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Author Topic: Technics 1500 capstan motor speed problem  (Read 11593 times)

Offline analogfan

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Technics 1500 capstan motor speed problem
« on: February 07, 2011, 09:37:24 AM »
Hello there,

I have been bringing my recently acquired "cherry" 1500 back into the world.  Everything works well at this point after working the switches/controls and keeping it running on and off for a couple of week.  It sounds great, all controls work well, FF, Rewind, play, pause, etc. work fine.  The tape lifters work as they should, all controls work great (no drop-outs) and the tape path has virtually no wear and is clean as a whistle.  Pinch rollers have good pressure and do not slip.

The one issue is capstan speed in "Normal" that is, pitch control disengaged is a bit slow and getting flutter or wow (not sure which is which) per the strobe and listening.

With the pitch control on/engaged, the speed is locked in and steady as she goes, and adjusts to the proper speed (about middle of the pot range) and no speed issue at all.  I suppose I can use it this way but I have a bit of OCD!

I do not know if this is related (probably is) but the reel motors  (both) have more torque and turn more rapidly when the pitch control is on/engaged then when off.

I have looked at the circuit topology and the big difference I can see is that there is a quartz clock IC chip in the circuit with the pitch control off.

I have both volumes of the service manuals, a digital multi-meter, a freq counter and dual-trace Tektronics.  I was a radar tech many years ago and worked mainly on tube equipment, not much solid state.  If the IC is shot I hope I am not dead in the water as I know there are few parts around for Technics R2Rs.  I have not pulled the Techics apart yet.

Can anyone help or may have seen this problem before and can suggest a good place to start?  It is probably beyond my skills, so 2nd question is, can I send the capstan control board to someone for a look-see?

Thank you, Bob Edwards

Offline docb

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Re: Technics 1500 capstan motor speed problem
« Reply #1 on: February 07, 2011, 12:27:50 PM »
I would be inclined to check the speed control switch itself. Also, have you adjusted the capstan motor control circuitry? The procedure is in Vol 1. of the service manual, part E of section 5, measurement of tape speed, wow and flutter. It's on page 21 in the RS1500US manual.
Dan "Doc B." Schmalle
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Offline ironbut

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Re: Technics 1500 capstan motor speed problem
« Reply #2 on: February 07, 2011, 01:05:24 PM »
I think Doc's right about just adjusting the speed control pot.
Usually if one of the IC's is bad than the capstan usually won't work for very long at certain speeds. So in other words, they either work or they don't.
Since the machine may have sat unused for decades, it may need a little "running in". It's a good idea to use a tiny bit of contact cleaner on the switches and pots too.
If you do need to replace one of the IC's, I have some.
steve koto
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Offline analogfan

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Re: Technics 1500 capstan motor speed problem
« Reply #3 on: February 07, 2011, 05:53:41 PM »
Thank you both, I will check out the switch as you suggested, hit it with Deoxit and go through the alignment.

More later, and thanks again!  Bob Edwards

Offline ironbut

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Re: Technics 1500 capstan motor speed problem
« Reply #4 on: February 07, 2011, 06:40:13 PM »
Bob,
Be sure and check the "internal" speed control adjustment not just the speed selection and pitch control. There have been a couple of members here who've had problems that could be traced back to iffy pitch/selection control contacts but they're usually more "gross" speed problems.
Fine speed control issues are often the result of tape path problems too. This can be from the tape itself (sticky or oily contaminants or damaged tape edge) or sticky/oily contaminants on the guides/rollers/bearings/heads. The tape tension also needs to be high enough to overcome the normal friction encountered in the tape path but not high enough to make the tape slip in the pinch roller/capstan interface.
steve koto
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Offline mossfred

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Re: Technics 1500 capstan motor speed problem
« Reply #5 on: March 01, 2011, 01:00:52 AM »
I have just experienced the exact same problem on another 1506 i have just picked up. I doubt its the pitch control knob as i would assume its out of circuit when swithed out. In any case when swithed in the speed is dead stable . I have adjusted the speed pot to get it right when switched out and its stays good for a few minutes and then starts to run slow or fast again.
Fred

