# Thread: -12 Volt unregulated low

1. ## -12 Volt unregulated low

Hello all,

I've been troubleshooting my Series I Boss 6 that keeps blowing the X-axis final drive transistors, and I tested the voltage coming from the power supply. The manual says to check the output of the -12V UR terminal. We went from there to the GND test point. We also went from the -12V UR test point to the +12V UR, and from the -12V UR test point to frame ground and got the same result on each test of around 10 volts. The manual said that we should have between 21-28.5 V measuring from the -12V UR test point. It doesn't say, however, what to place the other probe on.

Can someone tell me where the other probe goes, and what may be causing this low voltage? Thanks very much!!

Ian

2. Look at + & - voltage as simply "frames of reference".

Where you have 24volt potential difference measureed between the -12 and +12 terminals.

Where you have 12 volt potential difference measeured between Ground or Common point and +12

Where you also have 12 volt potential difference between -12 and Ground/Common.

The above readings assume you hook the red (+) lead of your VOM to the + side of the voltage and the black (-) lead to the more negative side terminal in.

Another way to look at a +/-12v power supply is in terms of a pair of 12 volt car batteries hooked up in series. The series voltage is 24 volts. The junction of the + side of the one battery to the - of the other is the COMMON point.

If you then put the negative terminal of the VOM on the COMMON terminal, you'll see +12v when you touch the RED lead to the unconnected positive terminal and -12v when you touch the RED lead to the unconnected negative terminal.

The fact remains that you have a 12 volt potential difference between the terminals of the battery. The + or - simply tells you which side terminal is at a higher potential difference of one side with respect to the other.

This same analogy works if you do +/-15vdc or +/-5vdc or whatever bipolar voltage that is needed or being supplied by the cells/batteries/PS.

3. The -12 VDC is not that critical in the older BOSS machines.
Where is the wild leg of your phase converter hooked in. Remember T2 is a 3 phase transformer and powers the 3 axis. I wonder if you put the wild leg on another terminal, if you would start blowing transistors on a different axis.

George

4. Glad to hear that the -12VDC issue is not a likely cause of my problems.

I think that the wild leg must be on the Z axis. Below are the readings that I got from the output of the T2 transformer. Note that 5C to 6C (or terminals 3-6) are out of limits at 70 volts.

5A-6A= 58V
5A-6B= 57V
5A-6C= 53V
5B-6A= 53V
5B-6B= 58V
5B-6C= 55V
5C-6A= 68V
5C-6B= 66V
5C-6C= 70V

Do you suggest that I change anything in view of the above numbers?

Ian

5. The 5A to 6A is normal (Xaxis)
5B to 6b is normal (Yaxis)
5C to 6C is not normal.
If you research my old posts, you will find that my experience has been that the break point is 63 VAC. Above that and you will blow transistors like a young child eating candy.

George

6. I was under the impression that the 5C to 6C terminals of T2 feed a different axis than X. I've only had problems with the X axis, but I am building another RPC using a Ronk Roto-Con II and some 50MFD run caps. I'm hoping that it will seriously clean up my power, and that wild leg. I get the caps tomorrow.

7. Yes, 5C to 6C should feed the Z axis. Unless someone has been playing with the wires. If I remember correctly, wires 11 and 12 are the X axis and are typically on the 5A and 6A terminals of T2.
The good thing about these machines is that you have 3 independent circuits that can be compared against each other for abnormalities. This is a great aid in trouble shooting. But if it is a instantaneous event that blows a transistor, it may be very difficult to find. Fortunately all of my customers are companies that do not use phase converters so I am spared the situation you are in.

George

8. Ahh, that makes sense. Always better to have 3 phase delivered to your door than have to make it yourself