Need help with a strange and extremely loud Hum/Buzz ??

[Missing Link]

Hi folks first a little back ground on this, I believe its a 1982 Sunn Beta Lead Digital C-Mos solid state 100W combo guitar amp.
I bought this amp a weeks ago after giving it a hard test with my guitar for at least 30 minutes without any troubles, and while talking over price with the seller and some of its history/small talk it was left on for at least 45 minutes and still without any troubles. The garage had some florescent lights in the area so it had what I presumed was some interference, hum.
After loading the amp in my truck face up, it was unloaded and set on the flood f my garage without any bumps or bangs.
I set all the knobs on the face of the amp to zero to include the vol knob and plugged the amp in and checked the jumper cable for the head to speakers to make sure it was not damaged in transport. Without an instrument plugged in I went to turn the power on and as soon as I did we got this extremely loud buzz sound like a bad ground but loud. I turned it off as fast as possible to save what I could just in case it was to blow up. So we checked all the wiring from the cab and anything else that was obvious that may be the cause. Turned it on and the same, very loud hum, buzz. I called the gent I just bought it from and explained the situation gong on with the amp. He never had a problem with the loud hum when turned on, but he did offer to take it back if I wanted. While on the phone with him I wanted to let him hear the sound, and yep you got it it was gone, no noise, no loud buzzing, nothing.

Fast forward to today - I had the head out of the cab to clean this 41 year old amp which when is played plays very nice but with that semi loud hiss in the reverb controls when at 9:00 and up. I wanted to check the reverb tank and its wire shielding to see if it was the cause of the extreme loud hum that have not happened since I go it home. The springs still have some tightness and not sloppy so I put it back in cleaned and installed the head back into the cab, plugged it in and yep externally loud hum so I turn it off as fast as possible.
I apologize for the long winded description but when asking folks on what maybe a problem I found it best to describe it as best as possible, sorry.
Anyway I love the amp and would like to keep it, but I unfortunately do not have a amp tec or service in my area (40 miles away) so I wanted to ask if from what you folks have read what would you suggest to check or take a closer look at.
I keep thinking that when I transported the amp on its back maybe the reverb tanks was bumped or messed up someway, and also when I removed it today to clean/check on it, it did the same thing as before. I am baffled that it seems when maybe 30 minuets goes by it is fine as it is now, no loud buzz or hum, sounds fine, plays fine. I also left the power on for two hours to see it its a heat related problem but it turns off and back on without a problem and no heat felt anywere on the head??
Would a Reverb spring tank cause this??? Any constructive information and questions are more than welcome.

Thanks

 

[Peegoo]

@Missing Link

The first thing to suspect with this sort of noise is a bad reverb cable or bad reverb pan.

Next time the amp does this, pull the reverb send and return cables from the jacks on the chassis and see if the noise goes away.

If it remains, then it's something else...perhaps a bad switch on an input jack or some other fault prior to the preamp stage.

 

[Missing Link]

Peegoo - thanks for the reply. I took a deep look at the reverb RCA plug ends and I am not impressed in the least with what I found.
Crappy workmanship if someone tried to repair the plugs. Second picture - looks okay but the barrel around the center pin is loose and swivels about a 1/8" turn or a bit more. The first picture with a closer look under a magnifier, well that is just crap, I wear glasses and did not even see that. That's just sad. I will have to look harder to see what else is not don right.

So you think this would cause some kind of a reaction in the spring tank? I say this as a little while ago I had the amp on and bumped the top of the amp with my hand and all hell start up again, got it in time to shut off. So now I know if I bump the top of the head it sets it off.
I will now have to see about finding a plug kit for the spring tank, maybe with shielded wire. I was thinking maybe just replacing the tank anyway as when played the reverb really is not that good, kind of weak not full.
You mentioned bad reverb pan, what does that in detail, or what should I look for, bent cover ??
Well I will get to work on the cable fix, good call, thank you.

 

[jtcnj]

Does this happen with a guitar plugged in?
Maybe the input jack shorting bar is not making good contact?

 

[Missing Link]

No guitar was plugged in on any of the cases. Strange that if the reverb cable ends are as bad a they seem that when the amp is in a static mode and turned on that the loud noise is not constant, seems like it needs a bump to the head to set things off then after a bit of time with power off and sitting the loud noise will be gone next time its turned on. Not sure if cable would cause this. I wonder if the spring tank is with a problem internally other than the crappy RCA plugs.
I have looked at all the input jacks and wiring now with a flashlight and magnifier with no external negative finding.
Found the wiring diagram schematic on line but it is not as informational as I would like. I am curious if the input and output cables from the PC board are correct.

Cheers

 

[Missing Link]

Hi guys, well did a bit more checking on the PC board and came across what looks to be a heat-sink problem. I am not to sure what the white item is and what it does but it looks pretty bad along with the one cap next to it. Looks as I will have to maybe take a 50 mile one way trip to find a qualified amp tech. Would anyone know what I would refer to this item as so I could at least sound like I know something when talking with a tech -.
I also had a RCA cable that I cut the ends off one side to maybe use as a replacement to the crappy cables for the reverb tank. Maybe splice into the wire prior to the PC board. I'm really good at soldering so this would not take much to do unless someone thinks that the cables need to be a different type.
Again any help and guidance is welcome, thanks.

