What does it mean when your amp goes

hhhmmmmmmmm all of a sudden?

And it doesn’t do anything else? ??? no guitar, nothing but a very clean hhhuuummmmmmmmmmmmm…

More than likely something screwed in the power supply - bad filter cap or open diode maybe.

The drag is that it is my Vox Pathfiner, a 70 dollar special from the fiend, and I really like how it sounds. But what’s a bench fee going to be to fix it? By the time it would be fixed I might as well have bought another…

Thanks for the info, tempus! :)

Yeah, that’s a problem that plagues us all - the repair could well cost more than replacing. I suppose you can check around, but I know here a lot of the guys charge $75 just to walk in the door. There are some techs, though, who charge about $20 to see what’s wrong with it, then deduct that from the final cost if you decide to get it fixed.
Based on information you’ve given, though, it sounds like it should be a fairly easy fix.

I suppose I should ask - how handy are you with a soldering iron? If you’re thinking about dabbling in electronics, maybe now’s the time (if you’ve got the time) to learn a bit and maybe try to fix it yourself. I myself got started that way, and while I’m by no means an expert, I’ve been able to fix a lot of stuff myself.

One word of caution, though - make sure you know the safety protocols before you start working on anything that uses AC line power.

Here’s the new version for $79



I seriously doubt it holds a candle to the old ones.

I picked up an old Pacemaker at a pawn shot a long time ago. It was cool for just about anything that could stand solid state distortion. For the most part I use a Tube Screamer for distortion, but it was great for a clean amp and even for bass at low volumes.


I haven’t heard the any of the new ones from Korea yet, but since the new Pathfinders are so cheap, they probably either sound like crap or would be good alternatives to today’s repair costs.

Sometimes that hum does wonders for sound creation. I was noodling around for some interesting guitar ambience fx and the hum going through a leslie rotary simulator or a delay and large reverb has some wicked sounds. :;):

Unfortunately, all the amp produces now is hum! Won’'t pass a signal. :( Is that good for sound creation? Mind you, it is a very pure hum, almost a sine wave, sounds like.

Tempus, I’m OK with electronics - I understand the fundamentals of power supplies, e.g., and I can read a schematic mostly, and I have a pretty good multimeter, that sort of thing - you know, the kind of stuff you get in a high school electronics course plus a bit I’ve picke dup here and there. I’ve several projects I’ve been warming up to, one of these days when I get time… well, OK, this one might have to be the one where I actually learn something substantial. I WANT to be technical, but I’m condemned to being a humanist, I think… :)

It could be your Pwr Supply. Filter cap would be one of my first checks. Also the rectifier diodes.

More likely it would be the output(s). I see more shorted outputs causing this problem than power supplies.

Since this is what I do.

Authorized warr for Korg, Ampeg, Vox, Digi, Marshall, Vestax, Yamaha, Allen & Heath, Peavey…etc.


Shorted outputs - you mean like if I plugged a cable into the speaker out that shorted? What would that do to the electronics? That may well have happened…

Oh yea Tom that would definately do it. 0 ohm on the output…BAD…16/8/4 and sometimes 2 ohms GOOD! Depends on the amp. :slight_smile:

I will have to wait til I get to work to check the service manual but if it is an integrated circuit power amp the cost should very reasonable and if you are handy with a soldering iron not a big thing to replace it. KorgUSA is the place to call for parts. They built it they have the parts. Just to be sure you get the correct ones.


Edited: I KNEW it WAS KorgUSA that handles VOX!!! I was just testing…ahem!

I had an old Gorilla practice amp that cut out on me. I was able to fix it by replacing the “high power” transistor in the power amp for $14. I am sure that you could do something similar with this amp if’n it isn’t full of surface mount components!

If you are getting no signal and only hum on the output (even when you short the guitar plug?) it actually sounds like some virtual short in the preamp or in the 1st stage of the power amp. If you had a scope, it would be easy to diagnose. LOL!

- Ben

Is the hum loud? Can you vary it with the master vol (not familiar with that model of amp)?

I’ve fixed a couple of the smaller Marshall valvestate amps which use a single IC power amp device that fries fairly easily if the amp is cranked for a while or you use a dodgy external speaker. Symptoms are loud hum/buzz, no controls work, no input. One of the output transistors on the chip short-circuits. A couple of pounds/dollars to buy a replacement, swap it out and it’s going again. I added some output overshoot protection diodes as well, and changed the Zobel network (amplifier output damping) to the IC manufacturer’s data sheet recommendation. Looks like Marshall skimped a bit on the circuit design, but that’s not unusual for any lower end amps.


The hum is pretty loud, no buzzes or pops, no crackles, or snaps for that matter. Just hhhmmmmm… And nothing changes it - neither the volume nor the gain nor shorting a cable plugged into the input - nothing… well, I didn’t check the tremolo…


Tom, as you can read a schematic, then it’s just a question of following the signal path.

Of course, an oscilloscope is ideal for this, but you can detect audio with a good analogue multimeter too.

So, you know the i/p is OK, but the o/p ain’t, so just look half way through the circuit, then subdivide and subdivide until you isolate.

But, are you sure it’s not something stupid like a cable or connector?


Quick and fairly easy check to see if it’s the output stage or not:

Plug a cable into the ext speaker jack, connect a multimeter (set to read DC volts) across the cable, and turn on the amp briefly. If the voltmeter shows plus or minus several volts (maybe 15-20 volts), the output stage of the amp is certainly damaged. Switch off quick.

Alternatively: Use a multimeter set to “diode check”, or “ohms” if there’s no diode check feature. Unplug and open up the amp chassis. Plug a cable into the ext speaker output. Locate the positive power supply rail (positive side of the bridge rectifier or big filter capacitors) and measure between positive supply and speaker jack tip, with the positive meter terminal to the positive power supply. Do the same with the negative supply rail and speaker output, this time with the meter positive terminal to the speaker jack tip. If one (or both) measurements shows zero (or very low) ohms (or the diode check beeps), the output stage is probably dead. If not, some more probing is required…

The Marshall 10 watt and thereabouts amps use the TDA2050 power amplifier IC, which looks like a metal tab transistor with 5 legs, probably bolted to a heatsink fin or the chassis. If it’s the same, and it’s dead, and you’re moderately handy with a soldering iron (and the amp is out of warranty!), you might only have to spend a couple of bucks and some spare time…


Tom, I think he is referring to an output transistor shorting out. A lot of times, when a transistor or transistor like device conducts too much current, it fails in a “shorted” state. Essentially, instead of being a semi-conductor, it is now a full-blown conductor. :) Take a multi-meter set to read DC volts and hook up to your speaker leads. If you see DC voltage there, you probably have a shorted output transistor. Oh, yes, a shorted speaker cable can cause this to happen if the amplifier has no protection circuitry.



PS I may have this totally screwed. So don’t shoot me if I’m wrong! :D :D

Right. If nothing changes the hum, you can assume that the problem occurs after the volume control in the signal path. As others have mentioned, the 1st thing to check is for DC at the speaker output.

Good, thanks guys, I have a free afternoon today, so down to the basement it is… :)