Fender Frontman 212R Volume 80% down problem

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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.

Re: Fender Frontman 212R Volume 80% down problem
On 14/10/2015 16:18, snipped-for-privacy@gmail.com wrote:
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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?

Re: Fender Frontman 212R Volume 80% down problem

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.pdf
thing

I don't have a signal generator, at least not now. I will put the pre amp o
ut to another guitar amplifier that is working 100% and see what happens. I
f this works with a good sound I think it safe to say that the problem is n
ot 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 sa
y 20W.

Re: Fender Frontman 212R Volume 80% down problem



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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.  


Re: Fender Frontman 212R Volume 80% down problem
On Wednesday, October 14, 2015 at 2:09:49 PM UTC-3, Ian Field wrote:


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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 origi
nal, but had been changed by someone (whom the owner cannot pinpoint), and  
this someone changed them for 22ohm resistors (one .22 ohm from the origina
l remained, and this is probably causing the imbalance). I will replace thi
s resistors with correct ones, and see if this fixes all the problems.

Re: Fender Frontman 212R Volume 80% down problem


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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.  


Re: Fender Frontman 212R Volume 80% down problem
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 bet
ween a 60 cycle hum and a 120 cycle hum. I can hear the effect of an open p
olarity in an amp hands down and go immediately to the problem which usuall
y 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 k
now 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

Re: Fender Frontman 212R Volume 80% down problem
Leonardo Capossio wrote:

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** 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  

Re: Fender Frontman 212R Volume 80% down problem
Well, in the end the problem were those resistors, now it works like it sho
uld. The only problem is that at high volumes the amp turns the sound off a
fter 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 pas
te, and in extreme situation adding a cooler.

Re: Fender Frontman 212R Volume 80% down problem
Leonardo Capossio wrote:
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** 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  

Re: Fender Frontman 212R Volume 80% down problem


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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.  


Re: Fender Frontman 212R Volume 80% down problem
snipped-for-privacy@ntlworld.com says...
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A center drill,tapered counterbore/sink or chamfer cutters are great for  
task like that.

 Make sure its metal grade..

Jamie


Re: Fender Frontman 212R Volume 80% down problem
Ian Field wrote:
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 ** 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  

Re: Fender Frontman 212R Volume 80% down problem


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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.  


Re: Fender Frontman 212R Volume 80% down problem

:
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eat from the output devices to the bottom of the case - right ?  
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the metal surfaces mating properly.  
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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.

Re: Fender Frontman 212R Volume 80% down problem

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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.





Re: Fender Frontman 212R Volume 80% down problem
El domingo, 29 de noviembre de 2015, 16:15:36 (UTC-3), Ralph Mowery wrote:
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Yes, I used a very thin layer.

Re: Fender Frontman 212R Volume 80% down problem
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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.


Re: Fender Frontman 212R Volume 80% down problem
snipped-for-privacy@gmail.com says...
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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.


Re: Fender Frontman 212R Volume 80% down problem
Leonardo Capossio wrote:

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 heat from the output devices to the bottom of the case - right ?  
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t the metal surfaces mating properly.  
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e shut downs.
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hey are fine, they are the same as in the schematic.
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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.
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** 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

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