So I checked out the new PL700II and all seems great...until I trun it off, then 10 sec. later one loud thump. Very repeatable. Input levels don't matter. No bad noise at shut off, just a delayed thump. Any ideas? Thanks
There was a 700 II service bulletin which was issued:
In the first year production runs of the model 400 and 700 Series Two PL36 driver boards we used a TO-99 packaged Fairchild or Signetics LF356 front end BI-FET op amp. This particular IC package was found to cause an unacceptable turn-off "thump" in some amplifiers. We later discovered and used in production a 9 pin mini-DIP packaged LF356 (National) which reduced this thump.
As of about May of 1979 all production runs of the PL36 driver board began using a National LF351 IC (Phase Linear part number 126-0114) which was found to have a better slew rate, slightly lower distortion, and substantially reduced the delayed turn-off thump. They are also cheaper.
If a model 400 or 700 Series Two amplifier has excessive delayed turn-off noise the front end IC's should be replaced with the LF351. These are a direct retrofit device and require no additional modification to the PC board.
NOTE: If you replace the LF356 with a LF351 you will also need to change out R1 and R2 (2.4K/5W) resistors to 7.5K/5W.
I also install 8 pin sockets with gold contacts.
If you have a handful of LF351's, you may be able to use trial and error replacement until the turn-off thump is reduced to an acceptable level. Changing out the IC's will also change the DC offset. (Factory spec was +/-10mV for DC offset. You should be able to get much better than that.)
So I checked out the new PL700II and all seems great...until I trun it off, then 10 sec. later one loud thump. Very repeatable. Input levels don't matter. No bad noise at shut off, just a delayed thump. Any ideas? Thanks
I don't know it that's too much or not but if Gepetto is curious it's probably a good place to look. Uncouple and post that same test if you would, or at least decrease the noise input a good deal and work up from that.