DC voltage @ speaker output

[methodofcontrol]

hey guys, i'm fixing a kustom KBA200 1x15 solid state amp for a friend. it started putting about 35vdc to the speaker output.

i've checked around the amp and the regulators and transformer all seem to be operating normally. i'm pretty certain the output transistor is the issue. it's an STK404-140, (data sheet here: www.onsemi.com/pub_link/Collateral/ENA2100-D.PDF)

i'm hoping someone more skilled in the art of amp repair can chime in with some pointers, pins 12 and 13 are OUT (-) and OUT (+) respectively.
the voltage on 12 is equivelent to the (-) VCC which is -54dc. the voltage on 13 is the same, whereas i would expect it to be a positive voltage. i would also expect that the voltage on 12 and 13 be increased voltages from the VCC (-) and (+).

i haven't been able to find a schematic, so i'm kind of flying blind on this one. any thoughts? thanks in advance!

[Enzo]

The STK is not a transistor, it is a whole power amp in one piece.

Did you contact Kustom and ask if they could provide the schematic?

Look at the box with the "equivalent circuit" in it. Note the emitters if TR7, TR8 are wired to pin 13 and 12. V+ to pin 11 and V- to pin 10.

Now look just below to the "test circuit." That circuit will be very close to what is in the amp I'd wager. Note pins 13 and 12 are joined together by a couple of resistors, R8, R9, which you can look up in the chart as 0.22 ohm.


Now step back and look at the big picture. isn't that about the same as most any SS power amp? A pair of output transistors, one to each power rail, with 0.22 ohm ballast resistors joined at the output? So I expect pins 13 and 12 to have the same voltage ion them because they are wired together with the 0.22 ohm resistors. If pins 10,11 have their proper voltages and 12 and 13 are linked as described, then I;d be thinking bad STK myself.


The +OUT and -OUT refer to the fact they are on the + SIDE and the - SIDE of the circuit. They didn't just hook them together inside because the chip can be used various ways. A couple external resistors like the data sheet, and we haqve an amp, but I could imagine those two outputs being used to drive even larger transistors. So do not expect +30 and -30 or something, they SHOULD be about the same voltage.

[methodofcontrol]

hey enzo! thanks for the quick reply!
no, i did not contact Kustom yet. i figured i'd use what i know and ask a few questions before bothering them.

i follow most of what you're saying... the test circuit drawing, the equivalent circuit drawing and all that. you're correct in assuming the "test circuit" is largely similar to what's in the amp. (the .22 ohm resistors were on my check list of things to take a closer look at.)

here's where you lose me:

Now step back and look at the big picture. isn't that about the same as most any SS power amp? A pair of output transistors, one to each power rail, with 0.22 ohm ballast resistors joined at the output? So I expect pins 13 and 12 to have the same voltage ion them because they are wired together with the 0.22 ohm resistors. If pins 10,11 have their proper voltages and 12 and 13 are linked as described, then I;d be thinking bad STK myself.

what i hear you saying is that, "yes. pins 10-13 are operating normally." so, if that stuff is operating normally.... what evidence leads you to suspect the STK as the culprit?

it sounds like we came to the same conclusion, but my logic was flawed and turned out to be just a good guess.

[Jazz P Bass]

Read it again.
Pin 10 & 11 are the power rails.
Pin 12 & 13 are 0.44 ohms apart & should sit at zero volts dc.

Here is the KBA200 schematic.

[The Dude]

You're on the right track finding the datasheet. That's the first half of the equation. Now you can just check the part as if it were discreet components. Pins 8, 11, & 13 are base, collector, and emitter of a darlington transistor. Pins 9, 10, & 12 are base, collector, and emitter of another. Pins 8 & 9 of another, etc. The most common failure would be one of these parts shorted, so check for a short between any of those pins where there shouldn't be. My usual method is this. I write down all of the pins that are shorted together and then look at the datasheet and see if they're supposed to be.

[Jazz P Bass]

I received an STK module from a reputable seller for a Yamaha mixer repair.
Soldered in the module, turned on the power, applied a signal &........

Nothing.

I used the datasheet to convince the seller that the module was 'blank'.
There was not any resistance from any one pin to any other.

They sent another one.

[methodofcontrol]

thanks for the additional input guys. now this makes sense. i don't do this often enough to not need the help! just often enough for my friends to give me their broken stuff.

[methodofcontrol]

got the new STK today and put it in. fired the amp up and played through it— everything sounds great. before i close it up and bring it back to my buddy i'd like to ask why the amp makes a loud popping sound when i turn it off? seems like there's something else i should fix while i've got it open. any ideas?

[Jazz P Bass]

Check that IC7 is working.

