blown output transistors

[[email protected]]

[email protected] wrote:

Practically all the amps I work on are dc coupled. Not a problem in most
cases.
Virtually all your Kenwood, Pioneer,Sansui, etc going back to 1970 or so are
DC coupled.
Modern amps like Denon, Yamaha, Onkyo, etc are all pretty similar in
topology. Certain components tend to go out along with the outputs
(resistors and driver transistors for example) and symptoms such as
premature clipping, DC offset etc have predictable causes.
As to the OP, it does seem his piece is quite possibly a "piece" all right,
but I'm not at all sure I'd classify it as such a difficult repair simply
because it may be a "DC" amp. As I say, I fix such amps all the time. No
problem.

Mark Z.

which puts you in a very different position to the OP.


NT

 

[Dave]

Practically all the amps I work on are dc coupled. Not a problem in most
cases.
Virtually all your Kenwood, Pioneer,Sansui, etc going back to 1970 or so
are DC coupled.
Modern amps like Denon, Yamaha, Onkyo, etc are all pretty similar in
topology. Certain components tend to go out along with the outputs
(resistors and driver transistors for example) and symptoms such as
premature clipping, DC offset etc have predictable causes.
As to the OP, it does seem his piece is quite possibly a "piece" all
right, but I'm not at all sure I'd classify it as such a difficult repair
simply because it may be a "DC" amp. As I say, I fix such amps all the
time. No problem.

Mark Z.

As I noted in my post, I'm willing to spend some time on this and would love
to learn something.

I've checked all of the driver transistors inasmuch as is possible using the
diode check function of a DMM. The ones that didn't pass were replaced.
I've checked all of the diodes and resistors in-circuit, everything passed,
resistors all "in the zone", i.e. within 40% of their stated values. My
multimeter also has a transistor check but it's pretty flaky especially with
small transistors.... I've tested known bad transistors and the meter
happily gives me an hfe value for them.

I can power up the amp with output transistors removed, as soon as they go
back in the two in-line 5A fuses on the transformer secondaries blow. there
are no dead shorts across the outputs. I get proper voltages at the inputs
to my IC's (really the only values I can check as I have no service manual
or schematic), voltage regulators are working at +15VDC/-15VDC/-6VDC.

what could cause this overcurrent situation?

The new output transistors are NOT fried, they're rated at 10A peak and the
new fuses are fast-blo at 5A. I'm sure they don't LIKE a big pulse of
instantaneous current but they're brand new and likely can handle it a few
times. ALL but two (drivers) of the transistors in the amp circuits are
2SC945/2A733's which should vaporize with anywhere near 5A of current..
theyr'e only rated for 150mA.

Almost makes you think there's a short before the amp section if the fuses
blow but not the drivers/outputs... but the thing runs fine with outputs
transistors removed.

I'm going to do something I should have done a few posts ago... install the
outputs one pair at a time which will at least localize my problem to one of
the three channels.

I know there's no substitute for experience, but does anyone know if any
books or online publications that cover DC coupled (audio in particular)
amplifiers and the troubleshooting thereof? I know you're all busy and
probably tend to lose interest when you see posts and think "this guy ain't
never gonna' fix this" but I'm a relatively smart guy with a couple of years
of Electrical Engineering schooling (although that was more than 20 years
ago we were still in the transistor age).

Dave

 

[[email protected]]

As I noted in my post, I'm willing to spend some time on this and would love
to learn something.

I've checked all of the driver transistors inasmuch as is possible using the
diode check function of a DMM. The ones that didn't pass were replaced.
I've checked all of the diodes and resistors in-circuit, everything passed,
resistors all "in the zone", i.e. within 40% of their stated values.

Most Rs should be within 5%, a few 1%. Anything as far out as 20 or 40%
needs replacing.

My
multimeter also has a transistor check but it's pretty flaky especially with
small transistors.... I've tested known bad transistors and the meter
happily gives me an hfe value for them.

then why do you say those trs are bad?

I can power up the amp with output transistors removed, as soon as they go
back in the two in-line 5A fuses on the transformer secondaries blow. there
are no dead shorts across the outputs. I get proper voltages at the inputs
to my IC's (really the only values I can check as I have no service manual
or schematic), voltage regulators are working at +15VDC/-15VDC/-6VDC.

what could cause this overcurrent situation?

a fault of any kind anywhere in the power amp that results in the
output trs being turned on.

