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Topic: Help... Session 400 Problem |
Jerry Heath
From:
Harrah, Oklahoma, USA
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Posted 26 Oct 2007 5:51 pm |
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I just purchased a 1976 Session 400 with a BW speaker. The parts for doing Brad Sarno's modification are on the way. Upon looking inside the chassis, I noticed that someone had already replaced the Filter Caps, what's strange is the size of capacitors that was chosen. Someone chose to use 4700 uF, I understand that the proper capacitor should at least be 6000 uF. The problem with the amp is that a light distortion or fuzziness can be heard that seems to be somewhat harmonic. Could this be the result of the filter caps being too small? UPDATED 11/4/07... I've done the Cap mod...Still have the light distortion...It is also accompanied by a random spitting sound. Any suggestions
_________________
Jerry Heath
Sierra Sessions U-12
MSA Vintage XL U-12
Nashville 400
Sessions 400
Profex II
Pod XT
Hilton Volume Pedal
Last edited by Jerry Heath on 4 Nov 2007 8:19 pm; edited 1 time in total |
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Brad Sarno
From:
St. Louis, MO USA
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Posted 26 Oct 2007 9:29 pm
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Jerry,
there are a lot of smaller filter caps in there too. On a 31 year old amp, it's time for all of the electrolytic cap's to be replaced, at the very least. I'd start there and then listen to the amp.
Brad |
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Jerry Heath
From:
Harrah, Oklahoma, USA
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Posted 2 Nov 2007 3:47 pm
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OK, replaced all the caps and the distortion is still there. So, I thought maybe it's a bad speaker. So I tested the amp section using my NV 400 BW and there's still distortion. I'm starting to think that maybe I need to replace all the transistors. Any suggestions?
_________________
Jerry Heath
Sierra Sessions U-12
MSA Vintage XL U-12
Nashville 400
Sessions 400
Profex II
Pod XT
Hilton Volume Pedal |
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Bob Cox
From:
Buckeye State
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Posted 2 Nov 2007 4:35 pm
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Those amps are famouse for their ability to give a growl,that distortion may be the differance in the
sound of it and another amp unless it is real noticable. |
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Jerry Heath
From:
Harrah, Oklahoma, USA
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Posted 2 Nov 2007 4:47 pm
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Bob,
It's noticeable to me, but I'm very particular about tone and sound quality. This amp just seems to be a little noisier than what I would expect, but I would have to admit I've never played through a SS 400. I played through a SS 500 a few weeks back and just loved the rich tone that it had. I came across this amp at a reasonable price and I thought I would try it. Looking at past post about the SS400, it seems like a lot of people are happy with the amp especially after the capacitor upgrades.
_________________
Jerry Heath
Sierra Sessions U-12
MSA Vintage XL U-12
Nashville 400
Sessions 400
Profex II
Pod XT
Hilton Volume Pedal |
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Jerry Heath
From:
Harrah, Oklahoma, USA
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Posted 2 Nov 2007 5:04 pm
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After evaluating this for a little while one might even call it a buzz rather than distortion. I still have no clue what it might be. It doesn't seem to be related to the Reverb, it it present without any reverb although it seems a little more noisy with the reverb.
_________________
Jerry Heath
Sierra Sessions U-12
MSA Vintage XL U-12
Nashville 400
Sessions 400
Profex II
Pod XT
Hilton Volume Pedal |
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Don Brown, Sr.
From:
New Jersey
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Posted 3 Nov 2007 10:14 am
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Jerry, Don't waste your money changing transistors. Transistors are either good, or bad. Not like tubes that go soft, etc.
But you might want to also check out or change the smaller Electrolytic Caps. They dry out with age, and even moreso, if the amps ever sat around not being used for long periods of time. |
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Bob Snelgrove
From:
san jose, ca
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Posted 3 Nov 2007 11:51 am
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| Jerry Heath wrote: |
| After evaluating this for a little while one might even call it a buzz rather than distortion. I still have no clue what it might be. It doesn't seem to be related to the Reverb, it it present without any reverb although it seems a little more noisy with the reverb. |
Have you tried the amp in different environments?
Line voltage?
bob |
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Jerry Heath
From:
Harrah, Oklahoma, USA
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Posted 3 Nov 2007 3:37 pm
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Don,
Thanks for the idea about the other capacitors. That's something I can try, at least it will eliminate all the capacitors.
Bob,
Different environments? I've evaluated the amp only in my studio/office. The same studio/office that I've used my NV400 and Session 400 Limited in. Neither of these amps sound like this one, they are crystal clear at all volume levels. Can you enlighten me on your different environment idea?
_________________
Jerry Heath
Sierra Sessions U-12
MSA Vintage XL U-12
Nashville 400
Sessions 400
Profex II
Pod XT
Hilton Volume Pedal |
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Jerry Heath
From:
Harrah, Oklahoma, USA
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Posted 4 Nov 2007 8:24 pm
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TTT... I know someone out there has experienced the same problem with their Session 400.
