Pioneer RT-701 Speed Control Issue

audiojones

audiojones

Jonesin' for audio
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Hi -
I originally posted this in the Pioneer forum, but this might be a more appropriate place for it. I've got an RT-701 reel to reel deck that has speed control problems. It's running way too fast and very warbly sounding. I cleaned the pots and the record button (all the buttons, actually) but it didn't help. I did adjust the trimmer pot on the servo board marked "co" and with it dialed all the way in the unit just barely slows down enough (and it's still all warbly sounding). Anyone have any ideas? Should I just start replacing servo board transistors and caps, or are there test points that I can check to narrow it down? I know I should get the service manual and do it right, but I just thought I'd check here first due to the wealth of knowledge and experience (plus kindness) of AKers.

I should mention that the PO did not take care of this machine at all and that it hasn't been stored properly, so hopefully I can bring it back from the edge. Any help greatly appreciated!
Thanks
 
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I remember when I used to work on those, the capstan motor itself, specifically the FG coil that generates an AC signal that the servo uses for motor speed control, would go open. If that goes open the motor runs out of control, too fast. I used to keep a spare RXM-033 motor on my bench to plug in and make sure before replacing. When running there should be an AC signal present maybe 6 to 7 V P-P (according to the manual Fig 38 pg 79) at motor connector pin 8 and IC901 pin 6 on the servo amplifier board. You can probably unplug the connector and check the continuity of the FG coil with an Ohm meter on a X1,000 setting. You will likely need an oscilloscope and the service manual that's downloadable to troubleshoot the circuit if the motor checks out OK. Probably with the age of these decks now, other things are possible too, like electrolytic capacitors. Good luck!
 
Thanks Ed - That gives me a great starting point. I'm assuming that the brown shielded wire coming form the motor is the FG lead, I'll start there. I don't see any IC chip on this servo board - all discrete devices. I see a space reserved for an 8 pin IC, but nothing is there - maybe this is an early model or the chip was for the auto reverse version? Serial # YD3607249
 
audiojones said:
I don't see any IC chip on this servo board - all discrete devices. I see a space reserved for an 8 pin IC, but nothing is there -
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Take a look at the attached pic from the RT-707 manual from the AK manuals database. (Don't see a 701 manual) The FG amp IC is 7 pins inline.

You may be seeing the empty space where the motor reverse relay would be on a 707.
 

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Correct-o-mundo, Ed. I see the IC, and I was looking at the relay socket for the reverse version. Thanks again, I'll use the 707 servo diagram as a LOOSE reference for the 701.

*ADD-ON* - I assume that the FG IC is unobtanium?
 
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I'm certainly no expert, but you might check to see that the pinch roller isn't headed or already gone *south*...gummed up. I just had to get mine rebuilt for my 909. It's the capstan-pinch roller-tape relationship that controls the speed, check the easy stuff out first before tearing into the electronics to see if the motor runs too fast. Just my opinion.
 
Confirming the pinch roller pressure should have been your first move. Adjusting or replacing components should only be attempted after resistance or voltage tests are made.

The pinch roller pressure sets the speed as the tape runs past the heads. The pressure MUST be set to spec. From there, tension confirmation is necessary.

Ron
 
jblmar said:
Confirming the pinch roller pressure should have been your first move. Adjusting or replacing components should only be attempted after resistance or voltage tests are made.

The pinch roller pressure sets the speed as the tape runs past the heads. The pressure MUST be set to spec. From there, tension confirmation is necessary.

Ron
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Thanks - I should have mentioned that I did check out all of the mechanical components first. I was hoping that's all that was causing my problem, but it is electronic in nature. I've taken voltage readings and something's way off, also the voltage is dropping/peaking very rapidly so I'm investigating further...we'll see :scratch2:
 
audiojones said:
Thanks - I should have mentioned that I did check out all of the mechanical components first. I was hoping that's all that was causing my problem, but it is electronic in nature. I've taken voltage readings and something's way off, also the voltage is dropping/peaking very rapidly so I'm investigating further...we'll see :scratch2:
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10-4. I know how easy it is to omit details from these kind of 'help me' posts.

Keep us posted!
 
Got it!

I replaced all the Q's (10) and caps (12) on the servo board of the RT-701 (a total of about $5.00) and much to my amazement it works! I really was expecting a puff of white smoke or a flash, but it's playing beautifully. This deck looks awful - all of the circuit boards have a thick layer of crud on them and all of the wire wraps are green and corroded. It was mounted in a rack, so I suspect that it was carried around to gigs and abused - it's amazing that it works at all. It looks as though it's been under water or at least left out in the rain. The frequency response it outstanding - I've never had one of these Pioneer decks before...I've seen them around and I remember when they were new, but I've never threaded one up or heard one. I must say that I'm really impressed (and that's coming from a dyed-in-the-wool TEAC fan). I couldn't finish putting it back together because I was really enjoying the tape that was playing (Thelonius Monk). It's going to take alot of compressed air, contact cleaner and Dremeling to uncrud this deck but it's totally worth it. Now I really want an RT-909!

Here's a shot of it playing with it's guts hanging out!
 

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audiojones said:
I replaced all the Q's (10) and caps (12) on the servo board of the RT-701 (a total of about $5.00) and much to my amazement it works!
Click to expand...

Congratulations! I wonder which one of the parts you replaced was actually the culprit. Fortulately is wasn't a bad motor after all!
 
