Classic 50 4x10 / Crackle Noise /Update: Solved

[plexi50]

I picked up another Classic 50 but the 4x10 version this time. I have a crackle that grows,then dissapears and then comes back intermitantly

I have ruled out all tubes as being the problem
If i turn the master volume up with the clean and dirty channels controls muted i can still hear it come and go even with no input jack being used

I am thinking plate resistor some where in the power section

I have to look at the schematic to see how the master control is wired and if it is the power section i need to concentrate on

I just pulled V1 & V2 and the noise is definatley coming from the power coupling section

If i pull the PI tube all is silent

[DummyLoad]

plate resistor or bad solder joint or socket connection or molexia.

--DL

[plexi50]

Changed PI plate resistors and it is a whole lot quieter but still just a tad of it is there. Barely noticable now. I was going to change out the coupling caps but i didnt. I dont they are the problem.  Going back in/

[Dave]

Two different times I have had a problem like that and was sure it was the plate resistors, and after changing them to no avail and scratching my head a while, it turned out to be the cathode resistors.

Dave

[plexi50]

Um Dave! Im running into the same situation here. Pulled and went over both the preamp and power boards. Dont really feel like changing every resistor to find it. I though i might use my DVM and test each plate resistor under load and see if there would be any voltage fluxuations while the crackle appeared. No luck diagnosing anything as far as that goes. Resoldered both boards real nice

But 85% of that crackle,occasional popping has dissapeared since i changed out the PI resistors
It's just at a lower level now but still there. 9 times out of 10 it useually is a tube or bad socket connection
Not the case this time. Starting to bug me. Is there a chance that even with no hum that a power supply cap could be going bad and make this noise? I can imagine it is possable but ive never seen that situation without a noticable hum

I have yet to replace the PS node resistors 22k and 10k.  My other Classic 50 head is stone quiet
It is absolutley,positivley not the tubes or there pin connections in the sockets

I will check the cathode resistors tommorro and any bypass caps that they may have BUT!
I can pull V1 & V2 leaving the PI tube in place and still hear that crackle
Please dont make me pull the power board again and replace the bias and grid resistors

Hey what about those (2) 100r screen resistors? Does a ceramic resistor exhibit the same characteristics of crackle and pop when going bad? They both read out at 100r

With what knowledge i have i have to believe it is in the power section of the amp

[plexi50]

Screened all the EL84 tubes with 470r while i had the power board out again
Crackle still present. I feel safer having screens on all 4 power tubes
Why did Peavey only screen (2) of the EL84 tubes anyway?

[plexi50]

Ok i need to re think where my crackle problem is coming from.
I have eliminated the preamp board as a possable source by pulling V1 & V2. Preamp board has been resoldered
PI tube circuit and all parts are good and i see no problem with it

Power board has been resoldered and all resistors replaced in grid and 220k coupling
All that leaves me is the EQ board. Thats where the master comes in here and leads me to believe my problem is either the treble peak cap,slope resistor or FX Loop. If i patch the FX send and return the crackle gets louder and worse

The master runs straight to a .1 cap and into the FX Loop joining a 470KR pair being fed -14V and into a MPSA63 transistor which i have already replaced

My thought is the .1 cap coming off the master is bad or the 470K pair R69 or R73.  -14V feed is getting back into the master pot at will (whenever it feels like showing itself)

I would really like to find this problem seeing as many other Peavey Classic amps are having the same problem with crackle
Can anyone explain or confirm my theory?
Last thing: Cant i just bypass the FX Loop and run the signal from the master to V3

[punkykatt]

Plexi50, Did you ever build yourself Doug`s listening device?  You will probably find that crackle in a matter of minutes.  Punky

[plexi50]

Plexi50, Did you ever build yourself Doug`s listening device?  You will probably find that crackle in a matter of minutes.  Punky

No i didnt. I quess it's time to do just that. Do you have that info handy?
I have a Fender amp chassis mounted under my work bench to work on pedals so all i need to do is make the probe and capacitor arraingement

[Willabe]

Here it is Plexi;


http://www.el34world.com/Hoffman/tools.htm

[plexi50]

Thanks Willabe. Now i need someone to get me off of Craigslist

[dpm309]

Plexi, here is a picture of the Amp Stethoscope I built for about $12 in parts (not pretty but it works!).  Just follow Doug's schematic.  You plug a 1/4" instrument cord from the device to your listening amp and attach the ground (alligator clip) to the amp you are testing.  You will also need a dummy load for the amp you are testing.  These work great as I just used it on the Lancer 35RVT I am working on.

Dan

[plexi50]

Thanks dpm309 for the info. Im getting the test gear i need slowly but surely. Im sure once i get the hang of using it i will kick myself for not building one sooner

[plexi50]

I solved my Classic 50 crackling problem
The FX Loop is the problem. I dont use an FX Loop so i removed the harness from the FX board and just cut the cable to length and connected one wire from the master pin on the EQ board and over 2" to the Preamp board plug on pin

Dead quiet and i believe a tad louder as well. You still have footswitch capability

If i were to quess what the real problem with the FX Loop was, it would be the el cheapo plastic tin foil metal tip and ground construction of the return and send jacks themselves