Re: Yamaha CT-7000 Mods and filters
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A few observations I would like to share with you all re the CT7000...
On more than a few occasions, and in many products that use the early seventies Alps rotary trim pots, I frequently see oxidation on the brass center wiper screw and the PCB terminal lead that can either can totally open the wiper connection , or , cause a substantial resistance increase in the wiper to element .
Also one other comment. Having serviced many early Tuners using polystyrene Caps in various circuit applications, I often find the device value to have significantly drifted from the chosen value. I have on many occasions given a PS cap a gentle squeeze between two fingers with the result of a circuit restored to improved operation.
Film caps do age, and there are certain brands and construction that don't age so well. As an example, the orange film mylar caps used in many Nakamichi cassette decks in the record and playback EQ circuits, and in the Dragon auto azimuth circuit. For those of you that service Rotel products, you may have noticed that many of the PS caps used are dipped in a clear blue sealant to delay the oxidation of the lead connections to the foil elements. PS caps are almost extinct these days , fortunately in most applications, polypropylene film caps in tuned circuits is an acceptable replacement.
I can see your dilemma, as the manual says you should see 55 dB separation in both narrow and wide, and you are only getting 45 dB. I think the only thing left would be to go back to the stock compensation cap and double check all MPX adjustments. Mostly because I can't understand how replacing the two stock orange filters with two 280's would require that much change from the stock factory setup. Perhaps double check the other MPX adjustments, especially the 19 kHz transformer. The setting for this one is not always peaked.