Ampeg Noise

[Valvehead]

pm me with your email addresses if you want to view the scem-- thanks ! ( and please ! )

[Valvehead]

hmmm...having troubles ....lets try this ...



Micro_CL_2037630-XX_Rev-100.pdf

[DrGonz78]

First thing first... Q2 and Q6 should be referred to as Q109 and Q110, respectively to the schematic, as they are jumpered off from the board. Also, R121 and R122 are not 5W 1k cement resistors and are really 0.22ohm 5W cement resistors. Are those the ones that are smoking? I don't think they are the ones you are referencing, but you had said 5 watt so that made me look at those on the picture originally.

Second thing... The VCC and VEE are the B+ & B- voltages where you should find +/-50v. The A+ & A- voltages should be the where you find +/-15v rails. Those 1K 3W resistors are R21 and R23, so are those the ones smoking? I am betting on these being the resistors you were referencing.

We need to know by component names what we are referencing now and this will get the ball rolling a lot faster. Also, you have not told us if any DC voltage is present on the output of the amp. Once we find out the answers to these questions then we will understand where to search next. I am wondering about that component U1 4558 chip for starters since it was missing -15 volts on pin 4. Or rather let us know for certain that is true ok? Good luck.

Edit: You had said R23 earlier on post #7.

Look like R23 and the res right next to it R? (cant see #) are getting very hot. they are 3 or 5watt , 1K , wirewound

But those are not wire wound resistors as far as I can tell. I think they are just flame proof Carbon Comps. That's where I got the notion to look at the picture and identify the cement wire wounds.

So definitely pull out U1 from the board and fire it up. Do the resistors still get hot? Check the board voltage to see if pin4 and pin8 are getting correct voltages +/-15vdc without the 4558 in place.

Double Edit: Well those must be wire wounds but non-inductive (or very close to non-inductive). In this case those to me would seem to not generate a lot of heat by the nature that they are non-inductive. Please anyone tell me if I am even close on that one... lol.

[Enzo]

R121 is a 5w 0.22 ohm, but there is a pair of 1k 3w resistors, R21 and R23 that drop the +/-50 to the 15v zeners. They will get hot normally. They look like metal film, and no, not wirewound. I don't see where he referenced R121.

Even if they were wirewounds, the inductance wouldn;t matter. After all, they are series elements in a power supply filter, exactly where you would put a choke. But the inductance of a wirewound resistor is teeny, not enough to bother a guitar amp. We use them in the screens of power tubes all over the place.

They get hot because they have 35v dropped across them. Ohm's Law tells us how much current must flow. inductive or non-inductive resitance winding won;t change the resistance.

I'm sticking with film resistors though.

[Valvehead]

ok guys-- yes , the 3w resistors are the ones getting hot -- r21 / r23 , NOT the cements

there is no DC at the output.

i pulled U1 from board and the resistors still get hot ---i checked the holes where U1 was , i still have the 15v at number 8 , and NOW, i have -15v at hole 4 !

[52 Bill]

i pulled U1 from board and the resistors still get hot ---i checked the holes where U1 was , i still have the 15v at number 8 , and NOW, i have -15v at hole 4 !

And now do the resistors still start to smoke after acouple of minutes?

[Dr Kaos]

My first thought was main filter caps. Check for dry joints on the two large capacitors, they often break away because of their weight and size that is why there is glue on them.

Dr Kaos

[Valvehead]

And now do the resistors still start to smoke after acouple of minutes?

they start smoking after 30 seconds !

[Enzo]

Power down and discharge. Unplug from the wall. Get some isopropyl alcohol and Qtips and clean the things. See if they still smoke after any extraneous goo is removed.

The 50v seems right, and the 15v is set by zener, so each resistor has 35v across it. For a 1k resistor I get about 1.22 watt dissipation. That is enough to get pretty hot, but well within the 3w rating of the resistors.

[Valvehead]

while its apart Enzo-- i have some 5w , 1K cements sitting here-ill just throw those in--what do you think about the -15v coming back with the 4558 removed ? would that the op amp itself ? I do have a few 4558 sitting here to throw in

[Enzo]

Oh wait. I wasn't thinking the -15 was missing, I was thinking it didn;t get measured right before. If the IC kills the -15, then the IC is bad or there is a bad resistor in series with the leg.

[Valvehead]

Well boys........ so far so good..

i replaced the 4558 , and the 2 , 1K with some 5 watts..

no smoke...the resistors get warm--but not hot.. and so far the buzz is gone .

I thank you all for your time and brains

i need to fire it up a few more times to be sure , but I think she is good to go


edit--- now i have +15 AND -15v at the 4558