Fender Frontman 212R Volume 80% down problem

Hello,

I was wondering if you could help repair a guitar amplifier, it is a Fender Frontman 212R, schematics are here:

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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.

Reply to
capossio.leonardo
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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?

Reply to
N_Cook

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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.

Reply to
Leonardo Capossio

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.

Reply to
Ian Field

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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.

Reply to
Leonardo Capossio

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.

Reply to
Ian Field

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 f***ed up amps...

LOL LOL

Reply to
jurb6006

** 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

Reply to
Phil Allison

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.

Reply to
Leonardo Capossio

** 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

Reply to
Phil Allison

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.

Reply to
Ian Field

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.

Reply to
Jeroni Paul

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

Make sure its metal grade..

Jamie

Reply to
M Philbrook

** 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

Reply to
Phil Allison

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.

Reply to
Ian Field

El jueves, 15 de octubre de 2015, 18:11:29 (UTC-3), Leonardo Capossio escri

hould. 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 pow er dissipator. But this can be solved by cleaning and re-applying thermal p aste, 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 heats ink) to blow hot air out of the amp head, the amp still behaves the same. I t 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 ?

Reply to
Leonardo Capossio

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

Reply to
M Philbrook

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

Reply to
dansabrservices

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

Reply to
makolber

rote:

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

ad speaker?

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 w orking like a charm, the sound isn't fading away or anything, and the sudde nly 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 s eems very strange that it abruptly cuts out the sound.

Reply to
Leonardo Capossio

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