PlasmaFire
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A follow-up, finally. It's a long one...
As of two days ago, I finally got the radio repaired. It's now playing with its warm mono sound once again in my dorm. Between that day and September 3, however, has been pretty much silence. The condition of the radio started to degrade a few weeks after my first post, and the drift-to-noise issue steadily worsened until one night, the radio cut out completely. The tubes would glow, but the neon pilot lamp was dark, and there was a louder humming noise as the heaters powered up. No sound came out otherwise.
So finally, about two weeks ago, I brought the radio to my physics department's demonstration/equipment man, who also turned out to be a guru with repairing vintage tube equipment! Wow, I learned so much more about my Model Eight just from watching him diagnose and fix the thing. Coming in before as well as after physics class, I kept tabs on the radio as progress was being made...
Somerset.net's Model Eight page was extremely helpful to us, as it provided printable schematics and a layout diagram: http://www.somerset.net/arm/fm_only_klh_8.html
First, we took the chassis out and probed the circuitry with a multimeter for voltages. We soon saw why the Model Eight completely died: the 4x-electrolytic power supply capacitor was “out of steam†from age (seems they were only supposed to serve for 5000 hours on-time maximum), and since the resistors attached to that unit were no longer receiving an even voltage load, the first one literally split from overheating—a bonus “safety feature,†perhaps (which explained why the neon lamp wouldn't light at all—reading the schematics, the 107-volt tube section from the power supply's B terminal received no voltage!). The bad cap set was also the reason for the louder 60 Hz humming sound on power-up; temporarily fixing a modern electrolytic in parallel with the bad cap can reduced the hum a lot.
The fade-to-noise issue originated from that 4x capacitor because of age as well; as the set powered up, the ancient electrolyte's chemistry would cause a significant drop in voltages downstream, causing the low volume levels. Once the internal temperature rose, the voltages would fall even further, silencing the tuner section and hence causing the noise. We saw drops as much as 30-40 volts from the normal schematic levels upon immediate warmup, all due to that single dying component!
Note that the rest of the radio's capacitors looked healthy...what a tedious task it would've been to replace them!
We tested the tubes out with a small tube checker. Almost all of the tubes were weak from age, it seemed, but the worst were two of the three 6AU6A, the 6U8A, and the 6BS8. We left the 6BM8 power amp tubes alone even though they showed weak too, because that might have been due to the old age of the tester's electronics.
The demo guy then told me he'd order four new 60 μF, 200-volt electrolytics from Digi-Key as a substitute for the power supply capacitor. The reason for a 200-volt rating over 150 was because it'd allow for a margin of safety (turned out that the first section [terminal A] was already regulating 145 volts, which might've been a bad thing for Kloss' original design!). Even then, the whole repair process bothered me a little bit: I wanted this unit to look as close to original as possible once repaired. Then very good news: he told me that some of the machine shop guys nearby will chuck the original cap's base in a lathe, cut it off from the rest of the can, and carefully machine the upper can portion so that it'll fit another similar base (from the collection of multi-capacitors he has).
Once the new 60 μF units arrived, he set to work and wired a new ground wire to the chassis as per the original cap's design. Higher-wattage power resistors for the cap were soldered on to the new set, and the necessary new/strong tubes were also installed. A new rectifier diode for the power supply was soldered on (because the old diode might not have been able to take the new capacitor set's faster charging speed).
The biggest surprise came for me yesterday: the machined upper can for the cap was so well-made that I didn't even realize it wasn't epoxied down yet—until he pulled it off and surprised me with a neat wiring job! I might glue it in one of these days. To complete the repair, the old, brittle white power cord was replaced with a new black cord. Oh well, I can't get perfect looks, right? The power cord antenna circuit still worked well after that, though.
To finish it off, I re-assembled the chassis and knobs, polished the cabinets, and “ceremoniously†turned the Model Eight on again. No re-alignment was necessary; in fact, replacing the tubes and capacitor actually brought the radio _back_ into the correct FM scale again!
So to summarize it all, yes, it was the power supply filter capacitor that caused the majority of issues, mainly due to the large voltage drop after warm-up. Weak tubes might've contributed to the low tuner output and volume levels. One of the filter cap's blown resistors killed the set in the end.
Wow, I am extremely thankful for people like my physics demo head. Thank God, too, that this radio is now playing again. Now all that needs to be fixed is the sad state of FM programming! :)
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