On Wed, 18 May 2005, Philip Pemberton wrote: > In message > Peter wrote: > >> Tape playback preamp is in f.ex. LM387 datasheet. Recording is done >> without ac bias in those things. Sometimes dc bias is used (resistor >> from +Vcc feeds through head). > > The one thing I need to work out is the bias current. 5V Vcc, with a > record/playback head impedance of 92.5mH. Same thing for the erase head - > except that's 92.28mH (yeah, I know, big difference). Something between 1 and 100kOhms should do it. The impedance is not relevant, shoot for dc current. The series resistor at the output of the recording amp can give a clue. Try a bias resistor between 10 and 0.5 times that resistor. >> The opto is nearly certainly open >> collector and needs a pullup (or pulldown!) between 10 and 100k. It is >> used to sense tpe end. > > I figured it would be for something like that. Might be good for determining > the rotational speed too. Speaking of which, I need to find out what speed > these tapes are usually run at (in normal rec/play mode). 1.2 or 2.4 cm/s . Imho forget about the solwer speed, the fast one is bad enough. In answering machines only 2.4 cm/s was used afaik. The motor usually has a built-in regulator, but only if not reversible. It used to acceptable to run it off straight dc with no regulation, the mechanical load being nearly constant. Peter -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist