Fender Frontman 212R Volume 80% down problem

[Leonardo Capossio]

El miércoles, 25 de noviembre de 2015, 12:26:48 (UTC-3), mako...@yahoo.com escribió:

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

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

Don't have a scope, best I can do is a DMM.

Just to clariphy, the audio from the amp gets cut abruptly. Everything is working like a charm, the sound isn't fading away or anything, and the suddenly it stops outputting audio. Everything else seems to be working.

In this case, the only thermally dependent element I see is the PTC100. The circuit surrounding the PT100 seems to be compensating the bias current of the power amp, though I do not understand it fully, if anyone can point to a source that explains how this circuit work it would be great. To me it seems very strange that it abruptly cuts out the sound.

 

If there are ultrasonic frequencies being amplified, you wont hear them. But... they will cause the amp to work really hard and generate more heat than usual. This could cause the amp to shutdown due to the heat. Other than using a scope, there is no way to confirm this.

Dan

 

[Leonardo Capossio]

El miércoles, 25 de noviembre de 2015, 13:13:52 (UTC-3), Leonardo Capossio escribió:

El miércoles, 25 de noviembre de 2015, 12:26:48 (UTC-3), mako...@yahoo.com escribió:
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

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

Don't have a scope, best I can do is a DMM.

Just to clariphy, the audio from the amp gets cut abruptly. Everything is working like a charm, the sound isn't fading away or anything, and the suddenly it stops outputting audio. Everything else seems to be working.

In this case, the only thermally dependent element I see is the PTC100. The circuit surrounding the PT100 seems to be compensating the bias current of the power amp, though I do not understand it fully, if anyone can point to a source that explains how this circuit work it would be great. To me it seems very strange that it abruptly cuts out the sound.

To clarify, the heatsink is working correctly, but I cannot measure the temperature because I don't have anything reliable to do it with.

 

[Leonardo Capossio]

El miércoles, 25 de noviembre de 2015, 14:24:18 (UTC-3), dansabr...@yahoo..com escribió:

If there are ultrasonic frequencies being amplified, you wont hear them. But... they will cause the amp to work really hard and generate more heat than usual. This could cause the amp to shutdown due to the heat. Other than using a scope, there is no way to confirm this.

Dan

If there are ultrasonic frequencies (which I cannot measure), then what could cause this ? Where might they be generated ?

The amp is shutting down via the PT100 circuit, that should also do some temperature drift compensation. If I bypass this compensation, would the amp sound very bad ?

 

On Wednesday, October 14, 2015 at 10:18:15 AM UTC-5, Leonardo Capossio 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.

Did you check the 42v and -42v once the amp went into the low volume state?

 

[Leonardo Capossio]

El miércoles, 25 de noviembre de 2015, 18:18:24 (UTC-3), jerry...@gmail.com escribió:

On Wednesday, October 14, 2015 at 10:18:15 AM UTC-5, Leonardo Capossio 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.

Did you check the 42v and -42v once the amp went into the low volume state?

No, but I don't notice the LED brightness to be down not even a little bit, or the cooler.

 

[Phil Allison]

Leonardo Capossio wrote:

In this case, the only thermally dependent element I see is the PTC100.
The circuit surrounding the PT100 seems to be compensating the bias
current of the power amp, though I do not understand it fully, if
anyone can point to a source that explains how this circuit work it
would be great.

** PTC100 operates the "mute" function of the power amp by shutting off Q7 and so the base voltage to Q8 - which is a 2mA current source for the input pair Q9 & Q10. This results in no current in the driver stage - transistors Q11, Q12 & Q13 and so no amplification.



..... Phil

 

[Leonardo Capossio]

El jueves, 26 de noviembre de 2015, 0:23:16 (UTC-3), Phil Allison escribió:

Leonardo Capossio wrote:




In this case, the only thermally dependent element I see is the PTC100.
The circuit surrounding the PT100 seems to be compensating the bias
current of the power amp, though I do not understand it fully, if
anyone can point to a source that explains how this circuit work it
would be great.


