I recently came into possession of a Fender FM212R. It's got (what appears to be a common issue) a faulty component(s) causing the power amp transistors to overheat. I've read through the other thread detailing a user's experience in trying to troubleshoot the problem and it seems like a bit more trouble than it's worth.
Now I'm thinking of replacing the power amp section with one of these 100W Class D amplifiers. I'd just run the pre-amp out into the peripheral power amp and then one speaker to each channel of the new power amp's output.
I'm trying to think of the best way to power this amp using the existing transformer in the FM212R, that is, finding existing points on the board to use for power. Here's a copy of the schematic for reference. The Class D board needs 14-39V at a regulated 6A. Can anyone suggest a good place to tap on the board that would meet this requirement?
Any other flagrant issues that this might raise?
Thanks in advance!
Now I'm thinking of replacing the power amp section with one of these 100W Class D amplifiers. I'd just run the pre-amp out into the peripheral power amp and then one speaker to each channel of the new power amp's output.
I'm trying to think of the best way to power this amp using the existing transformer in the FM212R, that is, finding existing points on the board to use for power. Here's a copy of the schematic for reference. The Class D board needs 14-39V at a regulated 6A. Can anyone suggest a good place to tap on the board that would meet this requirement?
Any other flagrant issues that this might raise?
Thanks in advance!
Some thoughts:
First, sure, why not? If there is room for it. The FM212R schematic says the main rails are 41v. Before you tear into it, what do the power rails in yours sit at? (Oddly enough it says 42v and the test point says 41) Wouldn;t be hard to lose a few volts from that amount.
I won't try to convince you to just fix it, but i will say that someone else's online repair saga really says nothing about how much it will take. Ever set up a childs swing set in the back yard? Not really difficult. But now imagine telling someone who had never seen one how to do it, and make it via internet forum you visit daily. Now we have to describe a piece of pipe and make sure the guy gets the right pipe, then we need to describe how to fit it etc etc.. It could take weeks. Just so this amp. If it came to my shop, I;d have it fixed within the hour most likely. But when each voltage measurement takes a day to report back, and then the person not understanding the significance of those readings, it gets a lot more complicated than it ought to.
And as to "common issue" I have to say no, it isn;t. WHo hasn't met a solid state amp that ran hot? it isn;t just this one. A few of these run hot and someone comes online with his for help. The vast majority of the thousands and thousands of them do not run hot. They just work right along. There are only so many failure modes for an amp, blows fuses, runs hot, no sound, distorted sound.
First, sure, why not? If there is room for it. The FM212R schematic says the main rails are 41v. Before you tear into it, what do the power rails in yours sit at? (Oddly enough it says 42v and the test point says 41) Wouldn;t be hard to lose a few volts from that amount.
I won't try to convince you to just fix it, but i will say that someone else's online repair saga really says nothing about how much it will take. Ever set up a childs swing set in the back yard? Not really difficult. But now imagine telling someone who had never seen one how to do it, and make it via internet forum you visit daily. Now we have to describe a piece of pipe and make sure the guy gets the right pipe, then we need to describe how to fit it etc etc.. It could take weeks. Just so this amp. If it came to my shop, I;d have it fixed within the hour most likely. But when each voltage measurement takes a day to report back, and then the person not understanding the significance of those readings, it gets a lot more complicated than it ought to.
And as to "common issue" I have to say no, it isn;t. WHo hasn't met a solid state amp that ran hot? it isn;t just this one. A few of these run hot and someone comes online with his for help. The vast majority of the thousands and thousands of them do not run hot. They just work right along. There are only so many failure modes for an amp, blows fuses, runs hot, no sound, distorted sound.
Thanks for the response Enzo, your posts in the thread mentioned previously were always very elucidating. At the risk of starting another enormous thread chronicling my attempt to fix this amp, you've inspired me not to give up on it just yet. Unfortunately I blew a fuse on it yesterday so I can't get a reading on that "high" rail voltage.
What would your methodology be in terms of assessing the problem? The symptoms I've noticed so far have been that a short time after playing, the amp turns off. This seems to happen faster if the overdrive channel is on or if the volume is turned higher. I thought I'd try and disconnect the preamp from the PA and see if it still over heated. It ran fine for a while but when I engaged the overdrive channel I started smelling something burning and saw a little wisp of smoke come off the board. (Think that's where I might've blown the fuse). It's worth mentioning that I didn't have the pcb bolted to the chassis via the power transistor's heatsink so it could have just gotten too hot that way. I also just had a dummy plug inserted into the Pre Out jack to try and disconnect the two. I know not having a load can be bad for tube amps, not sure if the same would apply here.
