Carlsbro GLX100 popping internal fuses

[TomHoward]

Hi all

I have a Carlsbro GLX100 which is popping internal fuses - got given it dead, swapped the fuses out and the new ones lasted about quarter of a second.

Before I start digging around too much is there a common or likely cause anyone knows?

Thanks in advance

[Jazz P Bass]

The amplifier part is a TDA7293 iC.
It is probably bad.

[TomHoward]

Thanks

I assume there's no way to test other than swapping out?

I can't see any other damage but are there any other components worth swapping at the same time that could have caused or been damaged by a bad chip?

Thanks, Tom

[Jazz P Bass]

When these IC's go, they usually go short from pin 13 (-Vdc) to pin 15 (+Vdc).

That is what blows the fuse.

Often it will also short those two pins to the output pin (14).

Ouch.

[TomHoward]

Thanks for that. I have ordered a replacement chip & I will desolder and check it out. Should the 3.15A protection fuses be quick-blow or delay do you think? I'm assuming quick unless that causes nuisance tripping.

[g1]

Depending on whether it's internal protection also fails, it could possibly fry the speaker.

[TomHoward]

Thanks for all the advice. I have swapped the IC and all now seems to be working (cross fingers).

There was a little bit of heat scarring around the two ceramic resistors in the power section so I resoldered some of the joints that looked to have been dulled by it.

The IC change was a bit of a pig because there is a capacitor floating on the underside of the board between two of the legs.

For reference Pin6 appears to be floating (no attachment to anywhere) and Pin3 is only connected to the -ve leg of the cap (which runs between 11 and 3)

Thanks, Tom

[TomHoward]

Got some more problems with this amp. I got it working in Dec by replacing the IC but it's gone again.

Same symptoms, popping the fuses, and a loud hum/bang when it's switched on, fuses last about a second.

If the IC has gone again, is there anything that could be causing this that you would check?

[Jazz P Bass]

If the IC has gone again, is there anything that could be causing this that you would check?

The back plane of the IC is Hot/ B- voltage.
Not too sure on this amp but was there a mica insulator between the ic & the heatsink.
If it did have one & you reused it, it may be bad.
Also, you have to apply heatsink compound to the ic & the heatsink.

Did you drain all of the power supply voltages before you hooked up the ic or the power connector?
For whatever reason, these ic's do not like stray voltages.

[TomHoward]

Thanks for your reply.

I didn't drain all the power supply voltages but this was off for several days during the repair so I don't expect there would have been much charge left in anything?

Also - this did work for two months before popping again - it didn't pop the new IC straight on startup, it was working fine and now it is displaying the same original problems.

I don't believe there was an insulator between IC and heatsink - I didn't see one as far as I am aware on the IC removed, and it didn't stay on the heatsink definitely as I cleaned that up.

I did use thermal compound on fitting the new IC.

How long would you have expected it to last without the insulator if it did need one? Could that be a problem that it would have survived a couple of months of regular use before popping?

[TomHoward]

Thinking about it, I'm pretty sure there wasn't any kind of bush or insulation on the nut & bolt between the IC and the heatsink, as I would have noticed that, so being insulated from the heatsink seems unlikely - as there must be if it was?

[Jazz P Bass]

If the heatsink is at ground potential, then you must use an insulator.

[TomHoward]

If the heatsink is at ground potential, then you must use an insulator.

Thanks, I will check the heatsink, I would imagine it is as it is quite a lump so I don't think it will be insulated from the chassis so probably is at ground.

Would you have expected the new IC to last as long as it did if this was the case? Or would you have expected it to last a matter of minutes?

Is there anything else I should check in case?

Thanks, Tom

[TomHoward]

I have checked the heatsink, and there is continuity between the heatsink and the backplate of the IC, but at the moment there is still continuity even after I have removed the nut & bolt and put a piece of card between the backplate and heat sink, so if the backplate is hot then there is a something wrong here somewhere.

I'll look further.

The heatsink is at ground as expected.

[Jazz P Bass]

The screw should have a plastic isolator on it.