** PTC100 operates the "mute" function of the power amp by shutting off Q7 and so the base voltage to Q8 - which is a 2mA current source for the input pair Q9 & Q10. This results in no current in the driver stage - transistors Q11, Q12 & Q13 and so no amplification.



.... Phil

That claryfies things a little bit.

The mute coming from the input jacks seems to pull the "mute" point to ground, hence setting the base voltage much lower than 10v+0.7v, and thus shutting down Q7.

Now I don't fully understand how the PTC100 shuts it down. At 25deg the PTC100 has 110ohm, according to some tables, and at 100deg it has a little less than 140ohms. I don't know how to calculate Q6's Q point (Vec or Ie), since Veb/PTC100 seems to control Ie directly (unless the only way out is using Shockley's equation). Also the shut down seems way to abrupt, like Q6 is open, and then suddenly it is a closed switch.

 

Is the heat sink hot when the unit shuts down?
Use your hand, if you can't keep your hand on the
Heat sink, its too hot.

 

[Phil Allison]

Leonardo Capossio wrote:

** PTC100 operates the "mute" function of the power amp by shutting off Q7 and so the base voltage to Q8 - which is a 2mA current source for the input pair Q9 & Q10. This results in no current in the driver stage - transistors Q11, Q12 & Q13 and so no amplification.



Now I don't fully understand how the PTC100 shuts it down. At 25deg the PTC100 has 110ohm, according to some tables, and at 100deg it has a little less than 140ohms.

** There is a device called PT100, a platinum resistance sensor that is quite linear and rises about 0.4% for each degree C. The thing on the schem labelled "PTC100" is not one of them.

It is a PTC "positive temp coefficient" thermistor (aka posistor) with a steep rise in resistance at 100C. The mute circuit in the Fender trips when the PTC device reaches 800 ohms and is rising steeply - so operates with a snap action.

FYI: "PTC100" is not a part number, but a code used on the schem referring to the parts list.


.... Phil

 

[Leonardo Capossio]

El viernes, 27 de noviembre de 2015, 1:50:10 (UTC-3), mako...@yahoo.com escribió:

Is the heat sink hot when the unit shuts down?
Use your hand, if you can't keep your hand on the
Heat sink, its too hot.

It is hot...which makes me wonder. I will make more checks when I get the chance.

 

[Leonardo Capossio]

El viernes, 27 de noviembre de 2015, 3:05:29 (UTC-3), Phil Allison escribió:

Leonardo Capossio wrote:



** PTC100 operates the "mute" function of the power amp by shutting off Q7 and so the base voltage to Q8 - which is a 2mA current source for the input pair Q9 & Q10. This results in no current in the driver stage - transistors Q11, Q12 & Q13 and so no amplification.



Now I don't fully understand how the PTC100 shuts it down. At 25deg the PTC100 has 110ohm, according to some tables, and at 100deg it has a little less than 140ohms.


** There is a device called PT100, a platinum resistance sensor that is quite linear and rises about 0.4% for each degree C. The thing on the schem labelled "PTC100" is not one of them.

It is a PTC "positive temp coefficient" thermistor (aka posistor) with a steep rise in resistance at 100C. The mute circuit in the Fender trips when the PTC device reaches 800 ohms and is rising steeply - so operates with a snap action.

FYI: "PTC100" is not a part number, but a code used on the schem referring to the parts list.


... Phil

That explains a LOT. Thanks.

 

[Phil Allison]

Leonardo Capossio wrote:

Now I don't fully understand how the PTC100 shuts it down.
At 25deg the PTC100 has 110ohm, according to some tables,
and at 100deg it has a little less than 140ohms.


** There is a device called PT100, a platinum resistance sensor
that is quite linear and rises about 0.4% for each degree C. The
thing on the schem labelled "PTC100" is not one of them.

It is a PTC "positive temp coefficient" thermistor (aka posistor)
with a steep rise in resistance at 100C. The mute circuit in the
Fender trips when the PTC device reaches 800 ohms and is rising
steeply - so operates with a snap action.