The new output transistors are NOT fried, they're rated at 10A peak and the
new fuses are fast-blo at 5A. I'm sure they don't LIKE a big pulse of
instantaneous current but they're brand new and likely can handle it a few
times.

very unlikely. If they handle it once you've been lucky.

ALL but two (drivers) of the transistors in the amp circuits are
2SC945/2A733's which should vaporize with anywhere near 5A of current..
theyr'e only rated for 150mA.

Almost makes you think there's a short before the amp section if the fuses
blow but not the drivers/outputs... but the thing runs fine with outputs
transistors removed.

so its your output trs that short. Or the speaker wiring.

I'm going to do something I should have done a few posts ago... install the
outputs one pair at a time which will at least localize my problem to one of
the three channels.

I know there's no substitute for experience, but does anyone know if any
books or online publications that cover DC coupled (audio in particular)
amplifiers and the troubleshooting thereof? I know you're all busy and
probably tend to lose interest when you see posts and think "this guy ain't
never gonna' fix this" but I'm a relatively smart guy with a couple of years
of Electrical Engineering schooling

OK first thing you need to do is put current limiting Rs onto the
collectors and bases of all these big expensive trs. This will stop
them dying instantly, and enable you to measure whats going on. Your
psu is 15v, so if we say 15v 1.7A thats 10 ohms 23 watt Rs for the
collectors. You can make those out of a reel of resistance wire, or buy
high power Rs if you've money to waste.

Base R values will depend on hfe of the TIPs.

Next thing you need is a circuit diagram. Without that its a waste of
time. Then you should be able to follow the cct through, check voltages
all over and narrow down where its going wrong.


NT

 

[Dave]

Most Rs should be within 5%, a few 1%. Anything as far out as 20 or 40%
needs replacing.

I mean that when I test them in circuit I never get the exact correct value.
I figured that if I was looking for a gross over-current-causing resistor
fault, it'd likely be open or shorted. or close.

then why do you say those trs are bad?

Because they don't work in circuit. Case in point I replaced a PS
transistor in a tuner awhile back which was putting out completely
out-to-lunch voltages (I had a circuit diagram for this one). Problem
solved. Stuck transistor into my DMM transistor tester and it gave it an
hfe which was in the range of what the data sheet might lead one to expect.

a fault of any kind anywhere in the power amp that results in the
output trs being turned on.



very unlikely. If they handle it once you've been lucky.

These are 6A continuous rated. Why shouldn't they be protected behind a 5A
fast-blow fuse? Or isn't "fast-blow" fast enough?

Damn, probably used up a bunch of luck on those trannies, then. I have a
feeling I still am going to be in need of some down the road on this one...

so its your output trs that short. Or the speaker wiring.

The amp was powered up with no load (no speakers connected). So unless I
got some bad brand-new TIP's I don't know how they could be shorting.

OK first thing you need to do is put current limiting Rs onto the
collectors and bases of all these big expensive trs. This will stop
them dying instantly, and enable you to measure whats going on. Your
psu is 15v, so if we say 15v 1.7A thats 10 ohms 23 watt Rs for the
collectors. You can make those out of a reel of resistance wire, or buy
high power Rs if you've money to waste.

Can I cheat and use a 25W light bulb?

Base R values will depend on hfe of the TIPs.

hfe at 3A is

min=15
max=75.

Do I use avg=45? 1.7A / 45 = 38mA, 15V / 0.038A = 400 ohm 1/2W.

Next thing you need is a circuit diagram. Without that its a waste of
time. Then you should be able to follow the cct through, check voltages
all over and narrow down where its going wrong.

I've got it mostly drawn out... another hour or two and I should be there
with a schematic.

 

[[email protected]]

I mean that when I test them in circuit I never get the exact correct value.
I figured that if I was looking for a gross over-current-causing resistor
fault, it'd likely be open or shorted. or close.

measuring them in circuit is going to give you all sorts of out there
readings. And resistors dont normally short, so I'm not sure if you'll
find much that way.

