fender frontman212r

[Jazz P Bass]

The often refered to " Rail" to me is a power buss.
Logic: + & - 15Vdc.
Power: + & - 75Vdc.
I don't think I have ever heard it used if it's a Bias voltage.
It is just a term. Slangy term.
From the battery operated days you have the A, B & C voltages.

[Enzo]

I could be wrong, but I think the term rail came from the early days of logic boards, or almost certainly early days of integrated circuits. You had a board full of ICs all in neat rows. In the case of something like TTL logic, there would be +5VDC and ground traces between the rows. The +5 trace ran along the top end of the ICs, and the ground trace along the bottom end. SO there would be a row of ICs, with parallel lines of copper running down the space between rows, looking like a pair of train tracks, or "rails." From each IC, a short trace of copper ran out to the +5 rail, and another ran down to the ground rail.

And in something like a mixer that used a lot of op amp ICs, you might see a vertical row of dual op amps, and the twin parallel lines of copper running up the middle under the ICs with +15 and -15 side by side. Agian the parallel copper stripes resemble train tracks or rails.

SO as Jazz said, it just means power buss.

generally it refers to a voltage supply that is distributed, like 15v going to a bunch of ICs. I don;t recall seeing a bias supply called a rail either, perhaps because it just goes one place usually. But if someone said that, I would take it to mean the common conductor delivering bias voltage to various tubes.

[andyjazz]

Thanks for responding to the perhaps overly semantic request for the "rail" definition. What I find a bit confusing is that there can be several "rails" indicated on a given schematic. On the Frontman 212r schematic you have a +16v and -16v rail powering a bunch of IC's and I'm guessing this supply is coming from the little network coming from p3, p4, and p5? Also you have this +42v and -42v supplying what looks to me like the output or power amp section? Am I on the right track here?

[Enzo]

Sure, substitute "power supply" for "rail" and there you are. means the same. Your got 4 power supplies, you got 4 rails, simple as that. Your op amp ICs will run on the +/-15v rails, while the powr amp will run on the +/-42v rails.

P3,4,5? P for plug or maybe push-on terminal. In any case, look at the schematic, the power transformer secondary says "to P3,P4,P5" Those Ps are on the circuit board, and that little network IS your powr supply. P4 is the center tap, see how it goes to ground? (over by C75) Then the other two are the hot AC for the bridge rectifier. In this case, the bridge is four discrete diodes, but could just as easily have been one of those square bridge things. Same thing. That makes the +/-42v. Now from each of those, there is a "dropping resistor" to a zener diode. One for each polarity of 16v. R144 limits current through D57, 16v zener to make the +16. and R145/D58 work their magic to make -16v.

SO there is no one THE rail in an amp like this, there are several, and they will either be specified, or marked with differing symbols, or be understood. For example, look right center, just above the power amp. See that row of ICs there? Not that all through the amp, there are op amp ICs, but none of them show power connections. It is understood that technicians will know there are power connections, but in this drawi9ng they are spelled out here. That row of ICs is the same ICs that are schattered throughout, but this shows their power connections only. See the parallel lines of +16 and -16 going down the row... like rails? Note also that the supply pins of U1 are isolated by little resistors. Doesn't affect the voltage much, but it serves to isolate the part from the rest enough to improve stability and noise. In some other amop, you might find the op amp IC in the power amp has its own +/-15, and the preamp ones are on separate rails.

Rail=power supply bus = power distribution oline = etc. Note the power amp, has a +42 and -42 rail running along the top and bottom, and various parts connect to them. Not much different from the outlet strip behind your amp rig.

Over in the power supply, there are the arrows labelled +42, -16, whatever. Those arrows just indicate that the +42 there goes to wherever it is needed. Could be one place, could be 20 places. The +42,-42 seem to only go to the power amp. The +/-16 goes to that row of ICs, but it also goes other places. Now look for U1 in the power amp, below it is Q6, see the nearby connection to -16? And above U6, see Q24 needs -16. And above that Q25 needs +16. Instead of drawing lines all over the page getting 16v connections to everything, they just invoke the rail concept with arrows.

"Am I on the right TRACK here?" DId you really say that?

[andyjazz]

Hey thanks alot Enzo for taking the time for the tutorial. Alot of information that I kind of figured on intuitively but it's great to have it spelled out. just being able to understand the road map I think is half the battle - is for me anyway. I kind of have a "big picture" way of wanting to understand things. So much of what I remember doing in electronic school was all "little picture" theoretical circuits that were mostly just about the math-that's the way I understood them anyway, I mean they weren't related to anything that actually DOES something as far as I could see. I got A's because I was good at the math but I didn't really have an understanding of what was actually going on half the time. Guess it's kind of hard to teach. I'll report back in a little bit when I have time to set up my scope and freq generator and start poking around. Thanks again- you're the best!

