Circuit board help Low voltage?

[M. Moore]

Slightly off topic but hopefully I can get some help here.
The pic shows the circuit board from my massage bed, an essential tool to keep me working!
It is a Ceragem.
The control is beeping and then showing a code, OD, which means motor overload according to the manual.
I am no electronics expert but I did check the voltage going to the motor and it is showing only 12 volts. Motor name plate says 1.4 amps and 24 volts.
The power supply puts out 5,13 and 24 volts and those voltages are incoming correctly measured at the connection just to the left of the motor wires. However the 24 volt wires are showing 29 volts, not sure if this matters. Low voltages are right on 5 and 13.

So what to check? Simple fix?

Thanks in advance for any help.

Michael

 

[M. Moore]

I see the software took my very good photo and made it small and fuzzy.

 

[Ziggy2]

Slightly off topic but hopefully I can get some help here.
The pic shows the circuit board from my massage bed, an essential tool to keep me working!
It is a Ceragem.
The control is beeping and then showing a code, OD, which means motor overload according to the manual.
I am no electronics expert but I did check the voltage going to the motor and it is showing only 12 volts. Motor name plate says 1.4 amps and 24 volts.
The power supply puts out 5,13 and 24 volts and those voltages are incoming correctly measured at the connection just to the left of the motor wires. However the 24 volt wires are showing 29 volts, not sure if this matters. Low voltages are right on 5 and 13.

So what to check? Simple fix?

Thanks in advance for any help.

MichaelView attachment 330475

I would suspect one of the driver power transistors is bad. Can't really tell but it looks like a typical H driver configuration, probably PWM for power control. This would short one corner of the H to the opposing corner. The PWM would prevent seeing magic smoke leakage.

 

[M. Moore]

Ziggy,
Thanks for the reply and sorry for the fuzzy photo.
I don't really understand what you said but should be capable of testing if you could walk me through it like I was an apprentice.....

I think what you are saying is that the 12 volts is what the motor is actually seeing because of the fault in the PWM (?) and if I test that I might find the problem?
How do I identify the power transistors?

 

[CarlBoyd]

Ziggy,
Thanks for the reply and sorry for the fuzzy photo.
I don't really understand what you said but should be capable of testing if you could walk me through it like I was an apprentice.....

I think what you are saying is that the 12 volts is what the motor is actually seeing because of the fault in the PWM (?) and if I test that I might find the problem?
How do I identify the power transistors?

Starting with simple question first.
"How do I identify the power transistors?"
1) they will be attached to a heat sink
2) fairly large
3) may be 4 ( or maybe 6) transistors or a single large part with 4 transistors inside (better pictures)
4) if 4 transistors they will have 3 leads each
4) one lead will be control, one connected to +24V or ground, one connected to motor, one to the PWM control

We do need more info, and a better pictures
1) Picture of motor name info plate
2) Picture of the board

If you can find the transistors, and identify the leads,
1) unplug from power
2) disconnect the motor
3) measure the resistance across the motor pin and power/ground pin on each of the 4(or 6) transistors
4) report back

CarlBoyd

 

[JST]

........................
3) measure the resistance across the motor pin and power/ground pin on each of the 4(or 6) transistors
4) report back

CarlBoyd

Do that "both directions" (reverse position of the test probes). The power devices have a diode in parallel, which may give a low reading in one direction even if all is fine, depending on what sort of meter you use.

 

[M. Moore]

Excellent, thanks for the detailed instructions.
I will have a look at the board later today and if I can’t get the tests done I will post up some better pics.
What range of numbers am I looking for in the resistance test?

 

[Scottl]

Ziggy,
Thanks for the reply and sorry for the fuzzy photo.
I don't really understand what you said but should be capable of testing if you could walk me through it like I was an apprentice.....

I think what you are saying is that the 12 volts is what the motor is actually seeing because of the fault in the PWM (?) and if I test that I might find the problem?
How do I identify the power transistors?