And that the relay is not fused closed.

[Enzo]

Loud pop at turn off? Since you got the unit not working, perhaps it always popped at turn off? Turn the master to zero first as well as any reverb. Does it still pop?

And leave the power switch on and just unplug the thing from the wall. Does it still pop that way? it might just be a noisy power switch.

I don;t generally consider pops at turn off to be a problem.

[methodofcontrol]

Check that IC7 is working.

And that the relay is not fused closed.

yes, the relay is always closed.
i would think the relay is supposed be open while at rest so that turning amp power on sends power to the relay, closing it and
allowing the speaker connection. thus avoiding pop sounds upon throwing the power switch.

if IC7 is controlling the relay or maybe the timing of the relay, how do i determine if the relay is bad or if IC7 is just controlling it poorly?
i'm not clear on what IC7's function is or how to test it? i checked for shorts and only found the obvious one between pins 3 and 5. i also
checked for voltage to the relay between D8 and R11 and came up w/ 55vdc which seems fine. makes me think the relay is bad.

also- the relay is rated for 24vdc, why is it ok for this circuit to be feeding it 52v? seems like that's asking for trouble.

[methodofcontrol]

sorry enzo, didn't see your reply until after i replied to jazz p.
i understand that SS amps are common to pop when turned off, my first thought was that it was normal but maybe just a tad excessive. but yeah, it pops a little when turned on and a lot when turned off. the cone probably moves 3/8" when turned off.
i did try turning it off w/ volumes on zero before i posted and just now i tried pulling the plug from the wall instead of using the switch and i heard no improvement in either case.

[52 Bill]

yes, the relay is always closed.
i would think the relay is supposed be open while at rest so that turning amp power on sends power to the relay, closing it and allowing the speaker connection. thus avoiding pop sounds upon throwing the power switch.

Correct, so if the relay is always closed there will be turn on and turn off pops. Is the relay closed even when the power is off and the supply voltages have been drained down? If it remains closed, suspect a bad relay. If it opens and closes normally, suspect a bad IC7.

also- the relay is rated for 24vdc, why is it ok for this circuit to be feeding it 52v? seems like that's asking for trouble.

You need to read the voltage across the relay coil. If there is say 52 volts on one side and 30 volts on the other end there is only 22 volts across the coil, safe for a 24 volt rating.

IC7 monitors the voltages at the speaker output and should turn off the speaker when there is any condition that could cause damage to the speaker, like having 35 volts dc on the output. It should also turn on and off the speaker so that there will be a minimum of pops heard. Monitor the voltage at pin 8 to see what happens there at turn on and at power down. This pin should turn on and off the relay.

[Jazz P Bass]

IC 7 is a 'speaker protection' ic.

C1237HA pdf, C1237HA description, C1237HA datasheets, C1237HA view ::: ALLDATASHEET :::

It turns the speaker relay On & Off.


What I asked was whether or not the relay contacts are fused closed or not.
You can check that with a meter with power off.
If you can read the speaker resistance with power off, from pin 12 or 13 to pin 5, then they are fused closed.

[methodofcontrol]

Correct, so if the relay is always closed there will be turn on and turn off pops. Is the relay closed even when the power is off and the supply voltages have been drained down? If it remains closed, suspect a bad relay. If it opens and closes normally, suspect a bad IC7.


You need to read the voltage across the relay coil. If there is say 52 volts on one side and 30 volts on the other end there is only 22 volts across the coil, safe for a 24 volt rating.

IC7 monitors the voltages at the speaker output and should turn off the speaker when there is any condition that could cause damage to the speaker, like having 35 volts dc on the output. It should also turn on and off the speaker so that there will be a minimum of pops heard. Monitor the voltage at pin 8 to see what happens there at turn on and at power down. This pin should turn on and off the relay.

hey bill, thanks for chiming in!
yes, the relay is always closed even when power is off. i should have made that more clear in previous posts. thanks for explaining the voltages on either side of a relay- i checked that and got 55v on one side and 24v on the other side. so, the relay is STILL seeing too much voltage based on the above math. (55-24= 31v) also, the "52v" on the schematic to the right of the relay, i thought that was where the relay was getting supplied from.

as for IC7, pin 8 hangs out at around 23v. it shoots right up to 23v when powered on and on the way down, when i powered the amp off, it seemed like it stopped for a second around 12v. but i don't suspect that's anything to dwell on. i also checked pin 6 which starts at 54v and very quickly changes to .6v shortly after powering on. again upon powering off, it jumps back up to around 50v just before the circuit dies.

this makes me think pin 6 is responsible for controlling the relay and that the relay itself is just deceased.