FYI: "PTC100" is not a part number, but a code used on the schem
referring to the parts list.




That explains a LOT. Thanks.

** BTW, the NTC10-15 shown in series with the AC power is used as an anti-surge device. It is mounted on the PCB, not inside the transformer.

At room temp it's a 10ohm resistor, falling to 0.2ohms at 5 amps current.

At switch-on it simply adds 10 ohms to the wiring on the primary side, limiting inrush surge currents so the fuse does not blow regularly.


..... Phil

 

[Leonardo Capossio]

El viernes, 16 de octubre de 2015, 1:54:16 (UTC-3), Phil Allison escribió:

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.

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


... Phil

Right.

I'm starting to think this is the problem. I checked the trasistors and they are fine, they are the same as in the schematic.

Y removed them from the heatsink, cleaned everything with alcohol (even tried not to touch it with my fingers) and put fresh thermal paste (with isolation) between the metal plate that holds the transitors, between the plate and the heatsink, and between the heatsink and the case, but still the metal plate is hot as hell (can't even touch it) and the heatsink is very hot, but not so much.

I wonder if there is a video on how to do it properly.

 

[Ralph Mowery]

"Leonardo Capossio" <capossio.leonardo@gmail.com> wrote in message
news:14fca79b-407a-43dc-93c7-99e24240bcdf@googlegroups.com...

Y removed them from the heatsink, cleaned everything with alcohol (even
tried not to touch it with my fingers) and put fresh thermal paste (with
isolation) between the metal plate that holds the transitors, between the
plate and the heatsink, and between the heatsink and the case, but still
the metal plate is hot ?>as hell (can't even touch it) and the heatsink is
very hot, but not so much.

When putting the paste on, did you use the thinist layer you could ? The
paste is not that conductive to heat, just beter than air. The ideal
ammount (which probably can not be done) would be to have the metel surfaces
touching each other on the high points and just the voids filled with the
paste.
Less is often beter than more.

 

[Dave Platt]

In article <14fca79b-407a-43dc-93c7-99e24240bcdf@googlegroups.com>,
Leonardo Capossio <capossio.leonardo@gmail.com> wrote:

Y removed them from the heatsink, cleaned everything with alcohol (even tried not to touch it with my
fingers) and put fresh thermal paste (with isolation) between the metal plate that holds the transitors,
between the plate and the heatsink, and between the heatsink and the case, but still the metal plate is hot
as hell (can't even touch it) and the heatsink is very hot, but not so much.

You might want to check the metal plate and heatsink for flatness.

If the plate was warped at all, it won't be making good contact with
the heatsink - there would be a few small points or lines of good
contact, and a significant air-gap everywhere else. Filling the gap
with thermal paste won't help very much. Thermal paste helps thermal
conductivity only when it's a very thin layer - just thick enough to
bridge the occasional thin gap between two surfaces which are
otherwise in good direct physical contact. A thick layer may be worse
than not having it at all.

CPU overclockers used to (maybe still) "lap" the top of the CPU and
the contact surface of the heatsink, in order to get the both as flat
as possible. When this is done, the layer of thermal paste between
the two is *very* thin... put a tiny dab on one surface, and then
spread it out thin-and-even with a single-edged razor blade.

Over-torquing the screws which hold down TO-220 or similar
plastic-package transistors to the heatsink can similarly cause
problems. Apply too much torque, make the transistor case and tab
flex a bit, and you end up pulling most of the case away from the
heatsink and thermal transfer gets worse. If your transistor tabs are
bent or warped, lapping them might help.

[Lapping of this type can be done with a sheet of wet-and-dry carbide
paper, mounted on a flat plate of some sort - a high-quality sheet of
plate glass will serve. The usual finishing technique of "start with
a coarser grit, then work your way up through the grits until you get
to 1000" would be appropriate.]

The fact that the heatsink itself is very hot does suggest that
something else may be going amiss... maybe the transistor quiescent
bias is too high, or maybe there's a parasitic oscillation taking
place. Using a scope (or an old-style grid-dip meter in absorbtion
mode) to look for RF where it doesn't belong might be a good idea.

