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amplifier troubleshooting

D

Dave

Jan 1, 1970
0
Awhile back I posted about an amp I am trying to repair. The amp is
composed of a +/- rectified and filtered power supply, an LM1036N
volume/tone/balance IC (two-channel), and a pair of LM1875T 20W TO-220 amps
which use bipolar power, no output filter caps. A bunch of various
resistors and caps and diodes. Very simple. I tested the resistors and
diodes in circuit, they look to be okay. I have no reason to suspect the
caps although I did replace one which appeared to be open circuit.

The symptoms: No sound on the right channel. When I turn the volume up
quickly it makes a "thump" on the bad channel. So. I started at the
outputs and worked backwards. First off, I bridged the inputs on the
LM1875's with a 1uF cap. Voila! Sound. So, the speaker works, the amp
works.

Another poster suggested that I use an analog meter to measure input and
output voltages of the LM1036N IC. So last night I soldered on some test
leads. I measured AC and DC voltages, and they varied a bit from channel to
channel, maybe 5V on one side and 4.2 on the other, no catastrophic failure
there. As an aside I kind of expected my analog multimeter to look like a
VU meter, moving up and down to the music, but didn't see that at all, very
steady. Then I noticed something: when I measured the potential between
the bad channel's INPUT to the LM1036 and ground, I heard a scratchy sound
come out of the bad speaker. Hmm. As the meter is basically a big resistor
in parallel with the circuit, I figured I'd try a smaller resisiter. I
changed the scale of my meter from 30V to 10V. Now I get tinny sound.
Changed to 3V. Meter was off-scale but I now had pretty good sound. Didn't
have the balls to try shorting the two, I have been burnt (actually my
components have been burnt) by trying dumb-ass things like that using the
axiom "if a little is good, a lot must be REAL good".

I have no formal training in electronics, but have done a lot of reading on
the good ol' internet and am keen to learn more. I am completely perplexed
as to what is happening. The only explanation I can conjure up (and sorry
if it sounds stupid but...) is maybe an excess of DC current on my signal
input which would screw up the bias of the transisters in the mixer IC.
There is a small electrolytic in series with the mixer output and the amp
input, plus a bunch of smaller ceramics from the signal trace to ground so I
don't think DC can be getting to the amp itself... and if it were my little
experiement with the bridging cap, above, wouldn't have worked so well.

I have another LM1036 mixer IC, but don't want to replace something that's
not broken.

Does anybody have any ideas? How can bridging my input signal to ground
with a resistor make sound come out? I've looked at this circuit long
enough that I can probably draw up a schematic if I can find a few hours.

THanks

Dave
 
M

M.Joshi

Jan 1, 1970
0
Dave said:
First off, I bridged the inputs on the
LM1875's with a 1uF cap. Voila! Sound. So, the speaker works, th
amp
works.

That's the key!

The problem lies somewhere before the power amplification stage
probably with the pre-amp if there is one.

Try connecting the 1uF capacitor further back on the right channe
circuitry and keep testing points further back until you cannot hea
any sound - there may be a capacitor which has gone open circuit o
possibly a dry joint
 
A

Arfa Daily

Jan 1, 1970
0
Dave said:
Awhile back I posted about an amp I am trying to repair. The amp is
composed of a +/- rectified and filtered power supply, an LM1036N
volume/tone/balance IC (two-channel), and a pair of LM1875T 20W TO-220
amps
which use bipolar power, no output filter caps. A bunch of various
resistors and caps and diodes. Very simple. I tested the resistors and
diodes in circuit, they look to be okay. I have no reason to suspect the
caps although I did replace one which appeared to be open circuit.

The symptoms: No sound on the right channel. When I turn the volume up
quickly it makes a "thump" on the bad channel. So. I started at the
outputs and worked backwards. First off, I bridged the inputs on the
LM1875's with a 1uF cap. Voila! Sound. So, the speaker works, the amp
works.

Another poster suggested that I use an analog meter to measure input and
output voltages of the LM1036N IC. So last night I soldered on some test
leads. I measured AC and DC voltages, and they varied a bit from channel
to
channel, maybe 5V on one side and 4.2 on the other, no catastrophic
failure
there. As an aside I kind of expected my analog multimeter to look like a
VU meter, moving up and down to the music, but didn't see that at all,
very
steady. Then I noticed something: when I measured the potential between
the bad channel's INPUT to the LM1036 and ground, I heard a scratchy
sound
come out of the bad speaker. Hmm. As the meter is basically a big
resistor
in parallel with the circuit, I figured I'd try a smaller resisiter. I
changed the scale of my meter from 30V to 10V. Now I get tinny sound.
Changed to 3V. Meter was off-scale but I now had pretty good sound.
Didn't
have the balls to try shorting the two, I have been burnt (actually my
components have been burnt) by trying dumb-ass things like that using the
axiom "if a little is good, a lot must be REAL good".

