power amp advice sought

[Dan K]

Hi group

I have a 240w single channel solid state transformer coupled power amp that
isn't working. There is not a whole lot to the power amp board, but I do
not have a matching schematic. I did find a schematic to a more recent
version of this amp and its close enough so I was able to figure out the
main power path, but not the front end, feedback, and protection circuitry.
It looks like the company re-designed the front end, feedback, and
protection circuitry on this later model. The power amp board had 2 open
transistors, 3 shorted transistors, and showed signs of burns. The amp blew
fuses when turned on.

I replaced all the bad transistors and an open resistor (burned). I
reinstalled the large heat sink that connects to the two power transistors,
the two pre-driver transistors, and the bridge rectifier. I verified that
all transistors connected to the heat sink were electrically isolated from
the heat sink and powered the amp up. It blew a fuse.

I removed the heat sink and the power transistor pair and powered things up.
I found very high voltages throughout the output stages (like around 60 vdc)
with a large amplitude distorted signal riding on it that was the input
frequency I was feeding into the amp. The amplitude of this distorted
signal followed the volume control. I assumed that this particular amp
cannot be run w/o the power transistors, so I put them back in. I powered
the amp back on and it worked. The only difference being I was running
without a heat sink now.

There is a fan in this amp. One fan wire is connected to a ground wire that
is connected to the power supply. These two wires are connected via a screw
that screws into the heat sink. I found that as soon as this ground from
the power supply is connected to chassis ground (via the heat sink
originally) I lost all audio (but did not blow a fuse or apparently damage
anything from the momentary contact). Anyway, I was running this way...no
heat sink, fan wire not grounded, just audible output...thinking about what
I should do next when snap, crackle, pop I had flames shooting out of the
board. Both pre-driver transistors are physically cracked, I presume the
power transistors are shorted, a couple of resistors have burned up, and the
pc board really shows burn signs now. So, here's my question:

Could this failure be due to just running for 3-4 minutes at extreme low
volume without the heat sink? I would guess not, but I don't have a lot of
experience with class B power amps. If it could be, it might be worth
replacing the transistors and trying again, otherwise I think the garbage is
the best place for this amp.

Thanks

Dan

 

[Arfa Daily]

Hi group

I have a 240w single channel solid state transformer coupled power amp
that isn't working. There is not a whole lot to the power amp board, but
I do not have a matching schematic. I did find a schematic to a more
recent version of this amp and its close enough so I was able to figure
out the main power path, but not the front end, feedback, and protection
circuitry. It looks like the company re-designed the front end, feedback,
and protection circuitry on this later model. The power amp board had 2
open transistors, 3 shorted transistors, and showed signs of burns. The
amp blew fuses when turned on.

I replaced all the bad transistors and an open resistor (burned). I
reinstalled the large heat sink that connects to the two power
transistors, the two pre-driver transistors, and the bridge rectifier. I
verified that all transistors connected to the heat sink were electrically
isolated from the heat sink and powered the amp up. It blew a fuse.

I removed the heat sink and the power transistor pair and powered things
up. I found very high voltages throughout the output stages (like around
60 vdc) with a large amplitude distorted signal riding on it that was the
input frequency I was feeding into the amp. The amplitude of this
distorted signal followed the volume control. I assumed that this
particular amp cannot be run w/o the power transistors, so I put them back
in. I powered the amp back on and it worked. The only difference being I
was running without a heat sink now.

There is a fan in this amp. One fan wire is connected to a ground wire
that is connected to the power supply. These two wires are connected via
a screw that screws into the heat sink. I found that as soon as this
ground from the power supply is connected to chassis ground (via the heat
sink originally) I lost all audio (but did not blow a fuse or apparently
damage anything from the momentary contact). Anyway, I was running this
way...no heat sink, fan wire not grounded, just audible output...thinking
about what I should do next when snap, crackle, pop I had flames shooting
out of the board. Both pre-driver transistors are physically cracked, I
presume the power transistors are shorted, a couple of resistors have
burned up, and the pc board really shows burn signs now. So, here's my
question:

Could this failure be due to just running for 3-4 minutes at extreme low
volume without the heat sink? I would guess not, but I don't have a lot
of experience with class B power amps. If it could be, it might be worth
replacing the transistors and trying again, otherwise I think the garbage
is the best place for this amp.

Thanks

Dan

Refailure of this kind is common. You really need to be able to run the amp
up on a variac to see what's going on without risk of re-frying all the
parts that you have replaced. It is possible that the transistors could have
failed as a result of having no heatsink on them, but probably only as a
result of something else still being wrong. With the amp running at just
above idle, the output stage dissipation should be minimal, but not if there
is a problem with the bias setting, if this is variable, or bias supply if
fixed. Once the transistors start to get hot, they will quickly go into
thermal runaway without adequate heatsinking, and rapidly fail.

If you can lay hands on a variac, it might be worth having one more go, but
after that, without proper schematics, you will probably struggle to get to
the bottom of it, and finish up throwing good money after bad

Arfa

 

[jakdedert]

Refailure of this kind is common. You really need to be able to run the amp
up on a variac to see what's going on without risk of re-frying all the
parts that you have replaced. It is possible that the transistors could have
failed as a result of having no heatsink on them, but probably only as a
result of something else still being wrong. With the amp running at just
above idle, the output stage dissipation should be minimal, but not if there
is a problem with the bias setting, if this is variable, or bias supply if
fixed. Once the transistors start to get hot, they will quickly go into
thermal runaway without adequate heatsinking, and rapidly fail.

