Hello,
I could have put this in the repair section maybe, but i plumped for here. I have this Pioneer ct-f700 deck which has a slightly low output on the main rail which should be 24v.
(spec are figures in black pen) i'm getting 22.7v (figures in blue ink) on the o/p. Downstream, when i try to calibrate the playback level on the Dolby board, i'm 30mV short - the trimpot is at the max it can go.
So my thinking is that if i get back to spec on the power supply board, then i should be in with a chance back on the Dolby board.
I replaced all the electrolytics in the attached circuit after initially only replacing the 33uF one under the base of Q1. Just replacing that one gave me 1/3v more on the output. However, replacing all the caps brought me back where i started at 22.7v o/p.
Ok, so my brain finally twigged the Zener wz-250 was measuring 24v across. It should be a nominal 25v according to the little info i can find on it. With a bit more current it might be up to 25.2v in circuit, which, with the two base/emitter transistor drops would output 24v hooray.
So i'm wondering whether to go looking for a replacement Zener, or to add two diodes in series with it to bring the base of Q1 up.
And a general question about the notion of old Zeners having a drooping voltage in old age (1978 deck)?
Thanks