Because they don't work in circuit. Case in point I replaced a PS
transistor in a tuner awhile back which was putting out completely
out-to-lunch voltages (I had a circuit diagram for this one). Problem
solved. Stuck transistor into my DMM transistor tester and it gave it an
hfe which was in the range of what the data sheet might lead one to expect.

perhaps the meter just reads dc collector current, and on faulty goods
its reading leakage? Try measuring a resistor from C to E connections
on the meter, see if it gives an hfe reading.

These are 6A continuous rated. Why shouldn't they be protected behind a 5A
fast-blow fuse? Or isn't "fast-blow" fast enough?

Everyone asks this one.
1. Fuses dont limit current, so way above 5A flows when a 5A fuse blows
to clear a short
2. Fast fuses are very slow compared to what a tr can survive. Fuses
can not protect trs to a significant extent.
3. Fuses dont blow at 5A, 5A is the current they'll pass for decades.
Blowing one takes much more.

Look up fuse curves to see what it takes to pop one.

The amp was powered up with no load (no speakers connected). So unless I
got some bad brand-new TIP's I don't know how they could be shorting.

Yabbut theres wiring from the pcb to the external connectors, plus
maybe a headphone jack etc. Remove the audio output connection at the
pcb, if convenient, to isolate all that crp.

Can I cheat and use a 25W light bulb?

Filament bulbs have a power on surge of 8-10x run current, their cold
resistance is about a tenth of hot resistance. If you take this into
account you can use a bulb with the right cold resistance. 10 ohms cold
is 100 ohm hot. If you're on 110v thats a 100watt bulb.

hfe at 3A is

min=15
max=75.

Do I use avg=45? 1.7A / 45 = 38mA, 15V / 0.038A = 400 ohm 1/2W.

If you use average hfe, half your trs wont get enough base drive to
deliver the required collector i. So use min hfe of 15. If you want to
run 1.5A thru collector, you need at least 1.5/15 = 0.1A into the base.

I've got it mostly drawn out... another hour or two and I should be there
with a schematic.

cool


NT

 

[Mark D. Zacharias]

As I noted in my post, I'm willing to spend some time on this and
would love to learn something.

I've checked all of the driver transistors inasmuch as is possible
using the diode check function of a DMM. The ones that didn't pass
were replaced. I've checked all of the diodes and resistors
in-circuit, everything passed, resistors all "in the zone", i.e.
within 40% of their stated values. My multimeter also has a
transistor check but it's pretty flaky especially with small
transistors.... I've tested known bad transistors and the meter
happily gives me an hfe value for them.
I can power up the amp with output transistors removed, as soon as
they go back in the two in-line 5A fuses on the transformer
secondaries blow. there are no dead shorts across the outputs. I
get proper voltages at the inputs to my IC's (really the only values
I can check as I have no service manual or schematic), voltage
regulators are working at +15VDC/-15VDC/-6VDC.
what could cause this overcurrent situation?

The new output transistors are NOT fried, they're rated at 10A peak
and the new fuses are fast-blo at 5A. I'm sure they don't LIKE a big
pulse of instantaneous current but they're brand new and likely can
handle it a few times. ALL but two (drivers) of the transistors in
the amp circuits are 2SC945/2A733's which should vaporize with
anywhere near 5A of current.. theyr'e only rated for 150mA.

Almost makes you think there's a short before the amp section if the
fuses blow but not the drivers/outputs... but the thing runs fine
with outputs transistors removed.

I'm going to do something I should have done a few posts ago...
install the outputs one pair at a time which will at least localize
my problem to one of the three channels.

I know there's no substitute for experience, but does anyone know if
any books or online publications that cover DC coupled (audio in
particular) amplifiers and the troubleshooting thereof? I know
you're all busy and probably tend to lose interest when you see posts
and think "this guy ain't never gonna' fix this" but I'm a relatively
smart guy with a couple of years of Electrical Engineering schooling
(although that was more than 20 years ago we were still in the
transistor age).
Dave

This is basic and may have already been covered, but DO check the DC
resistance between the collector of the outputs and the heat sink. Should be
open-circuit, an insulator could be damaged or missing.
Also check resistance between each emitter resistor and both the + and -
supplies. Should not be a short - should show open or cap charging. If you
show a short here, then an output transistor is probably bad, despite
earlier testing.

The hFe showing on a leaky transistor is common with these checkers. Lift
the base lead. If the transistor still conducts with only the E-C connected,
it's bad.

Mark Z.