DaveC wrote:
In my 400W audio power amp, a power MOSFET died, I think. The speaker output
from one of the channels is 5.5v DC, whereas it's supposed to be zero.


Dave,

I used to work fixing this kind of equipment (many moons ago). Here's
what I suggest.

Disconnet the unit from the AC power - do not simply turn it off!!!!

You need to disconnect at least 2 of the 3 leads on the mosfets and use
and ohm meter to check for leakage. This will find you the bad part.
Be carefull to keep yourself grounded to the unit's chassis or you may
kill one of the mosfets.

Visually determine if each bank is basically just paralleled - source,
gate, and drains. If so, you can just remove one bad one and run the
unit at reduced power.

Here is the important part: Rig a 40 watt 110 volt light bulb in series
with the AC power feed each time you test the unit after replacing
parts. If the light comes on full, you have other problems to go after.
If it comes on bright but settles down to less than full intensity,
you can then run full (AC) power to test it out.

I used to have one of these wired permenantly into my testbench along
with a switch to bypass the light bulb. Anything I worked on was
plugged in there. Low tech, but very useful.

--
Luhan Monat, "LuhanKnows" At 'Yahoo' dot 'Com'
http://members.cox.net/berniekm
"The future is not what it used to be."

"DaveC" wrote...
In my 400W audio power amp, a power MOSFET died, I think. The speaker
output
from one of the channels is 5.5v DC, whereas it's supposed to be zero.

Configuration is 3 parallel MOSFETs connecting the output to the plus
rail,
and 3 connecting it to the minus rail.Rails are plus and minus 68v,
nominal,
no ripple.

Can I just remove the failed plus-side MOSFET from the circuit? If I can
do
this, should I also remove one of the minus-side ones from that channel,
as
well?


I recently repaired a couple of high-power MOSFET amps. Take a look to see
if there are low-value resistors in series with each MOSFET's drain, before
they are paralleled. If the sources and drains are paralleled without any
resistors, then you have to replace all three MOSFETs, with matched units;
otherwise one MOSFET will hog all the current and blow up. (For some
reason, for a period of time, designers had got it into their heads that
MOSFETs were immune to this problem, and so they stopped using the
precautions they had used for BJT's. Unfortunately, this was a mistake.)
The sad news is that to get three matched MOSFETs, you're going to have to
buy a dozen or so and then hand-pick the best-matched ones.

Make sure that the gate resistors are still intact (and the rest of the gate
drive circuitry). Sometimes when a MOSFET fails it takes out the gate
resistor too; and then, when you connect up the replacement without fixing
the gate resistor, the new MOSFET has a dangling gate, meaning it waits a
random period of time and then turns full on, thus blowing itself up again.
Ask me how I know.

You may find that it is hard to get exact replacements to your MOSFETs: some
of the ones used in older audio amp designs are no longer commonly sold.
For one set of MOSFETs, I had excellent results buying equivalents from
Profusion PLC, in the UK (even though I'm in the USA, they were very
inexpensive and low-hassle, and shipped incredibly quickly).

IF you're lucky enough that just one of the MOSFETs is blown, then yes, you
could just remove it and operate at lower power. Indeed, there's probably
not even any point in removing one from the other side.

Note that if you're seeing DC on the output, then either the amp has no
protection circuitry or the protection circuitry is also fried. In neither
case would I feel very comfortable about connecting that amp to any speakers
I owned...

DaveC wrote:
|| In my 400W audio power amp, a power MOSFET died, I think. The
|| speaker output from one of the channels is 5.5v DC, whereas it's
|| supposed to be zero.
||
|| Configuration is 3 parallel MOSFETs connecting the output to the
|| plus rail, and 3 connecting it to the minus rail.Rails are plus and
|| minus 68v, nominal, no ripple.
||
|| Can I just remove the failed plus-side MOSFET from the circuit? If I
|| can do this, should I also remove one of the minus-side ones from
|| that channel, as well?
||
|| Eventually I'll look into exactly what has failed (MOSFET, driver
|| FET, or some passive component), but right now I just want to get
|| the amp's output back to zero and use it right away.
||
|| How can I easily determine which of the three MOSFETs has failed?
||
|| This amp is a little-known brand (VSP Labs) from a small, long-closed
|| company, so no schematics or repair documentation is available.
||
|| Suggestions?
||

Dave,

what gives you the idea that (only) one of the transistors "died"? Do you
have any measurement gear to verify this?
I would advice you to bring this amp to a musical instruments repair shop,
because your skills do not seem to be adaequate for a repair. :-(
Or let it Rest In Peace.

ciao Ban

Walter Harley wrote:

"DaveC" wrote...
In my 400W audio power amp, a power MOSFET died, I think. The speaker
output
from one of the channels is 5.5v DC, whereas it's supposed to be zero.

