Capstan motor problem in reel-2-reel tape recorder - Page 1
Forum section: electronics / repair

author: ascomm
author_rank: Contributor
timestamp: February 07, 2019, 11:24:26 am
content: HiI'm in the middle of confusion with capstan motor problem in 80's Akai reel to reel recorder. The motor starts when powering up the unit but stops shorty after and starts randomly since then for short times. The motor is split phase AC motor with capacitor. I have measured that the power supply has constant voltage (81V AC) so that can not be the problem. To my knowledge the motor should spin always when the tape recorder is powered on regardles of the operating mode of the player/recorder.What can be the problem?

author: ascomm
author_rank: Contributor
timestamp: February 07, 2019, 11:26:32 am
content: Btw: All of the components shown in the schematics are ok.

author: soldar
author_rank: Super Contributor
timestamp: February 07, 2019, 11:31:09 am
content: If everything is OK then the signal at the base of the transistor is what is controlling the motor. What signal do you have there?

author: paschulke2
author_rank: Contributor
timestamp: February 07, 2019, 05:22:53 pm
content: The components shown are most likely part of a frequency servo (there should be an inductive sensor for the speed of the capstan motor). Knowing the exact model and seeing the schematics of the speed regulator would be helpful …My first suggestion would be to check the speed switch and/or the trim pots for the speed.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: February 07, 2019, 05:41:57 pm
content: What are the impedances of the motor coils? Do they change when you tap the motor?

author: ascomm
author_rank: Contributor
timestamp: February 07, 2019, 06:36:47 pm
content: Model is Akai GX 625. The impedances are: left side: 100ohm, right side 158ohm. Neither one doesn't change when tapping the motor. There is no signal at the transistor TR13 base if the motor is not spinning.Here is the schematics of the whole servo control circuit.The upper capacitor (motor cap) consists of two parallel connected capacitors: 3,5uF and 1,5uF /180V AC capacitors.

author: ascomm
author_rank: Contributor
timestamp: February 07, 2019, 07:23:39 pm
content: I can make the motor speed up randomly if I touch the TR13 base with wire connected to nothing. The base voltage is around 8 volts (DC) when the motor is not spinning and it drops to somewhere around 1 volts when the motor is speeding up. There is somekind of waveform then but I've been unable to get that in scope screen as it lasts only a short time.

author: ascomm
author_rank: Contributor
timestamp: February 07, 2019, 07:50:16 pm
content: Seems pretty weird waveform on the junction of C18 and R30. Motor is not spinning at this time.

author: philipz
author_rank: Contributor
timestamp: February 07, 2019, 08:21:52 pm
content: Quote from: ascomm on February 07, 2019, 07:23:39 pmYou make some mistake. It is not possible to has 8V DC on base (on end of multimeter connected to base of TR13 and other end to ground or to emitter of TR13)! On the other hand 1V DC is still to high (if the TR13 is not Darlington). Please, check one more time voltages and share results.

author: ascomm
author_rank: Contributor
timestamp: February 07, 2019, 08:45:18 pm
content: Quote from: philipz on February 07, 2019, 08:21:52 pmYes it is. I just checked it once more. Emitter --> Base: 8,0V and Emitter --> Collector: 108,2V.

author: soldar
author_rank: Super Contributor
timestamp: February 07, 2019, 09:04:36 pm
content: Quote from: ascomm on February 07, 2019, 08:45:18 pmSomething is wrong here.

author: Gyro
author_rank: Super Contributor
timestamp: February 07, 2019, 09:28:34 pm
content: It sounds as if the Base connection of TR13 has gone open-circuit internally, unless there's a dry joint between the Base lead and the measuring point.Edit: Looking at the schematic, there's something special about the resistor between TR12 Collector and TR13 Base - FR1 100 ohms. It could be a fusible resistor, check that it's not open circuit. It wouldn't explain the 8V on TR13's Base though.

author: Nusa
author_rank: Super Contributor
timestamp: February 07, 2019, 11:01:55 pm
content: Intermittant cracked traces and/or solder joints?

author: SilverSolder
author_rank: Super Contributor
timestamp: February 08, 2019, 12:49:28 am
content: Are you able to measure the strange transistor voltages on the actual legs of the transistor?

author: Armadillo
author_rank: Super Contributor
timestamp: February 08, 2019, 03:26:56 am
content: You should check C16 and C17. Why don't you probe or measure there as well.

