Troubleshooting

[vahmer]

hi i'm a first timer working on my peavey mark III.  i've used the amp (for guitar, though it is a bass amp) consistently for 3 years without a problem.  last week at practice i hit a chord and loud, awful hum came out.  the heat sink was red hot.  the hum persisted with or without a guitar plugged in until i shut the amp off.  today i tried it again and the amp functioned for about a half hour and then the same hum.  i opened her up and found one burned looking resistor.  if i'm reading right it's 4.7 ohms.  could this be all or part of the problem?     thanks.

[benzer]

a 4.7 ohms? that should be some kind of output stage resistor
, maybe when you hit the chord hard n load too much current passed by this little resistor and burnt it,,, change the resistor,,you should if you are sure its burnt, and yea it can be only it ,, as long as its an output stage resistor it would have a big effect
although im not quite sure i think if you can provide a schematic we can help more

[LJ King]

I don't have a Mark III schem, but if it's similar to a Musician 400...

I only see one 4.7 ohm resistor - part of the preamp voltage regulator, which it's in series with the regulator output. The regulator is powered directly from the positive rail that goes to the power amp.

So this is the thought:

If the resistor is getting burnt it is drawing too much current. A primary cause of that may be a short after the resistor, and the most likely suspect to short is a 50 micro/50 volt electrolytic between the resistor and ground.

If the electrolytic shorts, the resistor would draw too much current and an excessive current draw would load down the positive rail. This could cause the hum since the power rails to the power amp would now be unbalanced.

Look for an electrolytic cap in the area around the resistor - there should be two of the same value.

Since the amp works for awhile before developing the symptom, it sounds thermal related - and I have seen electrolytics develop an internal short when they become heated sufficiently and appear to be functioning correctly after they cool down.

[vahmer]

so i replaced the 4.7 ohm res. (had to find one) and that wasn't all of it.  the hum is there right when the amp powers up.  how would i be able to tell if the electrolytic capacitor was shorted- just replace it?  also i got the schematic from peavey.  it's too big to post.

[Andy]

Did you manage to sort this out?
If so, what did you do?

I ask as I'm having the same problem with a '79 Beta Lead Combo. I just got given it and it's an absolute beauty - cleaned up the pots and it sounds great until..... the huge hum that will only stop when you turn the power off.

Andy