10-09-2015, 07:02 PM
(This post was last modified: 10-09-2015, 07:13 PM by Old Sparky.)
Sorry about the poor resolution, Mark. Having to limit the file size to post on the forum doesn't help. In the meantime, thanks for all the advice. Over the years I've made up quite a few 'Radio/Aux adaptors for other people, and fitted them to several different makes & models of radio. AFAIK, there have been no problems, but I will consider the mods. you recommend. One thing puzzles me with the circuit as it stands, which is, having, as I usually do, disconnected the radio/tape o/p from the top of the volume control, and inserted the c/o switch at this point, I expected to read a DC resistance between the 'aux-in' point and ground, since, with the switch in the 'aux' position, this is connected directly to the top of the Volume control, which, in the case of the 108SR(an 8-track player incidentally)is 25k Log, with a 1k fixed r. to ground from the loudness tap (At least that's what's shown in the diagram!). With a DMM I don't read a constant DC resistance, but rather a rising resistance going up to infinity as if a capacitor was charging, Why would this be? If I inject, say, a 1kHz sinewave at around 200mV, the volume of this can still be controlled in the usual way, so no real problem, just a puzzle for me, and I've been servicing audio gear for almost 45 years, and used to repair 108SRs 'back in the day' (early to mid 1970s)!! Incidentally, the way I connect these 'aux-in' devices -i.e. via a switch to the top of the volume control is essentially identical to the method used by manufacturers back in the 70s to allow a car tape player or recorder to be used via the o/p stage of the radio. Apart from the source medium, the only difference is that the tape players had an o/p at nominal 'line' level & impedance, whereas of course most, if not all portable CD< MP3 players, I-pods, etc., only, AFAIK, have a headphone level o/p.