author: Armadillo
author_rank: Super Contributor
timestamp: February 08, 2019, 02:11:55 pm
content: The attached picture 1 & 2 show the problems as you have measured. The voltage between C18 and R30 will soon dies down after the capacitor is charged fully.Picture 3 show the actual situation of a proper current and drive....The problem is...........

author: Armadillo
author_rank: Super Contributor
timestamp: February 08, 2019, 02:20:01 pm
content: The problem is,You need a voltage high enough at the base of TR13 to activate the drive.Possible FRL100 is open or C17 is leaking.To verify the TR12 and upper stream is the problem, you can hook a resistor so that 2V is applied at FRL100, the motor should rotate continuously.

author: StillTrying
author_rank: Super Contributor
timestamp: February 08, 2019, 02:54:52 pm
content: With the strange AC and floating voltages which will be at the collector of TR13, I'd think the DC voltage across C17 would be a good place to monitor what the motorshouldbe doing, similar to what Armadillo is saying.If the voltage across C17 rises when the motor is stops, the problem is around TR13, or the motor.If the voltage across C17 falls when the motor is stops, TR13 is doing the right thing, and the problem is around TR12 or before.

author: Armadillo
author_rank: Super Contributor
timestamp: February 08, 2019, 02:57:39 pm
content: If upper stream is good then test TR13 by jumper collector of TR13 to GND.If still don't rotate, then suspect motor.

author: StillTrying
author_rank: Super Contributor
timestamp: February 08, 2019, 03:18:08 pm
content: Quote from: Armadillo on February 08, 2019, 02:57:39 pmYeah, a DVM on the 10 Amps DC range collector to emitter would do, don't forget to switch theback to volts right after the test.

author: ascomm
author_rank: Contributor
timestamp: February 08, 2019, 04:56:02 pm
content: Voltage across C17 is 10V when the motor stops (shorted TR13 C-E) and 0V when motor is spinning.I removed the TR13 and it did'n change anything. It measures ok, though, with MM diode test.Motor itself seems ok as it is spinning (and it has torque) when the TR13 bypassed with jumper.

author: Armadillo
author_rank: Super Contributor
timestamp: February 08, 2019, 05:07:30 pm
content: Did you measure the FLR100 whether it is Open circuit. How many ohms did you measure across it?Otherwise, if tested good then,Anyway Change a Transistor for TR13 and try again.

author: ascomm
author_rank: Contributor
timestamp: February 08, 2019, 05:15:29 pm
content: Quote from: Armadillo on February 08, 2019, 05:07:30 pmIt measured 100,5 ohms. So it's ok.I just wonder why the motor randomly spins up. It may be quiet for 5 minutes and then suddenly spin up for a second or two. What can cause that?None of the switches (Tape speed select, reel size) or (tape speed adjust, pitch control) potentiometers are making any difference to the randomness.

author: schmitt trigger
author_rank: Super Contributor
timestamp: February 08, 2019, 05:25:45 pm
content: When I see these late 70s, perhaps early 80s circuits, I marvel at the functions implemented utilizing only discrete semiconductors.Nowadays such a circuit would only be implemented utilizing VLSI ICs.My two Yen on the subject....I agree with Philip's statement:It is not possible to has 8V DC on base (on end of multimeter connected to base of TR13 and other end to ground or to emitter of TR13)Very likely the base has a high AC ripple component which is confusing the DMM. Can you re-measure with a scope.

author: ascomm
author_rank: Contributor
timestamp: February 08, 2019, 05:43:57 pm
content: I changed the TR13 and guess what. The capstan motor now spins like a new!Thank you all for the help diagnosing this issue. I think I'll change the C18 also because it was "overvolted" to 115V due to TR13 working incorrectly. Also I have to find 2SC2336 from somewhere (not available at my usual places where I buy electronics) or a substitute for it. Now there is just some random old NPN which I found in my "transistor box".And also the VR901 (pitch control) pot needs some serious cleaning or even replacing.

author: ascomm
author_rank: Contributor
timestamp: February 08, 2019, 05:47:00 pm
content: And here is the correct signal at TR13 base.

author: Armadillo
author_rank: Super Contributor
timestamp: February 08, 2019, 05:57:01 pm
content: Quote from: ascomm on February 08, 2019, 04:56:02 pmSo, your "MM diode test technique" needs refinement. I strongly suggest you buy the less than $15 component tester. Make life so much easier.

author: ascomm
author_rank: Contributor
timestamp: February 08, 2019, 06:02:49 pm
content: Quote from: Armadillo on February 08, 2019, 05:57:01 pmIndeed!Quote from: Armadillo on February 08, 2019, 05:57:01 pmI'm already at the eBay...