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Old 25th May 2012, 09:55 PM   #1
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Default Punch 300s

I dont think theres alot of problems with this one. I think a ground wire was hooked up to the remote lead and ruined the turn on circuitry. I can run power to it and no smoke or excessive heat. The transformers warms to and idle temp but no light comes on. The voltage on Q6 and Q7 are from left to right

Q7 MTP75N05HD Q8MTP75N05HD
pins
1. 13.03v 1. 12.09v
2. 13.20v 2. 13.16v
3. 0v 3. 0 v
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Old 25th May 2012, 10:47 PM   #2
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Those transistors are blown.
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Old 25th May 2012, 10:48 PM   #3
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okay so I replace them? is there a suitable substitute as I have somecomponents laying around

Last edited by hifonics hugh; 25th May 2012 at 10:59 PM.
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Old 25th May 2012, 11:05 PM   #4
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I'd use IRF3205s.

If the gate resistors are out of tolerance (you'll have to remove the FETs before you can check the resistors), use 47 ohm replacements.

You'll also need to check the driver transistors.

You need to replace all of the power supply FETs.
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Old 25th May 2012, 11:12 PM   #5
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driver transistors are Q5,Q1,Q2,Q4? coming off TL494 feeding the irfz40?
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Old 25th May 2012, 11:45 PM   #6
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Yes they are marked 1A and 2A replace all 4.
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Old 27th May 2012, 04:27 PM   #7
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Havent checked the transistors yet as Im not home(away from schematic) but what is the model number of the transistors Q5,Q4, Q1,Q2 ?
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Old 27th May 2012, 04:55 PM   #8
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Tracked down the schematic ok bt3904's on the high side bt3906's on the lowside of each pair. Mr. Babin the schematics show IRFZ40's I have some Z44's will they work?The 75N05HD which are whats on the amp now have that HD- is that heavy duty? Are they possibly more robust than z40's or your suggested IRF3205's? I appreciate all the help. I guess I need not to be so lazy and do an ac/dc analysis of the circuit which would tell me what I should see at any node. Very time consuming. I guess if Im really lazy I could redraw the circuit in multisim and really play around with that to see what will work.

Perry you deserve a medal for saving folks all that time and effort on repair- nothing like real world experience!
Thank you very much.

Hugh.

Last edited by hifonics hugh; 27th May 2012 at 05:06 PM.
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Old 27th May 2012, 08:07 PM   #9
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Z44s = less reliable
3205s = more reliable
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Old 29th May 2012, 12:53 PM   #10
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R20 is 69 ohms
R19 is 68.7 ohms
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