Fender Frontman 212R Volume 80% down problem

Guest
Hello,

I was wondering if you could help repair a guitar amplifier, it is a
Fender Frontman 212R, schematics are here:
http://support.fender.com/schematics/guitar_amplifiers/FM212R_schematic.pdf

The problems are:
-Low volume. It is a 100W amplifier and at full power is delivering
something like 20W tops at max volume.
-Paper crackling sound when guitar is played very hard. It is very
subtle, but is annoying.

I know that:
-Power supply voltages are ok (including the power amp stage)
-Speakers are ok (it has two speakers of 8 ohms in parallel as load)
-Nothing seems to be burnt on the PCB, or any cap seems bloated or something

It is unknown to me what the PTC (RT2) is doing, I know the NTC (RT1)
is for mains thermal shutdown, but the PTC seems to be for thermal
shutdown of the power stage only (because it is thermally coupled to
the power transistors power dissipator with some paste).

The next time I see it I will try to see if it is a pre-amp problem or
a power amp problem (because fortunately it has a pre-amp out and pwr
amp in)

Thanks.
 
El miércoles, 14 de octubre de 2015, 12:28:01 (UTC-3), N_Cook escribió:
On 14/10/2015 16:18, capossio.leonardo@gmail.com wrote:
Hello,

I was wondering if you could help repair a guitar amplifier, it is a
Fender Frontman 212R, schematics are here:
http://support.fender.com/schematics/guitar_amplifiers/FM212R_schematic..pdf

The problems are:
-Low volume. It is a 100W amplifier and at full power is delivering
something like 20W tops at max volume.
-Paper crackling sound when guitar is played very hard. It is very
subtle, but is annoying.

I know that:
-Power supply voltages are ok (including the power amp stage)
-Speakers are ok (it has two speakers of 8 ohms in parallel as load)
-Nothing seems to be burnt on the PCB, or any cap seems bloated or something

It is unknown to me what the PTC (RT2) is doing, I know the NTC (RT1)
is for mains thermal shutdown, but the PTC seems to be for thermal
shutdown of the power stage only (because it is thermally coupled to
the power transistors power dissipator with some paste).

The next time I see it I will try to see if it is a pre-amp problem or
a power amp problem (because fortunately it has a pre-amp out and pwr
amp in)

Thanks.


Is there a send/return pair, that you can put a test signal in the RET
to narrow to PA or PREA.
Suddenly happen or previous intermittant level change?
Click to expand...

I don't have a signal generator, at least not now. I will put the pre amp out to another guitar amplifier that is working 100% and see what happens. If this works with a good sound I think it safe to say that the problem is not in the pre-amp.

The crackling sound seems to appear when strongly strumming the guitar. The volume does not go up or down, it just never reaches a level above lets say 20W.
 
On 14/10/2015 16:18, capossio.leonardo@gmail.com wrote:
Hello,

I was wondering if you could help repair a guitar amplifier, it is a
Fender Frontman 212R, schematics are here:
http://support.fender.com/schematics/guitar_amplifiers/FM212R_schematic.pdf

The problems are:
-Low volume. It is a 100W amplifier and at full power is delivering
something like 20W tops at max volume.
-Paper crackling sound when guitar is played very hard. It is very
subtle, but is annoying.

I know that:
-Power supply voltages are ok (including the power amp stage)
-Speakers are ok (it has two speakers of 8 ohms in parallel as load)
-Nothing seems to be burnt on the PCB, or any cap seems bloated or something

It is unknown to me what the PTC (RT2) is doing, I know the NTC (RT1)
is for mains thermal shutdown, but the PTC seems to be for thermal
shutdown of the power stage only (because it is thermally coupled to
the power transistors power dissipator with some paste).

The next time I see it I will try to see if it is a pre-amp problem or
a power amp problem (because fortunately it has a pre-amp out and pwr
amp in)

Thanks.
Click to expand...

Is there a send/return pair, that you can put a test signal in the RET
to narrow to PA or PREA.
Suddenly happen or previous intermittant level change?
 
