Summit Audio TBA 200B preamp

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joe333

Active member
Joined
Nov 28, 2009
Messages
39
Hi
I have a problem in my TBA 200B preamp but i m not sure if its a 990 opamp problem.
there's too much dc noise in one channel that makes the channel very noisy (crakles) but the amp works well...
I have schematics and i ve made dcv readings of the components around the two 990 opamps and there's a 0.4 DCv that i dont have in the channel working properly...in the other readings are around 35 mv...
 
Unfortunately 990 are not mounted in sockets...so quite hard to swap..
this is the output section and readings are in VDC
R86...0.5mv
C78...300mv
R84....280mv
R83...2.45V  same in both channels
R80...330mv

it seems that all the resistors reads the same in both channels...no signs of any resistor or capacitor damaged....it started crackle few days ago after some minutes warm up and is getting worse during this days...but the channel is still working regularly...
the 24v and 15v voltage values for ic and opamp are the same in both channels...
 

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tested LF412 in another circuit and works fine...
I cant really find any resistor or capacitor damaged...
there's a procedure to get both channels between -+200mv at pin1 of lf412 regulating R110 but the dc in the left channel is unstable and i can't keep it stable...
pics of the readings of both channels without signal and with 440hz sine
 

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I am sure you have done this, but whenever working on a unit first step is to make sure the power supply is working properly.

crackle could be something as simple as a faulty solder joint, reflow the joints.

another other noise and such, feed in 1K and scope it out with an o-scope...
 
Power supply has been checked at first...the voltages are fine...+24.2, -23.4, 15.06, -14.95 and 5.02
Also in the tube section everything is fine...the dc start to be unstable after....as you notice in the pic, when you feed a signal both channels work the same....440hz or 1khz ...
 
Yes,its a trim pot...R110..it changes the amount of DC and maybe regulate the servo amp...but is not able to keep the value between -+200mv in the left channel...the trimpot and the 1M resistors are not damaged...
 
Unfortunately 990 are not mounted in sockets...so quite hard to swap..
this is the output section and readings are in VDC
R86...0.5mv
C78...300mv
R84....280mv
R83...2.45V same in both channels
R80...330mv

it seems that all the resistors reads the same in both channels...no signs of any resistor or capacitor damaged....it started crackle few days ago after some minutes warm up and is getting worse during this days...but the channel is still working regularly...
the 24v and 15v voltage values for ic and opamp are the same in both channels...
joe ~ i'm encountering a very similar issue with my TPA-200A & curious if you have any advice from your repair. i'd looovveee to get ahold of any more schematics you'd be willing to share! that little crop you posted is the only schematic i've ever been able to find for this unit. all the best :)
 
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