Ampeg Noise

[Valvehead]

I have an Ampeg Micro-CL head and its got a "buzz" . This can be affected by placing my fingers over certain parts---like the fx loop pcb and 1 of the caps. I did change out this cap and it didnt help. If i grab the fx loop cable , nothing changes.. There is nothing on the fx pcb except 3 jacks . Ive checked all the solder joints in the amp .

it does this at idle even with the volume on 0 and no input---but gets way louder as i mover my hand around the parts mentioned

Take a look at this video-- it will show what Im talking about

ampeg video by solrizzo | Photobucket

[olddawg]

If you a cable from the effects out to effects in does it go away? If you run a clip lead from the shield on the cable to the chassis does it go away? Is it a parallel effects loop with bad jacks?

[Valvehead]

I fogot to mention-- it does this intermittently . I just fired up the amp and its quiet now......, Next time it makes the buzz , i will try the stuff you mentioned. dont know if its a series loop or not , I dont have the scem

[Valvehead]

ok-- touching the cable shield to ground does nothing. putting a cable in the loop does nothing. also grounding the jack grounds = nothing

if i ground the hots of the loop jacks or the hots of the cable going to that board , the buzz gets louder

[olddawg]

ok-- touching the cable shield to ground does nothing. putting a cable in the loop does nothing. also grounding the jack grounds = nothing

if i ground the hots of the loop jacks or the hots of the cable going to that board , the buzz gets louder

Did you run a short cable from the in to the out?

[Valvehead]

yes--no change

[Valvehead]

UPDATE---

the amp has developed a problem...i noticed smoke rising out of it tonight.
Look like R23 and the res right next to it R? (cant see #) are getting very hot. they are 3 or 5watt , 1K , wirewound

does anyone have the scem for this amp ?

What would cause those 2 to start burning up ?

[Valvehead]

Guys--please take a look at this --

I cant find a scem so I took some measurements hoping this might help .

At the 2 , 5w/ 1K resistors that are getting hot , i have 50vdc / 15vdc on one and -50/-15 on other.


There are 2 output chips with one tiny one in the middle --on the big heat sink .

On Q2 I get .6v / .4v / 60mv on the legs ,

On Q6 I get -300mv / -50v / 57mv at the legs .

on the tiny chip in the middle Q104 , i get -0.9v / 1.2v / -280mv


There are 2 large electrolytic caps at the end of the board , on those i get 0v/ -50mv and on the other 0v / 50mv



Does any of this help ? the 2 output chip readings should be close right ? they are way off from each other .

[DrGonz78]

Are any of those transistors heating up??? It looks like the positive rail on the Q2 is nearly shorted to ground or something here...? Is there DC on the output?

Really hard to help here without something picture wise or the schematic to understand what parts we are referencing. Q2 and Q6 are what type of transistors..? Q104 is a single driver or something here? Are those large electrolytic caps the main filter caps? What rating are these caps? Only having -50/+50 mv on those caps seems strange in my mind. Or maybe I am reading that part wrong, but still confusing.

The two wire wound resistors(these might be hot normally? not sure...) appear to have the main filter caps feeding them +/- 50v power and they drop to the 15v rails. So are the the two large electrolytic caps on the "far side of the board" the main filter caps?? Makes no sense here to me if the wire wounds don't follow after the main filter caps and you see that -/+ 50v. Later we have lost the +50v on one of the output transistors. No fuses are popping so this is really strange huh? Especially when these wire wounds are heating up really hot... Something is pulling it to ground almost... Something is failing but not quite popped major components yet.

Call Loud and get the schematic.

Edit: So those large caps(again good to know ratings on these caps) at the end of the board might be there to help support the 15v rails??? Look for shorted opamp chips and bad zeners that might be shorted. I don't think the main output transistors are blown yet. I think they are being dragged down somewhere on the 15v rails... If there are opamps tells us voltages there too.

[Valvehead]

the 2 large caps are right next to the rectifier-they are 3300u / 63v

the op amp is a 4558 and very low mv at the legs and pin 8 has 15v

the output transistors do not get hot

the 2 zeners test ok

here are some pics of the amp -


(who is Loud" ? )

http://img.photobucket.com/albums/v2...rizzo/amp3.jpg


http://img.photobucket.com/albums/v2...rizzo/amp1.jpg

[52 Bill]

The two resistors that look like they have gotten hot are part of the circuit that reduces the 50 volt B+ supply down to plus and minus 15 volts for the preamp. The get hot because they are dropping 35 volts across them. This is normal.

There are 2 output chips with one tiny one in the middle --on the big heat sink .

On Q2 I get .6v / .4v / 60mv on the legs ,

On Q6 I get -300mv / -50v / 57mv at the legs .

Until I see a schematic I'm not certain, but on Q2 I would expect to see +50 on one of the legs.

the op amp is a 4558 and very low mv at the legs and pin 8 has 15v

Again, I would expect to see -15 volts on pin 4.

(who is Loud" ? )

"Loud" is the owner of Ampeg, from whom you should get a schematic.

Originally you were trying to find a reason for a hum, now what is wrong with the amp, does it pass signal at all?

[Valvehead]

the resistors have smoke coming off them....

the amp still passes signal with normal volume, but there is a loud buzz --like a ground noise. But sometimes the amp is dead quiet

[52 Bill]

the resistors have smoke coming off them....

the amp still passes signal with normal volume, but there is a loud buzz --like a ground noise. But sometimes the amp is dead quiet

If they are smoking, re-read the voltages on both sides of them. There still should be +/- 50 volts on one side and +/- 15 volts on the other side. If either side is missing (reading low or 0 volts), then check the circuit for shorted diodes, ICs or caps.

If you hit the amp can you make the buzzing start and or stop?

Are any of the ICs getting warm or hot?

[Valvehead]

nothing else seems to get hot--those resistors start smoking after 2 minutes. yes there is +50/15 and -50/15 on them

i should have the scem tomorrow from Ampeg

[52 Bill]

nothing else seems to get hot--those resistors start smoking after 2 minutes. yes there is +50/15 and -50/15 on them

i should have the scem tomorrow from Ampeg

If the voltages across the resistors remain stable even when they start smoking, then they may be failing and need replacement. Or something has dripped on them and is burning off with the heat. If the voltages start to drop when they start to smoke, then there is something in the preamp that is causing them to overheat.