switching issue on fender fm 212r

[black_labb]

i've been using a fender frontman 212r at band practice for vocals, but its also the drummers guitar amp. the amp started cutting in and out due to switching between channels (i was watching as the led's that tell you what channel you are on start flicking on and off. this also meant for some pretty horrible mic feedback when it stayed on the heavier gain channel for a few seconds).
there is a footswitch that is for the channel switching, so i plugged that in thinking that the switches themselves were going bad, and the footpedal bypassed the switches. this did not fix anything.

here is the schematic (actually the fm212, the 212r has reverb, but im assuming this is the same in terms of the switching. probably uses the same circuitboard anyway)

http://www.fender.com/support/amp_sc..._Schematic.pdf

is this a relay switching design? i've never had a look at how relay works, but this amp seems to work by varying voltages, changing the channels.

the issue tended to be worse when there were vibrations from the bass amp and drums, so it may be a bad solder joint thats finally come loose (amp is ~5 years old).

the channels switch between normal, gain and more gain. it doesnt just switch between 2 channels. in terms of the switches, there is a switch that changes from gain to more gain, and one that changes from clean to gain (whichever is engaged with the other switch). this seems to suggest that the issue is not switch related, but in the circuit.

is there a certain chip that would be suspect to be at fault, or should i just touch up the solder joints in the area hoping that thats the problem?

[black_labb]

just had a look, and r144 or 145 has a cold solder joint that is completely loose. the resistor looks a bit brown so i may replace it, even if it is not blown. mine seems to have 330ohm resistors instead of the 200 on the schematic.

could this be the issue? i guess it would effect anything running off the lower voltage tap. either way i'll fix it, as its deffinately not a good thing.

[black_labb]

after thinking abut it im pretty sure that that would be the problem. what i thought was the amp going into the clean channel was the voltage not powering the amp properly and making the led go off. i thought this was switching into the clean channel (its volume was turned off). the amp going into the more drive would be related to there not being constant voltage drop across the resistors, making more/less voltage which was changing the channels.

i got some new resistors as he problem one seemed to have the resistor section seemed to be separated from the cement(?) casing and they were both a bit brown. hopefully when i get home and put it back together it will work fine.

[Robert M. Martinelli]

Hi Black Labb,

I had the same kind of problem on an Hot Rod Deluxe belonging to a friend.

Those resistors generate enough heat to ruin the solder joints, when this happen, the voltage becomes erratic and the switching issue is one of the symptoms.

The problem is that Fender took a shortcut on the PS section, you have about 42 Volts before those resistors, and about 16 on each zener, this means some 24 V are dropping across those resistors. A voltage drop of 24 V means that a current of 0.2 Amps is flowing, and if you multiply 0.2 Amps by 24 V you get 4.8 W, which is very close to the resistors' rating. If you simply change the resistors with new ones you' re likely to have the same problem in the future.

You could simply use 10 W resistors, or, if you want to do something better, you could change the resistors' value from 200 Ohms to 91 Ohm 5W; with a current of 0.2 Amps the voltage drop will be about 18 V so the dissipated power will be 3.6 W, then change the zeners with voltage regulators ( a 7815 on the positive rail and a 7915 on the negative rail - be careful as the pinouts are different ) and a couple of diodes in series between the regulators' 0V and the actual GND ( to elevate them some 1.2-1.4 V from ground ). The 7815-7915 will have to dissipate some 1.8 W so a small heatsink could be needed.

I calculated the values starting with the schematic's value, 200 Ohms; If your amp has 330 Ohms resistors simply re-calculate the new resistors' value according to the drop ( and current ) you're measuring.

The amp will be far more reliable and your preamp and switching circuitry will get a cleaner supply.

( BTW that's what I did on my friend's HRD ).

Hope this helps

Best regards

Bob

[black_labb]

thanks bob.
already replaced the resistors, so i might see how they go for a bit. i was considering getting some 10w, but the closest shop i could get them had the electronic parts packed up for christmas shopping, and the guy couldnt find a 10w one. i dont go there for anything other than the most basic parts.

havent measured the voltages, but if the resistors were still dropping 24v, then the current would be only 0.072A, which would mean ~1.7w of dissipation from each resistor. considering they were brown i doubt this was the value, but it may be a better case than the fm212 (non R model) from the schematic.

