Motor control board repair

[cyclones]

I have a motor control board I am trying to repair. I've been told the motor is fine, but the control board will only make the motor go one direction but not the other. Power comes in through a linear transformer and then a rectifier bridge and then filter caps. No schematics for the board. Relays on the board are 24V so I started by setting my bench PSU to 24V and injecting voltage. There are several LEDs. All but one LED lights up. There are test points labelled +15V, 0V and -15V. When checking voltage potential between +15V and 0V, I get a perfect 15V. For -15V to 0V, my meter shows approximately +1.15V. There are two voltage regulators with large heat sinks on the board. One gets warm, and the other is room temp. I think i will check for input to the cold voltage regulator next, but I'm trying to figure out why I'm getting the +1.15V reading on the -15V.

[sam_sam_sam]

Post a picture of this device so we can see how this is made so we might be able to figure out what might be going on

[cyclones]

Here's a picture. There's a second board on the bottom that's almost entirely diodes and transistors. I checked all those with a meter and they seem good. I'll get a picture of the bottom board if needed.

[R_J]

What are the numbers on the regulator ic's? especially the negative one. Doe the transformer only have 2 wires supplying the board or is it center taped?

There are two voltage regulators with large heat sinks on the board. One gets warm, and the other is room temp

you might want to indicate which one gets warm, is it the positive one or the negative one?
If you don't have a balanced circuit +15 and -15, you could see some positive voltage on the negative supply.

[cyclones]

Originally Posted by R_J

What are the numbers on the regulator ic's? especially the negative one. Doe the transformer only have 2 wires supplying the board or is it center taped?
you might want to indicate which one gets warm, is it the positive one or the negative one?
If you don't have a balanced circuit +15 and -15, you could see some positive voltage on the negative supply.

The two regulators which have heat sinks both have the following on them:
on the board they are labelled AVR1 and AVR2.

on the IC packages themselves, there is
3125p
6n27

on both. they seem to be identical, but only one is getting warm. The main transformer seems to have multiple taps.

[R_J]

It's nice top know one is getting warm, it would be nicer to know WHICH ONE? I will guess and say the +15 is getting warm and the -15 is not.
If both components have the same numbers it is unlikely they are both positive and negative voltage regulators

Do both regulator ic's have a common ground? have you checked the IN on the -15 ic to see if it has any voltage?
The transformer secondary is likely center tapped and the center tap goes to ground, and one part of the winding supplies each ~ on the bridge rectifier, then the + of the bridge supplies the positive IN for the +15v regulator and the - of the bridge supplies the negative IN of the -15 volt regulator

[cyclones]

Originally Posted by R_J

Do both regulator ic's have a common ground?

I checked tonight in diode test mode between the AVR1 ground to the AVR2 ground and got a diode voltage drop -- about .595 volts.

[eccerr0r]

Odd both regulators are TO-3P and the board silkscreen says out-in(tab)-G which is typical for the negative rail regulator (should be in-G(tab)-out for positive). Make sure the "in" is the right voltage in any case.

These regulators are probably NOT for the motor but rather for the zillion op amps on the board that may control the motor drivers.

[redwire]

Either the -15V rail has wimped out and collapsed, or something is overloading the -15V regulator. It's not good for IC's and semi's power if a -ve rail goes +ve. I would put a reverse diode from each of the two rails to GND, to keep working on this.

The two VREG's look like on the main board two big heatsinks, the IC's are TOP-3 packaged, one says G/IN/OUT.
The second board has a bunch of dual diodes and transistors, I don't see a voltage regulator in the line up.

[CapLeaker]

Originally Posted by redwire

Either the -15V rail has wimped out and collapsed, or something is overloading the -15V regulator. It's not good for IC's and semi's power if a -ve rail goes +ve. I would put a reverse diode from each of the two rails to GND, to keep working on this.

The two VREG's look like on the main board two big heatsinks, the IC's are TOP-3 packaged, one says G/IN/OUT.
The second board has a bunch of dual diodes and transistors, I don't see a voltage regulator in the line up.

Another possibility is the +15v is shorted with the -15v rail somewhere.

[cyclones]

I applied 120vac to the primary and here's I measured with green as ground:

grey one is 10V
blue one is 40V
purple is 2.25V
yellow is 5V
orange is .5V

[sam_sam_sam]

What type of motor does this have and what components are on the second board other than the ones on the heat sink

[cyclones]

Originally Posted by sam_sam_sam

What type of motor does this have and what components are on the second board other than the ones on the heat sink

What redwire says -- a bunch of dual diodes and transistors.

[eccerr0r]

photo of the tracks on the back side of the board? does not seem right that two apparently "GND" ports are not grounded/tied together - unless the company was playing tricks with virtual grounds and using the same BOM device for both rails... Or the silkscreen is wrong and the chips on the heatsink are different, still don't know what they are...

[cyclones]

I'm going to check the linear input transformer by disconnecting it from the board and applying 120VAC to the primary windings and see what all the outputs are. If everything looks good there, I'll connect up the board to 120VAC through an isolation transformer and see what happens. Got to get the supply voltages worked out first before checking anything else.

I've gotten more information about the problem. The outputs for one motor work, but not for the other motor. The maintenance guy switched the non-working motor to the contacts for the working motor and the motor worked, so it's probably something in the output, although it might be a problem on the input side too.

[R_J]

When you checked for the missing -15 volts, was that using the main transformer with 120vac applied or was it when you used your 24vdc bench power supply?

[cyclones]

Originally Posted by R_J

When you checked for the missing -15 volts, was that using the main transformer with 120vac applied or was it when you used your 24vdc bench power supply?

That was with the power supply. I finally got a chance to try the board in the machine and the input voltage was 107VAC.. not sure why it's 107V because that is just outside the typical 110-120VAC. Either way, in that case the -15V was also missing, and I was still getting about +1V on that rail, so my first task will be to get that figured out and repaired.

[R_J]

107vac is likely that they have 3 phase power supplying the building and they use one phase to supply the board, this is normal.
Locate the source for the -15v, trace the -15volt point back, the ac for this circuit is likely supplied from the transformer and then through a diode or bridge to give a negative voltage.

[cyclones]

Originally Posted by R_J

107vac is likely that they have 3 phase power supplying the building and they use one phase to supply the board, this is normal.
Locate the source for the -15v, trace the -15volt point back, the ac for this circuit is likely supplied from the transformer and then through a diode or bridge to give a negative voltage.

It looks like the -15V goes back to SR3 and the voltage regulator that doesn't get warm, AVR2. From looking at the trace lines and from the meter, it seems like the AVR2 is in parallel. SR3 has a + and - and two middle pins that go to the AC input, so it seems like a rectifier bridge, but I don't know what the SR3 designation could mean. Any ideas?

[R_J]

Originally Posted by cyclones

It looks like the -15V goes back to SR3 and the voltage regulator that doesn't get warm, AVR2. From looking at the trace lines and from the meter, it seems like the AVR2 is in parallel. SR3 has a + and - and two middle pins that go to the AC input, so it seems like a rectifier bridge, but I don't know what the SR3 designation could mean. Any ideas?

Are you getting a/c voltage to SR3? It is likely a bridge rectifier, why it is labeled SR3, who knows.
If you are not getting any a/c to the bridge, check the resistance of the traces and the power supply winding that supplies that circuit.
What is the part number on SR3?