A year or so ago, I got interested in picking up old LP's from thrift stores. So, I dug out my early
80s Sharp RP-113 linear tracking TT. Hadn't been used in at least 15yrs and the belt was turned
to sticky goo. I got a new belt and put it into use again:



It seems to work "mostly" fine, but the tone arm usually does not auto-return to "home" at the end
of playing the record. Once in a while it will but most often I need to press the "start/cue" button to
get it to retract. I don't remember it doing that when I used it back in the 80s.

Also, I noticed that the tone arm up/down button when pressed works but the tone arm usually "pops" backwards about 1/4" when it lifts off the record and the needle is out of the groove. I don't recall it doing that
way back when either.

This makes me concerned the servo mechanism is not tracking the record correctly and is lagging behind and being "dragged" by the record grooves too much. Also along those lines, the sound is often a bit staticy/stratchy sounding with certain types of instruments like horns and trumpets and this is with very nice cond. clean records that are otherwise very quiet.

Is there some sort of adjustment or ?? I should be looking at? I have the original owners manual that includes a schematic but there are no other service info. I know from the schematic this unit uses LEDs and phototransistors to detect where the tone arm is for the servo mechanism.

Thoughts? and thanks.

You already know the platter drive belt was bad.

There's another belt for the arm-drive - most likely a very thin belt.
And lubication of the motors are probably a good thing too.

Thanks,

I took a look under the hood and there is a 2nd "belt" but its a thin steel cable. The tone arm runs back and forth on a rod, being pulled by this cable. There is a brass pulley on one end (see pic) and the other end has a nylon pulley with a gear and motor. The tone arm up/down is controlled by a solenoid.

I had the TT running with the lid off and its was easy to see how the LED opto coupler worked when the arm angle was enough to cause the motor to advance. The motor does not seem to have any problem moving the arm back and forth.

The only thing in there that remotely looked like some adjustment was the tall nylon screw I've marked. Appears it may adjust the angle the "base" of the tone arm pivots on. I've not tried to mess with it yet, but maybe will after getting a closer look at it. I don't know how it could have gotten out of adjustment if that is the problem. This unit has never been banged around as I have owned it since new and I had it stored in the original box and foam inserts.

The other thought was dust on the LEDs but it looks pretty clean in there.

You really don't want to go messing with any adjustment screws.
They're factory set and critical.

Due to age, some capacitors fail, go out of tolerance - causing the system control to malfunction.

Chances are the lube on the carriage rail(s) and bearings has firmed up with age. Clean it all off with q-tips, paper towels and isopropyl alcohol. Then relube. This cures about 9 out of 10 of these type of problems.
Rick

OK, I think I "fixed" it. At least its working better now.

I took a closer look at that nylon "screw" noted earlier. It does have a screwdriver slot in it but a modest attempt to turn it didn't move it and I was not going to force it! Left it alone.

Then, I noted that there was a small gap (as in ~.015" or so) on either side of the screw (that is more like a post). The metal bracket the LED assembly is mounted to is held on with 1 screw that has a fibre washer under the head. See picture. A careful shove on the bracket and it was possible to rotate it a small amount in either direction before one of the forks on either side of the nylon post would stop it.

So, with the platter removed, I got the unit running so that the servo would follow me slowly dragging the tone arm like a record would. I found out that with the bracket pushed to either extreme of motion against that post, I could get the arm to slightly lead or lag being perpendicular to the direction of travel. So, I tweaked around on it a bit until it appeared to be square.

Then, I put the platter back on and tried a record and verified that it was tracking square as I could determine and that it DOES now pickup the tone arm and go back to home at the end.

Maybe this thing was always marginal or it did get bumped at some point. I think that if the tone arm is pushed on manually too far, it may tweak this adjustment since the arm has stops and would cause that bracket to turn under that fibre washer screw. Could have happened when cleaning I suppose.

Yes, I did take a look at the small amount of grease on a roller and the up/down solenoid. It was not dried up. I am not sure the slider has any lube on it at all. There may be a brass bushing that runs on the slider rod. Looks a lot like the mechanism on old dot matrix printers and those were dry lubed too I think.