[Robert M. Martinelli]

havent measured the voltages, but if the resistors were still dropping 24v, then the current would be only 0.072A, which would mean ~1.7w of dissipation from each resistor. considering they were brown i doubt this was the value, but it may be a better case than the fm212 (non R model) from the schematic.

Hi again BL,
I strongly doubt you have such a low power dissipated on the resistors; if this was the case, they would never have overheated.....I agree with you, if they were to dissipate a mere 1.7W they would have never gone "brown".

Since you have already replaced them, my advice would be to measure the actual voltage drop and calculate the actual current and dissipated power, this way you'll be able to figure out if they're going to last or not.

As to the schematic, I think your situation could be worse, not better, as the reverb circuit ( driver and recovery ) your amp has will draw some more current from the preamp PS section.

All in all, my vote still goes to the VR mod

Hope this helps

Best regards

Bob

[black_labb]

i guess the only thing to do is to see how much voltage it is dropping.

just judging on the different values for the resistor, i dont think the circuit follows the schematic as closely as i was assuming. if it was dropping more voltage, then it wouldnt likely be running the same circuit. if it had more circuitry to draw current, then it would need a lower value resistor to get the same voltage drop.

i'll measure the voltage drop before putting it together to see wheter i need to do something about it, or just see if it lasts another 5 years.

[black_labb]

the resistor is dropping 28v, giving ~2.4w dissipation. got pretty hot after 10mins of playing, but apparently they should cope.

[sparrks]

Hey Thanks for the hint. I just got an FM212R for 50$ non-working of course. I saw this post while searching for a schematic. I had the same issue. The source resistors that feed the zeners became unsoldered. One touch of the iron and it is back in business! I do like the idea of replacing the zeners with linear regulators. May do that some day.

[Robert M. Martinelli]

You're welcome!

(BTW, welcome to the forum, too!)

Best regards

Bob

[Vintagelover]

Hey Thanks for the hint. I just got an FM212R for 50$ non-working of course. I saw this post while searching for a schematic. I had the same issue. The source resistors that feed the zeners became unsoldered. One touch of the iron and it is back in business! I do like the idea of replacing the zeners with linear regulators. May do that some day.

Hey Guys!

I'm totally new here on this forum. But I noticed you really help people with their electronic problems, and that's what I search for

My knowledge is very basic of electronic components. Still i find it very interesting and would like to learn more.

The reason I reply on this post is because I have about the same issue. But mine is a bit different. This is the problem:

At the clean channel I have no problems, everything works fine.
The drive channel on itself works also perfect.
But when I switch to the more drive channel, the Red en orange LED's constantly switch on and off.
So after measuring a few hours and googeling, i found this topic.
But on my circuit board the 5W resistors are fine. The connection is a little bit brown, but they measure also 330 ohms instead of 200 and the solder joints are really stuck, totally not loose.
Still, my voltage at the opamp U7.A is on the more drive channel still -6V instead of 16V.
Can you please help me? I'm a bit stuck because of my limited knowledge.

Thank you in advance!

[g1]

How about your TP24 voltages? (U7 pin2) And TP26?
The box in area B7 of the schematic shows what you should see. It depends whether you are using the panel switch or footswitch.

[Enzo]

Hi Vintage, welcome to the forum.

But good form is to start a new thread for yours rather than adding to the end of a different repair. That just gets confusing.

[Vintagelover]

Hello!

Sorry to reply on another thread instead of making my own. I wanted to show I had about the same problem, so everybody knew the background.
I'll remember to start a new one in the future

Thanks for your help! The voltage on the TP24 point wasn't correct, but I forgot the value.
To give it a try I first changed the 2 5W resistors who handle de PS for the preamplifier. I changed them for 7W 200 ohm. The ones who were in there were 5W 330 ohm (not like in the schematics)
I tried the amplifier and everything worked again!!!!

I'm so happy, thank you for your knowledge and advice! I should have read this much sooner, I would have saved me a lot of misery.

My clean channel was ok though, my more drive channel also, but I think they only got messy when the more drive circuit asked more power, resulting in voltage variations.

Thanks again! And sorry for this wrong posting Regards, Vintagelover