What would your methodology be in terms of assessing the problem? The symptoms I've noticed so far have been that a short time after playing, the amp turns off. This seems to happen faster if the overdrive channel is on or if the volume is turned higher. I thought I'd try and disconnect the preamp from the PA and see if it still over heated. It ran fine for a while but when I engaged the overdrive channel I started smelling something burning and saw a little wisp of smoke come off the board. (Think that's where I might've blown the fuse). It's worth mentioning that I didn't have the pcb bolted to the chassis via the power transistor's heatsink so it could have just gotten too hot that way. I also just had a dummy plug inserted into the Pre Out jack to try and disconnect the two. I know not having a load can be bad for tube amps, not sure if the same would apply here.
You are blowing fuses, so start by disconnecting any and ALL loads from the amp. Until we know the amp is stable and does not put DC on its output, we will work WITHOUT a load.
Please don;t say "can;t get a reading". That to me means your meter goes dead or that you cannot connect your meter to the circuit. A reading of zero is still a reading. An infinite resistance reading is still a reading.
And never operate that amp without the heat sink screwed down. It needs it for thermal stability.
And look up "light bulb limiter" and make one. This will stop fuses from blowing and protect the amp from some further damage. The bulb limiter is very useful at the blowwing fuses stage. Once the amp stops trying to blow fuses we then ditch the bulb so it does not confuse the amp operation.
Please don;t say "can;t get a reading". That to me means your meter goes dead or that you cannot connect your meter to the circuit. A reading of zero is still a reading. An infinite resistance reading is still a reading.
And never operate that amp without the heat sink screwed down. It needs it for thermal stability.
And look up "light bulb limiter" and make one. This will stop fuses from blowing and protect the amp from some further damage. The bulb limiter is very useful at the blowwing fuses stage. Once the amp stops trying to blow fuses we then ditch the bulb so it does not confuse the amp operation.
Hi,
Class D amplifiers requiring a single rail are not good for dual rail as here.
FWIW goood guitar power amps are hardly ever "hifi amplifiers" and
I can see some current feedback in the Fenders arrangements.
Fixing it is a far better option, better result, and you'll learn a lot more.
rgds, sreten.
Class D amplifiers requiring a single rail are not good for dual rail as here.
FWIW goood guitar power amps are hardly ever "hifi amplifiers" and
I can see some current feedback in the Fenders arrangements.
Fixing it is a far better option, better result, and you'll learn a lot more.
rgds, sreten.
Sorry but you killed it.
In those amps,power transistors are bolted to a relatively thin aluminum plate (some 3mm) which is bolted to a solid block aliminumheat spreader (some 30mm x 30mm x 200/250mm) which is bolted to the full size aluminum chassis.
That is a powerful amp and it needs up to the last square cm.
Enzo, Fahey, and Sreten all know their stuff; you're getting the best advice there; I'm an amateur.
OK "212" as in 2X12 combo...'FM' FrontMan? So I guess that's a newer blackface brother to the older 1-tube silverface 1000 RocPro I was thinking of, which came either way: as a combo or with the GE412 bottom. Looking at the schematic they are not that closely related. About the same power output.
Those FM212R amps are not expensive used (for a Fender). Right now on ebay there are 15, and a few more of the DSP "emulator" version. You can bid on another entire combo currently at $40, working, but without speakers, and the competition bids will likely be light. There's also a good, working, chassis with a buy-it-now price of $130 so that's not going any higher and you can just decide what your time is worth. There's 4 more complete ones currently under $150 each, another "buy-it-now" option at $195. How clean are the cosmetics on yours? You don't know what some might bid up to, but you could set several snipes via bidnip to hide your hand and cancel the rest when one hits. I had to mention that because, well, I'm not enzo, fahey, or sreten and it takes me 10 times longer to be half as careful. So I only take on projects of passion or economy.
Of course you learn by doing it yourself and it's still going to be cheaper for the parts to fix yours, but I had to point out your obvious alternative options.
Is RT2 PTC100 a thermistor for over-temp sensing? Maybe you got lucky and overtemp protection worked, unless that's mounted on the heat spreader, in which case you might need a few more parts now like Fahey says.
