Servo 150 circuit board repair?

[T. J.]

I’m looking for advice on the repairability of this circuit board. It is from an inoperable Servo 150 power feed that I acquired in an auction.

The circuit board appears to have gotten hot, as evidenced by the discoloration seen here:


On the flip side, that discoloration is associated with these 3 blue resistors:


When checked with a multimeter, the top one reads 9.66 kOhms, which seems to be correct based on the color code (10k +/- 5%). The other two both read 5.05 kOhms, which doesn’t seem correct, but the color bands are hard for me to interpret.

Can anyone tell me what these two resistors should be? I have limited experience with electronics, but replacing these resistors is probably within my skill set. I know I can get a whole new circuit board for $112 from H&W, but if I can fix it with a couple of 10 cent resistors, I would rather do that.

 

[Weldingrod1]

Those are 3 or 5 Watt rated resistors. When you replace, leave more room under them. Go one size more powerful if you can fit it.

OR, go to remore mounted chassis mount resistors. That will give you more like 10+ Watts of power handling and get it away from the PCB. But, more challenging install...

Sent from my SM-S911U using Tapatalk

 

[markba633csi]

WAG the third band used to be red, so they would be green, black, red = 5K
You might want to start checking diodes near the hot areas

 

[Winegrower]

Resistors in general don't overheat as the root cause of a problem. Something elsewhere causes this. I predict replacing resistors will not fix your board.

 

[markba633csi]

Winegrower is right, it's rarely burned resistors but instead, semiconductors or diodes, or broken wires or solder joints
Check the motor for continuity
As I recall, there is a limit switch assembly that is needed to run the unit- without it the motor won't operate
Here is the only data I could find:

 

[T. J.]

Thanks guys. I’ll keep looking for other causes.

 

[tq60]

The resistors are high wattage, so they are expected to generate heat, look elsewhere.

Electronics 101, always startat power supply!

What are the symptoms?

You have a couple switches internally, do not forget, you have an external limit switch pair, if not connected it may act dead!

Sent from my SM-G781V using Tapatalk

 

[T. J.]

I do have the limit switches, and did verify that they work properly.

I have actually identified the reason why I was getting nothing from the motor - the mechanical link between the control lever and the internal forward/reverse switches is broken. That's simple enough to fix - it's just a broken pin. By directly actuating the switches, I was able to run the motor in both directions and verify the speed control and rapid button both work - it's alive!

However, the circuit board does get hot when running the motor, and I can smell it getting toasty. Based on Mark's advice above, I checked the diodes, and I believe two of the five are bad. With the diode testing function on my Fluke meter, I get voltage drops of 0.4v - forward bias, and 0.6v reverse bias.

 

[markba633csi]

You may have to lift one end of each diode to get a valid reading- in circuit there may be stray paths. They are likely ok if the unit is working.
Tip: snip the diode lead, test, then scrape and resolder. Less chance of board damage. Diodes are cheap.
Anyway, it's probably the resistors you smell- kinda normal. Replace with higher wattage parts if possible

 

[markba633csi]

Power resistors can get ferociously hot even tho they are operating within their wattage rating