_________________
Jerry Heath
Sierra Sessions U-12
MSA Vintage XL U-12
Nashville 400
Sessions 400
Profex II
Pod XT
Hilton Volume Pedal |
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Jerry Heath
From:
Harrah, Oklahoma, USA
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Posted 6 Nov 2007 6:05 pm
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Anyone have any idea what the tolerance should be for the voltage coming out of the transformer. Nominal voltage should be 50 vdc. I'm seeing about 38 vdc. Is this a problem? I'm still tracking down a slight distortion. I have improved it somewhat, but there some still there.
Thanks,
_________________
Jerry Heath
Sierra Sessions U-12
MSA Vintage XL U-12
Nashville 400
Sessions 400
Profex II
Pod XT
Hilton Volume Pedal |
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Brad Sarno
From:
St. Louis, MO USA
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Posted 6 Nov 2007 6:33 pm
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Jerry,
what is the voltage after the big filter caps? The transformer is putting out AC. The rectifier diodes turn that into DC, and then the power supply filter caps and resistors smooth it all out. That's where you want to check the DC voltage.
Brad |
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Jerry Heath
From:
Harrah, Oklahoma, USA
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Posted 7 Nov 2007 4:09 am
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Brad,
Ok, I'm seeing 51 vdc after the big caps. I'd say the transformer is good. Is there anyway to send the power amp section a signal from a different pre-amp? Any ideas on how to diagnosis this one?
Thanks,
_________________
Jerry Heath
Sierra Sessions U-12
MSA Vintage XL U-12
Nashville 400
Sessions 400
Profex II
Pod XT
Hilton Volume Pedal |
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Ken Fox
From:
Nashville GA USA
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Posted 7 Nov 2007 5:32 am
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The first thing I would do is check the filter caps for a clean sawtooth pattern on a scope. If there is a "church steeple" spike on either cap it needs to be replaced.
Probably the single most difficult repair is the noisy power amp in these amps. As the signal is feedback it can be difficult to trace with a scope.
I suspect a noisy transistor. Transistors are not just good/bad! They can be leaky and also have thermal noise associated with them. |
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Jerry Heath
From:
Harrah, Oklahoma, USA
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Posted 9 Nov 2007 7:52 pm
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Thanks for all the help guys.... After finding all the cold solder joints, replacing various components and help from you great folks the amp now sounds wonderful. Again thanks to all that posted, especially Ken and Brad.
_________________
Jerry Heath
Sierra Sessions U-12
MSA Vintage XL U-12
Nashville 400
Sessions 400
Profex II
Pod XT
Hilton Volume Pedal |
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Bill Moore
From:
Manchester, Michigan
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Posted 24 Nov 2007 7:18 pm
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| Jerry, I wonder if you could explain exactly you did that took care of this problem. I'm having something similar with an LTD 400, there is an intermittent distortion, along with a slight reduction in volume. Most of the time the amp has no distortion, then it begins to distort for a while, then it will sound clean. Any advice would be appreciated. Thanks. |
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Ken Fox
From:
Nashville GA USA
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Posted 25 Nov 2007 8:49 am
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A must do on these older amp with Black Widow speaker is to check the speaker. Remove the magnet and make sure the gap is clean. Often times the foam rubber piece in the back of the magnet has rotted and pieces are in the magnetic gap and stuck to the voice coil as well.
Cold solder joints on the power board, loose ground to chassis. The ground to the chassis form the power amp board is made via four small brackets that are riveted to the chassis. These can get loose at the chassis. If so, drill out the rivets and replaced with screw. lock washer and nut.
A loss in power would make me look for cold solder joints, dirty or loose Molex connector from the Preamp bard to the power board. I had one LTD come in here and pots fell out of the preamp board right onto the bench! Almost every pot had bad joints on it. It was a very early unit. The later unit were wave soldered much better.
I have serviced over 80 of the N-400, Session 400 and LTD 400 amps this year. I have seen many of the issues and been able to resolve them. These amps come in for the Intense mod and Sarno mod. Usually every amp has some issues to resolve. |
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Jerry Heath
From:
Harrah, Oklahoma, USA
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Posted 26 Nov 2007 5:25 pm
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Ken is right, you've got to eliminate the obvious. I found 2 different cold solder joints. One wire was the wire that comes from the input jack onto the preamp board. The other was a wire that attaches to one of the output transistors. Although, these particular defects wasn't the root cause of my problem, they would of eventually caused a problem. My problem in particular was a couple of open resistors on the power amp board. Two of the white brick 1 ohm resisters were wide open. I replaced the two resistors and the two 761 transistors in the power amp section under the advise of another amp technician. I do have plans to replace all the transistors in the box. I've got them all gathered up and I'm just waiting on a day when I bored and have nothing to do. I know some people might think that this would be a waste of time and money, but I know that these are the only components left that can and do cause such problems. Again before you start buying transistors and start replacing them, I recommend that you throughly check all your solder joints.
_________________
Jerry Heath
Sierra Sessions U-12
MSA Vintage XL U-12
Nashville 400
Sessions 400
Profex II
Pod XT
Hilton Volume Pedal |
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