[Enzo]

Well, you have that whole Bret Favre thing up that way, you never know which direction to look for him, so I figured you needed something else to think about.

[andyjazz]

enzo, Aside from a bunch of sine waves on my scope I found 2Vdc on the speaker outputs. Is this telling me something?

[Enzo]

More than I expect there. Is that with or without a load? If without, does connecting a load bring the DC offset to zero?

[andyjazz]

Some more info: 10hz signal applied. Amp is hooked up to load and I've got 2Vdc on the speaker outs.

[km6xz]

What is the DC offset with no load and no signal, Does varying the gain control influence the offset level with no signal applied? A very low frequency test signal can mislead many of your readings, try a higher frequency like 1khz. Distortion seen on the scope will also be a more realistic indicator of amp distortion without the supply lack of stiffness causing distorted waveforms at low frequencies and slew rate limiting distortion at the high frequencies

With an amp that you are concerned over stability or defects, keeping the load off to to a higher value like 100 ohm non-inductive power resistors until you are confident in the amp having been returned to normal operation. That will prevent a lot of blown test speakers, output transistors, emitter resistors and fuses. Any well designed solid state amp will run well without loads and not have more than a few millivolts offset.

[Enzo]

I generally start with 100Hz for ear fatigue reasons, personal preference, but I agree, 10Hz is not a good test signal for a guitar amp. In fact it isn't a very good test signal for much anything, unless you need a 10Hz driver for something, or maybe are working on some sort of Low-cut or rumble filter.

[andyjazz]

The freq gen is at 100hz. It's an old one and I was looking at the wrong row of presets. The dc at the output varies with level of signal applied. with 100hz at the input I set the scope to .5sec and .2v and get a nice tall sine wave at the input that is about +3 to -3. I poke around the input section and see the same wave at various place and then I can see where it jumps to very big at R7. All this is kind of moot because I've already demonstrated that the problem is with the outpuit section not the input. So anyway I put the scope leads on the speaker outs and set the V to 5 and get this square wave thats got a hook on one shoulder - consistant with what my ears are telling me. My problem now is should I start wroking my way back from here? How do I test Q20 and Q21 the big output transistors? Any help is greatly appreciated

[andyjazz]

OK I figured out how to work my way back from the outputs. I see the same square wave with a hook on it's shoulder at Q20, Q21. When I work my way back it's the same shape but a little smaller (Which is to be expected I guess) at Q18 and Q19, Q16 Q17, etc.... I keep working back and at Q12 and Q13 I get the square wave at Q13 but Q12 just shows a bit of dc offset (I guess thats what you'd call it) the flat line of the scope just moves a little to the +. i guess I'm thinking the problem is just this little transistor at Q12. Any thoughts?

[andyjazz]

Just in case anyone is still reading this thread - I've been over this amp a number of times. I've replaced a couple of transistors where I thought they might be bad - same problem. Traced the sine wave to the very input of the power amp section and replaced a couple of zener diodes that I thought might be bad - same problem. Just today I noticed that, with the guitar plugged in, when you power off just for an instant, you get a normal sound from the amp. I'm thinking it has to be a connection problem - probably a bad solder joint somewhere. From what I've read on other threads this amp is notorious for shitty connections - oh well. I'm almost ready to hand it back to the guy and tell him "go buy a real fucking guitar amplifier." I've had it screwing aroung with this POS.

[km6xz]

One of the points that is confusing your readings is that the power amp section uses 10-20 db of negative feedback. That means any distortion seen at the output is also going to be reflected in the frontend of the power amp, in the differential amplifier stage. With a great deal of distortion of the wave form, the feedback will try to correct it by generating a compensation signal that is the difference between the incoming clean signal and the output high level signal. but the difference signal is out of phase with the output signal which normally forces the out put to correctly follow the undistorted input. The result is you will see something that looks like the distorted output everywhere in the signal path of th power amp section. Some will be out of phase acting as that correction signal but on the scope, triggered from the signal, it will look the same. If you use external trigger, say, the input signal, you can see the phase difference between stages.
Have you thought of taking the amp to someone who is familiar with them?
Replacing a lot of good parts, as I warned of in my first post, just puts you further back since you have added ambiguity to the equation. A new untested part has a higher likelihood of being defective than any random part in an amp that has been working for a long time. The problem will turn out to be a single part that is breaking down or has failed. When massive shorts occur, there is usually collateral damage such as emitter resistors and driver transistors when power output transistors short, but other than that most repairs involve a single part that causes the symptom.

Can you lower the signal level so the it is not squared off? Or does it stay squared even at low levels. Can you take a photo of the scope trace. The appearance of the waveform can be instructive if forming a diagnosis. Please do not change any more parts unless you can prove they are defective and know how it impacts the symptom.