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Old 28th December 2004, 08:21 AM   #1
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Default Help - Tuner string replacement questions

Guys I have a couple questions about refurbishing an (analog tuner) Marantz receiver for my teen daughter's use in her bedroom. Thanks a ton in advance for any help you can provide!

1) The frequency indicated on the tuner dial readout did not match the actual frequency as heard through the speakers. Upon looking into it, the frequency indicating pointer had somehow moved relative to the tuning gang. I figured all I needed to do was to get them together again. So, I removed the pulley from the tuning gang so I could move the pointer and string independently of the gang, hoping to match them up once again. Well in so doing the string broke <poof>, easy as you please. And I had gently set the pulley aside to prevent just such a thing from happening, and it broke anyway. So - what do I use as replacement string? What is the trick to gettting the correct tension in the string, or is that not a big deal? Seems to me if I get it too tight it will overload the tuning gang gear drive mechanism; too loose, and the string will slip on the flywheel.

2) Before the string broke, I had it lined up and working - sort of, at least on one radio station. I set the indicated tuned frequency to match the actual station as heard from the speakers. This was in the center of the FM freq range, at about 98.5 MHz or so. Problem is, if I moved the dial away from that point, within about 1 or 2 inches to either side of that calibration setting, the station heard through the speakers did not match the dial reading. The further I got away from the station where I "calibrated" it, the worse the discrepancy became. What is going on here? It's as if the ratio that the string mechanism turns does not match what is needed by the tuning gang. Is this a tuner alignment issue, i.e. not something for a DIY guy?

Thanks for your help guys ! ! ! Pete in Bakersfield
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Old 28th December 2004, 09:36 AM   #2
Duo is offline Duo  Canada
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Well, this is highly interesting to me, as your profile states that you are a mechanical engineer.

The mechanics of the marantz gyro wheel tuners is relatively simple. As are most radio tuning systems. What maintains the string tension is a small spring in the wheel on the tuning gang. This allows for changes in string tension which occur at the ends of the dial, and also, for the friction winding on the thumbwheel to keep adequate grip on the string. The latter allows you to continually rotate the tuning dial even after the gang stops without undue tension on anything.

The real trick here is to get the same amount of string into the loop as before. As for the type of string, you can buy spools of it. It's normally a nylon cord, quite thin, and with a very good tensile strength, and little stretch at all. I happen to have a few rolls of slightly different gages that I use to re-string radios.

Normally, in order to get the length correct, I take out all of the old string, paying attention to how it was originally routed, and I measure the piece, or pieces of string in order to get the same amount of new string, allowing for enough to tie a knot in each end to hook onto the various parts. Normally, both ends of the string are in the plate on the tuning gang, and one will have a spring on it.

Basically, I start with a knot in one end of my new string, and I hook it onto the springed end of the gang plate, and route it through the various pulleys and cranks in the system. Once I'm back to the gang again, I have to make a knot in the string in an appropriate place so that I'll have some good tension on the spring, thus keeping everything tight. You'll usually have to give a good pull on it to hook it on if everything's done right. Needlenose pliers are a must here.

As far as the accuracy of the dial placement goes, this is made far easier if you have a signal generator and can thus determine any frequency you want along the dial. Here in Canada, I have found that many radio stations which are re-brodcast over the cable tv network do not attain their original frequencies. We receive many American stations here, but on different frequencies on the dial. Perhaps this may be what's happening to you when you find discrepancies in the indication. If the radio is actually accurate at one end and not on the other, this is an unusual alignment problem; one with which I don't have enough experience to explain at length. Essentially, in most circumstances, the dial will, once aligned, be relatively accurate over the entire scale. If not, I don't normally bother with it too much unless it is terribly out at certain spots.

I'm sure others here will have help for you as well. Hope I helped anyway
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Old 28th December 2004, 10:59 AM   #3
SY is offline SY  United States
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There's usually a trim cap adjustment which sets the span and linearity of the tuning cap's range. Often that trimmer is located right on the big variable cap. You DO need a couple of frequency standards to make that adjustment- a variable-frequency FM generator is optimal.
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