Samson Servo 550 issues, multiple amps, same issue.

cwahls

Member
2008-02-12 3:42 pm
Good morning! Wonderful forum!

We have 8 Samson Servo 550s and 3 of them are having the same issue. Won't come out of protect, no DC on the speaker outputs (before the relays), thermistor is good, yaddada yaddada.

There was obvious discoloration on the PCB and resistors themselves that buffer the TA7317P protection IC I replaced these even though they tested in spec OUT of operating conditions.

What are the chances I have a bad TA7317P? In searching I read that they are the most common IC unnecessarily replaced ;)

The amplifiers ARE in an area that runs warm, that is getting changed, they are not run hard and 95% of their life they spend idling or barely working (Classroom use.) Is this chip prone to failure in high temperatures? I've just not dealt with this protection scheme before and hoping someone who has can give me a nudge in the right direction.

Any ideas? Unfortunately I don't have any spares of these IC's to just shotgun in there. Is this a common issue. Samson tech support was of ABSOLUTELY no help. I figured if I have 3 of 8 doing this then certainly they may have seen it before ;)

I can post a schematic if someone wants one.


Many thanks! Chad
Chad
 

cwahls

Member
2008-02-12 3:42 pm
Nope, just the caps IIRC, I MAY have replaced a resistor that got a little warm, it's been a while. That's a HOT area in the amplifier. I used a quality hi-temp cap and all 4 of them have been going strong for 4 years now. I just shotgunned all of them instead of repairing them as they fail, usually when it's most inconvenient.
 

cwahls

Member
2008-02-12 3:42 pm
see if there's a reason it's in fault, as in check to see if there is DC on the output before the speaker relay, and check for appropriate rail voltage on both LV and HV also filtering on the HV rails.

How do the rail caps look, those are heat persnickity too. are they bulged up at all (press down on the plastic top and feel for metal bulging)
 
All my voltages coming from the transformer are correct but in my poking around I noticed a pair of burnt resistors that I somehow missed before. They measured at about four times the impedance they should so I am working on getting them. unfortunately there are no electrical component suppliers in my city for some reason. I'll replace those and see where I'm at.

unfortunately I do not have a scope.

Also I did find the datasheet and the protection IC looks for +/-1.1 V so that is in spec for that. The resistors appear in the schematic like they may be preventing the correct voltage to the relays so hopefully replacing those helps.
 

cwahls

Member
2008-02-12 3:42 pm
OK, Amplifier output goes into relays, is there DC there? On the guzinta of the relays. Not the comezata, they are not energized in protect.

Did you look at the HV and LV rails? With a Meter, set for DC, using the negative speaker lead as a ground reference. A chassis screwhole probably will work too.

You can use the finals to check HV rail voltage and pin 4 and 8 on an op-amp to check LV
 
Okay now that I feel like a rookie and have that cleared up after a little reasearch...

I tested the four transistors that had their own individual heat sinks and they are good. (though i thought it strange that two of them had +75V on the heat sink and two of them had -75V on their heat sinks, probably just me being a rookie..)

Is my next step testing the output transistors?