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 Post subject: Zenith Royal D7000Y
PostPosted: Jan Sat 05, 2019 8:16 pm 
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Location: Fredericton, Canada
My radio works fine for the most part but I am having a problem on the FM Band. As I turn the dial the signal will drop out at different points as if the tuning capacitor plates were momentarily touching. It doesn't happen on the other bands so I am wondering if there is a separate tuning cap for FM in the FM tuner. I don't want to get into it if there isn't something there to check.

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 Post subject: Re: Zenith Royal D7000Y
PostPosted: Jan Sat 05, 2019 8:31 pm 
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Location: Sunnyvale CA
Copiertech wrote:
My radio works fine for the most part but I am having a problem on the FM Band. As I turn the dial the signal will drop out at different points as if the tuning capacitor plates were momentarily touching. It doesn't happen on the other bands so I am wondering if there is a separate tuning cap for FM in the FM tuner. I don't want to get into it if there isn't something there to check.



It is not tuned with a capacitor, it is tuned with slugs in coils, so that it not it. Try rocking the bandswitch when it is dropped out and see if that makes it come back, also the same with the AFC switch.

Brett


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 Post subject: Re: Zenith Royal D7000Y
PostPosted: Jan Sat 05, 2019 8:36 pm 
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Brett: So, even though the 3 gang tuning cap is rotating when I turn the dial knob it has no effect on the FM tuning ? I should also restate the problem. The stations do not totally drop out but decrease is strength as I tune. In addition there is a skratchy noise happening too as I rotate the knob.

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 Post subject: Re: Zenith Royal D7000Y
PostPosted: Jan Sun 06, 2019 6:18 pm 
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Copiertech wrote:
Brett: So, even though the 3 gang tuning cap is rotating when I turn the dial knob it has no effect on the FM tuning ?


Correct. All the bands are on the same dial with the same mechanism, so they all turn at once, but the FM tuning is accomplished by moving a plate, with the ferrite slugs attached to it, running inside coils. If you have the radio apart, you can see the end of the tuning shaft from behind, and you can see it has a pulley and cord attached to turn the tuning cap. That shaft passes from the front where the knob it, to the back, passing through the FM RF chassis (in a sealed box). That has a worm gear on it that actuates the FM tuning. If the chassis is out, you can see three holes just to the left and below the tuning shaft. In each of these holes, you will see what looks like a ~1/16" "rod" that moves in and out when you turn the shaft. They are actually tightly-twisted springs that are attached to each tuning slug, and are rotated in place (screwing them in or out) to perform the FM RF alignment. This is much more stable at FM oscillator frequencies than air-variable capacitors, and is considered a superior method, and was the best they could do at the time. Most if not all Zenith FM radios had this sort of arrangement from the start, and early on, the Zenith FM radios were far, far, superior in terms of stable alignment and performance as a result.

Much later, it was found that polyethylene-dielectric tuning caps (Polyvaricon and other brands) were more than stable enough to do FM, and that displaced the slug-tuned tuners.

Your problem is still unclear. If the volume/sensitivity varies smoothly as you vary the tuning, I would guess that the three elements of the RF tuning are not synchronized with each other (TRF, Oscillator, and Mixer) and that affects it as you select different frequencies. That's unlikely to me, I have seen them WAY off and have no obvious problems aside from failing to tune weaker stations. Noise is far less likely, there may be some other odd issue or crosstalk through the IF stages (which use the same amplifiers and are connected in parallel with AM). Cleaning everything never hurts, but keep cleaner out of the FM subchassis.

Brett


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 Post subject: Re: Zenith Royal D7000Y
PostPosted: Jan Sun 06, 2019 7:53 pm 
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Thanks for the info Brett. The scratchy noise and drop out occur at the same marks on the dial so I am thinking it has something to do with that FM tuning mechanism. So, you think spraying some contact cleaner in there will damage it? Would getting in there and examining it do much harm?

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 Post subject: Re: Zenith Royal D7000Y
PostPosted: Jan Sun 06, 2019 11:32 pm 
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Location: Sunnyvale CA
Copiertech wrote:
Thanks for the info Brett. The scratchy noise and drop out occur at the same marks on the dial so I am thinking it has something to do with that FM tuning mechanism. So, you think spraying some contact cleaner in there will damage it? Would getting in there and examining it do much harm?


There are no contacts to clean inside the sealed FM subchassis, so don't spray anything into that. I speculate that you do have shorts in the tuning cap, that cause noise that gets into the IF system, since it is engaged all the time, and uses the same transistors as the FM. Blow it out with air/canned air/etc if you want.

Brett


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 Post subject: Re: Zenith Royal D7000Y
PostPosted: Jan Sat 19, 2019 1:07 am 
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I took this issue over to the restoration forum because that was really where it belonged. I found the fix for the problem . Here is the link.http://www.antiqueradios.com/forums/vie ... 6&t=352705

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 Post subject: Re: Zenith Royal D7000Y
PostPosted: Jan Sat 19, 2019 1:48 pm 
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Once a topic is started here and restoration begins, it's best to continue here so that all information about a particular radio is kept in the same place. This is in keeping with our "one radio, one topic policy". This applies to all of the forum sections. A moderator may move a topic if deemed necessary and leave a "shadow" link in the original forum. The Electrical/Mechanical Repair and Restoration forum concentrates mainly on tube radios.

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