On Wednesday, October 14, 2015 at 2:09:49 PM UTC-3, Ian Field wrote:
"Leonardo Capossio" <capossio.leonardo@gmail.com> wrote in message
news:7cdfcf24-c1a0-4479-8006-bea580bc263d@googlegroups.com...
El miércoles, 14 de octubre de 2015, 12:28:01 (UTC-3), N_Cook escribió:
On 14/10/2015 16:18, capossio.leonardo@gmail.com wrote:
Hello,

I was wondering if you could help repair a guitar amplifier, it is a
Fender Frontman 212R, schematics are here:
http://support.fender.com/schematics/guitar_amplifiers/FM212R_schematic.pdf

The problems are:
-Low volume. It is a 100W amplifier and at full power is delivering
something like 20W tops at max volume.
-Paper crackling sound when guitar is played very hard. It is very
subtle, but is annoying.

I know that:
-Power supply voltages are ok (including the power amp stage)
-Speakers are ok (it has two speakers of 8 ohms in parallel as load)
-Nothing seems to be burnt on the PCB, or any cap seems bloated or
something

It is unknown to me what the PTC (RT2) is doing, I know the NTC (RT1)
is for mains thermal shutdown, but the PTC seems to be for thermal
shutdown of the power stage only (because it is thermally coupled to
the power transistors power dissipator with some paste).

The next time I see it I will try to see if it is a pre-amp problem or
a power amp problem (because fortunately it has a pre-amp out and pwr
amp in)

Thanks.


Is there a send/return pair, that you can put a test signal in the RET
to narrow to PA or PREA.
Suddenly happen or previous intermittant level change?

I don't have a signal generator, at least not now. I will put the pre amp
out to another guitar amplifier that is working 100% and see what happens.
If this works with a good sound I think it safe to say that the problem is
not in the pre-amp.

The crackling sound seems to appear when strongly strumming the guitar.
The volume does not go up or down, it just never reaches a level above
lets say 20W.


For that big a drop, I'd think maybe half the push-pull is cut off and only
making it intermittently on big peaks. RoHS solder can cause things like
that.

It it turns out to be the low level stages, look for any discrete transistor
(or JFET) stages, a high ESR emitter bypass cap can allow a lot of
unintended nfb in that stage, so its gain will be a lot less than designed
for.

Any electrolytics coupling one stage to the next are also a possibility - if
you find any tantalum beads, study the circuit and satisfy yourself that
they can't under any circumstances be subjected to reverse voltage, they can
go leaky at the drop of a hat! If you find any tantalum caps in signal
paths, its not a bad idea to replace them with multilayer ceramic chip
types - some manufacturers offer them up to 180uF, and a few can supply
resin dipped leaded types.

Generally speaking; start with the soldering, then check electrolytics
(especially small ones). Resistors can go high in value or open circuit -
high resistance parts are particularly prone to increasing, the lower values
(especially wire wound) tend to fail open circuit, these are also slightly
more likely to show a burn mark.

If you find any scorched or burned open resistors, there's a good case to
start checking for leaky or shorted semiconductors.
Click to expand...

Hi all, thanks for all the replies. I have tested the pre-amp with another PA, and it seems to work correctly, even for the distorsion channels.

Taking out the PCB revealed that three 5W 0.22 ohm resistors were not original, but had been changed by someone (whom the owner cannot pinpoint), and this someone changed them for 22ohm resistors (one .22 ohm from the original remained, and this is probably causing the imbalance). I will replace this resistors with correct ones, and see if this fixes all the problems.
 
"Leonardo Capossio" <capossio.leonardo@gmail.com> wrote in message
news:7cdfcf24-c1a0-4479-8006-bea580bc263d@googlegroups.com...
El miércoles, 14 de octubre de 2015, 12:28:01 (UTC-3), N_Cook escribió:
On 14/10/2015 16:18, capossio.leonardo@gmail.com wrote:
Hello,

I was wondering if you could help repair a guitar amplifier, it is a
Fender Frontman 212R, schematics are here:
http://support.fender.com/schematics/guitar_amplifiers/FM212R_schematic.pdf

The problems are:
-Low volume. It is a 100W amplifier and at full power is delivering
something like 20W tops at max volume.
-Paper crackling sound when guitar is played very hard. It is very
subtle, but is annoying.

