Amp Repair - Stuck to negative Rail

[Ivan Barberis]

Hello!
I have to repair an old Cabotron amplifier.
The amp is a common class A-B, with MJ15015 e MJ15016 power transistors.
I have the output stuck to negative rail: checked all the transistor searching for an "opened" one, every transistor is ok (also in the driver part).
What may cause this kind of fault? Fuses are not blown.
Thanks a lot for the answers!
Ivan.

 

[N_Cook]

Ivan Barberis <bigground@gmail.com> wrote in message
news:2fb9c656-125f-42ac-bc4b-edf5d66dff7c@googlegroups.com...

Hello!
I have to repair an old Cabotron amplifier.
The amp is a common class A-B, with MJ15015 e MJ15016 power transistors.
I have the output stuck to negative rail: checked all the transistor
searching for an "opened" one, every transistor is ok (also in the driver

part).

What may cause this kind of fault? Fuses are not blown.
Thanks a lot for the answers!
Ivan.

Does it have a preamp section ? if so break it at that point and check for
any DC offset , perhaps from failed opamp in the preamp

 

[Arfa Daily]

"Ivan Barberis" <bigground@gmail.com> wrote in message
news:2fb9c656-125f-42ac-bc4b-edf5d66dff7c@googlegroups.com...

Hello!
I have to repair an old Cabotron amplifier.
The amp is a common class A-B, with MJ15015 e MJ15016 power transistors.
I have the output stuck to negative rail: checked all the transistor
searching for an "opened" one, every transistor is ok (also in the driver
part).
What may cause this kind of fault? Fuses are not blown.
Thanks a lot for the answers!
Ivan.

Check that the positive rail is actually present at the NPN output
transistor collector, and that the emitter resistor of the NPN is not open.
Then check that the positive rail is present at the opamps in the preamp
section - assuming that it uses opamps, of course. Even if it doesn't, it
will probably still have transistor stages fed from dual polarity rails. The
rails here are usually + / - 15 or 12 volts, and they are usually derived
from the main output stage rails via some kind of regulator. This might be a
three terminal monolithic type like a 7812, or a series regulator using a
transistor, or often, just a resistor and zener. Supply to the regulators is
sometimes fed via a fusible resistor, and these often fail for no apparent
reason.

A couple of weeks ago, I had an amp with the output slammed over to one rail
just like yours, and that was down to a rail missing from the preamp opamps,
and that was in turn due to an open feed resistor.

Arfa.

 

[Dave Platt]

In article <laIat.16633$Rb7.971@fx24.fr7>,
Arfa Daily <arfa.daily@ntlworld.com> wrote:

I have to repair an old Cabotron amplifier.
The amp is a common class A-B, with MJ15015 e MJ15016 power transistors.
I have the output stuck to negative rail: checked all the transistor
searching for an "opened" one, every transistor is ok (also in the driver
part).

What may cause this kind of fault? Fuses are not blown.

Thanks a lot for the answers!

Check that the positive rail is actually present at the NPN output
transistor collector, and that the emitter resistor of the NPN is not open.
Then check that the positive rail is present at the opamps in the preamp
section - assuming that it uses opamps, of course. Even if it doesn't, it
will probably still have transistor stages fed from dual polarity rails. The
rails here are usually + / - 15 or 12 volts, and they are usually derived
from the main output stage rails via some kind of regulator. This might be a
three terminal monolithic type like a 7812, or a series regulator using a
transistor, or often, just a resistor and zener. Supply to the regulators is
sometimes fed via a fusible resistor, and these often fail for no apparent
reason.

A couple of weeks ago, I had an amp with the output slammed over to one rail
just like yours, and that was down to a rail missing from the preamp opamps,
and that was in turn due to an open feed resistor.

I recently fixed an old McIntosh MA5100 with a "stuck to the positive
rail" fault in one channel. One PNP transistor in the power stage's
input "long-tailed" differential pair had failed open (open base).
This had the same effect on the differential pair and feedback loop as
if the base had been pulled all the way positive (although it hadn't
been) and the feedback loop drove the outputs upwards.

--
Dave Platt <dplatt@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

 

[Phil Allison]

"Ivan Barberis"

I have to repair an old Cabotron amplifier.
The amp is a common class A-B, with MJ15015 e MJ15016 power transistors.
I have the output stuck to negative rail: checked all the transistor
searching for an "opened" one, every transistor is ok (also in the driver
part).
What may cause this kind of fault?

** As others have said, resistors can go open - film types with values of
10 kohms or higher and WW types can suffer from internal corrosion when
large DC voltages are present.

Of course, check all zeners for shorts and ceramic caps too.

I use to see a lot of " Cerwin Vega " brand amps which had gone DC and
taken out expensive speakers because of a faulty batch of 0.1uF disk
ceramics shorting the + or - 15V rails to the input op-amp.



..... Phil