Kustom 250 amplifier, well done

[Lenny]

Hi there
I'm working on a model 250 amplifier that came into our shop for
repair. The customer reported that the unit smoked when it was powered
up. So far I have found the following defective components:
1. Q4, Q5, Q8, Q9, drivers open
2. R35, R25, .51 ohm resistors open
3. Q12, Q13 output transistors shorted
4. R27, 681 ohm base resistor for Q7 open
This seems to be an an inordinate amount of component failures on this
unit. I've never seen all the drivers on a unit open. Is this a usual
occurance on this model? Also, are these parts still available or can
I substitute?
Some other questions; Can I get away with just replacing the two
shorted outputs or should I change them all? Is it critical that these
outputs and the drivers are matched and how tight is the spec on that
681 ohm resistor? I have checked everything else on the output board
and have not found any othr problems as of yet however I was wondering
if given what I've already mentioned, its felt that I've covered all
the bases, or might there be other things I should look at as well?
Thanks very much for any help and advice.

Lenny Stein, CET.
Barlen Electronics

 

[Asimov]

"Lenny" bravely wrote to "All" (19 May 04 06:44:27)
--- on the heady topic of "Kustom 250 amplifier, well done"

Performance amps often have pretty minimal circuit protection and
when the outputs blow they take everything else with them on down the
line. They mostly depend on the robustness of the output devices for
protection. Put in the beefiest replacements that will fit.

When any power amp shorts their outputs the drivers should be
considered suspect and replaced along with them as a matter of
principle. They are often found to be leaky with reduced hfe, etc.

The parallel outputs that didn't short may be okay but only thorough
testing can tell for sure. Usually the emitter resistors are supposed
to equalize the output current but transistors from a totally different
lot may hog the current. This current hogging could be what happened to
those that shorted.

BTW 0.51 ohms is a pretty large value, are you sure? The 681 may be
code for 680, i.e. 223 = 22K etc, or that 1 is simply charred black
that turned brown?

Key phrases with high power devices is heat sinking and good bias idle
current control. If the bias goes wild the output devices won't last
10 seconds. Use resistors in the collectors to limit current when
first powering up in case of problems like an uncontrollable bias or a
huge offset voltage. A variac on the powerline may help but I don't
trust that as much. Idle bias in the range of 20 to 60ma per device.

Don't forget to check the power supply rectifiers, filter caps, etc. I
just had a bad amp where the user replaced the burnt 3 amp fuse with a
10 amp slow blow. The 10 amp fuse didn't blow but the rectifiers left
a crater in the pcb. I'm guessing the house lights must have dimmed
when he let out the smoke.

Check the zobel network in case it oscillated itself to death. I can't
think of anything else for now. Good luck.


Le> From: [email protected] (Lenny)

Le> Hi there
Le> I'm working on a model 250 amplifier that came into our shop for
Le> repair. The customer reported that the unit smoked when it was powered
Le> up. So far I have found the following defective components:
Le> 1. Q4, Q5, Q8, Q9, drivers open
Le> 2. R35, R25, .51 ohm resistors open
Le> 3. Q12, Q13 output transistors shorted
Le> 4. R27, 681 ohm base resistor for Q7 open
Le> This seems to be an an inordinate amount of component failures on this
Le> unit. I've never seen all the drivers on a unit open. Is this a usual
Le> occurance on this model? Also, are these parts still available or can
Le> I substitute?
Le> Some other questions; Can I get away with just replacing the two
Le> shorted outputs or should I change them all? Is it critical that these
Le> outputs and the drivers are matched and how tight is the spec on that
Le> 681 ohm resistor? I have checked everything else on the output board
Le> and have not found any othr problems as of yet however I was wondering
Le> if given what I've already mentioned, its felt that I've covered all
Le> the bases, or might there be other things I should look at as well?
Le> Thanks very much for any help and advice.

Le> Lenny Stein, CET.
Le> Barlen Electronics

.... A fail-safe circuit will destroy others.