Configuration is 3 parallel MOSFETs connecting the output to the plus
rail,
and 3 connecting it to the minus rail.Rails are plus and minus 68v,
nominal,
no ripple.

Can I just remove the failed plus-side MOSFET from the circuit? If I can
do
this, should I also remove one of the minus-side ones from that channel,
as
well?

I recently repaired a couple of high-power MOSFET amps. Take a look to see
if there are low-value resistors in series with each MOSFET's drain, before
they are paralleled. If the sources and drains are paralleled without any
resistors, then you have to replace all three MOSFETs, with matched units;
otherwise one MOSFET will hog all the current and blow up. (For some
reason, for a period of time, designers had got it into their heads that
MOSFETs were immune to this problem, and so they stopped using the
precautions they had used for BJT's. Unfortunately, this was a mistake.)
The sad news is that to get three matched MOSFETs, you're going to have to
buy a dozen or so and then hand-pick the best-matched ones.

Would it be practical/possible to add an equalizing resistor in
series with each drain in the event he can't get matched MOSFETs
to allow using an unmatched replacement? Would that be
effective?



Make sure that the gate resistors are still intact (and the rest of the gate
drive circuitry). Sometimes when a MOSFET fails it takes out the gate
resistor too; and then, when you connect up the replacement without fixing
the gate resistor, the new MOSFET has a dangling gate, meaning it waits a
random period of time and then turns full on, thus blowing itself up again.
Ask me how I know.

You may find that it is hard to get exact replacements to your MOSFETs: some
of the ones used in older audio amp designs are no longer commonly sold.
For one set of MOSFETs, I had excellent results buying equivalents from
Profusion PLC, in the UK (even though I'm in the USA, they were very
inexpensive and low-hassle, and shipped incredibly quickly).

IF you're lucky enough that just one of the MOSFETs is blown, then yes, you
could just remove it and operate at lower power. Indeed, there's probably
not even any point in removing one from the other side.

Note that if you're seeing DC on the output, then either the amp has no
protection circuitry or the protection circuitry is also fried. In neither
case would I feel very comfortable about connecting that amp to any speakers
I owned...

"DaveC" wrote in message
al.net...
Suspect that it is probably not, as I had first assumed, the power MOSFET.

Symptoms:
1. This channel's MOSFETs do not get warm at all while amp is idling with
no
input. Other channel MOSFETS get plenty warm under identical conditions.
Presume that bias on MOSFETS for defective channel is incorrect.

2. Amp's output waveform is severely distorted (see below).

Poking around the input circuitry I put scope probe on emitter of 2N4033.

With 0.1v p-p input:
http://home.covad.net/~peninsula/Amp/Wave1.jpg

With 0.15v p-p input:
http://home.covad.net/~peninsula/Amp/Wave2.jpg

While I was watching this, the waveform increased in its distortion over
just
a 2-second period to this:
http://home.covad.net/~peninsula/Amp/Wave3.jpg

Those are weird waveforms. I would be looking in the feed back circuit
first. I'd suspect an electrolitic cap gone bad, especally after 20 years.


With 0.25v p-p input:
http://home.covad.net/~peninsula/Amp/Wave4.jpg

Does this look familiar? Is it typical of a common type of failure?

Without schematics (this is a 20-year-old amp from a long-since defunct
company), I'm left to poke around and try to follow the signal path
looking
for good signal shape and distorted shape, which I can use to guess at
suspect components.

Any help in getting to the bottom of this would be greatly appreciated.

Thanks,
--
DaveC

This is an invalid return address
Please reply in the news group

Will you do the same measurements in oscilloscope in DC mode ? Seems a
component is burned, but i need a simple diagram of output stage and DC
snapshots to judge.
Dario2




DaveC wrote:

Suspect that it is probably not, as I had first assumed, the power MOSFET.

Symptoms:
1. This channel's MOSFETs do not get warm at all while amp is idling with no
input. Other channel MOSFETS get plenty warm under identical conditions.
Presume that bias on MOSFETS for defective channel is incorrect.