I know that:
-Power supply voltages are ok (including the power amp stage)
-Speakers are ok (it has two speakers of 8 ohms in parallel as load)
-Nothing seems to be burnt on the PCB, or any cap seems bloated or
something

It is unknown to me what the PTC (RT2) is doing, I know the NTC (RT1)
is for mains thermal shutdown, but the PTC seems to be for thermal
shutdown of the power stage only (because it is thermally coupled to
the power transistors power dissipator with some paste).

The next time I see it I will try to see if it is a pre-amp problem or
a power amp problem (because fortunately it has a pre-amp out and pwr
amp in)

Thanks.


Is there a send/return pair, that you can put a test signal in the RET
to narrow to PA or PREA.
Suddenly happen or previous intermittant level change?

I don't have a signal generator, at least not now. I will put the pre amp
out to another guitar amplifier that is working 100% and see what happens.
If this works with a good sound I think it safe to say that the problem is
not in the pre-amp.

The crackling sound seems to appear when strongly strumming the guitar.
The volume does not go up or down, it just never reaches a level above
lets say 20W.
Click to expand...

For that big a drop, I'd think maybe half the push-pull is cut off and only
making it intermittently on big peaks. RoHS solder can cause things like
that.

It it turns out to be the low level stages, look for any discrete transistor
(or JFET) stages, a high ESR emitter bypass cap can allow a lot of
unintended nfb in that stage, so its gain will be a lot less than designed
for.

Any electrolytics coupling one stage to the next are also a possibility - if
you find any tantalum beads, study the circuit and satisfy yourself that
they can't under any circumstances be subjected to reverse voltage, they can
go leaky at the drop of a hat! If you find any tantalum caps in signal
paths, its not a bad idea to replace them with multilayer ceramic chip
types - some manufacturers offer them up to 180uF, and a few can supply
resin dipped leaded types.

Generally speaking; start with the soldering, then check electrolytics
(especially small ones). Resistors can go high in value or open circuit -
high resistance parts are particularly prone to increasing, the lower values
(especially wire wound) tend to fail open circuit, these are also slightly
more likely to show a burn mark.

If you find any scorched or burned open resistors, there's a good case to
start checking for leaky or shorted semiconductors.
 
wrote in message
news:b8a6f8b3-174b-47a6-add9-8d7e05c53f17@googlegroups.com...

Hello,

I was wondering if you could help repair a guitar amplifier, it is a
Fender Frontman 212R, schematics are here:
http://support.fender.com/schematics/guitar_amplifiers/FM212R_schematic.pdf

The problems are:
-Low volume. It is a 100W amplifier and at full power is delivering
something like 20W tops at max volume.
-Paper crackling sound when guitar is played very hard. It is very
subtle, but is annoying.

I know that:
-Power supply voltages are ok (including the power amp stage)
-Speakers are ok (it has two speakers of 8 ohms in parallel as load)
-Nothing seems to be burnt on the PCB, or any cap seems bloated or something

It is unknown to me what the PTC (RT2) is doing, I know the NTC (RT1)
is for mains thermal shutdown, but the PTC seems to be for thermal
shutdown of the power stage only (because it is thermally coupled to
the power transistors power dissipator with some paste).

The next time I see it I will try to see if it is a pre-amp problem or
a power amp problem (because fortunately it has a pre-amp out and pwr
amp in)

Thanks.




Plug a signal into the Power Amp In socket - this is switched and if the
switch gets dirty it can cause your symptoms.

If you take the main board out you will find a couple of discoloured hot
spots, particularly around the zener diodes providing the +- 15v for the
pre-amp.
These need sorting out before they become dry joints, if they haven't
already.

I had one of these not so long ago that had dry joints causing one half of
the push pull circuit not working, as Ian suggests.


The best solution here is to resolder EVERY joint on the mainboard. That
might sound like overkill, but I bet it would take you less than 10
minutes - probably less time than in would take to track down the single (at
present) dry joint.


Gareth.
 