If you can lay hands on a variac, it might be worth having one more go, but
after that, without proper schematics, you will probably struggle to get to
the bottom of it, and finish up throwing good money after bad

Arfa

It might also be useful to post the brand and model # of the amp.
Someone might have experience with--or be able to supply a schematic
for--it.

jak

 

[Dan K]

It might also be useful to post the brand and model # of the amp. Someone
might have experience with--or be able to supply a schematic for--it.

jak

Its a CSI P-240A, which is from Speco.

 

[GregS]

Its a CSI P-240A, which is from Speco.

Its possible that you heated the parts. Try again?

I ALWAYS use a light bulb in series with power amp testing.
60 - 200 watt or so. I sometimes also use a variac in
conjunction with the light bulb. That would not have cured a heat
problem if that fried the amp, but it may have prevented sparks and
fire.

I laughed, sort of, after fixing a brute of complexity of a Crown
amp. After "fixing" it, I sat it on the floor playing some soft music,
and started walking upstairs, heard a pop, looked around at the bright
light comming from inside the power amp, then some smoke, and
that was that.


greg



greg

 

[default]

Could this failure be due to just running for 3-4 minutes at extreme low
volume without the heat sink? I would guess not, but I don't have a lot of
experience with class B power amps. If it could be, it might be worth
replacing the transistors and trying again, otherwise I think the garbage is
the best place for this amp.

Like someone already suggested - a variac is necessary when fixing
solid state amps. Monitor current and keep an eye on the output stage
to see that it comes up with the voltage you expect. I lack
experience with transformer coupled amps (except a few old Sansui amps
from the 60's)-

If the bias is adjustable set it to minimum until the full voltage is
present then set it to some reasonable value. (like 20-30 watts for a
250 watt amp)

Class B or Class AB? I have a pair of 500 watt amps - class AB and
they work class A up to about 100 watts and above that start working
class B. (24 output transistors and massive fan cooled heatsink)

It definitely needs a heatsink if it is class AB. Without the sink -
the output stage gain increases dramatically when the transistors warm
up - the bias current tracks the gain of the output stage - unless
there's some thermal feedback to control it. (old designs usually
had a pair of diodes, thermister, or bias transistor that would mount
to the same heatsink as the output transistors)

I've seen amps with two pairs of Darlington transistors in the output
- and a small thing like too little heat sink compound on one will
cause it to overheat.

 

[default]

PS - old transformer coupled Sansui amps had a pair of series
connected bias diodes in a molded plastic case with a bolt hole for
mounting. That was the thermal feedback system for that amp. The
diodes would develop an intermittent open connection and an otherwise
perfectly working amp would crackle the speakers and start blowing
fuses and transistors.

 

[Mark D. Zacharias]

I powered the amp back on and it worked. The only difference >being I was

running without a heat sink now.

Perhaps the output transistors weren't insulated from the heat sink?

Mark Z.

 

[Arfa Daily]

Perhaps the output transistors weren't insulated from the heat sink?

Mark Z.

That was my first thought also, Mark, but if you go up 4 posts, you will see
that he makes a point of saying that he checked that the devices were
isolated from the heatsink. It's a mystery ... |:-{

Arfa

 

[Mark D. Zacharias]

That was my first thought also, Mark, but if you go up 4 posts, you
will see that he makes a point of saying that he checked that the
devices were isolated from the heatsink. It's a mystery ... |:-{

Arfa

Could have a damaged insulator...

mz

 

[Arfa Daily]

Could have a damaged insulator...

mz

Could be. Might just be the way we are interpreting " checked ". When I
check that devices are isolated from a heatsink, I prove it with a meter
reading. Perhaps he was just visually inspecting ...

Arfa

 

[Dan K]

Could be. Might just be the way we are interpreting " checked ". When I
check that devices are isolated from a heatsink, I prove it with a meter
reading. Perhaps he was just visually inspecting ...

Arfa

Nope, meter reading. Isolated. It wouldn't have worked for a while if they
wasn't isolated, Vee would be shorted to Vcc. Could always be a crack in
the insulator I suppose that only shorts at certain times...They look ok
though

Dan

 

[Arfa Daily]

Nope, meter reading. Isolated. It wouldn't have worked for a while if
they wasn't isolated, Vee would be shorted to Vcc. Could always be a
crack in the insulator I suppose that only shorts at certain times...They
look ok though

Dan

Yeah, it kinda sounded like you had a done a proper test when I first read
it, and you are of course right that it wouldn't have worked ok for a
while - I'd forgotten that bit ...

Arfa

 

[jakdedert]

Nope, meter reading. Isolated. It wouldn't have worked for a while if they
wasn't isolated, Vee would be shorted to Vcc. Could always be a crack in
the insulator I suppose that only shorts at certain times...They look ok
though

Sometimes you'll get a little 'burr' of sharp metal around the holes in
the heatsink. It reads fine when cold, but when it heats up the burr
expands enough to short through the insulator.

Clean off the sink, and feel for any sharp points.

jak