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Technics RS-1500 Repair Diary

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Bobs_Bench:
Hello All,

I?m new to this forum.  I?ve been lurking here for a long time but I decided to join now so I could post.

I love the sound of tape.  I have three Technics RS-1500 tape machines.  They are all stock and in working order.  The third machine I bought didn?t function properly at first.  I bought it with the intent of using it to learn how to repair and adjust these machines.  And if I couldn?t fix it then it would become a parts donor machine to support the other two.  I also have about 400 vintage commercially pre-recorded R2R tapes that I'm slowly playing my way through.

Well I ended up fixing it so now I have three fully functioning machines.  Then Charles King suggested I document my repair experience on this forum.  He and I met at a NNETG meeting; Thank you Charles.  Maybe this post could be part of an ongoing thread that would be a repository or diary of problems and solutions to debugging and repairing the RS-1500 machines right down to the component level.  Perhaps similar threads could contain this information for other brands.

Over time as problems arise with the machines I own I would add that repair experience here and maybe others could do likewise.

I have a background in electronics.  I?ve been working as an electrical engineer since 1982 with a heavy concentration in analog circuit design including CMOS integrated circuit analog design.  I?ve been a long time student of audio electronics although I?m not much of a do-it-yourself type since I get my fill of electronics at the day job.

Although I?m not a Tape Project subscriber I?m glad new master quality reel to reel tapes are being made available.  I think this helps to keep the format alive.  I hope the business continues to be successful.

I welcome your comments and suggestions on the content, level of detail, errors and so on.

The following assumes that you have a service manual with schematics.

OK, so here it goes:

Technics RS-1500 Repair Diary:
Symptom: Machine went into rewind mode immediately after power up and none of the turbo touch keys responded.

Solution: Replaced TR817.  This transistor was not switching as intended.

Result:  The deck powers up in the correct state and all controls respond normally.

Symptom:  Blown PCB pad at the ground lead of C807 in the supply reel motor torque control.  I discovered this by visual inspection.  This must have happened while I was repairing the control problem.

Solution:  Removed C807 and repaired the missing pad with a small piece of copper tape and solder.  Replaced C807 in the supply reel circuit and the corresponding C808 in the take-up reel circuit as a precaution.

Result:  The Torque Control sawtooth ramp looks normal on the oscilloscope.

Symptom:  Take-up tension arm oscillated up and down every once-around of the take-up reel in playback mode.

Solution: Replaced pre-driver transistor TR732.  It had low current gain (beta).  Also replaced pre-driver transistor TR734.  It had completely failed and wasn?t driving the base terminal of the motor driver power transistor TR740.

This did not fix the tension arm oscillation but now all motor control and driver waveforms are pro forma.

Next I checked if the motor had a ?flat spot? where the friction of rotation increased at any point in its rotation.  And sure enough it did.  A subtle change in torque could be felt that occurred once per revolution.  I did this experiment with the motor removed from the machine in anticipation of having to repair it and also so it could spin free of the brakes.

To inspect and lubricate the take-up reel motor shaft I remove the three screws at the rear of the motor. There are three layers held on by those three screws, the rear plate, a black spring plate and a plastic thrust plate.  I kept these together and in the same order after I remove them.  I didn?t touch the reel table height adjustment screw and thereby avoided having to readjust it later.

 A c-clip is now exposed that prevents the motor shaft from being pulled out of the motor when changing reels.  Once that and the washer behind it were removed the shaft pulled out easily by pulling hard enough to overcome the magnetic force.

Visual inspection of the shaft and sleeve bearings turned up nothing obvious. I applied a small amount of turbine oil (Zoom Spout) on the shaft, reinserted it, spun the rotor to work the lube in and tested for smooth rotation by hand.  I did this process several times until I could no longer detect any torque variation.  Once satisfied I reassembled the motor and installed it back into the deck.

Result:  The take-up tension arm remains steady in playback mode.  The machine is now fully functional and sounds great.

ironbut:
Hi Bob,

Welcome to the forum.
What a fantastic first post!
Do you have any idea why the issues with TR732/734 had cropped up?
One machine that I've worked on does exhibit distorted waveforms.
The next time I see the machine, I'll try replacing these two transistors. Are they commonly available?

Bobs_Bench:
Hi Steve,

I don't know why the transistors I replaced went bad.  I was surprised that I discovered any bad transistors at all.  I was expecting bad electrolytic caps more than I was expecting bad transistors.

I think the machine sat unused for perhaps a decade or two.  The reel motor I removed had a "foot print" of the brake pads on the brake drum.  The pads must have been sitting in one spot on the reel motor brake drum for a very long time.  I'm surprised I didn't have more electrolytic caps failing from lack of use the first time I powered up.

The transistors that I had to replace were not in the audio path.  They were in the motor driver circuit. I don't think they would have caused distortion in the audio, unless having one bad reel motor driver phase caused some sort of flutter in the tape tension.

I'm not sure if the transistors are commonly available but I was able to find them using Google.  B & D Enterprises is one place to look.  Modern bipolar junction transistors can also be found with similar enough specs to be replacements for the  unobtanium devices.

As for your distorted audio problem I would suggest you first use a DC voltmeter to check all of the DC bias voltages along the entire signal path in the audio playback chain.  The service manual has the DC bias voltages documented in the schematics.  If they all check out then I would suggest playing a tape with a continuous tone, say 1KHz, and follow the signal through each stage of that same playback chain with a scope.  And yes, this is all rather tedious but it's the only way I know of isolating problems.

All this assumes that there's nothing wrong with the heads or the tape tracking over the heads.

I can try to help with this inside this thread if you would like.

ironbut:
Hi bob,

The waveforms that were distorted were measured from the motor control board, not the signal.

I've never found a failed repro or record amp yet (that didn't respond to cleaning of contacts that is). Most all the issues are with the reel and/or the capstan motor drive.
I have found a few machines with problems which stemmed from the IC's on the main control board. Those are available from some of the large parts houses in China.
The symptoms I've seen from them is one or two of the speeds won't engage properly.

Bobs_Bench:
Hi Steve,

Sorry about that, I saw the word distorted and immediately assumed it was the audio.

Is there a machine you have that's having these problems right now?

If one or two of the three speeds works that implies to me that the basic speed control might be working.  That's potentially good news.

Is the problem showing up in the capstan motor or the reel motors or both?

Aside from the capstan motor the reel motor control circuits also receive a "programming" signal from the speed selector switch.  It's the magnitude of the current from the current sources formed by TR809 and TR810 that are set by resistors that are switched in based on the speed selected by the user.  This current is then translated into motor torque.  There are adjustment potentiometers to calibrate those current sources to get the correct tape tension.  Those pots could be a problem.

If you identify the points in the circuit where the waveforms are distorted and how they're distorted we can try to work this problem together if you want.

Going through this would also add information  to the knowledge base on this site and hopefully enable folks to be more self sufficient in servicing their own machines.  The prospects of subjecting a fragile 70 pound machine to the rigors of shipping to some place halfway across the country and back is a recipe for high anxiety and possibly a machine that's beyond repair.

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