Repair BST-863

[blackbart]

Looking for some assistance from the community here.

I have a Best BST-863 that has stopped heating.  I've read some of the threads on this particular unit and haven't seen anything that aligns with the problem i'm currently having. I'll describe the problem the best I can.

When the unit turns on, the blower kicks on for a second or two as it should. When lifting the handle out of the stand, the blower kicks on, the little thermometer shows the red image to go up and down like its trying to heat up but no heat comes out. The little moon symbol stays lit unless you press the touch screen, then comes back on and the screen dims a second or two later.   The reed switch is functioning as it should, tested with stand and with a stand alone magnet. The temp screen stays showing 000*c or 032*f, I applied heat with a hair dryer and this value does go up to 004*c or 040*f with the unit running, handle out of stand.

I've removed the cover, looked everything over, and do not see anything amiss. I've taken a few readings from the unit and was hoping someone could compare them to their working unit. I cannot find these values anywhere i've looked on the web.

With the unit turned on, handle in the cradle, (assuming AC voltage by looking at the board) voltage reads 54.18v on the blue & brown wires on the heater board. When lifting the handle out of the stand, blower kicks on, and the voltage changes to 123.15v. I took the handle apart, removed the wires from the heating element side and tested voltage there as well, it shows roughly 14.5-15v regardless of handle status in stand or out of stand with blower running at the pins. There's an electrical component soldered in line with the blue wire at the handle but I cannot make out the markings to determine what it is (maybe a resistor?). The lack of voltage change is leading me to believe that component on the blue wire may have something to do with my problem.

Heating element seems ok, measuring across the pins show 16.5

I tried to measure resistance across the pins of what I believe the temp sensor to be inside the heating element, 3 pin side, center ground. Value across the two outside pins shows ~1 and fluctuates around 2-3 when heated with the hair dryer.

If anyone here can assist, i'd greatly appreciate it. This unit was functioning properly, had been shipped, and arrived in its current state. I'd love to get it back in working condition.

[Alex_Baker]

I don't have one of these devices or any real context, but I just thought I would just throw a couple of thoughts out there.

There's an electrical component soldered in line with the blue wire at the handle.

Could this be a non-resettable thermal fuse of some kind?

You mentioned that there was 1-3 ohms across the "temperature sensor", that sounds like a thermocouple of some sort, I have some K-type ones that measure around 1 ohm. It definitely seems like there is a problem with the temp sensing though, since the display only shows 0c.

Just to clarify, the ac voltage you measured was across the wires to the heating element correct?


 

[blackbart]

More than likely it is a thermocouple rather than a sensor, sorry poor terminology on my part. There are no parts diagrams/scematics that I can find on this thing so i'm shooting in the dark.

Yes, I measured the ac voltage in two places with the handle in and out of the holder (ie when its in standby and when it should be heating). The voltage where the wires from the heating element attach to the circuit board gave me the 53v and 123v. At the heating element side (after the unknown component) is where I got the 14v measurement regardless of the handle status (standby or heating mode). It seems that the voltage changing so drastically at the board side and staying consistent on the element side should be a red flag.

Not sure if its a thermal fuse or what, doesn't seem like it should be a fuse as its still letting some voltage through. I'm going to desolder it and pull it out, there are markings on it but they are covered by the plastic handle housing and cannot be read until its removed. I'll try to pull it off tomorrow and report back with what I find.

[Alex_Baker]

If you have 53V at the wires going to the heating element but the heating element is not heating up then the circuit is open somewhere.

[blackbart]

Ok, I removed the mystery component.  Markings on it are y.c.w ry 121 tf 121*c 250v 10a

So its a thermal fuse, checked with a DMM and there is no continuity across each side. Interesting that when powered up, it will allow voltage through, inductance I suppose.

Anyway, bypassed the thermal fuse and it fires right up. Still shows the moon symbol and a dim screen while running but I suppose that may be normal when the touch screen isn't being messed with. Off to order some fuses.

[Alex_Baker]

Does it regulate temperature properly?

Those thermal fuses wear out and randomly blow, in fact I just replaced one in a toaster oven last week, but be cautious and make sure that it did not legitimately over heat.