 

[Phil Allison]

Leonardo Capossio wrote:

** 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

Right.

I'm starting to think this is the problem. I checked the trasistors and they are fine, they are the same as in the schematic.

Y removed them from the heatsink, cleaned everything with alcohol (even tried not to touch it with my fingers) and put fresh thermal paste (with isolation) between the metal plate that holds the transitors, between the plate and the heatsink, and between the heatsink and the case, but still the metal plate is hot as hell (can't even touch it) and the heatsink is very hot, but not so much.

I wonder if there is a video on how to do it properly.

** Two things are not clear:

1. Does the heatsink get very hot whenever the amp is on or do you have to PLAY something ?

2. Have you found and fixed any burs on the holes in the heatsink coupler and or the chassis ?

BTW:

That amp has an odd, non adjustable bias arrangement.

Transistors Q14 and Q 15 should be in thermal contact with the heatsink.


.... Phil

 

[M Philbrook]

In article <14fca79b-407a-43dc-93c7-99e24240bcdf@googlegroups.com>,
capossio.leonardo@gmail.com says...

El viernes, 16 de octubre de 2015, 1:54:16 (UTC-3), Phil Allison escribió:
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.

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


... Phil

Right.

I'm starting to think this is the problem. I checked the trasistors and they are fine, they are the same as in the schematic.

Y removed them from the heatsink, cleaned everything with alcohol (even tried not to touch it with my fingers) and put fresh thermal paste (with isolation) between the metal plate that holds the transitors, between the plate and the heatsink, and between the heatsink and the case, but still the metal plate is hot as hell (can't even touch it) and the heatsink is very hot, but not so much.

I wonder if there is a video on how to do it properly.

If it's getting hot like that with the amp on and not generating any
sound, you have other problems.

Put a volt meter on the output in DC mode first, see if you are getting
any DC above a few mVs

After that, put the meter in AC mode and see if you are getting AC
ripple of a significant level, which you should not be.

Do this in ideal mode, no sound.

Me thinks you have big ISSUES with biasing.


Jamie.

 

[Leonardo Capossio]

El domingo, 29 de noviembre de 2015, 16:15:36 (UTC-3), Ralph Mowery wrote:

When putting the paste on, did you use the thinist layer you could ? The
paste is not that conductive to heat, just beter than air. The ideal
ammount (which probably can not be done) would be to have the metel surfaces
touching each other on the high points and just the voids filled with the
paste.
Less is often beter than more.

Yes, I used a very thin layer.

 

[Leonardo Capossio]

On Monday, November 30, 2015 at 12:16:15 AM UTC-3, Phil Allison wrote:

Leonardo Capossio wrote:


** 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

Right.

I'm starting to think this is the problem. I checked the trasistors and they are fine, they are the same as in the schematic.

Y removed them from the heatsink, cleaned everything with alcohol (even tried not to touch it with my fingers) and put fresh thermal paste (with isolation) between the metal plate that holds the transitors, between the plate and the heatsink, and between the heatsink and the case, but still the metal plate is hot as hell (can't even touch it) and the heatsink is very hot, but not so much.

I wonder if there is a video on how to do it properly.



** Two things are not clear:

1. Does the heatsink get very hot whenever the amp is on or do you have to PLAY something ?

You have to play for a while at low volumes (10min tops), or play for a minute at the highest volume. In idle, with a plug connected (otherwise the amp mutes), it does not shut down.

2. Have you found and fixed any burs on the holes in the heatsink coupler and or the chassis ?

If by burr you mean what is described in this page: "http://www.nmri.go.jp/eng/khirata/metalwork/basic/bari/index_e.html", then I don't see any worrysome burr on the heatsink.

BTW:

That amp has an odd, non adjustable bias arrangement.

Transistors Q14 and Q 15 should be in thermal contact with the heatsink.

They are on the heatsink, but their tab is completely plastic.