2. Amp's output waveform is severely distorted (see below).

Poking around the input circuitry I put scope probe on emitter of 2N4033.

With 0.1v p-p input:
http://home.covad.net/~peninsula/Amp/Wave1.jpg

With 0.15v p-p input:
http://home.covad.net/~peninsula/Amp/Wave2.jpg

While I was watching this, the waveform increased in its distortion over just
a 2-second period to this:
http://home.covad.net/~peninsula/Amp/Wave3.jpg

With 0.25v p-p input:
http://home.covad.net/~peninsula/Amp/Wave4.jpg

Does this look familiar? Is it typical of a common type of failure?

Without schematics (this is a 20-year-old amp from a long-since defunct
company), I'm left to poke around and try to follow the signal path looking
for good signal shape and distorted shape, which I can use to guess at
suspect components.

Any help in getting to the bottom of this would be greatly appreciated.

Thanks,

DaveC wrote:

Suspect that it is probably not, as I had first assumed, the power MOSFET.

Symptoms:
1. This channel's MOSFETs do not get warm at all while amp is idling with no
input. Other channel MOSFETS get plenty warm under identical conditions.
Presume that bias on MOSFETS for defective channel is incorrect.

2. Amp's output waveform is severely distorted (see below).

Poking around the input circuitry I put scope probe on emitter of 2N4033.

With 0.1v p-p input:
http://home.covad.net/~peninsula/Amp/Wave1.jpg

With 0.15v p-p input:
http://home.covad.net/~peninsula/Amp/Wave2.jpg

While I was watching this, the waveform increased in its distortion over just
a 2-second period to this:
http://home.covad.net/~peninsula/Amp/Wave3.jpg

With 0.25v p-p input:
http://home.covad.net/~peninsula/Amp/Wave4.jpg

Does this look familiar? Is it typical of a common type of failure?

Without schematics (this is a 20-year-old amp from a long-since defunct
company), I'm left to poke around and try to follow the signal path looking
for good signal shape and distorted shape, which I can use to guess at
suspect components.

Any help in getting to the bottom of this would be greatly appreciated.

Thanks,
--
DaveC

This is an invalid return address
Please reply in the news group

Dave - Great pictures! I particularly like the .1 v input
resultant - looks like 1 kHz with "notches". It's almost
a shame to fix it! :-)

You may have thought of this already, but could you
lift the 2N4033 emitter and then drive it into a
power resistor instead of the MOSFETS? I'm thinking
that might isolate the source of the "notch generator".
Right now, its not clear if the Mosfets are pulling
down the signal, making the notch, or the notch is
actually on the input of the 2N4033.

It dies look like the Mosfets are shorting the output
on the emitter whenever it reaches .1 volt. Driving the
emitter into a resistor would eliminate that possibility.

What does the output from the Mosfets look like at
..1 v input? Identical wave but larger amplitude?
If not, and the output from the Mosfets appears
only when the notch appears, then it would appear they
are biased wrong, as you mentioned. Can you compare
bias levels with the other channel?

Someone else mentioned electrolytics - they are famous
for causing audio distortion, as I'm sure you know.
I would love it if you could scope it down to the failing
'lytic - if that is the problem - rather than just
replacing. I kick myself now for the ones I've replaced
for distortion problems - I never looked at them closely
with the scope. Maybe if I had done it as carefully
as you set your scope and generator, I'd have seen
notches, too. What I've seen looks more like your last
picture, and I've never bothered to look at it closely.

I hope you'll keep posting on this - it is very
interesting. I've never run into a waveform like
yours in the .1v picture.

DaveC wrote:
Suspect that it is probably not, as I had first assumed, the power MOSFET.

Symptoms:
1. This channel's MOSFETs do not get warm at all while amp is idling with no
input. Other channel MOSFETS get plenty warm under identical conditions.
Presume that bias on MOSFETS for defective channel is incorrect.

2. Amp's output waveform is severely distorted (see below).

Poking around the input circuitry I put scope probe on emitter of 2N4033.

With 0.1v p-p input:
http://home.covad.net/~peninsula/Amp/Wave1.jpg

With 0.15v p-p input:
http://home.covad.net/~peninsula/Amp/Wave2.jpg

While I was watching this, the waveform increased in its distortion over just
a 2-second period to this:
http://home.covad.net/~peninsula/Amp/Wave3.jpg

With 0.25v p-p input:
http://home.covad.net/~peninsula/Amp/Wave4.jpg

Does this look familiar? Is it typical of a common type of failure?

