need help troubleshooting solid state guitar amp

[brainwreck]

Hi.  First time posting here.  I found this forum looking for tips on troubleshooting a solid state guitar amp.  Hopefully someone here will be able to help me with this thing. 

A buddy of mine got a little 2x8 solid state guitar amp given to him but it has something wrong with it.  I tinker around with a lot of things so he dropped it off here hoping I could figure it out.  I really don't have a clue as to what's wrong with it.  It's an RMS500RC, made in Korea, 2x8" 2 channel amp with reverb and chorus.  I've never seen one before.  The problem is that when the 'high' knob is turned up, it starts crackling until about halfway thru the travel of the pot.  From half to full the amp just hums and there's no output other than the hum.  I took it apart and had a look around but didn't find any pcb cracks, bad joints, etc.  I did notice a burned plastic washer that goes between one of the ic's and it's mounting screw.  This amp has two TDA2030's mounted to the back of the chassis.  The burned washer (or insulator?) is white plastic.  That's the only sign of overheating that I could find.  All controls work good except this issue with the 'high' control.  Output to both speakers seems fine and output from the line outs seems fine. 

I recorded a sound clip by connecting one of the line out's to my sound card.  Maybe it will be helpful.  In the sound clip I flip on the amp, start playing some chords, and begin slowly turning up the high control from zero to halfway.  I'm strumming chords all along the way.  At 10 seconds you can hear the crackling starting.  At 36 seconds you can hear the humming.  I increased the volume of the humming to make it more apparent.  The hum is very apparent when running thru the amp's speakers but for some reason it's very low volume when using the line out's alone.  Sound clip is here (RMS500RC.mp3):  http://sellyoursoulformusic.googlepages.com/travis%27smp3%27s 

I can solder, I know how most components work and how to check them (except ic's), but I don't have a clue where to go from here.  If I fix, bonus for my buddy.  If I don't, bonus for me.  I'll have a complete chassis with a reverb tank and speakers for an amp project.          

[teemuk]

Replace the potentiometer.

[joecool85]

Yup, sounds like you just need a new pot.  Let us know if that fixes everything for you.  I couldn't really hear the hum on the clip, but my laptop has crappy speakers.  If the hum doesn't go away after replacing the pot, you may need to replace the power caps in the amp.  But I would say go with just the pot first as that is 99% of your problem right now.

[brainwreck]

Thanks guys.  I'm making a trip to radio shack today for a few things.  Hopefully I can find a suitable pot there.  I'll let you know how it turns out.

[brainwreck]

Strange.  I pulled the pot and checked it's resistance and it checked out fine.  The low pot was the same value so i swapped the high and low to see if the problem followed.  It didn't and everything is working fine now.  The problem must have been a bad solder joint.  I guess I should have went over that pot's joints with a solder iron before pulling it.  Thanks again.   

[joecool85]

Sort of, just because the value showed fine doesn't mean it was fine.  When moving it if the contacts are dirty/worn it will make noise but may still show the proper value of resistance on a multimeter.

[brainwreck]

I thought it was strange because I connected a meter to the pot and it checked out fine no matter where I rotated it to.  In other words, it had a steady resistance when connected to the meter.  That didn't seem to be the case when the pot was in the amp.  Turning it from just past zero to full resulted in no signal except scratchiness and hum.  Had the pot been bad I'd think that the resistance would have fluctuated, indicating an intermittent connection.  I'm just glad it was an easy fix.  My buddy decided he didn't want the amp and said he would sell it for $25.  It doesn't sound very good to my ears but I might take it and experiment on it.  Watcha think?  It has two channels, probably 30-40 watts into two 8" speakers, spring reverb and a really crappy chorus (both footswitchable), crappy osb cabinet, and no real heatsink for the chips.  The heatsink is a flat piece of aluminum that runs the length of the chassis.  Sound wise it's in the neighborhood of an old Crate (yackkk!).  It might be something good to tinker around with.           

[brainwreck]

I found the same model amp on ebay:  http://cgi.ebay.com/1990s-RMS-500RC-RMS_W0QQitemZ120118330641QQihZ002QQcategoryZ43375QQrdZ1QQssPageNameZWD1VQQcmdZViewItem

The auction says 6" speakers.  They looked like 8" to me.

[joecool85]

Pretty cool looking amp.  Try replacing the crappy stock speakers with decent ones and you may have a kickass little amp.  Quite frequently this is the issue.

[Trepan]

Another trick I've used before is if you have some contact cleaner spray around and the pot is not sealed, you can spray some in and rotate the knob around a bunch - obviously with the power off and the amp unplugged