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By Colonel Panic
#1835726
The lift & lower actuator on a "medical" bed wasn't working, so I open the control box & noticed that one resistor(?) looks a bit rough, with some scorch marks beneath. On the reverse side of the board I saw that the solder joint was "broken". I took a gamble and re-soldered the joint, and the actuator now works but I would be more comfortable if the repair was more thorough.

The board is 24V after the mains in goes through a transformer. The resistor gets hot during operation, so not sure if this is the cause or the effect of a fault.

QUESTION: Is there any way to fix this without what will almost certainly be a criminally expensive replacement control box? I've Googled the board number but nothing comes up.

EDITED TO ADD: The MD7C seems to refer to the box - which is one of these. http://www.moteck.com/ProductDetail.asp ... ductId=242

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By Gas Guzzler
FLYER Club Member  FLYER Club Member
#1835735
That looks to me like an overheated resistor. If it were me, now that it's working again I would measure the resistance and replace it before it fails completely. At the same time I would increase the power rating to prevent future overheating, it looks to be a 2.5watt wirewound resistor, I would replace it with a 5-6 watt device. Best of luck
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By Morten
FLYER Club Member  FLYER Club Member
#1835761
You need to ask yourself why it has failed - otherwise the replacement will probably fail again. The overheating event may have caused it to change resistance, so measuring with an Ohmmeter and buying the same may not give the desired result. Looks like it'd be difficult to read the rings, but try if you can. E.g. https://www.allaboutcircuits.com/tools/ ... alculator/

If it has lasted for many years then replacing it with a higher power rated item will not help anything - just like you wouldn't replace a 5 A fuse with a 10A fuse... If it's random failure then that will likely not take place again soon. If it's something else you've just made the problem warmer/potentially more dangerous.
You could probably do worse than looking at/replacing the diode next to it as well as it may also have overheated - they're dirt cheap.

(Edited to add: let me know what R and D you need and I may have some spare ones of the right value.)
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By Cessna571
#1835768
I wouldn't try and measure it, I can't tell from the picture, but I am presuming you can work out the colours when you cut it off. The rings go right round.

I also have the odd box of resistors here.. I am sure Morten or I can send you one if you like.

show us what the tracks / joints on the other side of the board look like though. If there was enough heat to delaminate a track, a repair that just replaces that resistor might not be what you want.

Pretty sure there is enough expertise on this forum if it needs a better repair.
By Aerials
FLYER Club Member  FLYER Club Member
#1835781
Once the value of the resistor has been determined and a suitable replacement arrives, I'd add small ceramic spacing tubes to the vertical sections of the leads before inserting the leads through the board. This will keep the body of the resistor from affecting the board by radiant heat. Quite where one would buy these tubes these days, I have no idea but it's a technique I've seen before in older equipment.
I am wondering if the diode is used as a temperature sensor as it is placed close to the resistor body. This might be used in some sort of feedback control to reduce current through the resistor for some reason. If it is, a dollop of heatsink paste to bridge the gap would be good.
By cockney steve
FLYER Club Member  FLYER Club Member
#1835859
I'd think that lengthening the "tails" on the resistor , would allow the heat from the body to dissipate, before it heated the (very short) tails enough to melt the print-solder. IMO, It's designed to run hot, otherwise they wouldn't have used a wirewound.

Are you sure the user isn't activating a switch for extended periods, thus overloading the circuit? IE shoving control under a pillow and accidentally causing a long-term button-press?
By Colonel Panic
#1836604
Thanks all; don't waste a huge amount of time on this as a whole new control box is only £75 (but it would be fun to fix rather than replace).

Herewith some more photos. I have tried to measure the resistance, but (& I am not brilliant with a multi-tester) whatever Ω scale I use it always shows 0.583. (It shows 1 when "open", and 0 when I touch the two probes together). It is very difficult to read the lines, but I _think_ it is Brown Black Black Black (which would be 100 Ω ?).

I have a load of resistors ranging between 0 Ω > 1M Ω, including 100 Ω, but they are all physically much smaller / thinner. (They were sold by a Raspberry pi shop).

As for the cause of the overheating I would lay blame on the remote button being pressed for too long; there are several labels indicating "Fct 2 min / Pause 18 min", and I can imagine that this wasn't always followed by a previous user ...

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By Flying_john
FLYER Club Member  FLYER Club Member
#1836614
So here is my two penneth !

I think the two blue wires on the white two pin plug are A.C input (not DC) and this is the input to the Bridge rectifier. The DC output of the bridge rectifier ( the Black thingy) has the large black electrolytic across it. Then the positive rail passes to the burnt out "resistor", which I think is a virtually zero ohm wire wound component acting as an inductor and in-line current limiter and ultimately fuse, from here there is another electrolytic, finally smoothing out the supply that is used to power the motor.

Reason its got hot is that it has been used beyond its duty cycle. I suspect it was 10 ohms.

Its a cheap way of providing a reasonably good DC source.
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By Morten
FLYER Club Member  FLYER Club Member
#1836624
As John says, the black square "BD1" next to it is a bridge diode rectifier fed by the input which would be AC, otherwise the rectifier serves no purpose. The 2 blue wires being the same colour are also a bit of a giveaway. The colour code does look like Brown Black Black with a Silver tolerance, i.e. a 10Ω resistor with 10% tolerance. Not sure whether it's 0.5 or 1 W rated - If it's 5mm Ø it's probably the latter, 3.5mm Ø it's the former.

I'd still have a quick look inside the hand held remote - if the resistor has overheated, there may well be another component in there which has also overheated - that current went somewhere... If indeed it's hand held, it'd be good for it not to get too hot ;)
By Colonel Panic
#1836629
Noted re: 24vAC, not DC.

The scorched resistor is ~4.3mm Ø. I have opened the remote, and there is absolutely no sign of any electrical damage. The only component within it (other than the buttons and an LED) is a tiny (2mm Ø) resistor (PROB90 = Brown Black Orange Gold, so perhaps 1.3Ω ??? - or am I reading that wrong)? The resistor goes from the wire in & the LED.
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By stevelup
FLYER Club Member  FLYER Club Member
#1836641
You can't measure the resistors reliably when they are in circuit - you're probably measuring some other component there.

Brown Black Orange is 10k (the gold will be the tolerance)

I'd replace your burnt up resistor with a 10 Ohm 5W one and everything may well be fine.
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By Colonel Panic
#1836652
Thanks @stevelup - the burnt out resistor is not Brown Black Orange Gold (that is the handset one.) I _think_ it may be Brown Black Black Brown (which is 1k Ω ??)

How does one know the Watt value of resistors?
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By stevelup
FLYER Club Member  FLYER Club Member
#1836653
Well, we really don't know the value of it with absolute certainty because of the heat damage. I can tell you though that it definitely won't be 1K. It's impossible for a 1K resistor in that circuit to dissipate any amount of heat - ohms law :)

Forget about the last band - that is the tolerance.

I suspect it's brown black black which is 10 ohm and that makes sense in terms of heat dissipation.

The wattage? Just by eyeballing them... I'm confident a 5W one will be suitable in that position. The original may have been 2.5W
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