stuck rail

[catstrat]

Hello all. I've heard the term "stuck rail". What does that mean, what are the signs,what causes it and what is the cure? Thanks

[Enzo]

If I ever said that it was in informal speech. A rail is a power supply voltage. +15v and -15v for op amps for example. Or +/-50v for a solid state power amp. I think I tend to just say B+ instead of B+ rail, but that would be OK too. But I think of it mainly as a solid state term. But in general it is easier to write "rail" than "power supply" when you are busy.

Stuck means stuck, as in not moving. A relay or an LED is stuck on, my amp is stuck in the clean channel. A voltage signal that controls something - an LDR for example - can get stuck, meaning the circuit can no longer turn it off and on. The term doesn;t specify WHY.

If transistors fail or something the output of a solid state amp can swing over to the DC voltage of one of its power rails - you get a steady 50vDC on the output. I might refer to that as stuck to the rail. Same thing with an op amp. If the IC is bad OR if the input to it is bad, the output can wind up stuck to the 15v rail.

Can you point to a specific example or two? I could then better explain what it meant in that context, perhaps the language was too imprecise.

[catstrat]

Hi Enzo. I know i'm a little late on this but i'm still getting the hang of this forum. The term stuck rail, i now know what it means and you explained it very well. I've just heard it mentioned in passing and since i'm not good with solid state i was just wondering. Like a 4558 op amp, in most of the stuff i work on you should have positive voltage at pin 8 and negative voltage at pin 4. If that voltage appears at any of the other pins, you can say it's gat a stuck rail and a shorted op amp. Right?
Thanks

[Enzo]

Sorta. Depends on the context. In general, with +15 on pin 8 and -15 on pin 4, we'd expect all the signal pins to be centered around zero volts. SO in general, if I found +15 on the output where signal ought to be, then I do indeed think the op amp would be bad. However, I always check once I remove it to see that its input pin spots on the board do not have somne voltage on them. If the input pin has 15v on it, then the output will too. But usually, rail on the output means bad IC.

However, there are other applications that signal amplification for an op amp. In some amps, an op amp drives an LED, for example a clip indicator LED. Or in many modern Fender amps, op amps are used in complex channel switching circuits. In those circuits it is normal for the IC output pins to sit at some DC voltage.

[catstrat]

Hi Enzo. I never thought of removing the op amp from the circuit and checking the voltage at the board. I'm beginning to learn some valuable information from this site. I have another post up now about a Hartke lh 500 blowing fuses. I turn down alot of solid state stuff because of my lack of confidence in troubleshooting it. I'm pretty good with tube stuff and tube amps are about 96 per cent of my business but i'm sure turning down alot of money by not working on transistor amps. Thanks for the help.

[Enzo]

Troubleshooting is troubleshooting, it doesn't matter what you apply it to. If someone brings you a tube amp blowing fuses, what do you do? If you are experienced, you first look for a bad output device - power tube. If those are not at fault, you look for maybe a shorted rectifier. Of course there are other things. But a SS amp is no different. Blows fuses?: Look for shorted output devices. Check power supply rectifiers. Troubeshooting is nothing more than a systematic approach to isolating the problem to a specific area. I don't have to really know HOW op amps work to know they need power supply and have inputs and outputs. Do you really have to know HOW a triode works to know it needs B+ and signal goes in the grid and (usually) out the plate? Does it matter if an amp is tube or solid state to isolate a problem to either power amp or preamp? No, the FX loop jacks let you try each section separately. Does it matter what technology amplifies the signal that a faulty input jack can;t pass? Loud hum? Filter caps maybe? Tube or SS, who cares. If you know how to troubleshoot, then the technology is just a detail.

[catstrat]

Well said Enzo. I think i created a mental block about transistors myself and it would stand to reason that what you say is right. Who cares how the signal is amplified and by what device, it's just amplified. And the fact i've not worked on that many solid state amps, but that's my own fault. Thanks for the pep talk of sorts. These last few days i've learned more info about solid state that will be filed away in my noggin and if i get stumped in the future i won't be so shy about asking you guys for advice. Thanks everyone.