David McGown wrote:
Walt,
By exactly matched, did you mean to the (a) original in-place components, (b) the values on the schematic/parts list and between left/right channels (i.e., after the 38kHz oscillator). Carbon comps will drift, of course, so (a) is unlikely. For (b), the values seem to be for 10% (some 5%) tolerance components, so using 1% resistors and 5% (or better) capacitors would be a lot better than what is there. Did you need to readjust the MPX section after you worked it? I have the service manual (of course), but lack some of the test equipment (I.e., RF signal generators) necessary.
Was there a technical reason for using polycarbonate vs styrene or PP capacitors. Is that what you had on hand?
David
The MPX section can make or break a stereo FM tuner since it is handling the audio signal directly off of the discriminator so the more noble the parts the better. The MAC MPX section is somewhat proprietary in that it does not have any critical adjustments to deal with. The chosen resistors, the Roderstein metal-film resistors, have a low self-inductance and I hand-matched them with my RCL meter to withing +/- 1/2%. The capacitors I used were stacked-film (not unlike monolythic ceramic capacitors) and they were Siemens polycarbonate stacked-film caps rated at 630-vac with immeasurable dissipation factor (DF) and very low self inductance. These also were hand selected to within +/- 1/2%. Typical polystyrene caps and polypropylene caps are not suited to the application (other than possibly the WIMA pulse caps) because they are wound and have measurable self-inductance since the MPX section is reaching into the low-RF realm.
The coupling cap (discriminator output to the MPX section) is a RTI Teflon cap not unlike the V-Cap Teflon. Anything to do with the de-emphasis network I went "whole-hog" with WIMA pulse caps, polycarbonate caps, Teflon caps, and silver micas depending on the portion of the circuit and what it was doing.
The audio circuit was a bit unorthodox where the main coupler was aluminum electrolytic with secondary aluminum and ceramic caps. ??? The main electrolytics were replaced with WIMA polyester stacked-film caps only because of limited space and the secondaries (aluminum electrolytics and ceramics) in the photo were replaced with polycarbonate and Teflon types.
The discriminator had two critical-path resistors that I replaced with hand-matched mil-spec metal-film resistors.
I added a heat sink to the RF input Nuvistor more to suppress induced mechanical vibration than heat removal and rolled the audio-output tubes to get the best sound possible.
Beyond that -- a liberal application of Teflon rings on the tube shields and bare tubes to suppress induced mechanical vibration -- I also installed a F-Connector for the 75-ohm antenna input and added a 1K-ohm shunt (Vishay bulk film) resistor across the 300-ohm antenna inputs.
As long as the ceramic caps used by MAC served the RF and IF sections they remained in place since the design intent relies on their thermal characteristics and suitability for use in RF circuits.
I went this far with the project because I got real lucky and the MR-67 I bought was in pristine condition with no observable evidence that it was ever used. WOO-HOO!
Plus -- these tube MAC FM tuners follow the original Edward Armstrong FM radio and GE-Zenith MPX design requirements (old school) so my thought was to optimize the MAC to see how far I could go with textbook FM stereo playback. It was worth the effort especially because several FM radio stations in this area that have upgraded their electronics over the years. WBJC-FM (95.1 mHz.) especially is only as good as the playback system they use for broadcast -- their live broadcasts are really cool!
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