Help deciphering bi-directional motor driver circuit - Page 1
Forum section: electronics / projects

author: amaschas
author_rank: Regular Contributor
timestamp: March 26, 2024, 06:22:56 am
content: I'm working on a repair to an old film processor (Jobo CPE2) for which there is no existing published schematic. I photographed the board and diagramed the circuit, hoping it would illuminate the issue, but I'm having a hard time determining how it works. The circuit is composed of three primary components, an M74HC4060 14-stage binary counter/oscillator, a BC546 NPN transistor, and a TCA 2465 dual power op-amp. The intended function of the circuit is for the connector motor, running at 24VDC, to repeatedly rotate for a few seconds, and then the reverse for a few seconds. I could probably replace the entire thing with an ATTINY and a cheap motor driver, but I kind of want to repair the original and document the schematic for others (assuming I actually got it right).I think the real puzzle for me is how the the time period is measured by this circuit. I would think that whatever switching that is happening between the two op-amp outputs, it would happen at the frequency that pin 3 (Q13) of U2 is outputting, which is occurring at ms per pulse, not seconds. I've attached the aligned photos I used to work out the circuit and my attempt at a diagram. Any help would be greatly appreciated, both by me and presumably anyone who tries to repair one of these things in the future.

author: moffy
author_rank: Super Contributor
timestamp: March 26, 2024, 07:57:56 am
content: You have a divide by 2^14 counter output on Q14, the clock for which is set by C1, R1, R2. Q14 is a 50% duty cycle square wave that feeds U1b and its inverted output through Q1 feeds U1a, so for every change of level on Q14 the motor changes direction.

author: Zero999
author_rank: Super Contributor
timestamp: March 26, 2024, 10:41:01 am
content: Yes, the 74HC4060 will provide a low frequency squarewave to change the direction of the motor.The circuit isn't quite right. I suggest checking it.The 74HC4060 will be powered off a lower voltage, of around 5V. D1 will be a reverse biased zener diode, likely 4.7V or 5.1V. The polarity is wrong in your schematic.Check the resistor values. They look off. I'd expect they're all E24 numbers i.e. base units of 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91, not random values like 44 or 55.The 10µF capacitors will be polarised.

author: amaschas
author_rank: Regular Contributor
timestamp: March 26, 2024, 07:33:59 pm
content: Thanks for the help, I think I was actually getting confused by the clock signal, because when I zoomed out I was able to see that there are two levels of oscillation on Q13, and one of them does in fact occur ever few seconds. I've gone back and checked everything and updated the schematic as well.I believe I've also chased down the fault in the board. Some solder joints appear to be loose, particularly a joint on the capacitor that sets the counter timing.

author: moffy
author_rank: Super Contributor
timestamp: March 26, 2024, 09:02:36 pm
content: Nicely done and thanks for the update.