Phase Linear 400 issue help!

They are on the back wall with the 3 wires running out nof them.
 
3403's are obsolete. We use the 2N5088's in their place. We knock he 3403 out of the P-strap and replace it with the 2N5088 and then solder and heatshrink the wires back on them. Voila--bias transistor!!
 
Picked up the test transistors at lunch today. Looking forward to popping them in and seeing if the bias adjusts.
 
Bias Transistor Substitution and Replacement
The original Q6 bias transistors are the 2N3403 devices on the back wall mounted in a small D shaped metal thermal bracket. Substitute matched 2N5088 devices as they are better than the original parts and have higher beta which is important for this Q6 location. The band clamps from the original 2N3403s are saved and reused. Once removed from the amp, the 2N3403s are relatively easy to tap out from the open top hat end of the metal band clamp. First slightly spread the overlapping metal portions of the band clamp in the area that the mounting hole is in using the blade of a small flat bladed jewelers screwdriver.
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Then clamp the part of the 2N3403 metal band clamp sideways in your vise so that you are holding it but not putting closing force on the band clamp (transistor leads facing away from you and parallel to the closing screw of your vise), you can gently tap the transistor out of the bottom of the strap using a small wooden dowel.
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The 2N5088 devices fit the band clamp opening that remains nicely. Put a small dab of instant glue on the face of the 2N5088 and slide it into the band clamp such that the base of the transistor where the leads exit is flush with the edge of the band clamp.
If applicable, rewire the original wires, heat shrink the connections and you are back in business. Note that the pinout of the 2N3403 is different from the 2N5088 so take that into account. The 2N5088 has a pinout of E-B-C when looking at the flat face of the transistor with the 3 leads pointing downward. The 2N3403 has a pinout of E-C-B when looking at the flat face of the transistor with the 3 leads pointing downward. The red wire connects to the transistor Collector, the brown wire connects to the transistor Emitter, the black wire connects to the transistor Base.
 
enoyes12 said:
Picked up the test transistors at lunch today. Looking forward to popping them in and seeing if the bias adjusts.
Click to expand...

Did you get the replacement pots too? That is the usual spot for problems on these old amps.
 
enoyes12 said:
Picked up the test transistors at lunch today. Looking forward to popping them in and seeing if the bias adjusts.
Click to expand...

Can I just solder the bias transistor to the board or do I have to mount it on the back wall of the amplifier?
It is required that you mount this transistor on the back wall in the original location using the original type aluminum thermal strap. This is NOT because this device needs heat sinking to cool it. It is because it acts as a thermal sensor to sense the heat being generated by the power dissipation in the 2 pre-driver transistors at the bottom of each pair of colums for the left and right channel. Transistor and diode junctions have negative temperature coefficient characteristics meaning that the forward P/N junction voltage of approximately 0.65V at room temperature decreases as the junction temperature increases. Thus if bias voltage was always held to a constant voltage value, the bias current through R38 and R39 would increase as the amplifier was worked hard putting out increased power to the speakers. When the bias current increases, it increases the voltage across R38 and R39. If this voltage was to get too high, it would risk turning on both the positive output drivers and the negative output drivers at the same time which would create a thermal runaway situation and damage to the output driver transistors. Instead, as Q6’s temperature increases by absorbing the temperature produced on the back wall by the pre-driver and output driver transistors, its P/N junction also decreases in forward voltage causing the bias voltage to go down in direct proportion to the decrease in the P/N junctions of the pre-drivers and output drivers, thus keeping the bias current constant and preventing the possible thermal runaway that might otherwise occur.
 
I only picked up the the transes for test purposes since the only place by me has the NTE brand. I will be just connecting them long enough to to see if I can adjust the bias. I did not get the pots because again they would be NTE but the pots are working 0-5K. I know that doesn't always mean under a load they won't fail but I should at least be able to see if I can adjust the bias. After putting in the test transes I can adjust the bias I will order the good transistors and pots.
 
Do yourself a HUGE favour and pick some probes similar to these when taking measurements in confined areas, indispensable.
 

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I have some leads with alligator clips on the ends. So I replaced the transistors and at least the readings are much closer at 67.8mV left, 46.2 mV right. Better than the 206mV left, 60mV right. The pots are still not adjusting the bias so I'm going to order new ones and the 2n5088 transistors.
 
Mouser is showing 6 different 5088's. Which one am I looking for? Also whats the model number of the pots I'm looking for?
 
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$HIT these new pots are a lot smaller than the originals. The originals are 5K right?
 
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