"Leonardo Capossio" <capossio.leonardo@gmail.com> wrote in message
news:c922cd09-fa98-4ccd-adf9-ea70dedd9d5f@googlegroups.com...
On Wednesday, October 14, 2015 at 2:09:49 PM UTC-3, Ian Field wrote:
"Leonardo Capossio" <capossio.leonardo@gmail.com> wrote in message
news:7cdfcf24-c1a0-4479-8006-bea580bc263d@googlegroups.com...
El miércoles, 14 de octubre de 2015, 12:28:01 (UTC-3), N_Cook escribió:
On 14/10/2015 16:18, capossio.leonardo@gmail.com wrote:
Hello,

I was wondering if you could help repair a guitar amplifier, it is a
Fender Frontman 212R, schematics are here:
http://support.fender.com/schematics/guitar_amplifiers/FM212R_schematic.pdf

The problems are:
-Low volume. It is a 100W amplifier and at full power is delivering
something like 20W tops at max volume.
-Paper crackling sound when guitar is played very hard. It is very
subtle, but is annoying.

I know that:
-Power supply voltages are ok (including the power amp stage)
-Speakers are ok (it has two speakers of 8 ohms in parallel as load)
-Nothing seems to be burnt on the PCB, or any cap seems bloated or
something

It is unknown to me what the PTC (RT2) is doing, I know the NTC
(RT1)
is for mains thermal shutdown, but the PTC seems to be for thermal
shutdown of the power stage only (because it is thermally coupled to
the power transistors power dissipator with some paste).

The next time I see it I will try to see if it is a pre-amp problem
or
a power amp problem (because fortunately it has a pre-amp out and
pwr
amp in)

Thanks.


Is there a send/return pair, that you can put a test signal in the RET
to narrow to PA or PREA.
Suddenly happen or previous intermittant level change?

I don't have a signal generator, at least not now. I will put the pre
amp
out to another guitar amplifier that is working 100% and see what
happens.
If this works with a good sound I think it safe to say that the problem
is
not in the pre-amp.

The crackling sound seems to appear when strongly strumming the guitar.
The volume does not go up or down, it just never reaches a level above
lets say 20W.


For that big a drop, I'd think maybe half the push-pull is cut off and
only
making it intermittently on big peaks. RoHS solder can cause things like
that.

It it turns out to be the low level stages, look for any discrete
transistor
(or JFET) stages, a high ESR emitter bypass cap can allow a lot of
unintended nfb in that stage, so its gain will be a lot less than
designed
for.

Any electrolytics coupling one stage to the next are also a possibility -
if
you find any tantalum beads, study the circuit and satisfy yourself that
they can't under any circumstances be subjected to reverse voltage, they
can
go leaky at the drop of a hat! If you find any tantalum caps in signal
paths, its not a bad idea to replace them with multilayer ceramic chip
types - some manufacturers offer them up to 180uF, and a few can supply
resin dipped leaded types.

Generally speaking; start with the soldering, then check electrolytics
(especially small ones). Resistors can go high in value or open circuit -
high resistance parts are particularly prone to increasing, the lower
values
(especially wire wound) tend to fail open circuit, these are also
slightly
more likely to show a burn mark.

If you find any scorched or burned open resistors, there's a good case to
start checking for leaky or shorted semiconductors.

Hi all, thanks for all the replies. I have tested the pre-amp with another
PA, and it seems to work correctly, even for the distorsion channels.

Taking out the PCB revealed that three 5W 0.22 ohm resistors were not
original, but had been changed by someone (whom the owner cannot
pinpoint), and this someone changed them for 22ohm resistors (one .22 ohm
from the original remained, and this is probably causing the imbalance). I
will replace this resistors with correct ones, and see if this fixes all
the problems.
Click to expand...

Those parts had probably been replaced because the originals blew. Check
whether any associated semiconductors show any sign of having been replaced
as well - it wouldn't hurt to test them anyway.
 
How are you measuring this power output to be 20 watts ?

And as for what is happening here, it is EXTREMELY hard to describe sounds with words, like second harmonic distortion n shit and how it differs from third harmonic.

Know what ? I got an idea and I can do it. Put up a website with different sounds describing what they are. People do not even know the difference between a 60 cycle hum and a 120 cycle hum. I can hear the effect of an open polarity in an amp hands down and go immediately to the problem which usually was a driver (not output) transistor with an open B-E or B-C junction. I could ID the problem by the sound.

Maybe what I should do is make these sounds and put them up on the net. I know how to disconnect this or that and make it sound like that exact fault.

^Yeah right. Bob Carver got his claim to fame mimicking the sound of super audiophile amps, well I am Cletus Carver and I am gong to have my claim to fame mimicking fucked up amps...