Offline ironbut

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Re: Technics 1500 capstan motor speed problem
« Reply #6 on: March 01, 2011, 03:16:51 AM »
Hey Fred,
You should be basing your results on measurements done with something like the MRL speed/flutter test tape and a good frequency meter. If you're going by the strobe on the machine, it's not what I'd call instrument quality. The markings on the strobe assume that tape tension is perfect, there is zero tape stretch between the rollers and that the reversing roller/tape interface have zero slip. These aren't very likely to happen at the same instant you're looking at the strobe.
The only way to know if the tape is crossing the heads at the right speed is with a test tape made specifically to make that test. And even with that, the IEC standard is still +/- .1% @15ips. That's equal to 2 1/2 feet for a 2500' reel.
I've found that the reversing roller can be the source of slipping that results in scrape flutter. In it's stock form, it's also the source of the force that runs the tape counter. So, between the bearing friction, the mass of the reversing roller and the need to turn the belt that turns a pulley that turns another belt, that turns yet another pulley to turn the counter, there's just too darn much resistance in that gizmo! On top of that, those belts get kinda hard over the decades and usually become egg shaped. If you watch an old one move you can actually see the two "humps" move where it should be flat.
Did I mention that the tape has a lubricant(s) built into the formula so the tape slides across the heads and through the guides as smoothly as possible?

BTW, I hope I'm not sounding like some smart-aleck. Once I started writing the reasons the strobe isn't accurate, they just kept coming! Yikes!
http://home.comcast.net/~mrltapes/
steve koto
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Offline analogfan

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Re: Technics 1500 capstan motor speed problem
« Reply #7 on: March 15, 2011, 06:47:11 PM »
To the guy with the 1506 speed problem like mine, I am going to try going through the 1st few alignment steps including checking brake tension and the speed adjustment - please feel free to call me if you want to share ideas.  I will also look into the belt suggestion though I think it is a circuit issue as there are two different circuits with the manual speed adjustment and the standard speed circuit.  I doubt if it is a mechanical problem because it would show up either way.  No disrespect intended, just an old (really old) tech with (maybe) still good instincts.

Thanks, Bob Edwards/617 335-0066

Offline mossfred

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Re: Technics 1500 capstan motor speed problem
« Reply #8 on: April 18, 2011, 05:32:33 PM »
Hi Bob, i never did solve the problem! I have a little tech knowledge and some equipment but am possibly likely to do more harm than good!
i suspect if one follows the instruction in the work shop manual it might do the trick but i been a little lazy/busy buying and servicing some other interesting decks.

Fred

Offline analogfan

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Re: Technics 1500 capstan motor speed problem UPDATE AND QUESTION
« Reply #9 on: November 15, 2011, 08:01:38 AM »
Hello all,

Finely got the speed problem resolved.  The tape tension  idlers needed cleaning/lube, pulled the capstan servo board, checked it for bad diodes, caps, resistors, put the board back in, reseated the connectors, including one the was loose on the reel control board and carefully went through the alignment procedures several times.  The oscillator pot is near the end of its adjustment range but the speed issue is resolved for both fixed and variable speed.  I was surprised that there was quite a bit of interaction between the fixed speed alignment and the variable speed alignment as I thought the circuits were pretty much independent of one another.  Thanks to the forum for the advice.

QUESTION:  I found a "replacement" board for the capstan servo control board but while about 3/4s of it matched, the connectors were different.

The one in my RS1500US is Technics part #:  QJI0745RP

The replacement board (which I could not use) is Technics part #: QJI0745RP E  The "E" seems to be a later revision or for use in one of the other RS15XX family.

Does anyone know which Technics model (maybe a 1506 or 1520?) this board is used in?

Thank you, Bob Edwards