Repair a old floppy disk drive - LM2917 faulty? - Page 1
Forum section: electronics / repair

author: gmc
author_rank: Regular Contributor
timestamp: July 08, 2022, 01:15:29 pm
content: I'm busying trying to repair a old floppy disk drive for a retro computer from the 80's On power up the drive should spin, but it doesn't.I have attached the speed controller schematic - this controls the speed of the spindle using a LM2917. I have tested all the resistors, diodes, and transistors and they are all ok. None of the caps are shorted either. I haven't checked their values yet but they appear physically ok.I'm thinking the LM2917 is faulty but before I order one, and find out it's not faulty is there any way I can test it either inline, or removing and building some test circuit on a bread board.On the schematic tach + and - connect to the motor spindle and it's a magnetic sensor. This tests good.Red is connected to the motor. I've removed the motor and applied 12V (current limited) and it spins.The collector (P8) of the LM2917 is always 11.8V. In order for the motor to run am I correct in thinking this needs to go low to turn on Q1 which will then power the motor?When the motor is supposed to be off the orange connector is at 1V.When it's supposed to be spinning it's then at 4.6VDatasheet for the LM2917 (I have the pin 14 version)https://www.ti.com/lit/ds/symlink/lm2907-n.pdfThe circuity around the LM2719 is a bit of a mystery to me.

author: MikeK
author_rank: Super Contributor
timestamp: July 08, 2022, 02:52:36 pm
content: Have you applied 12V to the motor to see if it spins? Next I would ground the output, pin 8, to see if that spins the motor. Pin 8 is open collector, so it's either off (floating) or on (pulled to ground).

author: gmc
author_rank: Regular Contributor
timestamp: July 08, 2022, 06:17:19 pm
content: The 12V motor spins when powered from a external source.I've also tried pulling pin 8 to ground and it spins up the motor.So I'm guessing it mean's the LM2917 is faulty.I've also since removed all the caps and measured them and they are all within spec.

author: Andy Watson
author_rank: Super Contributor
timestamp: July 08, 2022, 06:34:52 pm
content: What is the voltage at the inputs of the amplifier ? (pins 10 and 4 - I believe). And is there a power supply to the chip - check on the actual pins of the IC?

author: gmc
author_rank: Regular Contributor
timestamp: July 08, 2022, 06:45:52 pm
content: Power on the chip is 7.2VPin 4 IN+ = 2.6VPin 10 IN- = 0VPin 8 = 11.8VPin 2 CP1 = 2.8VPin 3 CP2 = 0VPin 5 EMIT = 0V

author: Andy Watson
author_rank: Super Contributor
timestamp: July 08, 2022, 06:58:41 pm
content: Quote from: gmc on July 08, 2022, 06:45:52 pmIt's dead Jim!If the voltage on pin 4 is higher than pin 10, the ouput transistor should be on. No emitter voltage an no current through the collecttor - it's dead Jim.

author: gmc
author_rank: Regular Contributor
timestamp: July 08, 2022, 07:00:19 pm
content: Thanks for the confirmation. At least that confims it.I'll order a replacement and report back, but I'm sure it will fix it.

author: gmc
author_rank: Regular Contributor
timestamp: July 13, 2022, 12:14:09 pm
content: Just a update on this in case anyone runs into this post in future.I replaced the LM2917 and its fixed the issue.Thanks for all the help.