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Old 16th July 2008, 02:16 AM   #1
Semms is offline Semms  United States
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Default mtx thunder 342 resistors

This amp had some FETs burned out in the power supply which also took out some surface mount resistors. Does anyone know the value of R628, R629, R636, R637? Was there originally four and two got blown right off the board? Its hard to tell and the remaining resistors are unreadable. The pictures posted on Ampguts weren't usable.
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Old 16th July 2008, 03:42 AM   #2
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If they go from the driver transformer to the power supply FETs and there are two groups of parallel resistors, I'd try 68 ohms in each location (68 x 4 resistors). This should get it to power up.

If the PZTA56s also failed, check the 10 ohm resistors connected between the base and emitter of each transistor.
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Old 16th July 2008, 03:55 AM   #3
Semms is offline Semms  United States
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yeah they go from the driver transformer to the power supply Fets (HUF75339). it looks like there might have been a total of two resistors in parallel for each group or just one for each group and empty pads. Only one in each group remain and its not clear if the pads were originally empty? The rest of the transistors all check out fine.

There are small weld like marks on the outer case of the amp right near the Batt connector, i'm guessing someone shorted +12V to the case.

i can throw some 68ohm resistors on it after i replace the two power supply fet's but would it be close enough to original to allow me to fully test this amp if it powers up ok?
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Old 16th July 2008, 04:02 AM   #4
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Monitor the temperature of the power supply FETs. If they don't get hot when you drive the amp hard, the replacement resistors are close enough.
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Old 16th July 2008, 02:38 PM   #5
Semms is offline Semms  United States
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Are these resistors 1206 size? i'm not very familiar with smd components.
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Old 16th July 2008, 04:05 PM   #6
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They're probably 0805s (0.08" x 0.05"). If you don't have a dial caliper to measure them, you can use the ICs on the board to estimate their size. the SMD ICs have a pin spacing of 0.05". The through-hole DIP ICs have a pin spacing of 0.1" If the length is slightly greater than the center to center spacing between adjacent legs of a DIP IC, it's a 1206. If the length is less than the center to center spacing between legs, it's an 0805.

http://www.bcae1.com/temp/ttIMG_7798b.jpg
http://www.bcae1.com/temp/ttIMG_7796b.jpg
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Old 16th July 2008, 08:26 PM   #7
Semms is offline Semms  United States
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Looking at the pics you posted (Thanks) i'm pretty sure they are 0805's. You gave a value of 68ohms which would be 34ohms overall since they are in parallel. I have some 75 ohm resistors in the proper size which would yeild ~38ohms in parallel. Would these be close enough or should i just order some 68 ohm resistors?
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Old 16th July 2008, 08:36 PM   #8
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The 75 ohm resistors will likely work as well as the 68 ohm resistors.
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Old 17th July 2008, 03:17 PM   #9
Semms is offline Semms  United States
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The 75 ohm resistors got it to turn on, i still have to run it hard and check the fet's. Thanks for all you help! BTW, very nice website!
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