Over the weekend I dusted off a stock 400 that had been sitting on a shelf with the intent of repairing it. I suspected issues with the front end in the right channel. I pulled Q1-Q4 and checked them on a curve tracer. They looked ok. I changed them out anyhow. Normally I don't go for the shotgun approach but I made an exception here.
Next was the 2 power supply caps. The originals were GE and to my surprise they were 7300uf/75 volt. Not the usual 5900uf. Date code is 7420 (20th week, 1974). They were slightly misapplied given the DC voltage runs at 80 volts and these are only rated for 75 volts.
For the heck of it I checked the ripple voltage on them with both channels operating at 40 volts output into 8 ohms. On a good cap the ripple should be less than 1 volt when measured with a true RMS meter. One was .9 volts and the other was about 1.1 volts. These caps are almost 50 years old and still functioning. I went ahead and put in the usual 12,000uf with Joe's boards.
Today I checked the old caps with a $20 transistor tester. Here is the link (no I have no affiliation). EBAY transistor tester I find myself using this thing more and more to match transistor gains and verify device types. It also does capacitors . So first I discharged the GE 7300's and then tested them. One was 7200uf and the other was 6800uf. ESR was in the neighborhood of .15 ohms. Did I mention they're almost 50 years old?
I also checked some 15,000uf/100v caps that I just got from Mouser. They tested 13,700uf with ESR of .09 ohms. The spec is +/-20% on capacitance and ESR of .22 ohms so they meet spec.
Several years ago electrolytics were always on the high side of the spec. Today, manufacturers are more consistent and you don't get anything for free.
Next was the 2 power supply caps. The originals were GE and to my surprise they were 7300uf/75 volt. Not the usual 5900uf. Date code is 7420 (20th week, 1974). They were slightly misapplied given the DC voltage runs at 80 volts and these are only rated for 75 volts.
For the heck of it I checked the ripple voltage on them with both channels operating at 40 volts output into 8 ohms. On a good cap the ripple should be less than 1 volt when measured with a true RMS meter. One was .9 volts and the other was about 1.1 volts. These caps are almost 50 years old and still functioning. I went ahead and put in the usual 12,000uf with Joe's boards.
Today I checked the old caps with a $20 transistor tester. Here is the link (no I have no affiliation). EBAY transistor tester I find myself using this thing more and more to match transistor gains and verify device types. It also does capacitors . So first I discharged the GE 7300's and then tested them. One was 7200uf and the other was 6800uf. ESR was in the neighborhood of .15 ohms. Did I mention they're almost 50 years old?
I also checked some 15,000uf/100v caps that I just got from Mouser. They tested 13,700uf with ESR of .09 ohms. The spec is +/-20% on capacitance and ESR of .22 ohms so they meet spec.
Several years ago electrolytics were always on the high side of the spec. Today, manufacturers are more consistent and you don't get anything for free.