 

[Peegoo]

Peegoo - thanks for the reply. I took a deep look at the reverb RCA plug ends and I am not impressed in the least with what I found.
Crappy workmanship if someone tried to repair the plugs. Second picture - looks okay but the barrel around the center pin is loose and swivels about a 1/8" turn or a bit more. The first picture with a closer look under a magnifier, well that is just crap, I wear glasses and did not even see that. That's just sad. I will have to look harder to see what else is not don right.

So you think this would cause some kind of a reaction in the spring tank? I say this as a little while ago I had the amp on and bumped the top of the amp with my hand and all hell start up again, got it in time to shut off. So now I know if I bump the top of the head it sets it off.
I will now have to see about finding a plug kit for the spring tank, maybe with shielded wire. I was thinking maybe just replacing the tank anyway as when played the reverb really is not that good, kind of weak not full.
You mentioned bad reverb pan, what does that in detail, or what should I look for, bent cover ??
Well I will get to work on the cable fix, good call, thank you.

The appearance of those RCA solder connections is actually pretty common. Rather that start shooting from the hip at possible causes of the problem, it's better to isolate the problem and then see what needs to be done. If the amp does not make the noise with the cables removed, then we know the issue is the reverb cables or the pan itself.

Replicate the noise and pull the cables to see if that makes the noise go away.

 

[Peegoo]

That big white item is a 120 Ohm power resistor, probably in the 10-watt range. These can sometimes look cooked like that (they do get got in use) and still work fine. But it would be a good thing to lift one end and test it for spec.

If you are not familiar with soldering on PC boards, do not attempt to repair this because you can easily do serious damage to the board or components.

 

[Missing Link]

120 Ohm power resistor, okay now when I call I can give a few bits of info. To me it looks to be a problem, maybe a tech can add something (shielding paste, ?) to help keep down the heat around the area with newer tech - knowledge and or modern parts.
Not a problem, I have worked on PC hoards before but not a tech, just minor repairs and such. I stay out of areas I am not familiar with such a this amps situation up to a point. If I can fix a few basics and do a few tests, no problem but I am not schooled in amp repair so hence and few qestion for guidance.
I will remove the RCA cables as suggested from the reverb tank and fire it up and give it a bump on the head to see if I reproduce that loud sound. I will post the outcome.
Thank you for you input.

 

[Peegoo]

@Missing Link

See if you can find a schematic for that. That way the tech won't have to guess on component specs.

 

[Missing Link]

So far all I can find is a bad reproduction of a block diagram or the Sunn Lead. No amp tech to be found yet.

Thanks

 

[trev333]

Give the RCA plugs a good cleaning/de rusting.... for a start..

 

[Missing Link]

Yes they are a bit tarnished from age. Would the RCA plugs be reusable (re soldered) ?

Thanks for the information.

 

[Missing Link]

Hi guys just an update on this problem. I did try the bump test without the reverb hook up and still got a real bad loud sound. So we now know its not the reverb but I would want to replace it for one that has a bit more reverb in it as this is not that reverb-y sounding. The good is I found a gent about 15 miles from me that does work on electronics and has worked with guitar amps before. So stopped in this morning with he head and he tested the head without the reverb tank and most defiantly has a problem in the board near the I think they are called power resistor the big 3300MFD blue things in the picture. He did a lot of looking and tapping but cannot find the problem. He tested a lot of the resistors and any thing in the troubled area with no bad parts and checked the board cracks. He did say that the Power resistors (blue caps) are original and could be failing on the inside without any outside changes as in leaky or budging. That is the only thing he suspects so he is ordering two new ones to replace the old one just for the being old. Regardless he says that if left in this condition it will blow the amp if not fixed. So I am hoping that the two power caps will be the problem.

Will advise.
Thanks

 

[Michael Smith]

Is he going to replace that big ceramic 120Ohm resistor that has burned the board? If so, the new one can be elevated a bit from the surface of the board.

The big blue reservoir capacitors, if original, should be replaced, but when they fail, they don't exhibit a noise that comes and goes. If hitting the cabinet causes the problem to come back, I would suspect a bad solder joint somewhere. Transporting it to your house probably loosened up a solder joint that was cracked, so now it makes intermittent contact with a component. It could be that big 120 ohm resistor.

 

[Missing Link]

We did talk about that 1200hm resistor and he stated that it actuly would not cause any problem as in odd sounds as it is kind of a separate part of the power to the amp. He said that it not uncommon to see the cracking and heated area around what he call a concrete resistor. They have a newer type now than what they had in the 80's.
He is going to see about finding a replacement for that as I personally don't like the looks of it regardless if its still works. I was more worried about the small resistor next to it. But he checked both and they are within specs and working.
Seems to know his electronics when it comes to amps, he had a few amps in his garage, one being an older Fender twin reverb that he completely rebuilt as the PO fried it somehow, I have my eye on that.
I do try to think of the way when in transporting the amp how it could of caused damage as it was not that far in travel and was never dropped or bounced in the back of the PU.
He put the PC board back after his checking the back side. He said he flexed the board while viewing all the areas with a set of mag glasses he was wearing and found no brakes in the solders or the board or the board trace.
Last words were I am stumped and will stop for now be resume looking at till Monday as he is heading out of town for the holiday. Older retired Navy electronics engineer.
So I do hope he is on some kind of track.
Your information will be in consideration and I will ask him to check again on that.

Thanks much