I have no formal training in electronics, but have done a lot of reading
on
the good ol' internet and am keen to learn more. I am completely
perplexed
as to what is happening. The only explanation I can conjure up (and sorry
if it sounds stupid but...) is maybe an excess of DC current on my signal
input which would screw up the bias of the transisters in the mixer IC.
There is a small electrolytic in series with the mixer output and the amp
input, plus a bunch of smaller ceramics from the signal trace to ground so
I
don't think DC can be getting to the amp itself... and if it were my
little
experiement with the bridging cap, above, wouldn't have worked so well.

I have another LM1036 mixer IC, but don't want to replace something that's
not broken.

Does anybody have any ideas? How can bridging my input signal to ground
with a resistor make sound come out? I've looked at this circuit long
enough that I can probably draw up a schematic if I can find a few hours.

THanks

Dave
My first checks would be on the decoupling caps connected to pins 3 (18), 5
(16), and 6 (15). It is quite likely that one of these is leaky or short
circuit ( figures in brackets are corresponding pin numbers for other
channel ). After this, check the caps in series with the input pins for
being leaky. Failing any of these being the problem, then looking at the
internals of the IC, and a typical application circuit, I can't see much
else it can be bar the IC itself

Arfa
 
A

Asimov

Jan 1, 1970
0
"Dave" bravely wrote to "All" (24 Feb 06 15:33:29)
--- on the heady topic of "amplifier troubleshooting"

Da> From: "Dave" <[email protected]>
Da> Xref: core-easynews sci.electronics.repair:359019

Da> Awhile back I posted about an amp I am trying to repair. The amp is
Da> composed of a +/- rectified and filtered power supply, an LM1036N
Da> volume/tone/balance IC (two-channel), and a pair of LM1875T 20W TO-220
Da> amps which use bipolar power, no output filter caps. A bunch of
Da> various resistors and caps and diodes. Very simple. I tested the
Da> resistors and diodes in circuit, they look to be okay. I have no
Da> reason to suspect the caps although I did replace one which appeared to
Da> be open circuit.
Da> The symptoms: No sound on the right channel. When I turn the volume
Da> up quickly it makes a "thump" on the bad channel. So. I started at
Da> the outputs and worked backwards. First off, I bridged the inputs on
Da> the LM1875's with a 1uF cap. Voila! Sound. So, the speaker works,
Da> the amp works.

Da> Another poster suggested that I use an analog meter to measure input
Da> and output voltages of the LM1036N IC. So last night I soldered on
Da> some test leads. I measured AC and DC voltages, and they varied a bit
Da> from channel to channel, maybe 5V on one side and 4.2 on the other, no
Da> catastrophic failure there. As an aside I kind of expected my analog
Da> multimeter to look like a VU meter, moving up and down to the music,
Da> but didn't see that at all, very steady. Then I noticed something:
Da> when I measured the potential between the bad channel's INPUT to the
Da> LM1036 and ground, I heard a scratchy sound come out of the bad
Da> speaker. Hmm. As the meter is basically a big resistor in parallel
Da> with the circuit, I figured I'd try a smaller resisiter. I changed the
Da> scale of my meter from 30V to 10V. Now I get tinny sound. Changed to
Da> 3V. Meter was off-scale but I now had pretty good sound. Didn't have
Da> the balls to try shorting the two, I have been burnt (actually my
Da> components have been burnt) by trying dumb-ass things like that using
Da> the axiom "if a little is good, a lot must be REAL good".

Da> I have no formal training in electronics, but have done a lot of
Da> reading on the good ol' internet and am keen to learn more. I am
Da> completely perplexed as to what is happening. The only explanation I
Da> can conjure up (and sorry if it sounds stupid but...) is maybe an
Da> excess of DC current on my signal input which would screw up the bias
Da> of the transisters in the mixer IC. There is a small electrolytic in
Da> series with the mixer output and the amp input, plus a bunch of smaller
Da> ceramics from the signal trace to ground so I don't think DC can be
Da> getting to the amp itself... and if it were my little experiement with
Da> the bridging cap, above, wouldn't have worked so well.
Da> I have another LM1036 mixer IC, but don't want to replace something
Da> that's not broken.