OK "212" as in 2X12 combo...'FM' FrontMan? So I guess that's a newer blackface brother to the older 1-tube silverface 1000 RocPro I was thinking of, which came either way: as a combo or with the GE412 bottom. Looking at the schematic they are not that closely related. About the same power output.
Those FM212R amps are not expensive used (for a Fender). Right now on ebay there are 15, and a few more of the DSP "emulator" version. You can bid on another entire combo currently at $40, working, but without speakers, and the competition bids will likely be light. There's also a good, working, chassis with a buy-it-now price of $130 so that's not going any higher and you can just decide what your time is worth. There's 4 more complete ones currently under $150 each, another "buy-it-now" option at $195. How clean are the cosmetics on yours? You don't know what some might bid up to, but you could set several snipes via bidnip to hide your hand and cancel the rest when one hits. I had to mention that because, well, I'm not enzo, fahey, or sreten and it takes me 10 times longer to be half as careful. So I only take on projects of passion or economy.
Of course you learn by doing it yourself and it's still going to be cheaper for the parts to fix yours, but I had to point out your obvious alternative options.
Is RT2 PTC100 a thermistor for over-temp sensing? Maybe you got lucky and overtemp protection worked, unless that's mounted on the heat spreader, in which case you might need a few more parts now like Fahey says.
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Sorry about the confusion re:meter reading, I just meant that with the fuse blown I was getting 0V at that rail (a tiny bit more as the cap charged). As you said though, even that reading is meaningful.
I'm going to build a light bulb limiter this weekend and buy a couple new 4A fuses to have on hand.
Bummer, I should've been more conscious of that. Any advice on how to get readings from test points on the underside of the board with the whole thing bolted to the chassis?
Unfortunately that one was also sitting on the heatsink.
Thanks all for the help, I appreciate it.
Not really.
The amp is good and well made but it was optimized for efficient manufacturing, not so much for servicing.
The not-best-but-practical thing to do is to remove board (there's no other way
) , with amp unplugged measure transistors for shorts, replace them, also measure and replace if needed ballast resistors (the rectangular ceramic ones), any parts visibly toasted or cracked, etc.So unlike the preferred approach of test first and measure voltages , then replace, we are doing the educated version of shotgunning
, so to speak.When everything seems to be fine, we plug the amp into the bulb limiter, turn it on without speaker, without signal, (so we are not forcing it to overheat trying to do its job) and measure "static" voltages.
Basically +/- rails, we expect somewhat less than normal, say +/- 30/35V ; speaker output rail , we expect less than 100mV DC ; bias current, we expect less than , say, 30 mA through the ballast resistors (say 15 mV across any and all of them).
IF everything looks normal, turn amp off, bolt heatsinks properly , turn amp on, check that everything still is normal, only then connect speaker and apply a little audio, say some music to have your hands free and not more than a couple watts.
All we want to know is that the amp works.
Post results.
Don't read too much into replacement only. It ONLY refers to paid warranty repairs. Fender can replace it cheaper than it can pay me $50 to fix it. The thing is warranted five years. it has nothing to do with how hard or simple it is to service. Once the warranty is up, they are not suggesting it not be repaired, the customer would pay for the repairs like any other amp.
Awww C'mon Really?
A Fender transistor amp... it's simply a non starter imo
Not a Fender, but some cheap imitation contraption... likely made in Korea
One can buy a ..Real/genuine.. tube Fender Amp for much less than 1K$... pretty well anywhere.
I've had my '64 Bassman for decades.. and I wouldn't even bother playing my Guitar if I had to use a some cheap imitation amp.
Sounds are simply wrong.
A Fender transistor amp... it's simply a non starter imo
Not a Fender, but some cheap imitation contraption... likely made in Korea
One can buy a ..Real/genuine.. tube Fender Amp for much less than 1K$... pretty well anywhere.
I've had my '64 Bassman for decades.. and I wouldn't even bother playing my Guitar if I had to use a some cheap imitation amp.
Sounds are simply wrong.
Exactly as you say about warranty repairs, Enzo. So do they even offer replacement boards or parts, and what do they cost? Is that an option? Does it encourage them to disregard designing for repair?
