Technics RS-1500US servo issues newbie post

[steveidosound]

First the good news. I WAS going to modify one of my older Ampex 350 series or Otari MX-5050 4channel machine (not 1/4 track) with another head block, or one of my other decks to be used with my Seduction kit for Tape Project tapes.
I already have 2 tapes so I am anxious to get something going !
Today I became the owner of a well used,  partially non-working Technics RS-1500. One of the toggle knobs on the right is missing and the deck is dirty but otherwise not that horrible. The heads look OK and it is, being a 1500 1/2 track play/record  with 1/4 track play. In initial testing, it powers up and fast forward and rewind work, but play does not. The pinch rollers engage and it starts to move then slows to a stop and then goes back into stop mode. So my guess is capstain motor and servo mechanism. Anybody know where to start with this particular malfunction? Otherwise it looks like it might work OK ! A related question is the idler which the tape wraps around at the bottom seems to have an inner and outer ring with 2 little dots on them that turn separately, then the flat spanner nut in the middle. What is that all about? I guess because of the red LED on the outer edge that this is part of the speed strobe sevo mechanism.
In any case, because the former owner was frustrated and the deck was not pristine by any means, I was able to pick it up for almost nothing. So I have some refurbishing room even within my limited budget.
Hope I am not going over material previously covered. I did try to look for this problem elswhere on the forum.
Thanks - Steve

[ironbut]

Hi Steve, welcome to the forum. I don't know if you checked out this other thread but it may be the same problem.
http://www.tapeproject.com/smf/index.php/topic,530.0.html

[steveidosound]

Hi, Thanks. I did see that thread. My issues are with the capstan motor. FF and rewind work properly. Take up reel torque and braking seem OK too. I have also found that the machine passes signal input to headphones and tape (if you help it along by applying more take up reel torque) to phones via either playback head. I have not tried to test record with it not moving tape properly. I hope it is not the capstan motor itself ! Hoping for a typical problem with these  machines with an easy fix. The capstan has never turned on it's own when fooling the machine into play mode with no tape. With tape threaded, it tries to pull tape but take up torque is not sufficient to cause it to keep going so it grinds quickly to a halt and goes back into stop.

[ironbut]

So, let me get this straight,.. without tape threaded, and with the machine on, if you pull down one of the tape tensioners and look at the capstan, does it move at all and if so, does it seem to be rotating in a steady motion ( you can put a piece of scotch tape on the face of the capstan and put black dots on it at opposite points, watching them will make it easier to see the rotation). If it doesn't turn at all, try unplugging the machine, unscrewing the 4 feet and 2 securing screws on the back panel. Once you get the back off, you'll see the huge flywheel on the capstan. Try turning it by hand. It should feel heavy but turn like 'budda' .
Also, a little background on diy might be important so we know how far you might want to go.

[steveidosound]

Hi, thanks for your help,
Yes the capstan is free and turns smoothly but does not rotate under electrical power on it's own. If you fool the tape tensioners with no tape threaded the take up spins the supply provides back tension, the pinch rollers engage, and the capstan does not turn. The cue control the variable speed in or out, pot up or down on any of the 3 speeds. I was asking if the red led and strobe markings on the idler at the bottom have anything to do with motor speed control as read off of tape speed or is that just there for show? I did remove the rear and tried to figure out where the capstan motor wires go, wiggled connectors etc.
I have the service manual on the way. I presume it to be something in the servo system for the capstan motor or (hopefully not) the motor itself. I was just hoping, since people had been restoring these machines, I would get someone to tell me a "typical" failure mode for this or something like "there is supposed to be a dummy plug in the remote socket" (which there is not BTW).
I have a fair bit of DIY experience. I am not afraid (with the service manual) to get in there with a volt meter and 'scope and try to figure it out.
Otherwise, with the exception of recording which I don't want to try by dragging a tape through the machine via take up torque, the machine passes signal through and passes signal from either playback head to phones out.

[docb]

In my experience it's not a common failure mode for the capstan motor to quit. Sounds like you are on the right track in checking for loose connectors first, then checking voltage on the capstan controller board against the manual. Check to make sure the switches are clicking when the tension roller arms are moved up and down too.

[steveidosound]

Thanks again. I was hoping for something simple like a remote plug or an optical sensor. I gather from what you are all saying that the strobe and LED on the idler has nothing to do with controlling the capstan servo  by directly measuring tape speed.
I am positive the tensioner switches work because releasing them in fast wind mode causes the machine to stop. In other words they seem to be functioning as control devices.
New Question. How does the machine control tape tension? I don't see the typical large/small reel switch. I thought perhaps the position of the tension arms but this does not seem to control reel torque as in some other designs I have seen. They seem to be simple switches. In fact I can see them being actuated with the back removed.

[ironbut]

The strobe is just that. It is only for checking the actual tape speed.
There isn't a large/small reel switch like some machines have. There are a couple of trim pots on the reel motor control board that adjust tension via the reel motors so I doubt if that would be your problem. The capstan control board is on the right hand side (looking in the back) kinda behind the remote/power cord area. When you get your service manual you should be able to trace the board to board connections. I think the flow would be power supply>main control board>capstan control board. The multi pin connectors aren't the most positive connections in the world so hopefully, it's just one of those or just a loose wire.
Hey, just for yuks, check to be sure the timer start button is in the out position. The deck shouldn't work at all if it's depressed (or rather, depressed and turned) but I noticed that it's mounted on the capstan board (along with that row of buttons/knobs on the front panel) so what the heck.

[cagedodger]

A related question is the idler which the tape wraps around at the bottom seems to have an inner and outer ring with 2 little dots on them that turn separately, then the flat spanner nut in the middle. What is that all about? I guess because of the red LED on the outer edge that this is part of the speed strobe sevo mechanism.
Thanks - Steve

Steve, I'm new to this forum but not Technics RS-1500s. The marks are for tape editing. When you slide the cue switch to the left, the arms that keep the tape off the heads (on either side of the head block) allow the tape to contact the heads. You can now roll the reels from side to side, moving the tape across the playback head manually to locate by sound a specific spot on the tape, which will be right on the head. Once you do this, turn the inner ring on the reversing idler to line it up with a dot. Now turn the reels manually so that the tape moves forward (left to right) so that the other dot lines up with the inner ring. The spot that was on the playback head is now at the small ridge (indexing mark on the illustration on the top of the cabinet) You can press it against the ridge with your finger to make a mark for cutting and splicing. This only works for forward (left to right) play as there is only one indexing mark.

Hope this helps.

Scott

[ironbut]

Well, what do you know! That answers two mysteries! Thanks Scott, nothing like first impressions. I figured that the marks on the reversing roller had to be there for something, but until now, I just had no idea what those two guys above the capstan and first guides were. Thanks a bunch,.. and welcome to the forum.