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Old 19th April 2011, 04:35 AM   #1
Dr Zeus is offline Dr Zeus  United States
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Default Crunch Ground Pounder GP1500D

I half took this amp apart and I forgot where the PS driver transistors are supposed to go. Its hard to dead all the descriptions on the PCB because a transformer coil is covering with glue, but I beleive they are Q7, Q7R, Q8, and Q8R.

Originally, this amp had either an A1275/C3228 or A1274/C3227 combination but I cannot remember which, or what parts I can use as better installation. I have a lot of MPSA06/56 but I'm also putting togeth an order at Mouser. Which would be the best combo for this style of amp? I fix a lot of these for one of my customers.

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Old 19th April 2011, 08:16 AM   #2
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In other amps, I use the BD139/140 when I need this pin configuration.

The collector of the PNP transistors will be directly connected to ground if you can't remember which ones are NPN and which are PNP.
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Old 25th April 2011, 10:02 PM   #3
Dr Zeus is offline Dr Zeus  United States
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OK I replaced the PS driver transistors with BD139/BD140. The BD140s ended up in the furthest possitioned components from each other. The amp does the same thing. When powered on, the PS fets get hot within seconds. The waveform on the fet gates while the fets are installed quickly goes from about a +8v square wave down to about a +4v square wave.

I pulled the rectifiers because I thought they were measuring up in-correctly, but then realized they are AC Doubler type diodes similar to the schematic listed as "Suffix D" in this datasheet:

http://www.fairchildsemi.com/ds/FE%2FFEP16DT.pdf

I'm not really too sure why amp is acting like this.
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Old 25th April 2011, 10:04 PM   #4
Dr Zeus is offline Dr Zeus  United States
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I should also mention, the PS fets were all dead in this amp, so I installed one IRF3205 into each bank with 47ohm gate resistors. Q13 and Q18. Q18 is getting much hotter and quicker than Q13, but the waveforms are still bad on both Q13/Q18 while IRF3205s are installed. Removing the IRF3205s and I get a nice +10v square wave on the gates.

After pulling the rectifiers, the power supply fets still get hot after seconds.

Last edited by Dr Zeus; 25th April 2011 at 10:08 PM.
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Old 25th April 2011, 10:06 PM   #5
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Is the square wave going to ground?

Post a photo of the gate waveform from the gate leg of the FETs.
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Old 25th April 2011, 10:25 PM   #6
Dr Zeus is offline Dr Zeus  United States
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Referencing amp ground;

With the fets removed and the scope set to 5v/Div and 10uS:

Click the image to open in full size.

With one fet installed, the waveform is not solidly defined, starts at about +8v Square and drops do +5v Square.

Click the image to open in full size.

I posted a very short video (<10 second video) of the action/change on the scope here. At ~3 seconds I power the amp, and then the waveform drops to ~5v I power the amp off to protect the fet.

mtx :: VID_20110425_182356.mp4 video by unclemeat2010 - Photobucket

Last edited by Dr Zeus; 25th April 2011 at 10:28 PM.
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Old 25th April 2011, 10:29 PM   #7
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What's the resistance from the base of the driver transistors to ground?
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Old 25th April 2011, 10:46 PM   #8
Dr Zeus is offline Dr Zeus  United States
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Resistance referencing ground to base of drivers:

All drivers measure 470ohms ground to emitter (Leg #1)
BD140 drivers measure 0 ohms ground to collector (Leg #2)
BD139 drivers measure OL ground to collector (Leg #2)

All drivers measure 1k ohms ground to base (Leg #3)
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Old 25th April 2011, 10:49 PM   #9
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Try reducing the 1k pulldown resistors to something between 220 and 330 ohms. Does that help?
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Old 25th April 2011, 11:21 PM   #10
Dr Zeus is offline Dr Zeus  United States
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Not totally thinking your suggestion though, I went and pulled out the pair of 3.2k resistors located between the drivers. I believe you are refering to the R30 and R31 right? These are connected to the PNP driver collectors.

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