A NEW USE FOR SORBOTHANE HAVING READ of the piezoelectric effects of capacitor
dielectrics, I decided to try an experiment. I had an Audio Research SP-12
which would occasionally go into high-frequency (15kHz and above) oscillation,
blowing the fuses or activating the thermal breakers on my Adcom 545 (modified).
Bypassing the V+ and V leads of the regulating ICs produced no real added stability
to the circuit. Adding sorbothane ‘tube dampers’ didn't help either.
Tapping the coupling caps while the preamp was powered produced a very large ‘thump ’ and a ringing sound. The addition of a small sorbothane strip under each coupling cap not only eliminated the oscillation, but improved the sound quality. The sound is clearer, as if an echo or ringing had disappeared, as though I had replaced silver mica caps with polystyrene.
Perhaps instead of using strips of a sorbothane sheet, sorbothane ‘tube dampers ’ could be used around the coupling caps to dampen cap vibrations. I use a local source to purchase sorbothane sheets: Grewe Systems, Ltd., 112 S. Port Crescent, Bad Axe, MI 48413.
RICHARD PAINTER; Ubly, MI 48475
FM-3 MODS
HERE ARE SOME FM-3 mods you may wish to try.
1. Desolder the rear lead of R22, that faces toward the rear of the chassis. R22 is the 1W 47k resistor between discriminator T5 and V6. Bend R22 so it is standing straight up. Beneath this was eyelet #29; connect a 1-inch wire from the free end of R22 to the eyelet (the discriminator may become slightly mistuned so you may have to realign it).
2. Replace R21, the 100 k-O carbon resistor, with a 150-270 k-O metal film type. This will lower the output slightly but it should be cleaner.
3. Replace/bypass C22 and C23, the 0.0047 uF disc caps on PC8 near V6, with 0.1-0.47uF, 100V plastic film types. Bass should be firmer and less boomy.
4. Squeeze the last bit of sensitivity out of those IFs by doing the following: warm the tuner up for 15 minutes. Connect the negative lead of a voltmeter to eyelet #26 on PC8, positive to chassis.
Tune to interstation hiss. Adjust the IFs and the trim capacitors on top of the tuning capacitor with a plastic alignment tool. The peaks are very ‘soft ’ and there is some flicker, so make sure you have the peak.
5. Very carefully remove C24 and C25 47pF NPO disk caps from PC8. Separation should improve. Please try one mod at a time and make sure your tuner is working for each one.
[Note: Be sure to keep the unit off to make changes 1, 2, 3 and 5. On No. 4, keep the other hand in your pocket .-Ed.]
VINCENT MOGAVERO; Maspeth, NY 11378
CD KIT KX-2
ONE STEP IN THE POOGE-4 kit is to re place the audio output electrolytic caps (C2366 and C2367, 220uF) with 10uF polycarbonate types (two 5uF in parallel).
When installing each of these capacitors, allow the leads which go to the former electrolytics' negative end to extend 1/8-inch after soldering, that is, don't cut the ends off short.
You can use these as tie points to anchor both the 100 k-O and 100-ohm resistor replacements for the chip resistors as follows:
C2366 negative tie point-R3360 (100k) and R3362 (100 ohm)
C2367 negative tie point-R3361 (100k ohm) and R3363 (100 ohm).
This eliminates some soldering to the board's foil, which is a tricky process at best.
MATTHEW J. JURICK; Silver Springs, MD 20906
MAGNAVOX CD MODS
IF YOU HAVE A MAGNAVOX CD player with the large heatsink (as I have on my CDB-650), removing the main circuit board from the chassis is difficult. It is more difficult, if not impossible, to re install it after the mods are completed.
The six central fins of the heatsink are blocked by the plastic lip of the back panel. The solution is to saw six slots about 1/4-inch deep into the plastic lip (one slot for each of the six heatsink fins). I used a wood saw, instead of a hacksaw, to cut the required thickness of the slots. With this accomplished, the main circuit board is easily re-installed.
MATTHEW J. JURICK; Silver Springs , MD 20906
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