Without schematics (this is a 20-year-old amp from a long-since defunct
company), I'm left to poke around and try to follow the signal path looking
for good signal shape and distorted shape, which I can use to guess at
suspect components.

Any help in getting to the bottom of this would be greatly appreciated.

Thanks,

The second image looks familiar. Could be parsitic oscillations at the
positive peak. If the frequency is way higher than the scope can
handle, then it can appear as a notch.


--
Luhan Monat, "LuhanKnows" At 'Yahoo' dot 'Com'
http://members.cox.net/berniekm
"The future is not what it used to be."

DaveC wrote:
|| OK, it's not electrolytic caps. I've replaced all dozen or so of the
|| small caps and the symptoms are unchanged.
||
|| Now I'll isolate each stage by lifting one of the legs of each
|| device and driving it into a resistor, as someone mentioned earlier
|| in the thread. This will help to isolate the symptom.
||
|| Any suggestions while I do that would be gratefully appreciated.
||
|| Thanks,
|| --
|| DaveC

Dave,
again an unuseful idea, you were on the right track before(with the bias!),
just first measure the gate voltage of both the upper and lower Fets (if
they are FETs at all) and then check the bias voltage pot(s), maybe replace
it, or adjust it to a value of 100-200mA idle current, or better check how
much that is on the other side. To mesure this current take out the fuses of
each side if there are any in the 65V line or just put the ampmeter into
this line. Start with a high range not to blow the fuse in the meter.

Since the feedback from the output will distort the balance of the input
stage, it was not the right place to measure. Go backwards from the output
stage in DC-mode with the scope, and compare each measurement with the same
point of the working side.
Repairing means a bit more than replacing parts, finding the reason of the
failure.
good luck

ciao Ban

"DaveC" wrote in message
al.net...
I've found several references to MOSFET matching to avoid
"current-hogging".

If they are in parallel it is quite necessary. If they have source
resistors, it is less necessary, because those provide a form of current
feedback.

I built myself a very simple rig to do the matching, but I don't remember
offhand the component values I used. I'll try to post the schematic later
tonight. It basically hooks the MOSFET up in a two-terminal current-sink
configuration (gate connected to drain) and sees how much current it draws.
In principle you're trying to match two curves, so you want to measure Ids
for a lot of different Vgs and get the best overall match. But in practice,
using a single point has worked okay for me.

Do you have a dummy load to test the amp against, once you've got it fixed?


Originals are 2SK1221 (TO-3) and its compliment (number not at hand right
now...)

Yeah, it's hard to find TO-3 replacements. You probably won't do better
than Profusion PLC. (And, I don't have any matched equivalents for those,
sorry!)

I read in sci.electronics.design that DaveC wrote (in
et) about 'Amp's
power MOSFET died. Can I just remove it?', on Tue, 18 Nov 2003:
Someone suggested a couple of Walmart electric heating coils in series.
Sound good?

No. You need an **8 ohm** resistor, of at least 250 W rating. (Assuming
you amp needs an 8 ohm load, which most of them do.)
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

I read in sci.electronics.design that DaveC wrote (in
et) about 'Amp's
power MOSFET died. Can I just remove it?', on Wed, 19 Nov 2003:
The person suggesting this solution stated that the heating coils (I
think they are actually electric tea kettles, IIRC) were each about 4
ohms.

Are these for use on vehicles, from 12 V, then? I didn't see that
article.

You can buy high-power resistors from Farnell/Newark and they are not
costly. They are even in the horrible UK Digikey catalogue, as 'Aluminum
Housed Chassis Mount Resistors. Eight 1 ohm TMC-50 type in series give
you 8 ohms at 400 W if you bolt them to a big sheet of 1/8 inch
'aluminum'.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

John Woodgate wrote:
|| I read in sci.electronics.design that DaveC wrote
|| (in et) about
|| 'Amp's
|| power MOSFET died. Can I just remove it?', on Wed, 19 Nov 2003:
||| The person suggesting this solution stated that the heating coils (I
||| think they are actually electric tea kettles, IIRC) were each about
||| 4 ohms.
||
|| Are these for use on vehicles, from 12 V, then? I didn't see that
|| article.

