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Old 18th November 2012, 03:35 PM   #21
Zaapp is offline Zaapp  United States
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email sent.

I accidentally destroyed one of the a1275's removing it for testing, but thats alright because I'm pretty sure it was bad after testing the other (is there a common set of transistors that I can replace these with or is it time to put them on the Parts order list?)

undamaged a1275
red lead on base black lead on collector I get 1.001v
red lead on base black lead on emitter I get OL

q7
red lead on base black lead on collector I get 0.6v
red lead on base black lead on emitter I get 0.6v
and OL on every other combination
q7a was just a repeat of all of the q7 readings
red lead on base black lead on collector I get 0.6v
red lead on base black lead on emitter I get 0.6v
and OL on every other combination
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Old 18th November 2012, 03:45 PM   #22
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I use the BD139 and BD140 as replacements for driver transistors. If you're going to do repairs, you should keep both on hand.

The 1275 is defective.

Using the form from the transistor testing page will require much less typing.
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Old 18th November 2012, 03:55 PM   #23
Zaapp is offline Zaapp  United States
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the FETs are blown too, I cut em out and tested a few, they all tested bad... again

I will go ahead and find and order bd139s, bd140s, and another set of irf3205s

looks like I'm on hold till further notice when parts arrive, probably after thanksgiving sometime.
I have lots of pieces parts on hand, but not a single spare replacement to anything I've pulled from this amp so far.
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Old 18th November 2012, 05:29 PM   #24
Zaapp is offline Zaapp  United States
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Thank you for the email Perry. I appreciate it. after looking over the schematic and my board I can see how these amps are quite alike. The power section is nearly identical, down to the component designations and type.

Is there anything else I can check right now with the board disassembled the way that it is?
I believe I found the bd139s and bd140s semi locally (more local to work) and will pick them up tomorrow evening perhaps. but I will have to wait on another order of the irf3205s
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Old 18th November 2012, 05:46 PM   #25
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After you get the drivers in place, you can check the drive circuit at the gate pads for the FETs.

Did you double-check all of the gate resistors to confirm that all are withing tolerance?
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Old 18th November 2012, 10:48 PM   #26
Zaapp is offline Zaapp  United States
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just checked... all 16 gate resistors are 46.5- 46.9 ohm. well within spec. will get the drivers tomorrow and we'll go from there.
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Old 20th November 2012, 11:56 PM   #27
Zaapp is offline Zaapp  United States
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I got the bd139s and bd140s, but I noticed that they were only rated at half of the power of the original a1275 and c3228s, so I also picked up another different set of mje340s and mje350s rated at almost twice the original power of the a1275s and c3228s (could not find the originals for the life of me tho)
which would be the better choice as replacements? the BDs or the MJEs?
the irf3205s will be here friday
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Old 21st November 2012, 02:39 AM   #28
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Power dissipation isn't the most significant factor in switching circuits. The current rating of the BD139/140 is 1.5A vs 0.5A for the MJE parts. I doubt that you'd have a problem with either one but the BD transistors are more likely to survive if the FETs fail again.
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Old 21st November 2012, 02:45 AM   #29
Zaapp is offline Zaapp  United States
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BD's it is then. I spent a good deal of time tonight de-soldering the board for all these fancy new parts... I've never cleaned anything electronic up as good as I did this board.

I will get the drivers in tomorrow after work and check to see if I have a good signal on the gate pads.
Good signal should be something like a 22.5kHz square wave right?
and bad signal would be no signal or just high or low voltage, probably very near 12v right?
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Old 21st November 2012, 02:48 AM   #30
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A good signal would have its falling edge go perfectly vertically down to within about 1v from ground (looking at it with a scope).
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