guitar amplifier going bad ..... why???

[dirtyb15]

Hey guys, dont know where to ask about my guitar amp, but maybe someone here has some ideas? I have a fender 1000 amplifier head. i bought it new about 5 years ago. After about a half hour to an hour, with loud volume the sound will start cutting in and out, which makes it hard to play with the rest of the band I have changed cords, guitars ect,, so i know it has to be in the amp somewhere. I also notice that when it does this, if i tap on the head, that sound also gets amplfied. Very strange. It has a pre-amp tube, but the rest is solid state. Any ideas?

 

[chemelec]

Possibly Your Probably Overheating it.
A Fan Might help.

Or Maybe that tube has gone "Microphonic"!
This happens sometimes. Try Replacing it.

Also could be Cold Solder Joints, just Showing up with years of heating and cooling. Not so easy to find.

 

[spuffock]

you can just pull the tube out, the amp should still work. It isnt used to amplify anything, just as a diode clipper.
If the amp still cuts out, its probably a dry joint. Tap things VERY gently with the handle of a small screwdriver to locate it.

 

[Nigel Goodwin]

you can just pull the tube out, the amp should still work. It isnt used to amplify anything, just as a diode clipper.
If the amp still cuts out, its probably a dry joint. Tap things VERY gently with the handle of a small screwdriver to locate it.

I just checked the circuit! - as you say the valve is only wired as a diode clipper! - what a rip off!.

I've got a PDF of the circuit if it's any help?, the Fender site seems to require a password now?, but I've got them all saved on my hard drive :lol:

 

[dirtyb15]

pdf would be great. Ill take it apart tommorow and pull the tube and if that does nothing ill hopefully find a cold solder joint. Are there certain areas i should check for the cold solder joints first?

 

[Nigel Goodwin]

pdf would be great. Ill take it apart tommorow and pull the tube and if that does nothing ill hopefully find a cold solder joint. Are there certain areas i should check for the cold solder joints first?

I'm not at work now, and the diagram is there, I'll dig it out Monday for you - PM me your email address!.

As for fault finding, you need to find which section the fault lies in for a start - an easy split is power amp/preamp, there's an effect send and return socket, so you can test it to some extent without taking it to pieces.

See if the output from the effect send socket goes off as well.

 

[spuffock]

There is a high voltage op amp in the driver stage. The negative feedback loop comes from the output to the NON INVERTING side of it. There is a capacitor in this loop, I cant remember the value, but I've had dry joints on it twice.

 

[audioguru]

I just checked the circuit! - as you say the valve is only wired as a diode clipper! - what a rip off!.

Hee, hee. :lol: You should be glad that it is hooked-up to something or anything! :lol: The valve is just eye candy, a diode with a series resistor would sound about the same.

 

[chemelec]

I remember back in the old days of transistor radios.

People were under the impression that more transistors in these radios were better.

So the manufacturers started using the transistor, bast to emitter junctions, in place of Diodes and were producing radios with 17 or more transistors. That Fooled a lot of people into believing they were better.

Sort of off the topic, But my Dad was a Watchmaker & Jeweller.
And generally the Public thought that more Jewels in a watch was also better.
For those persons unaware of what jewels are: "Jewels are Bearings".

So Numerous watch makers just put in more jewels. Just Anywhere, Not Doing Anything.

How to Screw things around for the sake of Sales!

 

[audioguru]

Stereos are sold saying their speakers are 3-way. They have a woofer, a midrange-tweeter and a photo-label for a super-tweeter.
My son's 1st stereo had perforated metal hemispheres for tweeters. There was nothing inside.

 

[dirtyb15]

Sorry to bring up the thread again, but had more questions. I am going to tear the amp apart this week too see if i can find any bad joints ect..
I am a bit confused though. If i play on the amp by myself (volume not too loud) it plays fine without any issues. As soon as i crank up the volume it will start cutting out after about 5-10 min. I tried a fan but no difference. I am just curious if it was a cold joint, why does it only show up at higher volumes?

 

[Nigel Goodwin]

Sorry to bring up the thread again, but had more questions. I am going to tear the amp apart this week too see if i can find any bad joints ect..
I am a bit confused though. If i play on the amp by myself (volume not too loud) it plays fine without any issues. As soon as i crank up the volume it will start cutting out after about 5-10 min. I tried a fan but no difference. I am just curious if it was a cold joint, why does it only show up at higher volumes?

It could be that the higher vibration triggers it?, or it could be the thermal expansion?, but it might not be a dry joint at all?.

First thing is to find what section the fault is in! - check the output from the 'effect send socket', this is essentially the output from the preamp. If it's OK from there the fault is most likely in the power amp, if it's faulty from there, the fault is in the preamp.

 

[ljcox]

Once you localise the general area where the fault is, as Nigel suggested, do a careful examination of the soldered joints. Dry joints can often be seen. It may not even have been soldered. Or the solder may have developed a hair line crack.

You can also use another amp as a signal tracer. Connect its input to various points in the circuit and find where the fault is first evident. ie. start at the output of the first amp stage, if no crackle in the test amp when there is a crackle in the amp uner test, move to the output of the next stage.

However, negative feedback may confuse the issue.

The fault could also be a hair line crack in a track of the PCB. Or it could be inside a component.

 

[ljcox]

If you use another amp as a signal tracer as I suggested, connect its input via a capacitor say 100 nF to block DC.

A simple test is - assuming you can make the crackle occur by tapping the amp - is to turn the volume control down to zero. Then tap again. If you can't make it crackle, then the fault is before the volume control.

 

[dirtyb15]

Thanks for the halp and advice guys, ill hopefully find something tommorow when i open her up.

 

[dirtyb15]

well i have yet to pen it up yet, but i was playing last night, and i tried tapping on the head with the volume all the way down and i can still make it pop. Hopefully ill have it apart this weekend and i can get her fixed up.

 

[dirtyb15]

Well i got it apart on my bench right now, and may have already found the problem,,, (I hope). The footswitch jack on the board is totally loose, and all 3 solder joints are broken. I never even realized it, but the nut holding the jack to the face plate was missing, which is probably why the joints broke in the first place. Hopefully this is the problem!!!!

 

[audioguru]

Good, you found it. :lol:
Put a dab of glue on the replacement nut. Don't tell my wifey, I steal her nail polish. Shhhhh.

 

[Nigel Goodwin]

Good, you found it. :lol:
Put a dab of glue on the replacement nut. Don't tell my wifey, I steal her nail polish. Shhhhh.

Yes, nail polish is great for sealing screw threads :lol:

 

[dirtyb15]

never tried nail polish, ill have to try that out. Well, as always, i somehow made things worse. I got everything together last night, and took it home and plugged it in. What do you know, i can tap and the crackle is gone. So i am thinking great, fixed. About 5 min later as i am getting my guitar out of the case the amp just starts humming. I plug the guitar in and no sound can be heard, just a loud hum. NO matter what channel i use or anything. I really dont know what could have happened. I was very careful removing the board and i know i did not have any solder drip. I did notice some discoloration around 2 diodes near the speaker output, but i never smelled any burning. Guess ill check them first, but I wonder what caused this..... Keep you posted