A 115V 1500W electric kettle apparently has 7.6 Ohms (when warm probably).
But you have to get an American one, difficult in Europe. Anyway, a simple
multimeter can verify this. Eventual inductive behaviour can be compensated
with a parallel R-C combination, but I do not think this will be needed in
the audio range. Of course the 200W will not heat up a lot of water, so put
just a minimum amount into it. Then you can make a nice Nescoffee after a
reasonably timed test. (Only English make tea).
Precision will not be possible, but for an amplifier test it will suffice.

ciao Ban

"DaveC" wrote...
I'm a bit confused about the connection of the FET:

The drain and gate are tied together (per your earlier post). These are
connected to Vdd through the 150 ohm resistor?

But then you provide a gate voltage via 470 ohm resistor?

Source is tied to ground?

I think the ASCII is screwing me up...

View the ASCII with a fixed-width font. It should look fine.

Source is tied to ground. Gate is tied to drain through a 470R resistor.
Drain to power supply through a 150R power resistor. The only reason for
the 470R gate resistor is to eliminate high-frequency oscillations that
could skew the results. It's probably not necessary, but I just don't like
connecting a MOSFET gate directly to something like a test lead that could
have arbitrary inductances, capacitances, etc.

This circuit is a "two-terminal current sink". It is a self-contained
negative-feedback regulated system. If current flow increases, that makes
the voltage drop across the 150R increase, which drops Vgs, which decreases
current flow; and vice versa. So, Id is constant; all you have to do is
*measure* Vgs, you don't independently control it.

The number you get doesn't really tell you a lot about the MOSFET. But two
MOSFETs with the same Id-to-Vgs curve will show the same Vgs in this rig,
and that's all you care about.

If you wanted to actually plot the whole curve of Id versus Vgs, you would
need to separately control Vgs. But there's no reason to do that, because
you're not going to buy enough MOSFETs to be able to do matching on that
level, so you'd only make yourself unhappy :-)

-walter

"John Woodgate" wrote in message
...
I read in sci.electronics.design that DaveC wrote (in
et) about 'Amp's
power MOSFET died. Can I just remove it?', on Wed, 19 Nov 2003:
The person suggesting this solution stated that the heating coils (I
think they are actually electric tea kettles, IIRC) were each about 4
ohms.

Are these for use on vehicles, from 12 V, then? I didn't see that
article.

You can buy high-power resistors from Farnell/Newark and they are not
costly. They are even in the horrible UK Digikey catalogue, as 'Aluminum
Housed Chassis Mount Resistors. Eight 1 ohm TMC-50 type in series give
you 8 ohms at 400 W if you bolt them to a big sheet of 1/8 inch
'aluminum'.

You could build a dummy load in a paint can (available at hardware stores)
filled with mineral oil to dramatically increase the power handling of the
resistors. The kettle elements sound like a good idea to me though.

"John Woodgate" wrote in message
...
I read in sci.electronics.design that DaveC wrote (in
et) about 'Amp's
power MOSFET died. Can I just remove it?', on Wed, 19 Nov 2003:
The person suggesting this solution stated that the heating coils (I
think they are actually electric tea kettles, IIRC) were each about 4
ohms.

Are these for use on vehicles, from 12 V, then? I didn't see that
article.

You can buy high-power resistors from Farnell/Newark and they are not
costly. They are even in the horrible UK Digikey catalogue, as 'Aluminum
Housed Chassis Mount Resistors. Eight 1 ohm TMC-50 type in series give
you 8 ohms at 400 W if you bolt them to a big sheet of 1/8 inch
'aluminum'.


I like the Dale NH-250 series but they were not cheap last the time I priced
them.
A freind of mine used to use toaster coils from the
realy old ones that used ceramic insulators.
They would glow pretty good on a larger amp.
Jeff


--
Regards, John Woodgate, OOO - Own Opinions Only.
http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go
to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!

"DaveC" bravely wrote to "All" (19 Nov 03 06:56:13)
--- on the heady topic of " Amp's power MOSFET died. Can I just remove it?"

Da From: DaveC

Da The person suggesting this solution stated that the heating coils (I
Da think they are actually electric tea kettles, IIRC) were each about 4
Da ohms. Not exactly, but close, and a resistive load. Two in series
Da seems quite a good idea. And I can fill them with water to dissipate
Da the energy, and make a pot of tea, as well :-)

Da I'll look around for a large wirewound resistor, but 200 W? I don't
Da hold out much hope, even here in Silicon Valley...

Try paralleling ten 160 ohm 20 watt resistors or 0.4 ohm resistors in
series. I made an 8 ohm dummy load using nichrome wire in an oil bath.

Asimov
******

.... Just a little force field zap.