LOL LOL
 
Leonardo Capossio wrote:

Taking out the PCB revealed that three 5W 0.22 ohm resistors were not
original, but had been changed by someone (whom the owner cannot pinpoint)
and this someone changed them for 22ohm resistors (one .22 ohm from the
original remained, and this is probably causing the imbalance). I will
replace this resistors with correct ones, and see if this fixes all
the problems.
Click to expand...

** That is bad error to make, the repairer must have been clueless or blind.

I see four 0.22ohm, 5W resistors on the schem - two in the power amp and two ( wired in parallel) in series with the ground link for the speaker.

The only time you need to change such resistors is when output transistors have failed and caused them to burn out - so make sure they are the correct type numbers and mounted properly to the heatsink.


.... Phil
 
Well, in the end the problem were those resistors, now it works like it should. The only problem is that at high volumes the amp turns the sound off after a while, I guess because of thermal shutdown of the PT100 on the power dissipator. But this can be solved by cleaning and re-applying thermal paste, and in extreme situation adding a cooler.
 
Leonardo Capossio wrote:
Well, in the end the problem were those resistors,
now it works like it should. The only problem is that
at high volumes the amp turns the sound off after a while,
I guess because of thermal shutdown of the PT100 on the
power dissipator. But this can be solved by cleaning and
re-applying thermal paste, and in extreme situation
adding a cooler.
Click to expand...

** That Fender model and similar ones uses a thick, alloy bar to couple heat from the output devices to the bottom of the case - right ?

Often there are burrs left around bolt holes in the coupler that prevent the metal surfaces mating properly.

Those holes need to be de-burred in a drill press.

Once done and regreased, output devices run cooler and there are no more shut downs.


.... Phil
 
I recall a similar case where I found a big heavy 25W resistor fitted in the line level audio path of a JAMMA adapter. When I found the schematic I understood, it.called for a .25W resistor but the decimal point was barely visible.
 
"Phil Allison" <pallison49@gmail.com> wrote in message
news:657c1a61-421d-4e07-8279-c120bdc170a5@googlegroups.com...
Leonardo Capossio wrote:


Well, in the end the problem were those resistors,
now it works like it should. The only problem is that
at high volumes the amp turns the sound off after a while,
I guess because of thermal shutdown of the PT100 on the
power dissipator. But this can be solved by cleaning and
re-applying thermal paste, and in extreme situation
adding a cooler.

** That Fender model and similar ones uses a thick, alloy bar to couple
heat from the output devices to the bottom of the case - right ?

Often there are burrs left around bolt holes in the coupler that prevent
the metal surfaces mating properly.

Those holes need to be de-burred in a drill press.
Click to expand...

If the parts are malleable metal, a drill press can be difficult to control
how much metal is removed.

Usually a 3/8" (10mm) drill bit held in the hand is quite sufficient.
 
In article <NW9Ux.42147$rO5.4079@fx39.am4>,
gangprobing.alien@ntlworld.com says...
"Phil Allison" <pallison49@gmail.com> wrote in message
news:657c1a61-421d-4e07-8279-c120bdc170a5@googlegroups.com...
Leonardo Capossio wrote:


Well, in the end the problem were those resistors,
now it works like it should. The only problem is that
at high volumes the amp turns the sound off after a while,
I guess because of thermal shutdown of the PT100 on the
power dissipator. But this can be solved by cleaning and
re-applying thermal paste, and in extreme situation
adding a cooler.

** That Fender model and similar ones uses a thick, alloy bar to couple
heat from the output devices to the bottom of the case - right ?

Often there are burrs left around bolt holes in the coupler that prevent
the metal surfaces mating properly.

Those holes need to be de-burred in a drill press.

If the parts are malleable metal, a drill press can be difficult to control
how much metal is removed.

Usually a 3/8" (10mm) drill bit held in the hand is quite sufficient.
Click to expand...

A center drill,tapered counterbore/sink or chamfer cutters are great for
task like that.

Make sure its metal grade..

Jamie
 
Ian Field wrote:
"Phil Allison"


** That Fender model and similar ones uses a thick, alloy bar to couple
heat from the output devices to the bottom of the case - right ?

Often there are burrs left around bolt holes in the coupler that prevent
the metal surfaces mating properly.

Those holes need to be de-burred in a drill press.


