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Old 20th January 2014, 01:54 AM   #21
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Quote:
Originally Posted by rfitts46 View Post
This was off of T Waits thread on resurrecting a Crown DC300A, the15003 are seriously compromised at the supply voltages available from this amp. 21193 would be much better in the higher voltage region of SOAR. The extra current from the 15003 is of no advantage.
The SOA limit on the 21193 is 4A at 60V, the 15003 is 3A at 60V
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Old 20th January 2014, 01:00 PM   #22
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Default 15003 transistors

jackinnj, i dont know much about how to select eqvlt transistors so I am relying on what I have read. Your response indicating "The SOA limit on the 21193 is 4A at 60V, the 15003 is 3A at 60V " what does it mean in laymen terms? From your experience are the 15003 transistors an acceptable replacement? Please provide quote for 20 pieces delivered to 03051, Hudson, NH. thanks Bob
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Old 20th January 2014, 02:46 PM   #23
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Here's what I mean -- the MJ21193 is a slightly more expensive and faster transistor, ($5.25@ in quantities of 10), ft 4MHz, higher power handling -- the MJ15003 is somewhat inferior with an ft of 2MHz but less expensive. I've also got a few hundred of the discontinued BU208A which was a horizontal deflection transistor -- http://www.st.com/web/en/resource/te...CD00000993.pdf nobody needs HV CRT transistors anymore!
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Old 20th January 2014, 03:20 PM   #24
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thanks for the reply. I decided to purchase MJ15024G at $2.49ea from US source guaranteeing not counterfeit.
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Old 20th January 2014, 07:31 PM   #25
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Good choice.

I have 3 of these amplifiers -- the only thing original in one is the transformer -- it uses the LM4702 as a MOSFET driver and will drive over 200W. I sold the transistors and driver board on EBay -- about 25% of the transistors were shot. The other 2 are going to go under the knife at some point.
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Old 24th January 2014, 01:39 AM   #26
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purchased 50 pcs of Sil Pad Classic 400 TO-3 insulator pads. Now have complete set of driver and output transistors for the Channel 1 output board. Last night while inspecting the input board I noticed that both Q106 and Q107 had both been shorted to the face plate, not surprising since the amps face plate was missing all the mounting hardware and had been taped in place. While removing Q106 I noticed that the collector printed circuit board trace had melted and opened. Makes sense since the metallic heat exchanger on Q106 is the collector. The collector shorted to ground.
Q106 tests okay for a double diode test. Will reinstall and repair trace. Without an output from the input board to the output board driver Channel 2 would not work. The previous owner started removing the two output drivers and two of the output transistors. There might not have been any output transistor damage since the driver transistor was not getting an output from the input board. We will never know since the removed transistors did not come with the amp, requiring a complete set of driver and output transistors to be purchased. I have enough to rebuild both channels so they match.
Instead of removing Q107 can I power up amp and measure emitter outputs of Q106 and Q107 using Q206 and Q207 (channel 2 is good) as a reference?
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File Type: jpg marked up lifted trace input board.jpg (142.9 KB, 176 views)
File Type: jpg resized Q106.jpg (83.5 KB, 174 views)

Last edited by rfitts46; 24th January 2014 at 01:51 AM.
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Old 24th January 2014, 12:21 PM   #27
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I suggest you remove base from all transistors one step back from any blown up ones, and do a leakage test. Double diode test with a 2v meter does not detect stressed transistors. Current leakage test with base shorted to emitter, at some elevated voltage (I use 17 vdc out of a car battery charger) does. I put a 47k resistor in series with the milliammeter so I don't blow up the DVM fuse if I get it backwards. + to C on npn, - to C on pnp. Takes a DVM, a wall transformer or other DC supply, some clip leads, a good transistor for reference on how much leakage is okay.
I found blown up stuff all the way back to the input opamp of my last amp. 50 v rated ceramic caps, resistors, diodes, all blown. 107 parts total. Hope you have better luck. The first amp I fixed, the idle bias current circuits weren't even blown up by the output transistors. Just a matter of luck, I guess, and how many times the owner pushes the breaker button back in and tries again.
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Old 1st February 2014, 02:24 PM   #28
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the negative predriver was pulled out 2N4929 (Vceo = 150V, Ic=0.05A, hFE 25-250, ft = 100mhz, 1W) and it failed double diode test. Looking for suggestions for substitute part. I have found references to 2N5415, 2N2216, 2N3637.
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Old 2nd February 2014, 09:53 PM   #29
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Purchased 5 ea 2N5415 to replace the failed neg. predriver Q107 . Pulling all the outputs and drivers out of Channel 2. Will install new drivers and outputs on Channel 2 output board.

Last edited by rfitts46; 2nd February 2014 at 10:19 PM.
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Old 3rd February 2014, 08:30 PM   #30
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Purchased 4 pieces of 2N3440 to replace the positive pre driver Q106, Q206.
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