by Bob01605 » Mon Aug 20, 2007 3:26 pm
In relation to what Sal mentioned above. The 40, 80, 30, 20 cap is a good upgrade for many older ST-70's - especially those that hum a little or create a situation in which the fuse blows but only after a period of time - a few seconds to a few minutes. If a fuse blows instantly then you may have a bad power tranny or a wiring dead short somewhere. Also, if that quad cap is "hot" to the touch after 15 minutes or so of play time and not just "warm" then one or more sections is starting to go.
Another thing to check out on a suspect cap is to measure the resistance of each section to ground. (the outer metal case of the cap). A new cap will ususally measure over 1 meg on each section but as a cap wears out the resistance to ground will start to drop. If you are only getting 5000 or 10000 ohms on a section then that section is weak and the whole cap should be replaced.
Sometimes as a stop gap measure you can disconnect that section and put in a single electrolytic cap of maybe 47 Mfd @ 525 volts or higher in it's place just to keep the amp playing. Connect everything that was connected to the bad section to the positive end of the single cap and then ground the other end of this single cap.
As to adding capacitance just to "see" how the increased capacitance will affect overall sound - Try adding the positive end of a 47 Mfd (@525 volts or higher) cap to the section of the quad cap that has the two red wires and one choke lead. That's one of the "20" sections and the one that faces the back of the amp. This is the section that powers the output transformers. Dynaco got away with just 20 Mfd here probably because vinyl records don't have as wide a dynamic range as modern digital sources like CD's. This is a good "tweak" if you notice a crackling sound on peaks when playing your 70 at high volumes.
Bob Latino