Bare, a new handwired '57 deluxe head lists for $1899 for a 12-watt amp without even a reverb. A new handwired vibro king is $3600 now and you still don't even get a reverb. These transistor amps are on eBay for about 1/10 of that. If you have the money for the Deluxe I agree I wouldn't prefer 10 of these, but if people don't then they buy one of these, or buy them for their kid who may not stick with their lessons. I usually buy broken unpopular tube models though I'd like that '57 Deluxe (but could get an equivalent kit for half that). I would never have wanted a Roland Cube 30 but I picked up a broken one for really cheap on eBay and it's been really handy for practice. It won't "emulate" my Champ practice amp, but the effects keep it interesting. One way to look at it is that right now if he fixes this it's nearly free, as it's worth nothing broken and the parts are cheap. Now if he's like me and buys an oscilloscope it's a more expensive education.
Bare, a new handwired '57 deluxe head lists for $1899 for a 12-watt amp without even a reverb. A new handwired vibro king is $3600 now and you still don't even get a reverb. These transistor amps are on eBay for about 1/10 of that. If you have the money for the Deluxe I agree I wouldn't prefer 10 of these, but if people don't then they buy one of these, or buy them for their kid who may not stick with their lessons. I usually buy broken unpopular tube models though I'd like that '57 Deluxe (but could get an equivalent kit for half that). I would never have wanted a Roland Cube 30 but I picked up a broken one for really cheap on eBay and it's been really handy for practice. It won't "emulate" my Champ practice amp, but the effects keep it interesting. One way to look at it is that right now if he fixes this it's nearly free, as it's worth nothing broken and the parts are cheap. Now if he's like me and buys an oscilloscope it's a more expensive education.
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And it sounds real good (clean).
Your favorite pedal will supply all the dirt you want.
It costs 1/10th what a Twin would, and weighs 1/4 as much .
All points to consider.
Did I say it's LOUD?
By the way, it's as "Fender" as any amp of that brand made after 1967, in either case the old company had been previously owned by Leo Fender himself.
Remark "previously".
It's not an imitation by any means, which would apply to amps made by others, such as most Boutique Makers.
Your favorite pedal will supply all the dirt you want.
It costs 1/10th what a Twin would, and weighs 1/4 as much .
All points to consider.
Did I say it's LOUD?
By the way, it's as "Fender" as any amp of that brand made after 1967, in either case the old company had been previously owned by Leo Fender himself.
Remark "previously".
It's not an imitation by any means, which would apply to amps made by others, such as most Boutique Makers.
Do they offer boards? I have no idea, probably not. people don't usually replace whole boards in pro audio repair, any more than automobile garages replace whole engines. Cost to the consumer would be high in any case there.
Parts? yes, they have the parts. They have the jacks and controls and speakers and power transformers. KNobs and trim. ANything else is pretty much generic components.
Again, "do not repair" ONLY refers to warranty billing, it does not mean the model is not supported nor does it mean it is more difficult to repair.
I don;t find these amps any less serviceable than any other solid state amp of the era. The circuit is more complex "than it needs to be", because they are trying to get every ounce of loud out of the thing as they can. But the same power amp approach is used in various other models in the fender line. The amp is one big circuit board, like a bunch of other models. I free it, flip it over with transformer leads still attached and i can solder anything i want. I don;t consider it less "designed for repair" than much of anything else. It is probably easier for most of us to service than a Hot Rod DeVille, a popular tube model from Fender. The Hot Rod is not very convenient to get the board out.
I'd suggest we forego the temptation to compare these entry level amps with $3000 boutique amps. They are not designed for the same purposes nor marketed to the same demographic. The cost of an amp is not in the parts.
Parts? yes, they have the parts. They have the jacks and controls and speakers and power transformers. KNobs and trim. ANything else is pretty much generic components.
Again, "do not repair" ONLY refers to warranty billing, it does not mean the model is not supported nor does it mean it is more difficult to repair.
I don;t find these amps any less serviceable than any other solid state amp of the era. The circuit is more complex "than it needs to be", because they are trying to get every ounce of loud out of the thing as they can. But the same power amp approach is used in various other models in the fender line. The amp is one big circuit board, like a bunch of other models. I free it, flip it over with transformer leads still attached and i can solder anything i want. I don;t consider it less "designed for repair" than much of anything else. It is probably easier for most of us to service than a Hot Rod DeVille, a popular tube model from Fender. The Hot Rod is not very convenient to get the board out.
I'd suggest we forego the temptation to compare these entry level amps with $3000 boutique amps. They are not designed for the same purposes nor marketed to the same demographic. The cost of an amp is not in the parts.
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