Session 400

Jerry Roller · Post by **Jerry Roller** » 7 Dec 2005 6:55 pm

I need advise from you electronics techs. My '81 Session 400 sound fine when I first strike a string but as the note fades it turns into a gravelly scratching hum thing. I can hit the 10th string pretty loud and get a clean note without distortion but just before the note fades away the note fades out and is replaced with this noise which actually slightly increases in volume. Any idea what to check? It will do it with a cord directly from the guitar to the amp input. It almost sounds like some sort of feedback starting up.
Jerry

Bob Knight · Post by **Bob Knight** » 7 Dec 2005 6:57 pm

SPEAKER!!

David Higginbotham · Post by **David Higginbotham** » 7 Dec 2005 7:44 pm

What Bob Said! Check closely for any rubbing around the voice coil or slight tears in the cone.

I lucked out one time with a Session 400 Limited and found a metal washer between the magnet and cone. Removed it and the noise was gone!
Dave

Jerry Roller · Post by **Jerry Roller** » 7 Dec 2005 7:45 pm

Bob, I failed to mention I unplugged the speaker and plugged in a speaker cabinet also with Black Widow in it and no change. I think I ruled out the speaker. Speaking of Black Widow, I pulled the chassis out a few minutes ago and there was a REAL Black Widow living in the chassis. It occured to me that guys who work on amps best be on lookout for spiders.
Jerry[This message was edited by Jerry Roller on 07 December 2005 at 07:51 PM.]

Peter Hart · Post by **Peter Hart** » 7 Dec 2005 8:14 pm

I worked on an older Session 400 recently and it had a hum problem along with similar type "just not sounding right" complaint. I found the filter capacitors (6800 uF @ 63 VDC) needed to be replaced which I did. But, the amp still hummed although not as much AND it would kinda come and go. What I found was three ground loops.
The older Session 400 amps (maybe all of them, I don't know) secure their pre-amp and power amp printed circuit boards to the chassis by way of these little "L" brackets. Much to my amazement, they are rivited to the chassis. AS time goes by, the electrical ground between the bracket and the chassis becomes compromised (to put it mildly). You can check to see if this may be contributing to your problem by removing the chassis and placing it on a bench. Plug in a speaker and crank up the controls to see how much hum you are dealing with. The hum should be little or none with a properly grounded amp. If you do hear hum, try carefully wedging a small flat bladed screwdriver blade between the bottom of one of the brackets and the chassis. If the hum level doesn't change go to the next one. Check them all in this way. If you find one with a bad ground, drill out the rivit from the top side, remove the bracket in order to remove the remaining pieces of the rivit from it. I had to put the bracket in a vice and finish drilling the rivit out there. Then replace with appropriate hardware including star washers for a good bite into the chassis. A little time consuming but well worth the time. I hope this helps.

Pete

jim milewski · Post by **jim milewski** » 7 Dec 2005 8:31 pm

the amp has an input to the power amp I think, maybe plug a pre amped signal into that and see if you have the same problem, at least it will determine if it's in the pre or power amp, it sounds like some poorly filtered DC

Jerry Roller · Post by **Jerry Roller** » 7 Dec 2005 10:09 pm

I could not think of a good word to describe the problem but I will try "flutter". As the note, more so a bass note starts to fade out it changes to a fluttering sound. I have the chassis on the bench, have probed all around with a small dowel wrapped with black tape and find nothing loose. With the knobs cranked up there is a pretty bad frying hum and when I tapped around I found that the whole chassis is microphonic. Is that normal? There is a row of 6 small resistors lined up directly behind the volume pot and these are all slightly microphonic when tapped. Right behind that row is another resistor which shows no color bands in the middle, just on the ends and that one is VERY sensitive, barely tap it and it is quite loud, but it will not move and the hiss does not change when it is tapped or push against. It looks like I need to send it to Mike Brown and partake of some of that great customer support I am always talking about. Any more ideas before I box it up?
Thanks for all the suggestions.
Jerry

Ken Fox · Post by **Ken Fox** » 8 Dec 2005 8:34 am

Filter caps have caused this problem before for me and it was cured with fresh ones! On a scope, a filtered DC pattern is a sawtooth. When the filter(s) start actng as you describe, there is a large and very narrow "church steeple" on the leading edge of that sawtooth! I have seen that steeple pulse over twice as high as the sawtooth wave from before! That pulse amplifies as a buzzing tone.

[This message was edited by Ken Fox on 08 December 2005 at 08:35 AM.]

Bill Crook · Post by **Bill Crook** » 9 Dec 2005 1:12 pm

A lot of amps use the metal chassis as a part of the "ground" circuit. Check and make sure that all screws (that secure the PCB to the chassis) and jacks are tighten good.

I have found several units really "cleaned up" good after a maintance check and cleaning.

Remember, We throw the amps around quite a lot in a year or so of gigging. They are like our cars, Ye got to pull a bit of checkin' on em'.

------------------
http://home.comcast.net/~crookwf/
http://photobucket.com/albums/v479/billcrook/

[This message was edited by Bill Crook on 09 December 2005 at 01:12 PM.]

Ken Fox · Post by **Ken Fox** » 9 Dec 2005 2:04 pm

Bill make a great point there! The power board has aluminum legs that are riveted to the chassis for the ground and board support! I have seen these loose many a time on these amps. Best to drill the rivets out and replace with 6/32" screws and nuts (maybe even 4/40" )

Jack Stoner · Post by **Jack Stoner** » 9 Dec 2005 3:28 pm

Screws, nuts and "Lock Washers".

jim flynn · Post by **jim flynn** » 10 Dec 2005 4:39 pm

Jerry, I had the same problem with one of mine and had a local check it out, he found so solder joints broken, said this was common to this model, you might want to have it checked, I believe he said it was at the pots on the front. Good Luck Jim

Brad Sarno · Post by **Brad Sarno** » 12 Dec 2005 7:28 am

Ken, you offered that tip to me a couple of years ago and it was great advice. Those little "Z" brackets that ground to the chassis. Jack, I like your suggestion. Maybe ditch the rivets and put some little screws with good lock washers on there to bite thru and stay put.

Brad

Mike Brown · Post by **Mike Brown** » 12 Dec 2005 7:54 am

Thanks for the great advice guys. You gave him several good points to approach. However, we offer a repair assistance line at 1-877-732-8391,then select the "repair assistance" option.

Mike Brown
Peavey USA

Jerry Roller · Post by **Jerry Roller** » 12 Dec 2005 8:45 am

Thanks for all the help everyone.
Please close this thread.
Jerry[This message was edited by Jerry Roller on 12 December 2005 at 09:23 AM.][This message was edited by Jerry Roller on 13 December 2005 at 08:40 AM.]