. Oddly, mine has the original selenium rectifier still electrically connected, but also has a later diode in parallel with it that isn't listed on the schematic. I figure the selenium must've failed at some point and they bypassed it. I'll be disconnecting both and replacing them with a 1N4004 and dropping resistor.
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Old 11-18-2018, 09:50 PM
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If s selenium remains in-circuit while paralleled with a Si diode, it can still have reverse leakage which could potentially lead to overheating/ stinky smoke.

Last edited by old_coot88; 11-18-2018 at 09:54 PM.
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Old 11-21-2018, 05:25 PM
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Well, I just wrapped the capacitor swap today. Removed the diode and rectifier from the circuit as well, and swapped in a new rectifier and dropping resistor. There's still a few things to do - the cabinet needs to be cleaned up, I need to fix that knob, replace the dial string, and, as I was warned, the alignment is pretty off. I've reached out to the shop a town over from me, so my hope is to go ahead and outsource the dial string and alignment to someone more experienced while I tackle the cabinet. I'm hoping to learn the ins and outs of radio alignment eventually, but from the looks of it, this isn't a great beginner set for that. Oh well.

I asked this before, but wanted to see if I could get any info. Does anyone have any recommendations for a good bakelite adhesive? One of my knobs has a split in the face, and I'd like to repair it. I'm thinking of cushioning a vice grip with a washcloth or something along those lines, and clamping it together to try and get a tight fit without doing any further damage. Is regular superglue the way to go here?
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Old 11-21-2018, 07:43 PM
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You can do both FM and AM alignments by ear (assuming you can get a station to pass to the speaker). Peak the RF amp adjustment (by adjusting the point the slug mounts to the board by turning the slug). Compare the reported freq of a station to the dial indicator and if substantially off tune towards the correct dial marking (till station becomes weak) and adjust osc for best reception walk dial and osc over till you have the station and correct indication (make sure you have stations on bottom and top of the band if you don't then the dial pointer is misaligned with the string and you may have walked the osc away from the correct value)...Readjust RF amp slug after osc if needed. Peak IFs on weak station for max volume and adjust FM detector for minimum distortion...If distortion still present reduce IF gain and try to get lower distortion thru staggering the IF a bit.

I actually have alignment gear and skill, but usually, do things by ear unless something is being difficult or is overly complex/finicky.
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Old 11-29-2018, 02:00 PM
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I'm back with good news. The radio is done!

I went ahead and took a risk, and was able to successfully restring the dial cord. It took more tries than I care to admit, but it's done. As Electronic M predicted, I was able to do the alignment by ear. I'm sure I could get it closer with equipment, but I'm not sure the difference would be noticeable in practice.

I did end up having to tear apart the tuner again. Apparently I hadn't slid the cores deep enough into the variable coils, and I was actually missing a few stations from the bottom of the band. Took me a few sessions to get everything lined up right, but it's finally there.

Thanks, everyone!
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Old 11-29-2018, 03:12 PM
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Bravo!
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Old 11-29-2018, 10:57 PM
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Good show!
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