Da> Does anybody have any ideas? How can bridging my input signal to
Da> ground with a resistor make sound come out? I've looked at this
Da> circuit long enough that I can probably draw up a schematic if I can
Da> find a few hours.
Da> THanks

Da> Dave


If the cap between the preamp/mixer to the power amp is leaky then it
might be biasing the amp's output to one voltage supply rail or the
other. The smaller value resistor to ground simply allows the output
to become biased closer to normal and sound can get through. Now that
we know a little more a good guess is to change that coupling cap.
The input resistor itself may be bad but that is easy to test on
the ohm range.

Your other symptom about the thump when turning the volume pot quickly
may be pointing to a leaky cap to the wiper of the pot. Check that.

A*s*i*m*o*v

.... If there were no electricity, we'd all be ohmless.
 
D

Dave

Jan 1, 1970
0
Asimov said:
If the cap between the preamp/mixer to the power amp is leaky then it
might be biasing the amp's output to one voltage supply rail or the
other. The smaller value resistor to ground simply allows the output
to become biased closer to normal and sound can get through. Now that
we know a little more a good guess is to change that coupling cap.
The input resistor itself may be bad but that is easy to test on
the ohm range.
But if that was the case then bridging the amps' inputs together with a cap
wouldn't help, right? I mean the DC would still be getting to the bad
channel amp's input to throw off the bias...
 
A

Arfa Daily

Jan 1, 1970
0
Asimov said:
"Dave" bravely wrote to "All" (24 Feb 06 15:33:29)
--- on the heady topic of "amplifier troubleshooting"

Da> From: "Dave" <[email protected]>
Da> Xref: core-easynews sci.electronics.repair:359019

Da> Awhile back I posted about an amp I am trying to repair. The amp is
Da> composed of a +/- rectified and filtered power supply, an LM1036N
Da> volume/tone/balance IC (two-channel), and a pair of LM1875T 20W TO-220
Da> amps which use bipolar power, no output filter caps. A bunch of
Da> various resistors and caps and diodes. Very simple. I tested the
Da> resistors and diodes in circuit, they look to be okay. I have no
Da> reason to suspect the caps although I did replace one which appeared
to
Da> be open circuit.
Da> The symptoms: No sound on the right channel. When I turn the volume
Da> up quickly it makes a "thump" on the bad channel. So. I started at
Da> the outputs and worked backwards. First off, I bridged the inputs on
Da> the LM1875's with a 1uF cap. Voila! Sound. So, the speaker works,
Da> the amp works.

Da> Another poster suggested that I use an analog meter to measure input
Da> and output voltages of the LM1036N IC. So last night I soldered on
Da> some test leads. I measured AC and DC voltages, and they varied a bit
Da> from channel to channel, maybe 5V on one side and 4.2 on the other, no
Da> catastrophic failure there. As an aside I kind of expected my analog
Da> multimeter to look like a VU meter, moving up and down to the music,
Da> but didn't see that at all, very steady. Then I noticed something:
Da> when I measured the potential between the bad channel's INPUT to the
Da> LM1036 and ground, I heard a scratchy sound come out of the bad
Da> speaker. Hmm. As the meter is basically a big resistor in parallel
Da> with the circuit, I figured I'd try a smaller resisiter. I changed
the
Da> scale of my meter from 30V to 10V. Now I get tinny sound. Changed to
Da> 3V. Meter was off-scale but I now had pretty good sound. Didn't have
Da> the balls to try shorting the two, I have been burnt (actually my
Da> components have been burnt) by trying dumb-ass things like that using
Da> the axiom "if a little is good, a lot must be REAL good".

Da> I have no formal training in electronics, but have done a lot of
Da> reading on the good ol' internet and am keen to learn more. I am
Da> completely perplexed as to what is happening. The only explanation I
Da> can conjure up (and sorry if it sounds stupid but...) is maybe an
Da> excess of DC current on my signal input which would screw up the bias
Da> of the transisters in the mixer IC. There is a small electrolytic in
Da> series with the mixer output and the amp input, plus a bunch of
smaller
Da> ceramics from the signal trace to ground so I don't think DC can be
Da> getting to the amp itself... and if it were my little experiement with
Da> the bridging cap, above, wouldn't have worked so well.
Da> I have another LM1036 mixer IC, but don't want to replace something
Da> that's not broken.

Da> Does anybody have any ideas? How can bridging my input signal to
Da> ground with a resistor make sound come out? I've looked at this
Da> circuit long enough that I can probably draw up a schematic if I can
Da> find a few hours.
Da> THanks

Da> Dave


If the cap between the preamp/mixer to the power amp is leaky then it
might be biasing the amp's output to one voltage supply rail or the
other. The smaller value resistor to ground simply allows the output
to become biased closer to normal and sound can get through. Now that
we know a little more a good guess is to change that coupling cap.
The input resistor itself may be bad but that is easy to test on
the ohm range.

