Orac Spindle Speed Control Problem

[the_mechanic]

I own an orac lathe 1984, and I am facing a problem with the spindle speed.
When the spindle speed ON button is depressed, the spindle increases speed up to a certain limit, and then it remains rotating at that speed. Pressing the + or - spindle control buttons will have no effect at all on the actual spindle speed.
The speed value on the display will change, the beep sound will be heard as well, however the motor remains rotating at the same speed.
Any ideas what might be my problem please?

[Steam-Chris]

Hi, i have recently been working on my Orac with spindle speed problems so while its fresh i'll suggest some points to check.

On the PC1690 board, bottom one of the stack of three on the left side in the rear of the machine, check the voltage on connector TBF No19 (last terminal to the left of the relays in the middle of the board) it is the 0-10v supply to the VFD, think pin 16 is the ground, its an iisolated supply so only check at these terminals, the voltage should change as you press the speed + - keys. if it does then ckeck wiring to VFD, if there is no volts then a problem with the PC1623 board,top one of the stack, check + and - 12 volt lines in, 3 black wires, check 10 volt zener, located between the 2 terminal blocks ( power in and signal in/out) check it like a normal diode in circuit, if it is short or open both ways, replace it.

hope this helps

Chris

[the_mechanic]

Thanks for your reply.
I followed all your points and following are my findings:

I checked the voltage at pin 19 with the machine switched on, but in idle mode, and the voltage at this pin was 0 volts.
I then started the spindle motor and the voltage on this pin 19 rised gradually from 0 to 102 volts! - it rised gradually with the motor speed, till the motor reached a fixed speed at which point the voltage at pin 19 was 102 volts. Pressing the speed + and - keys did not result in any differrences at all - neither in the voltage at pin 19 nor to the motor's speed.
At this point I did not follow the VFD wiring as it seems that the fault has to do with the PC1690 or oher cards on the stack. What do you think?

Then I moved on to check the voltage of the PC1623 and the voltage across the white and black wires was only 5V. Is this normal as you said that this should be 12V?

I then moved on to look for the Zener, however on my PC1623 board there is no zener between the 2 terminal blocks, but instead there are 2 zeners adjacent to each other underneath the power in block and a bit to the right of it. The voltage across these zeners was 1.5V on one, and 2.3 on the other.

Any ideas what might be wrong?

Thanks for the help.

[Steam-Chris]

Hmmm, 102v on pin 19 think you may be using the wrong ground to measure this, it should be between 0 and 10 volts, this is an isolated supply, the measured voltage will be well above 10 if you use the machine ground. the later boards where different to mine but the principal it the same, i will check the later drawings and see if i can come up with other suggestions.
Just checked drawings and you should check pin 18 + and 17 -, i think you have the later modified boards and the 10v zener is on the PC1690 somewere about the pin 18

Chris

[the_mechanic]

Thanks for your help.

Checked for any voltage across pins 17 and 18 as you mentioned and there was no voltage at all. Pressing the + and - speed buttons did not result in any 10V signals at all across these pins (pins 17 and 18).

Then looked for the zener and it was on the PC1690 board near pin 18 as you mentioned. I noted something very strange though. This zener was joined to another diode, with both cathodes joined together. The anode side of the zener was still soldered in the PC1690 board however the cathode was soldered to the cathode side of a normal diode with the joint resting in mid air. Then the anode of the diode was soldered in the board. It seems that whoever owned the machine before me had some problems and soldered this diode for some reason. (I bought this machine as second hand and never touched its electronics before).

Desoldered this joint and checked both the zener and the diode. They were good. I still wanted to ensure that zener is giving a correct 10V drop across it, so I removed the zener completely from the board, and wired it up in a simple resistor(150 ohms) circuit with a 12V battery to test the voltage across it. It was 9.3V when tested, and so I concluded that this zener is working fine.

At this point I resoldered the zener as it was, to the diode, and connected all the wiring again. Tried the machine again, with exactly the same fault.

I think that the problem has to do either with some other chip on the board, or else no signal is coming from the CPU of the machine.
Do you have any suggestions please as to how I should proceed? Is there a method how I can test wether the signal is coming from the CPU?

Your help is greatly appreciated.
Thank you very much.

[Steam-Chris]

https://www.denfordata.com/downloads/dos ... RCAL-1.zip

https://www.denfordata.com/downloads/dos ... RCAL-2.zip

If you down load these drawings you will find the diagrams of you boards, check out numbers, 1714, 1804, 1849, and 1848