A 24 volt H bridge with a non-functioning transistor will only see 12 volts at the motor. You need to measure (while operating) from both sides of the motor to the 24V plus and 24V minus. One will likely measure close to zero, meaning a transistor that is not passing current. Measure in both rotations (CW and CCW).

"both sides of the motor" of course means measure only one at a time.

An H bridge has 2 pairs of transistors. When rotating in one direction one transistor switches the + and the other switches the - and when rotating in the other direction the other pair does the same thing except that the motor polarity is reversed.

 

[JST]

...............
What range of numbers am I looking for in the resistance test?

With no power and the thing having been "off" long enough for voltage to drain off....... and the motor disconnected...

If your meter will measure a diode with conduction one way and nothing the other, you should measure essentially that, Nearly "open" one direction, and the same as a normal diode the other.

If your meter shows a diode as "open" both ways, then you should get a rather high resistance each way. I would think at least 100k ohms.

Many will have some circuitry that is connected across the transistors, and may give a resistance reading a bit lower, or much higher.

A very low resistance generally indicates a bad transistor.

 

[M. Moore]

Here are the photos.
There is a large heat sink on the left with a solid black attachment with a bunch of leads coming out the bottom. It appears that this connects to the motor leads based on the routing on the back side of the circuit board.
I have a fluke 179 meter, it should be able to take these readings.

 

[doug8cat]

Agree with all said so far. Look for shit stains on board esspecially around the transistors, as well examine traces and solder connections on the board.
Also when replacing a bad FET, transistor,or voltage reg. that has a heat sink, replace the thermal transfer compound, some manufacures don't always put enough on and it can dry up over time. I always go through and clean off the old and place a nice new layer down, not too much but just enough.
Hope your resting well soon.

 

[M. Moore]

Scottl,
To clarify your instructions, there are four incoming wires on the 24v + side and four on the - side so when I test across any pair I get 29ish volts. So from the red motor lead I can check to any of the 24 volt + wires?
Sorry for the dumb questions but I am not very good with electrical stuff.

 

[M. Moore]

Update,
I was able to locate the H switch and found the motor lead connections. I tested for voltage and found one side at 13 and the other at 29.
I did this several times to confirm and it was a bit hard to do as the unit wanted to switch itself off without much run time. I am fairly confident that the H switch is the problem so I ordered a new one from a firm in PA. It was only a $10 part, plus $20 for shipping, Toshiba part# TA8249HQ.
I will need some advice on how to properly remove the old one and install the new. There is some very fine soldering on these boards and I am not sure how to proceed and what tools are required.

Thank you all for the help, I would not have been able to figure this out on my own.

 

[rons]

Mistakes happen when assuming a dc level with a handheld meter, but the level may be pulse width modulated.

 

[M. Moore]

What does that mean?

 

[Tony Quiring]

In simple terms...

Imagine connecting a DC fan motor to your car battery with a push button switch momentary contact type.

Hold button on and motor runs full speed.

Tap button for short periods and motor runs slower.

The length of time you hold the button down is the pulse width.

Redo your measurement in AC VOLTS mode.

Sent from my SM-G781V using Tapatalk

 

[M. Moore]

Tony,
I looked up pulse width and that is used for speed control. This massage bed does not have speed control nor does it need it, the motor runs full speed all the time. There is no way to change the speed of the rollers that move back and forth. I am guessing therefore that there would not be a pulse width modulation circuit
with a 555 or 7555 timer control, I will check for one just to be sure.
Others have suggested that it most likely is the H switch and given the reasons, if you think it is something else and can guide me through the tests I would be all ears. Thank you for your posts I now know what PWM is and how it works.

 

[CarlBoyd]

guide me through the tests I would be all ears. Thank you for your posts I now know what PWM is and how it works.

You're getting lots of suggestions from lots of people, pick one (let us know who) and follow up.

My post 5, with JST good additions in post 6 will determine if the h bridge is bad. Other methods will work too.

An H bridge can be used to reverse directions as well as PWM.

CarlBoyd