Fender Frontman 212R Volume 80% down problem

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

Dan

Reply to
dansabrservices
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. Perhaps the system is working too hard on things you can not hear.

bad speaker?

working like a charm, the sound isn't fading away or anything, and the sud denly it stops outputting audio. Everything else seems to be working.

he 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 tem perature because I don't have anything reliable to do it with.

Reply to
Leonardo Capossio

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.

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

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

Reply to
Leonardo Capossio

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

Reply to
jerryrego03

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

Reply to
Leonardo Capossio

** 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 - transisto rs Q11, Q12 & Q13 and so no amplification.

.... Phil

Reply to
Phil Allison

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

7 and so the base voltage to Q8 - which is a 2mA current source for the inp ut pair Q9 & Q10. This results in no current in the driver stage - transis tors Q11, Q12 & Q13 and so no amplification.

That claryfies things a little bit.

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

Now I don't fully understand how the PTC100 shuts it down. At 25deg the PTC

100 has 110ohm, according to some tables, and at 100deg it has a little les s than 140ohms. I don't know how to calculate Q6's Q point (Vec or Ie), sin ce Veb/PTC100 seems to control Ie directly (unless the only way out is usin g Shockley's equation). Also the shut down seems way to abrupt, like Q6 is open, and then suddenly it is a closed switch.
Reply to
Leonardo Capossio

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.

Reply to
makolber

Q7 and so the base voltage to Q8 - which is a 2mA current source for the i nput pair Q9 & Q10. This results in no current in the driver stage - trans istors Q11, Q12 & Q13 and so no amplification.

TC100 has 110ohm, according to some tables, and at 100deg it has a little l ess than 140ohms.

** There is a device called PT100, a platinum resistance sensor that is qui te linear and rises about 0.4% for each degree C. The thing on the schem la belled "PTC100" is not one of them. It is a PTC "positive temp coefficient" thermistor (aka posistor) with a st eep rise in resistance at 100C. The mute circuit in the Fender trips when t he PTC device reaches 800 ohms and is rising steeply - so operates with a s nap action.

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

... Phil

Reply to
Phil Allison

El viernes, 27 de noviembre de 2015, 1:50:10 (UTC-3), snipped-for-privacy@yahoo.com esc

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

Reply to
Leonardo Capossio

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

ff 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 - tra nsistors Q11, Q12 & Q13 and so no amplification.

PTC100 has 110ohm, according to some tables, and at 100deg it has a little less than 140ohms.

uite linear and rises about 0.4% for each degree C. The thing on the schem labelled "PTC100" is not one of them.

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.

g to the parts list.

That explains a LOT. Thanks.

Reply to
Leonardo Capossio

** 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

Reply to
Phil Allison

eat from the output devices to the bottom of the case - right ?

the metal surfaces mating properly.

shut downs.

Right.

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

Y removed them from the heatsink, cleaned everything with alcohol (even tri ed not to touch it with my fingers) and put fresh thermal paste (with isola tion) between the metal plate that holds the transitors, between the plate and the heatsink, and between the heatsink and the case, but still the meta l 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.

Reply to
Leonardo Capossio

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.

Reply to
Ralph Mowery

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.

Reply to
Dave Platt

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.

Reply to
M Philbrook

heat from the output devices to the bottom of the case - right ?

t the metal surfaces mating properly.

e shut downs.

hey are fine, they are the same as in the schematic.

ried not to touch it with my fingers) and put fresh thermal paste (with iso lation) between the metal plate that holds the transitors, between the plat e and the heatsink, and between the heatsink and the case, but still the me tal plate is hot as hell (can't even touch it) and the heatsink is very hot , but not so much.

** 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 a nd 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

Reply to
Phil Allison

Yes, I used a very thin layer.

Reply to
Leonardo Capossio

le heat from the output devices to the bottom of the case - right ?

ent the metal surfaces mating properly.

ore shut downs.

they are fine, they are the same as in the schematic.

tried not to touch it with my fingers) and put fresh thermal paste (with i solation) between the metal plate that holds the transitors, between the pl ate 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 h ot, but not so much.

o PLAY something ?

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

and or the chassis ?

If by burr you mean what is described in this page: "

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", then I don't see any worrys ome burr on the heatsink.

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

Reply to
Leonardo Capossio

** When in "mute" mode, the power stage is completely shut down and cannot get hot. But does the heatsink get hot when active but not played, even if this take an hour ?
** Figure 7 is the appropriate one.

The best test is with a finger tip, the hole should not feel rough when you rub your finger over it.

** That does not matter, but they do need to be held down and have a smear of thermal grease under them.

What you are describing is called " thermal runaway " and I have seen it with other Fender models that use the same heatsink arrangement.

In those examples, careful de-burring of ALL the holes, re-greasing and making sure bias tracking devices were thermally coupled to the heatsink fixed the problem completely.

BTW:

You are lucky the Frontman 212 merely shuts down, the ones I worked on all blew up as there was no over temp sensing device fitted.

.... Phil

Reply to
Phil Allison

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