Kustom KBA-200 blown power amp AGAIN!

[methodofcontrol]

i repaired this amp about six months ago for my friend and it looks like the power amp (STK 404-140) has shorted.

the amp was blowing fuses and after some tests i discovered that the fuse stayed OK after
i unsoldered the jumpers leading from the rectifier to the (+) and (-) pins on the power amp (pins 10 and 11, IIRC).

i'm happy to order another power amp and install, but i don't want to have to do this every six months. i'm wondering what would
cause the power amp to short? the first power amp survived from the amp's birth about 7 years ago until 6 months ago, the 2nd power amp lasted only until now. is there another component that might be failing that would cause this?

[Jazz P Bass]

Which IC was in the amp originally?

The STK404-140 N or the 140S?

[Enzo]

And amplifers do not sit there working by themselves. Inspect your friends speaker cables. They are actual speaker cables, yes? Not guitar cords? How about the speaker loads, are the jacks in good shape? The inner wiring? Is he using a proper load or is he piling on the speakers?

[methodofcontrol]

i wasn't aware of the N vs S designation on the STK. the amplifier schematic doesn't make the distinction, it simply calls for an STK 404-140. i can look at what is currently in there when i get home, (i believe it's an N given that is all i can find on mouser) but as far as what was in there from the get go... i couldn't say as the part was thrown out months ago. do you happen to know the difference between the N vs S model STK?

as for the speaker cable, it's a combo amp. speaker is wired via internal harness. i'll check that harness for tightness! good thinking! when an extension cab is plugged in, the combo speaker is automatically disconnected. i can say with great certainty that he never uses extension cabs. he said he was playing and it died over the course of a song several songs into a set. he couldn't think of anything unusual that happened as to gear he used or sketchy power situation at the club.

[J M Fahey]

just in case add a 75 or 85 DegC thermal switch bolted to the heat sink.

Normal use is to open 120/240V power inmput but if you don't want th have live power cables where they are not expected, use it to cut speaker out.

Most "fried egg" thermal switches have 10A current capacity.

I routinely do that in amps which come back too soon, sometimes a customer gets somewhat pissed off at the amp muting in the middle of a great solo or whatever, but it self resets in 5 minutes or less and is less frustrating than a burnt amp.

[Jazz P Bass]

The STK404-140S replaced the N.

Does the amp have any sort of speaker protection circuitry?

The power rails should be checked for any Vac ripple.

These STK IC's, being a self contained output section, do not require much to run properly.
As far as 'killing' them is concerened, overloading them will do it.
If you do not have the schematic, consult the STK404 data sheet, as it was most probably copied by Kustom.

STK404-140N: http://www.onsemi.com/pub_link/Collateral/ENA2100-D.PDF
STK404-140S: HTTP 301 This page has been moved

[Tone Meister]

Just this morning dealt with a Kustom DE200 with this same IC (STK404-140S) that appeared to be bad. The amp was dead shorting and would light up the 150 watt bulb in my LBL to max brightness.

Turned out to be a shorted 6800/63 filter cap (C7).

[methodofcontrol]

if you look at the schematic for the KBA200, i lifted J1 and J3 at the STK to remove power from it. that rules out the large caps as shorted (C923 and C924) since they're still receiving voltage.

speaker protection circuit...? you mean like IC7 and the relay on the KBA schem? i can hear the relay working by way of the speaker making a tiny sound when the relay switches the speaker into the circuit. i don't know how to test IC7 or if that step is required.

jazz p mentioned overloading the IC, are you referring to just running too hot a signal or running it too hard?

my workshop and my internet access are separated by a huge divide currently, makes this a particularly frustrating process to find the proper information and have it accessible when i need it. the support of this tech community has helped me through so many head scratchers over the years and taught me a lot. thanks for all of the help and for bearing with me guys!

[Jazz P Bass]

Can you post the schematic?

[Tone Meister]

Here you go.

[Jazz P Bass]

if you look at the schematic for the KBA200, i lifted J1 and J3 at the STK to remove power from it. that rules out the large caps as shorted (C923 and C924) since they're still receiving voltage.

Yes the voltages are on the caps but are they relatively ripple free?

If the Protect IC is engaging the relay then it is working.
I like to measure the closed relay contacts to ensure that they are a very low resistance.

[methodofcontrol]

received and installed new STK404-140 today. amp works like a charm again!

however.... the speaker is blown. still makes noise, but it's terribly fuzzy. my next question is:

did the blown speaker likely take out the power amp?
or did the shorted power amp likely take out the speaker?

[Enzo]

Most likely the bad amp put DC on the speaker and burnt it out.

[Jazz P Bass]

I would suggest checking the speaker on another piece of gear.

If it is blown. then it's blown.

If not, you may have fried the relay contacts.

[methodofcontrol]

I would suggest checking the speaker on another piece of gear.

If it is blown. then it's blown.

If not, you may have fried the relay contacts.

i plugged a known good speaker into the ext. speaker jack on the back of the amp. the jack interrupts the signal to the internal speaker. sound from my external speaker sounded great. also, removed the grill and physically pushed on the cone of internal speaker, it's rubbing like crazy which explains the farty distorted sound.