A Blue Sky Sub 8 - fuse blowing

[kevinkace]

Having some trouble with this Blue Sky Sub 8 (mk-I)http://abluesky.com/products/sub-8-mkiii/
TL;DR: fuse is blowing on power up, visually looks fine though some gunk around 1 transistor. Not sure what/where to test.

- questionable transistors: https://i.imgur.com/gznHwPo.jpg
- right side: https://i.imgur.com/1UxzuP9.jpg
- left side: https://i.imgur.com/tbfTPKy.jpg

Tried contacting A Blue Sky without any luck.

Transistor looks like mospec tip32c - data sheet http://www.alldatasheet.com/datasheet-pdf/pdf/2783/MOSPEC/TIP32C.html

Whole story:
- playing music too loudly through Sub 8/2x Sat5
- blew fuse
- bought replacement fuse but then managed to lose fuse holder cap
- fruitlessly search for replacement cap
- finally I decide to just force open Sub 8
- replace fuse holder/fuse
- fuse immediately blows, twice in a row

 

[jwhmca]

Suspect failed/ shorted output.

How are you with electronics repair?

 

[joaquins]

Ask for a fuse that doesn't blow... problem solved! (please don't!)

Do you have the right fuse replacement, same current and speed? if you have a fast blow it may be blowing in the transient loading the PS caps... Current is always looked, I assume you get one for the same current, timing not so much.

It probably is due to failed output devices, as jwhmca said. If you are lucky are just the transistor and didn't took anything around with them sometimes when they die they kill something else, let's hope not. Start desoldering the power transistors and see if they are shorted, if the fuse blows immediately they must be, open failure won't cause that. Replace the transistors and you should be good to go, or at least make an smoke test.

I'd try to add a bigger heatsink to those transistors, the ones already there look quite small. If using one big heatsink for both you must insulate them. You could wire them out since there is no much space in the board for a bigger block of aluminum.

Good luck with that!

JS

 

[kevinkace]

Thanks for the tips. I'm pretty handy with soldering iron, just not the most adept with theory/formulas etc.

I was using slow-blow 2A 250v fuses (as was printed on the back of the device).

I thought those heat sinks looked pretty small as well! I guess I'll try ordering some replacement transistors and see if there's any change.

I should have mentioned that when powering on the unit would quickly power up, hum then buzz and die, about 700ms-1s for that to happen.

 

[JohnRoberts]

If it blows fuses repeatedly, the fuse is working and the circuit is broken.

If you are adventurous you can attempt the repair yourself, but may end up blowing up some of the same parts again if you don't get it completely repaired.  Either way it will be educational.

JR