well there goes about 1500 bucks :(

[supton]

Found the issue, not sure what this little board does yet, I have an idea but I need to trace it out. Cap is vented and resistor obviously was very hot. For those of you who noticed, I took the cover off the transistor on the end, it is not missing.

View attachment 149917
View attachment 149918

Not sure what that is, looks like some sort of band aid they tacked on? Diode bridge but missing capacitor for it… probably used on other designs, and only a portion of the board here.

Wondering if this is just an RC across the output, for noise suppression. Only one wire, and then some screws to the chassis?

My stab in the dark, the capacitor got leaky, and then ran away, absorbing more current as it heated up. The resistor wasn’t sized for full voltage across it (since it shouldn’t see that).

 

[BeerCan]

Not sure what that is, looks like some sort of band aid they tacked on? Diode bridge but missing capacitor for it… probably used on other designs, and only a portion of the board here.

Wondering if this is just an RC across the output, for noise suppression. Only one wire, and then some screws to the chassis?

My stab in the dark, the capacitor got leaky, and then ran away, absorbing more current as it heated up. The resistor wasn’t sized for full voltage across it (since it shouldn’t see that).

At first I thought it might be a crowbar, but I think it some kind of filter to block ac across the inputs. I was having some weird oscillation errors on the load at high amps before it smoked. The cap was probably bad.

I'm going to rebuild it and cross my fingers that it is the only damage.

 

[supton]

At first I thought it might be a crowbar, but I think it some kind of filter to block ac across the inputs. I was having some weird oscillation errors on the load at high amps before it smoked. The cap was probably bad.

I'm going to rebuild it and cross my fingers that it is the only damage.

RC across the output, as frequency rises, the capacitive reaction drops, and eventually it looks like the R only across the output. Thing is, V/R might be leading to ripple current that was outside of the rating of the capacitor. Or it was one of those junk caps that got made turn of the century, or just a junk cap period.

I’d be curious why it was oscillating… is the DC load not “DC” and instead using PWM? if the battery resistance was high (relatively speaking) then the voltage might have been bouncing (if the load was not constant), and then this RC sees that. That’s a stab though.

No choice but to try and see if it wants to still work. If it comes up and works at low loads, I might then rework this RC and use higher Pd resistors, or ones with high surge rating, and make sure to use a cap with high ripple current rating.