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Old 23rd January 2013, 03:02 PM   #1
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Default Problem with Totem Pole Amp

Hello and please forgive the long post tha follows.

I recently built one channel of Randy Slone's Totem Pole design (the one that can be found in his book, Fig. 6-21). When trying to test it, I notice that the output (with or without load) will start from a relatively high positive voltage, then slowly fall to zero before suddenly going to the negative rail. An then it slowly returns to the positive rail before doing the same thing over and over again.

The very first time I connected a power supply to it (not the actual power supply, but a smaller one for the purposes of testing) one of the resistors in the VA protection circuit (R28 for those of you that may have the schematic) gave out some smoke, but hasn't been destroyed (I assume, since it still measures the correct value). I have tested for a short circuit at the output but couldn't find one. I also tried to test the transistors for a short circuit between their leads but there isn't any (is that the indication that one has been fried?).

I'm currently thinking that the feedback loop is broken somewhere but I haven't found anything as of yet. I would greatly appreciate any ideas.
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Old 23rd January 2013, 06:59 PM   #2
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Is posting schematic an option ?
that can either be latching or oscillation
Of course it might also be a construction error
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Old 24th January 2013, 07:35 PM   #3
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This is the best quality schematic I could find. The resistor in red is the one that almost burnt.
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Old 24th January 2013, 09:17 PM   #4
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If its burnt change it out, for testing this circuit the s/c protection can be removed also this type of protection has ref point to the supply rails..

Check each part of this circuit on a lower ht and to save the any damage to the MOSFET's just leave one pair in. I've got this book also and will look over things..


Regards. A.
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Old 25th January 2013, 09:16 AM   #5
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Thanks all for your help. The resistor hasn't really burnt, I tested it with an Ohmmeter and it measures just fine.

Am I right to assume that a destroyed transistor will have (some of) its leads short-circuited? Because if that's the case, then there is no problem there either. I might post some pics of my amp later, in case someone can spot a construction error.
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