fender stage 112 "pop,crackle"

[raypist]

i have a fender stage 112 se. i went to turn it on yesterday and got nothing. took it home opened it up and the fuse was blown. changed the fuse, now it turns on but hardly amplifies the signal and gives me loud pops and crackles through the speaker AND the line out (so its not the speaker wires). both inputs are giving me the trouble.
the last time i used it i was feeling a little shock from the guitar strings, nothing major (i thought).
any ideas?

[Enzo]

Does it sit there popping and crackling? or does it only pop and crackle as it tries to pass the signal? Of the former, you have a noisy component most likely an op amp. If the thing does it with signal, I'd think you either have a failing or gone low voltage rail - +/-15VDC or an op amp failing with DC on its output pin.

Bad op amps are found by going chip to chip with a voltmeter. If the rails are bad, look to teh zeners and their dropping resistors.

[raypist]

it cracks and pops on its own, regardless of anything plugged into the input. what am i checking on the opamps? i checked resistance on each one and found they all give me a value.

[Anthony]

112 pop crackle

Hey Mike. I've noticed several problems occur when these amps exhibit the problems you are discribing. The first is that the PC mounted 1/4 jacks often time crack the solder joint underneath the board. It's hard to see from the naked eye...you'd need to remove the board and inspect it under a bench lamp or magnifier. If you have the board out, check all the solder joints for the amplifier transistors.
Next, there should be some large white ceramic resistors associated with the power section. I once repaired a stage 112 that had an intermittent pop and crackle due to a faulty ceramic wire-wound resistor.

Anthony

[raypist]

enzo- how does an opamp go bad? would this be related to the fuse frying?
"low voltage rail" is this an opamp term or something different?
how do you check an opamp with a voltmeter? from the research (google) ive done it seems like you need to build some kind of contraption and take the opamp out of the circuit board.

anthony- solder joints appear okay.

would using an un-grounded surge protector (the ground pin was ripped off) combined with a fishy power source lead to these problems im having? i dont want to burn out another amp at this practice space i rent.

[Enzo]

No, the lack of ground on the AC power cord is not going to hurt the amp. Might injure YOU, but not likely the amp.

In a circuit, there are power supplies to provide the operating voltages. In your amp there are positive and negative voltages for the power amp transistors, something like 35-50VDC. We refer to them as "rails" sometimes. You could call them busses I suppose, or just supplies. It is NOT how they are laid out, but imagine if we made the two power supplies, then ran a heavy piece of bare wire along the two sides of the circuit, one for each, and various components in the circuit could connect to them at convenient spots along their length. The two would sorta resemble the rails or a train track. Nothing cosmic.

The preamp circuits operate on a lower voltage, again positive and negative, but only 15VDC each. I refer to these as the low voltage rails or supplies, and the ones for the power amp as the high voltage rails.

How does an op amp go bad? How does anything go bad? The tiny integrated circuit chips have complete circuits inside them, as opposed to just one part. The circuits inside are made up of many microscopic transistors and other parts. Any one of those micro-parts fails, and the IC no longer works right.

The data sheet circuits are for charactoerizing the performance of the parts. Engineers do this to see if the parts will be suited to their applications. For us, we need to verify that the [power rails are getting to the IC, and then look at the output pins for voltages that don't belong there. The little 8-leg ICs all have their outputs on pins 1 and 7. We don;t want to see DC voltages there. Power we do want to see. Pin 4 has the -15 and pin 8 the +15. If one is missing the IC won;t work.

SO I was suggesting that you check for +15v and -15v getting to the IC chips, then look for unwanted DC voltage on the output pins of each.

[raypist]

so i have to plug this amp in, turn it on and poke around with my volt meter? any safety precautions i should take?

enzo, that explanation was great. thank you for taking the time. ill let you know how it turns out. if i dont write back, my landlord should be informed of a rotting body in apartment 5.

[mikeaaron_1999]

Ok, so a buddy of mine asks if I can take a look at his amp because it's making a buzzing noise. I bring it in, plug in my guitar, it sounds ok. I crank up the volume to about 3 and it sounds good but low and high notes give a crackle and pop when played. After about five minutes of playing with tones and volume and striking notes on the guitar, it starts to make a rythemic pop, pop, pop, and it didn't change when I disconnected the guitar or turned all pots to "0". So I pulled it appart to take a look. Someone has been in here before. The first obvious things I found were CR28 had been replaced at one time and also Q2. However, when they were replaced, the master solderer used too much heat, burned the board and thus, removed the pads from the solder runs. So he/she brilliantly soldered a "jumper" between one leg on the top of the board @ CR28 to one leg on the bottom of the board @ Q2. I was glad to see the jumper was at least insulated. Because this was something goofy, I checked CR28, found it was "OL" one direction with continuity the other as a diode should be. No problem. So I cleaned up the solder on both sides of the board associated with both components, re routed the "jumper" to be completly on the bottom of the board, put it all back together again and turned it on. Instantly, I got that pop, pop, pop, pop. Nothing plugged into it, all controls at "0". I manipulated the controls, plugged an instrument into it, and nothing changed. The pop, pop, pop continued. I unplugged the instrument, turned it off, pulled it all appart again and just the control portion sitting on the bench plugged it back in, turned it on. The pop, pop, pop, was still there, so I went for my DVM but before I could probe any test points the fuse blew and the amp went dead. Sooooo, now what???? Any help would be greatly appreciated!

[52 Bill]

Please start a new thread for your amp, and tell us what amp you are working on. Is it a Fender Stage 112?