Help repairing Samson Servo 550 amp

[p-daddy]

Thanks for checking out my post! Hopefully some of you guys can help me out...

A little background:
I have always dabbled in audio & electronics, but I'm not trained. I am a very logical thinker and love troubleshooting problems...I just don't have enough knowledge to know how to tackle an amp like this. My brother is a senior Computer Sci & Electronic Engineering major. We have an oscilloscope (sp?), voltmeter, and tools.

The test subject:
A Samson Servo 550 amp that has given nothing but trouble ever since I got it. After 2 trips to a Dallas authorized service center (who couldn't help), I fixed the first problem - cracked solder joints at the speaker outputs. Later another problem arose...'staticy' audio...either due to a voltage spike (storm) or slightly rough handling or a few raindrops getting inside (I know, that doesn't really narrow it down). We traced it to the IC chip on the small input board, and replace that recently, but that still left us with something wrong.

The current problem:
The amp has trouble on Channel 1 (Left). As you turn that channel up between 1/4 & 1/2 it begins to clip (causes the clip light to come on). That channel does not give as loud of output as the good channel either. When we run a test signal (sine wave) through, we can see that the wave is not smooth (like it is on the good channel) but squared. Obviously, there is something(s) not right with the mainboard of the amp.

Can any of you walk us through some steps to diagnose our trouble? Or point us in the direction of some website(s) that might help?

I will post a schematic tomorrow to a website if that would help (I'll update when I do). From the schematic, each channel appears to be a set of mirrored components running off a +15V DC / - 15V DC supply, and I can tell where there are 3 stages. The first appears to balance the incoming signal somehow by feeding through some transistors that feed back into the input voltages of the opposite + or - side. The second is the most complicated, I'm guess a pre-amp. The third is the power boosting section with 5 or 6 pairs of amping gizmos (sorry, not sure what the correct term is).

Thanks again for any help you can give. I hate to trash this amp as it has been primarily home studio kept and is practically in brand new condition. (don't think I'll ever buy another Samson, though!).

I don't do IM or anything, but feel free to email me if you want:
(my first name)@hot1.net
-Paul M

 

[EchoWars]

From the schematic, each channel appears to be a set of mirrored components running off a +15V DC / - 15V DC supply

The amp is rated at 275WPC, IIRC, and is gonna need a lot more than +/-15V to do it.

First things first...after looking for obvious signs of damage (burnt traces and /or resistors, etc., etc.), you need to locate ALL the supply voltages on the schematic, and verify them on your unit.

 

[p-daddy]

Thank you...I've already done most of that but will double check everything asap...this evening.

In the meantime, here are links to the schematic:

http://www.brazosmeadows.org/img/Samson_Servo_550_Schematic.tif (937 KB tif)

http://www.brazosmeadows.org/img/Samson_Servo_550_Schematic.jpg (2.72 MB jpg)

And of course, you were correct that the + & - supply is not labeled 15V... BUT...perhaps I'm not reading the diagram correctly, but I don't actually see anywhere that voltages are listed other than the +/- 15V places. Maybe there is something I'm missing about how to read the diagram...?

 

[jaycee]

The supply rails for the amp can be measured on the terminals of C201/C202.

Are you sure the amp isn't in bridged mode?

Check Q108/Q109 and make sure these transistors are not faulty - the VI limiter could be activating.

 

[p-daddy]

The supply rails for the amp can be measured on the terminals of C201/C202.

Are you sure the amp isn't in bridged mode?

Check Q108/Q109 and make sure these transistors are not faulty - the VI limiter could be activating.

OK, I double-checked for burned / bad connections - nothing found yet. I confirmed that the amp IS in stereo mode.

I also checked some voltages (again, the only voltages that I could find listed on the schematic are the + / - 15V ones. Here is what I found:

D200 (main bridge rectifier i think?): incoming 52, outgoing +/- 76 on respective sides (iirc, i left my notes at home).

D200-1 (x4): incoming 22, outgoing +/- 28 on respective sides

so R202 & R204 incoming is +/-28

BUT... R202 outgoing is -12 V, and R204 outgoing is +11 V

that looks suspect to me, but may well be w/in acceptable limits...and I guess that supplies both channels so since only one side is giving trouble I don't see how that could be the problem.

Can anyone comment on these voltages and/or advise me further? (other voltages to check?, what next?, etc?)

Also, I had to learn how to check transistors...and I understand basic resistance testing of them now...I can try to test them IN the circuit (haven't yet), but will I need to remove them to test properly?

(And can anyone confirm that I should start w/ the two Q108/Q109 mentioned above?)

thanks!