Beginners Guide to Signal Troubleshooting

[rmroza]

Ok, not sure if this has come up or not, but I think it's a great thread for newbies and everyone.

Ok, the situation is this...an amplifier and basically voltages are good, there is output to the speaker, it's biased, but no signal.

How would one go about diagnosing where the signal is getting lost? How does the signal flow through a standard preamp circuit? Where to test at??

Can someone explain and in detail? Is there a thread and/or youtube video out there??

I'm sure one would need a signal input. Probably a signal generator and a computer and there is free software, so this is easy. I'm sure an oscilloscope would be needed. Personally, I have a Fluke 123 2-channel.

Please let me and the world know. Thanks in advance.

[xtian]

You can get pretty far with no equipment other than your eyes and ears. When amp is on, can you hear hiss in the speaker? With chassis open, you can touch your multimeter probe to the PI input; does it pop?

Working backwards from the output is one way to diagnose an unexpected silence.

[Structo]

Well the first thing I do is give it a very thorough visual check.

Look for burnt resistors and the like.

For example, I worked on a Fender Champ which is a very simple design.

No sound but I could tell the power amp was working.

The input cable (coax) was shorted at the volume pot.
I measured between the conductor and shield and got zero ohms.

Replaced that and it's good for another 30 years.

A lot of guys trace an amp back from the power amp.

If you can tell if you are getting some hiss or air from the speaker, most likely the power amp is working.
As mentioned when you check the voltage on the PI plates it will and should put a bit of a pop in the speaker.

But one time I had a speaker cable fail when I was working on a amp head.
The amp was working before, I was just tweaking the circuit.
I nearly thought the OT went bad and finally checked the cable and it was open.
So if you get nothing from the speaker, be sure to check the cable first.

R.G. Keene has a pretty good routine outlined here.
http://www.geofex.com/ampdbug/ampdebug.htm

[tubeswell]

Hi rmroza

You say they 'are good' - so what are the voltages?

And did you check for DC continuity with your R-meter between all the circuit points where you want continuity (i.e.: in the signal path etc)?

[dorrisant]

An amp stethoscope really helps find the fault in a broken signal chain.

http://www.el34world.com/Hoffman/tools.htm

Tony

[rmroza]

Holy crap Dorrisant that's insane IMHO!

Ok, what I did was use a small computer I did and free signal generator software. I have a Fluke 123 2-channel oscilloscope. I injected the signal into the input and just followed it through the circuit running onto the grid of the valve when applicable and out the plates and continuing on!

I found the issue and where signal was lost, but there was another, but following the same methodology, I found it also!

I can report the amp is up and running well. It still needs a little tweaking as the bass notes and muddy and not well defined, but works, it quiet, channel switching fine, etc.!

Thanks for your support.

Equinox

[sliberty]

An amp stethoscope really helps find the fault in a broken signal chain.

http://www.el34world.com/Hoffman/tools.htm

Tony

I do something like this all the time, but my setup is a bit different. I use a capacitor taped to a chop stick as a probe, and I use a set of old computer speakers as the amplifier. Works great.