Pioneer RT-909 Trouble

S

snailman

Member
Recently my RT-909 stopped working. Everything lights up, but when a function button is pressed, the tape jumps foreword just enough to open the shutoff switch in the tension arm.

After taking the unit apart, I found that the capstan motor is not turning.

Any idea what could be causing this?

Thank you in advance for any help!
 
Capstan Belt driven on the RT 901 and RT 909. Capstan motors not the most robust on these machines either, DC and a bit light for the job of moving large reels of tape.
 
The belt was my first thought, but the belt is intact, and the capstans turn if the motor spindle is turned by hand.

When I listened to it yesterday, I noticed it was making a loud throbbing hum in during playback, which I haven't heard before.

Does this mean I most likely have a bad capstan motor?
 
Capstan belts must be correct, and must be done on time. Otherwise, those capstan motors get more stressed than is necessary. And need replacement every few years. This Pioneer performs well, deserves your special care and maintenance.
 
I replaced the capstan belt and pinch rollers when I bought it back in December, as they had become sticky.

Up until now everything was perfect with it. The heads look as though they had been relapped, electrolytic caps appear new, etc.

Other than some small scuffs, it looks like whoever owned it before me took good care of it, and it sure sounds as such:music:
 
Today I removed and tested the tension arm microswitches. The left switch was bad, so I jumped it to test, but now no functions engage.

I then tested for voltage across the microswitches, and got 0.5V.

Is this voltage normal?

Any suggestions are much appreciated!
 
Yes, I have a pdf of the service manual, and have been working through it.

As of now, I'm thinking it's probably going to be a bad power transistor.

As for the reel size, I am using 7" reels with the selector set to 7"

Just out of curiosity, even if it was set wrong, wouldn't that just increase the torque (and potentially damage the tape)?
 
If the capstan motor can't turn (seized), you need a parts unit. If, when changing the belt, the bearing balls at the rear of the capstan shafts were lost, and the unit got rigged to work without them (I've seen it), you could lose the capstan motor drive or motor or both. An old machine that has not been run in a long time should be freed and lubricated (tri-flow or something similar), or there could be problems with overheating. The possibilities are endless.

Switches should be tested with an ohm meter, with power removed. Micro-switches have no wiping action and should be replaced (not cleaned) if not measuring. If switches were bad, the heads would not lift for play.

If the hum is not in the audio but in the unit as a whole, and the capstans are not turning, but the heads do lift, my vote is for bad capstan motor and/or drive circuitry. I don't have time for remote troubleshooting. Maybe markthefixer is still doing remote troubleshooting. There is a logic chip that is only available from a parts unit, but the discrete transistors do have modern subs.

Good luck,
Rich P
 
Last edited:
Well, I found the faulty transistor!:D
Q214 has ~40V going in, and ~0.5V out. If an function is pressed, it increases to about 1V
When removed, and tested, it is indeed faulty.
Are there any other components that this transistor would've taken with it?

Rich, the motor turns fine, and the capstan system is mechanically sound. I gave this unit a good look-over while replacing the belt, and everything seemed to be in good shape.

The microswitches are indeed bad, as when they even close, they measure about 15 Ohms

As for the hum, I was making a recording off vinyl onto a tape for about an hour and a half and didn't hear it until the last 2-3 minutes. The hum was not in the audio, and I've run this deck for much longer periods of time without hearing it.
 
I don't remember the schematic now, but the 909 I've fixed some months ago with this same fault, had a bad voltage regulator feeding the capstan motor, I've replaced 2 transistors and one zener. I remember there was a zener at the base of one small transistor, biasing a larger transistor, I've changed all 3 parts.

Check the zener current limiting resistor just in case it was stressed and went burned.
 
Good info in this thread: http://www.audiokarma.org/forums/showthread.php?t=162638

OP never got back to us to tell if he fixed it, but the H-Bridge transistors and subs, as well as testing the motor itself are good. If it is the logic control chip (not likely) you are screwed until you find one from a parts unit.

Decent input voltage to a voltage regulator (Q214), with no output speaks of a bad voltage regulator (and/or assoc. circuitry) and/or a short on its output (H-Bridge and its control).

Good luck,
Rich P
 
Last edited:
Here is (from a 901, but the circuit should be the same)

I've replaced the darlington Q214, the other transistor Q245, and the zener ZD201. That should be 25V zener.

rt-901-capstanPS.png


I hope this helps.
 
Last edited:
Am in the process of removing/testing other transistors in the circuit.
Will post results when done.
Thank you for the all the help so far!
 
If you have 25V at the Q214 base (without the transistor installed) the zener is OK.

I think darlingtons just go bad because their age.
 
24V measured between the base and ground?
(It's been about 7 years since I last worked on something at the component level, so I'm a little rusty)
 
Yes, it should be 25V. If the zener were bad or shorted, you'd have less or zero....

If you have the part, replace just the darlington and try. If you need to order online, buy all 3 parts , since they are cheap.
 
Looking at the schematic, without the Zener and the darlington installed, you should have 37V at the Darlington base point (R241, both sides). If you don't have voltage at the Base point (R241 end), R241 is probably open.

Without the zener regulating at 25V, you should have the full 37V at R241, at both sides of the resistor.

ALL VOLTAGES REFERRED TO "GROUND", except when somebody tells you "measure across" xx device.
 
R241 reads exactly 2700 Ohms, but Q245 is dead.

Will Q245 being removed impact the reading at Q214 base?

Also, what modern equivalents are there for the 2sd686, the 2sc1382-Y, and the is2473 zener diode, and what is a good site to buy them from?

I am currently testing Q210 - Q213 and D221 and D222.
So far Q210 is good.

EDIT:
Accidentally typed wrong component #.
 
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