Way to go Jones!

I do like my 909, and I'm trying to get an X1000R going here now too, so then I'll have the Pioneer/TEAC comparison here too. What is it you like about the Pioneer sound over your TEAC, or is it just different in some way?
 
Not sure at all which of the components caused the trouble, but I did notice a 10uf cap coming off of pin 4 of the Toshiba TA7061AP ic chip that looked kind of bloated and leaking, although that might have happened when I was desoldering it. I did check all of the transistors that I removed as well as the ones I installed with a Q checker. None of them showed up bad, but the tester is very crude and only shows bad when there is a junction short. Usually a transistor that is misbehaving isn't dead shorted, it's just not working up to spec or it's intermittent.
The replacement Q's were kind of a PITA to install because the pin basing was different, but so many of them were the same number that I got used to pre-bending the pins and lining them up before placing them on the board.
Oddly enough the Toshiba TA7061AP 7 pin chip was commonly used in short wave radio transceivers as a frequency limiter - I think it's still available through surplus houses but you'd probably have to jump through hoops to get it and I hope I never have to.

What impressed me most about this deck (and I can't really say more than the TEAC decks without doing a side by side comparison) was the frequency response. The highs seem very crisp and clear and the whole frequency range seems to be very well represented. My TEACS are all of the A series, however I have had some X series decks and they all had very crisp, excellent frequency resopnse as well. I'm also very impressed with the build quality - it's right up there with the TEAC A series decks. Very well thought out and robust components that can obviously take a beating (you should see this poor thing!) I'm not 100% sure that I like the servo direct drive over the belt drive, only lots of usage and careful listening will determine that. I'll have to listen to some music with lots of sustained piano notes and compare the two drive systems. I don't know if I'll ever respect a deck more than the TEAC A series decks, but so far the Pioneers are right up there with them :thmbsp:
 
audiojones said:
Got it!

I replaced all the Q's (10) and caps (12) on the servo board of the RT-701 (a total of about $5.00) and much to my amazement it works! I really was expecting a puff of white smoke or a flash, but it's playing beautifully. This deck looks awful - all of the circuit boards have a thick layer of crud on them and all of the wire wraps are green and corroded. It was mounted in a rack, so I suspect that it was carried around to gigs and abused - it's amazing that it works at all. It looks as though it's been under water or at least left out in the rain. The frequency response it outstanding - I've never had one of these Pioneer decks before...I've seen them around and I remember when they were new, but I've never threaded one up or heard one. I must say that I'm really impressed (and that's coming from a dyed-in-the-wool TEAC fan). I couldn't finish putting it back together because I was really enjoying the tape that was playing (Thelonius Monk). It's going to take alot of compressed air, contact cleaner and Dremeling to uncrud this deck but it's totally worth it. Now I really want an RT-909!

Here's a shot of it playing with it's guts hanging out!
Click to expand...
Hi, by any chance do you have a list of the transistors you replaced them with, I'm having the same issue, I've already replaced the caps but I'm having trouble coming up with suitable replacements for the transistors. Thanks and much appreciated.
 
Sorry guys, that was 7 years ago and I just did it by seeing what was on the board and replacing in kind with appropriate subs. A search of the component numbers should reveal a suitable cross reference if the original isn't available (especially if you search AK, the new search function works really well). I don't recall any unusual transistor numbers in there, all pretty much garden variety stuff.

Man that seems like such a long time ago, I've had a number of RT-707's, RT-701's and RT-1011's come and go through here since then. Still got a few hanging around (actually one of em's not mine, it's waiting to be picked up).
 
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Hi,

the servo on the RT 70X machines is near enough digital in operation. It is governed by ons and offs. The input from the FG on the motor is high gain amplified in the TA chip so that it clips. This gives a fast edge to trigger a Monostable flip flop which has a variable time constant. The output from the flip flop is then filtered and the average used as the speed measurement. This is then compared to a reference from a diode/divider and the difference is the error which controls the loop.

I only have experience of the RT 707 version but the 701 should be similar. Looks to me that the main difference is the reverse circuitry.

There is a test point on the output of the TA7061. If you have a scope this should be a stable square wave which is capstan speed dependent. This MUST be stable and have no dropouts. Any errors here and the rest of the servo will try to compensate.

The next step is the operation of the Monostable comprising Q903/4. The time constant is derived from C917 and the speed control circuitry which is external to the PCB. Again these must have good connections. The speed switch and the record switch must be clean. The pot VR904 sets the span of the Variable. This must be adjusted first if it has been moved. The other 2 pots (VR902/3) can be set with a frequency counter or test tape. The pulses are effectively counted which gives the speed control. The pulse with for the slow speed will be twice the width of the higher speed to give an average voltage following the filter at Q906 base. This point is the control point. The long tailed pair measures the difference between the two bases and the error is then used to drive the power transistor Q901.

As already stated, the square wave at the TP is critical to the operation of the whole servo.

GPS16
 
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Thank You GPS

While reading post your post I felt a strong breeze and my hair standing up. It went right over my head.

The theory of operation present by you is welcomed ! In my past employment every procedure was accompanied with an explanation, a section on the theory of operation. IT is very useful when learning to trouble shoot diagnose a problem. I'm very ignorant on how these type of circuits work.

IT cause me to do some thinking which isn't bad thing. I looking forward to read more post of this type by you others who have writing chops, patience and time to invest.
 
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