Your other symptom about the thump when turning the volume pot quickly
may be pointing to a leaky cap to the wiper of the pot. Check that.

A*s*i*m*o*v

... If there were no electricity, we'd all be ohmless.
Am I missing something here ? I thought the OP said that he was starting to
get some sound of sorts from the bad channel, when he put his meter on the
input of the vol / tone control IC. He has already proven that both channels
of the power amp work correctly, by bridging that IC's inputs. Thus, the
problem must be in or around the LM1036 - see my post above. The volume
control thump is nothing to do with DC or leaky caps at the control. This is
a DC controlled electronic volume control IC. There is supposed to be DC on
the wiper, as there will be on the tone and balance controls also, as they
will all be fed from the internal zener reference on pin 17 of the IC.

Arfa
 
D

Dave

Jan 1, 1970
0
Arfa Daily said:
Am I missing something here ? I thought the OP said that he was starting to
get some sound of sorts from the bad channel, when he put his meter on the
input of the vol / tone control IC. He has already proven that both channels
of the power amp work correctly, by bridging that IC's inputs. Thus, the
problem must be in or around the LM1036 - see my post above. The volume
control thump is nothing to do with DC or leaky caps at the control. This is
a DC controlled electronic volume control IC. There is supposed to be DC on
the wiper, as there will be on the tone and balance controls also, as they
will all be fed from the internal zener reference on pin 17 of the IC.

Arfa
Can I check the caps in-circuit or must I remove them? I've been checking
them with a DMM at the highest-resistance setting. Electrolytics start at
zero and climb to infinite resistance. The smaller the cap, the faster the
rise in resistance. With film caps (tan, thin, round), though, I don't seem
to go to infinite resistance... on EITHER channel, the good or the bad. So
I conclude that my test method is inappropriate. Anyway, all of the
resistors and caps between the mixer IC and the amps measure similarly from
one channel to the other IN CIRCUIT.
 
A

Arfa Daily

Jan 1, 1970
0
Dave said:
Can I check the caps in-circuit or must I remove them? I've been checking
them with a DMM at the highest-resistance setting. Electrolytics start at
zero and climb to infinite resistance. The smaller the cap, the faster
the
rise in resistance. With film caps (tan, thin, round), though, I don't
seem
to go to infinite resistance... on EITHER channel, the good or the bad.
So
I conclude that my test method is inappropriate. Anyway, all of the
resistors and caps between the mixer IC and the amps measure similarly
from
one channel to the other IN CIRCUIT.
Tan, thin and round sounds like typical disc ceramic caps. These are often
used for decoupling purposes, and it is quite common for them to go leaky.
You need to lift one end really, to measure them. They should read nominally
open circuit both ways, as their value is usually small. As far as the
electrolytics go, the sorts of readings that you are seeing, look ok.
Ideally, they should be removed from circuit to check them properly, or
tested with an ESR meter, but for the problem you have with the amp, it is
unlikely that you have an ESR issue, or even one that requires the caps to
be removed and tested more than you have already. If one of these caps was
going to be the problem, it would more than likely be that it was short
circuit, pulling the pin on the IC that it was connected to, to ground.
Clearly, none of them are short circuit, and as you seem to get comparable
readings on each one, to its corresponding one on the other channel, it's
unlikely that you have any gone leaky. I think now, that there is little
else that you can do, bar risk replacing the tone / balance / volume IC.

Arfa
 
D

Dave

Jan 1, 1970
0
Arfa Daily said:
Tan, thin and round sounds like typical disc ceramic caps. These are often
used for decoupling purposes, and it is quite common for them to go leaky.
You need to lift one end really, to measure them. They should read nominally
open circuit both ways, as their value is usually small. As far as the
electrolytics go, the sorts of readings that you are seeing, look ok.
Ideally, they should be removed from circuit to check them properly, or
tested with an ESR meter, but for the problem you have with the amp, it is
unlikely that you have an ESR issue, or even one that requires the caps to
be removed and tested more than you have already. If one of these caps was
going to be the problem, it would more than likely be that it was short
circuit, pulling the pin on the IC that it was connected to, to ground.
Clearly, none of them are short circuit, and as you seem to get comparable
readings on each one, to its corresponding one on the other channel, it's
unlikely that you have any gone leaky. I think now, that there is little
else that you can do, bar risk replacing the tone / balance / volume IC.

Arfa
Thanks, I was heading that way... I've got a new tone/balance/volume IC, it
was <$4 from Digi-Key. Will let you know if it works.

Dave
 
D

Dave

Jan 1, 1970
0
I think now, that there is little
else that you can do, bar risk replacing the tone / balance / volume IC.

Arfa
Replaced the IC last night, problem solved. Thanks for your help.

Dave
 
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