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I acquired a Panasonic Re-7487 table radio in remarkably good shape. There is something strange with the AM tuning performance though that I'd like some troubleshooting help with.The tuner is a mechanical one with a string, pulleys and pointer arrangement and separate, parallel linear scales to show FM and AM station frequencies. The pointer is tied into the string and cannot (simply) be moved (slid) along the string.
When tuned into a station on the FM band, the pointer is pretty close to the numeric value of the station's frequency. That's not the case on AM though. The pointer ends up about 300 kHz higher than the actual frquency of the tuned-in station.
Since the pointer position seems to be correct on FM, this doesn't seem to me to be a mechanical problem with the string/pointer system. Instead the AM tuner seems to doing something funny.
I'd appreciate any thoughts anyone has on this.
Follow Ups:
Those other posters are pretty unsympathetic to your problem, which can be corrected.If the frequency tracking is relatively close at the bottom of the dial but not at the top, then adjusting the trimmer capacitor that is in parallel with (and possibly mounted to) the section of the variable capacitor for the local oscillator should have a substantial effect. If it is not, then there are two likely possibilities: either the IF stage is not peaked close to 455 kHz; or (more likely) the inductor in the local oscillator is out of adjustment. There's probably a variable inductor you can adjust for that.
Be aware that ferrite core antennas can gradually lose their permeability (i.e., mu) over time. When that happens in radios that use a separate winding on the loopstick as the local oscillator's inductance, the local oscillator's frequency will go up. If that's the problem, there's a simple solution: add more turns of wire.
In what part of the country is your radio? Do you have a schematic?
Thanks for the info.I'm in Milwaukee, WI.
There is a schematic pasted inside the cabinet of the radio. Its small and has faded somewhat. I've trying to figure out some way of either peeling it out intact or reproducing it. But being inside the cabnet, I'm not sure I can get a good enough shot with a digital camera.
The problem is with the lower end of the AM band. 620 comes in at about 920 on the dial.
If you had grown up during the time when such dials were the norm, you would know that such a very minor discrepency was nothing to be concerned about; many had much worse dial error right out of the box. If it works fine otherwise, leave well enough alone lest you make it much worse in performance; listen to the radio & look at the TV instead.
Ron,I'm 49. That's old enough to remember B&W TV as well as analog tuners.
The difference with the AM tuner is 300 kHz. I have to twist the pointer up to 920 kHz to find the 620 kHz station in town. I could live with that, but I wouldn't call it minor. And it would throw anybody else trying to use the radio for a loop.
Since the pointer is pretty much spot-on when switched over to FM, I'm wondering if there's some adjustment to the tuner circuit that could bring the AM frequnecy and the pointer closer together.
I'd still be hesitant to turning the IF slugs; there ought to be trimmer capacitor screws alongside the main tuning variable condensor (the closely-spaced metal fins rotated when dial moves), these screws will bear a metal strip against a mica or plastic film separating strip from main frame- note exact position prior to adjustment so that you may return if neccessary. Usually the FM & AM sections share the same basic tuning condensor BUT the FM portions will have far fewer fins & wider spaced while the AM portions will have many fins closely spaced. SO, trial turn only those trimer screws that are adjacent to AM fin sections. Your own body capacitance thru tool may affect adjustment zero so several small trial changes may be needed. With miniture plasic-encased variable tuning capacitors, the trimmer screws are located on the package rear & only require one full turn either direction for a complete adjustment range.
Thanks for the info.The tuning condenser is unenclosed.
Is adjusting the trimmer screws something that's personally safe to do with the radio on (electrocution)? Or should I adjust, turn the radio on to see what progress I've made, turn it off & try again?
You need an alignment on your AM dial part.If you want factory spec, you need to get a factory service manual to service your AM IF cans and/or your VC top screws.
You may try by trial-n-error by first getting a permanent marker pen and mark all the corresponding screw slot positions on your VC, and then while you have your selector set to AM, carefully adjust your screws until it "dials" in to proper frequency.
Anything goes wrong, you can always reverse-engineer your screw settings based on the permanent marker markings.
Q.
p.s. don't forget to check all other components for runaway or out of specs.
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