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My First Phase Linear Restore- New Guy to the Forum
- Thread starter mjstriker
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Thanks for posting the schematic.
I can't read the transistor numbers, but I assume that they are a NPN / PNP complimentary pair with diodes on the collectors, and a resistor / capacitor / diode network on the bases connected to the emitter resistors of two of the output transistors. This is a type of safe area protection for power transistors.
Germanium transistors have a greater NTC than silicon transistors and would offer better protection against thermal overload.
I can't read the transistor numbers, but I assume that they are a NPN / PNP complimentary pair with diodes on the collectors, and a resistor / capacitor / diode network on the bases connected to the emitter resistors of two of the output transistors. This is a type of safe area protection for power transistors.
Germanium transistors have a greater NTC than silicon transistors and would offer better protection against thermal overload.
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I see... well let me get my 400 up and running then we can talk more about getting the 700 series I a full makeover.
for the 2000 series I, what fuse size can I get away with? I know it asks for 1/8 but that would mean waiting for one to come in the mail. Was wondering if I could get away with a little larger so I can run to Ratshack and grab one to start testing the unit.
for the 2000 series I, what fuse size can I get away with? I know it asks for 1/8 but that would mean waiting for one to come in the mail. Was wondering if I could get away with a little larger so I can run to Ratshack and grab one to start testing the unit.
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Picked up a 1/4 amp fuse and gave the 2000 a test run. Light came on and the unit played some nice music hooked up to the 400. It appears most of the dials/push buttons are working but are very tempermental about being touched (very scratchy and intermittently dropping channel). They are going to need a good cleaning in the next couple of days along with the RCA input/outputs. Scrambling an order together with Mouser to replace the lytics and ceramic caps seeing as I have been needing a few other things from them.
Other than that I am waiting for my supply of wire to show up and a few other odds and ends for the 400 so I can start dismantling the unit. Until then the project for tomorrow is to populate the backboards.
Anybody have any input on silicon insulators for the outputs? I have been using the mica with the white goop for all my previous work but I know a guy who swears by them. I was a little weary as the 400 has potantial to get extra hot when pushed.
Other than that I am waiting for my supply of wire to show up and a few other odds and ends for the 400 so I can start dismantling the unit. Until then the project for tomorrow is to populate the backboards.
Anybody have any input on silicon insulators for the outputs? I have been using the mica with the white goop for all my previous work but I know a guy who swears by them. I was a little weary as the 400 has potantial to get extra hot when pushed.
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seem like a decent deal?
http://sanantonio.craigslist.org/ele/4322601577.html
Been looking around for one but not 100% exactly what I need but this is the first one that has popped up recently that looks like it would work for the stuff I am doing
thanks
http://sanantonio.craigslist.org/ele/4322601577.html
Been looking around for one but not 100% exactly what I need but this is the first one that has popped up recently that looks like it would work for the stuff I am doing
thanks
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Today I populated both backboards for the 400 with all wiring. Looks like I am about ready to tear down the amp and start the building up process. One mistake I have made is the bias transistor I soldered in and didn't realize it should have been left long to bend down to fit into bracket. Question is do I need the transistor to fit into the bracket or can I go without as now it is too short to bent into the bracket?
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.
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.
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.
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.
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.
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.
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.
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.
Thermal Interface Material
Opinions vary on Bergquist Sil-Pad 400 vs mica and grease. I only use Sil-Pads as it is a very good thermal interface material (TIM) from the industry leader in TIM material. They are so close in thermal transfer both right about 0.9W/m-K that it does not matter thermally. The grease tends to get everywhere and when it comes to solderability of the transistor leads to the boards, having grease on them is not a good thing for board soldering or sockets. The Bergquist pads are not inexpensive so do not look at their use as the cheap option.
Note:
Only use Sil Pads and the 400 or better material. Bergquist is the undisputed leader in thermal management materials. They have run extensive testing on their products and have supporting data. The entire power industry relies on the Bergquist thermal materials in multi kilowatt power applications. Do not rely on good thermal performance of run of the mill pads found coming out of China on Ebay. They are not the same.
At the end of the day, do what makes you feel good but technically both are very good solutions.
Opinions vary on Bergquist Sil-Pad 400 vs mica and grease. I only use Sil-Pads as it is a very good thermal interface material (TIM) from the industry leader in TIM material. They are so close in thermal transfer both right about 0.9W/m-K that it does not matter thermally. The grease tends to get everywhere and when it comes to solderability of the transistor leads to the boards, having grease on them is not a good thing for board soldering or sockets. The Bergquist pads are not inexpensive so do not look at their use as the cheap option.
Note:
Only use Sil Pads and the 400 or better material. Bergquist is the undisputed leader in thermal management materials. They have run extensive testing on their products and have supporting data. The entire power industry relies on the Bergquist thermal materials in multi kilowatt power applications. Do not rely on good thermal performance of run of the mill pads found coming out of China on Ebay. They are not the same.
At the end of the day, do what makes you feel good but technically both are very good solutions.
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