If the parts are malleable metal, a drill press can be difficult to control
how much metal is removed.
Click to expand...

** Done about a dozen of them that way, it was quick & easy with a light touch on the feed handle. There are 5 or 6 holes that need doing at both ends.

The cast alloy was a bit too hard for a drill bit held in the fingers.



.... Phil
 
"Phil Allison" <pallison49@gmail.com> wrote in message
news:0b2eab1a-cf45-4175-81cb-ad390b3fad95@googlegroups.com...
Ian Field wrote:

"Phil Allison"


** That Fender model and similar ones uses a thick, alloy bar to couple
heat from the output devices to the bottom of the case - right ?

Often there are burrs left around bolt holes in the coupler that
prevent
the metal surfaces mating properly.

Those holes need to be de-burred in a drill press.


If the parts are malleable metal, a drill press can be difficult to
control
how much metal is removed.


** Done about a dozen of them that way, it was quick & easy with a light
touch on the feed handle. There are 5 or 6 holes that need doing at both
ends.

The cast alloy was a bit too hard for a drill bit held in the fingers.
Click to expand...

Usually I find cast alloy heatsinks in automotive assemblies and power
tools, they seem to be the exception in most other things.

The common rolled sheet aluminium is very easy to cut too deep, I think
extruded is a little harder, but it still needs a light touch if you do it
in a drill press.

Anything other than a block can also snag on the drill bit if you
accidentally go right through.
 
El jueves, 15 de octubre de 2015, 18:11:29 (UTC-3), Leonardo Capossio escribió:
> Well, in the end the problem were those resistors, now it works like it should. The only problem is that at high volumes the amp turns the sound off after a while, I guess because of thermal shutdown of the PT100 on the power dissipator. But this can be solved by cleaning and re-applying thermal paste, and in extreme situation adding a cooler.

Unfortunately after re-applying thermal paste on the heatsink and the power transistors, and adding a cooler on the far side (not exactly by the heatsink) to blow hot air out of the amp head, the amp still behaves the same. It turns off the audio (LED and cooler are still working) after a couple of minutes of playing.

Anyone can point out to what might be the problem ?
 
Put a scope on the outputs and look for very high frequency harmonics. Perhaps the system is working too hard on things you can not hear.

Dan
 
In article <b9e7a5a8-3dd2-4875-9b1e-ddf77935405e@googlegroups.com>,
capossio.leonardo@gmail.com says...
El jueves, 15 de octubre de 2015, 18:11:29 (UTC-3), Leonardo Capossio escribió:
Well, in the end the problem were those resistors, now it works like it should. The only problem is that at high volumes the amp turns the sound off after a while, I guess because of thermal shutdown of the PT100 on the power dissipator. But this can be solved by cleaning and re-applying thermal paste, and in extreme situation adding a cooler.

Unfortunately after re-applying thermal paste on the heatsink and the power transistors, and adding a cooler on the far side (not exactly by the heatsink) to blow hot air out of the amp head, the amp still behaves the same. It turns off the audio (LED and cooler are still working) after a couple of minutes of playing.

Anyone can point out to what might be the problem ?
Click to expand...

Does the outputs on the sink actually feel like they are getting that
hot?

DO you have a IR tool to meaure it? Most likely not..

If you feel the unit isn't that hot and should be able to maintain
output at that temp? then maybe the PT resistor has degraded. They do
drift a little over time.

You could try checking all of the R's in that ciccuit to make sure they
are on track.

Looking at the schematic it looks like you have at least 2 R's that
could be suspect. That would be in the lower left about 1/2 the way
over.

If you have some kind of component cooler you can spray the PT while
its operating and see if it extends the on time. Or blow on it using a
straw.

Jamie
 
On Tuesday, November 24, 2015 at 6:10:30 PM UTC-5, dansabr...@yahoo.com wrote:
Put a scope on the outputs and look for very high frequency harmonics. Perhaps the system is working too hard on things you can not hear.

Dan
Click to expand...

good point


logical seteps
is it actually getting hot?
if not, the thermal shutdown system could be faulty

if it is getting hot you need to figure out why....
there could be an ultrasonic oscilation in the amp
the quiescent bias current could be too high in the amp
there could be some kind of short or overload at the output of the amp, bad speaker?

have fun , fixing stuff is a great way to learn

Mark
 
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