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Compact 5 CNC Feed Speed slow in Hand mode


newagedesire@...
 

I have a new to me Compact 5 CNC in the 5xxx serial range that has a bug I'm trying to locate. 
Feeds on both X and Z are running very slow only when in hand mode. 
Max speed is 2" per minute when the pot is set at the max setting.  Speed is still variable.
Single taping the X and Z buttons work to move each axis a step at a time.
The rapid traverse speed works fine.
Both axis work correctly at all feed speeds in CNC Mode. (eliminating the stepper driver circuit in my mind)
The speed adjustment pot checks out a from 0-98k ohms.
I have 13v and 50v at the 5 power pins for the steppers.
I suspect there is something after the pot but before the stepper driver circuit that has malfunctioned.
Does anyone have experience with the control circuit in hand mode?
Hoping someone has sorted a problem like this already.

Thanks,
Steve


J G
 

The drives are controlled by the CPU on the main controller board. The stepper windings are driven by the power amplifiers according to I/O line signals from parallel interface interface chips (6821 / 6520 PIO) in turn according to a sequential bit pattern from the CPU - whatever mode the machine is in.

In 'Hand' mode the CPU detects the +/- key taps and advances the pattern. When the +/- is held beyond a certain time the CPU uses pulses from a 555 (or equivalent) timer IC via another PIO. The 555 pulse rate is determine by an R/C (resistor/capacitor) circuit containing the jog speed potentiometer.

From what you say I understand the machine moves correctly in 'CNC' mode and also jogs one step per tap of the +/-.
That implies to me the CPU is reading the +/- buttons correctly and the drive circuit is good but that the pulse train is not correct.

My first test would be that the pot is in good order and that the connections to the board are ok. Pot spindles (knobs) can get bashed by mishandling leading to poor internal connections and/or the pot can come loose on the board which leads to poor/broken connections.
Less likely, any metal chips which may have found their way in to the main board past the shielding can also cause problems.
Regards JG


newagedesire@...
 

Thanks this narrows it down a bit while confirming some of my suspicions.  The resistance on the pot is good through the whole range and it doesn't seem to have any connection issues at the board.  Sounds to me like the other parts of the timer circuit might be to blame.  The circuit seems to be partial working as the pot still varies the speed it is just very slow.  The ratio in the voltage divider or capacitor charge/discharge rate seems to be off.  It does seem to rapid fine as well when holding the rapid button and +/- buttons. I'm not sure if that narrows the circuit down anymore...

I'll try blowing it out, but the case is very clean.  If that doesn't do it I'll pull the boards and get a look at the capacitors and check the resistor values. 

Thanks,
Steve


J G
 

Yes, its only the user variable jog rate between 'single step' and 'rapid'  that uses the pot/pulse circuit. Otherwise the velocity and acceleration  is handled by the CPU. Seems an odd fault to occur spontaneously unless it is a aged capacitor.

Definitely worth having the boards out to get eyes on, IMO. It's easy enough but a pain refitting due to space and access restrictions. Unlikely to be contamination but even heavy dust - esp around workshop activity - can upset digital circuits, I've found.  I use a dry paintbrush to clean them - although in fact the commonly recommended way of cleaning very dusty old boards is washing with (fairly pure ) water ! Very thorough drying obv.

The timer IC is near the bottom by the amplifier board connectors and the PROMs. Sadly all the board components are inaccessible without removal.

Happy probing.
Regards JG.