Background - I have basic replacement experience, but very limited troubleshooting knowledge. I've replaced caps, convergence chips, repaired game consoles and have a collection of soldering irons in varying wattage to work on boards.
I've read Perry Babin's multiple answers to kx1200 threads and have a few questions regarding some smoked power supply FETs on this (red board) amp.
1) The amp appeared sealed and original, but the power FETs are 3205's with 100 ohm gate resistors (were they ever built with this combination?). Is it possible to run 3205's with this resistor or do they need to be changed to 47 ohm resistors? The resistors all read within tolerance.
2) If the resistors need to be changed, what is a good part number from Digikey? I tried crossing part number 660-RK73B2BTTDD470J that Perry Babin suggested, but came up blank. Using their search engine I found so many options that I got lost.
3) How can I check the gate drive transistors? They don't cost much to replace but I question my ability to not jack up the tighter/smaller soldering in that area.
Thanks!
I've read Perry Babin's multiple answers to kx1200 threads and have a few questions regarding some smoked power supply FETs on this (red board) amp.
1) The amp appeared sealed and original, but the power FETs are 3205's with 100 ohm gate resistors (were they ever built with this combination?). Is it possible to run 3205's with this resistor or do they need to be changed to 47 ohm resistors? The resistors all read within tolerance.
2) If the resistors need to be changed, what is a good part number from Digikey? I tried crossing part number 660-RK73B2BTTDD470J that Perry Babin suggested, but came up blank. Using their search engine I found so many options that I got lost.
3) How can I check the gate drive transistors? They don't cost much to replace but I question my ability to not jack up the tighter/smaller soldering in that area.
Thanks!
Still works great...
http://www.diyaudio.com/forums/car-audio/162033-jbl-bp-1200-1-project.html
http://www.diyaudio.com/forums/car-audio/162033-jbl-bp-1200-1-project.html
Okay, I wondered. The other posts about this amp suggested 'upgrading' the FETs to 3205's but needed the gate resistors to be changed out to 47 ohm. I assumed this meant someone had attempted a repair and botched the job.
On that note, do you believe the (within tolerance) resistors lead to a high probability of the gate drive transistors being okay, or would their condition be unrelated. I'll probably try to replace the transistors anyway, just trying to bump my chances of a successful repair.
THANKS!!! (for helping a newbie hack)
The 100 ohm fate resistors are orginal . If they all test fine dont replace them.
If you switch to 47 ohm gate resistors you might have problems with the amp turning on to fast.
Your next question was how do you check the driver transistors for the power supply fets.
Set you meter to ohms and test between the legs of them 1-2 1-3 2-3
Post your results.
If you switch to 47 ohm gate resistors you might have problems with the amp turning on to fast.
Your next question was how do you check the driver transistors for the power supply fets.
Set you meter to ohms and test between the legs of them 1-2 1-3 2-3
Post your results.
Not sure if it matters, but these figures are with the bad FETs installed. (a repair shop wanted everything how I found it if I took it in, so I reinstalled them before I decided to attempt the repair myself.)
Q05:
1-2 = 11.1
1-3 = 29.6
2-3 = 37.4
Q11:
1-2 = 276.8
1-3 = 41.5
2-3 = 235.7
Sorry about the slow replies, I'm still a moderated poster so there is a bit of a delay.
Q05:
1-2 = 11.1
1-3 = 29.6
2-3 = 37.4
Q11:
1-2 = 276.8
1-3 = 41.5
2-3 = 235.7
Sorry about the slow replies, I'm still a moderated poster so there is a bit of a delay.
I should have known better.....
Anywho, here's the numbers:
Q05:
1-2 = 11.1
1-3 = 245.3
2-3 = 234.6
Q11:
1-2 = 31.56k
1-3 = 31.58k
2-3 = 235.6
I also check all the gate resistors again and found R16 now reads 111.3. All the others were still around 100.1 to 100.2. Yes, I had checked them earlier with the FETs removed. But, I guess I'll need to at least replace R16. I guess they are 1206 size, but what other specs do I need to know to order from digikey. (wattage, pulse withstanding, etc.)
Anywho, here's the numbers:
Q05:
1-2 = 11.1
1-3 = 245.3
2-3 = 234.6
Q11:
1-2 = 31.56k
1-3 = 31.58k
2-3 = 235.6
I also check all the gate resistors again and found R16 now reads 111.3. All the others were still around 100.1 to 100.2. Yes, I had checked them earlier with the FETs removed.
But, I guess I'll need to at least replace R16. I guess they are 1206 size, but what other specs do I need to know to order from digikey. (wattage, pulse withstanding, etc.)Q05 does appear to be defective. Order the MMBTA56 to replace it.
Yes, they are 1206. That and the resistance is really all you need.
Order several extras of all parts.
After replacing Q05 but before installing the new power supply FETs, measure the gate voltage on the gate pads for the power supply FETs. They should all read approximately 5v DC.
Have you checked the output FETs yet?
Do either of the output inductors appear to have overheated?
Yes, they are 1206. That and the resistance is really all you need.
Order several extras of all parts.
After replacing Q05 but before installing the new power supply FETs, measure the gate voltage on the gate pads for the power supply FETs. They should all read approximately 5v DC.
Have you checked the output FETs yet?
Do either of the output inductors appear to have overheated?
I haven't check the output FETs in any way. (don't know how) The board looks clean with no darkening of the inductors. There isn't a physical sign of any problem other than the crumbly power supply FETs.
I always order 100% backup for parts so I can (all thumbs) my way through repairs. I'll get the parts on order. BTW - I've never needed the extra parts..... yet.
Double checking... RMCF1206FT100R is what I came up with for a 100 ohm gate resistor. Sound good to you? (hxxp://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=RMCF1206FT100RCT-ND)
Again, thanks for the support.
I always order 100% backup for parts so I can (all thumbs) my way through repairs. I'll get the parts on order. BTW - I've never needed the extra parts..... yet.
Double checking... RMCF1206FT100R is what I came up with for a 100 ohm gate resistor. Sound good to you? (hxxp://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=RMCF1206FT100RCT-ND)
Again, thanks for the support.
Before ordering parts you need to check the output transistors to make sure none are shorted.
You may have shorted outputs causing the power supply fets to fail.
If you have a multimeter set it to ohms.
Place black probe on leg 1 of the output transistor place red probe on middle leg then place black probe on leg 1 and red probe on leg 3 then black probe on middle leg (leg 2) and red probe on leg 3.
You should read nothing near 0 ohms. If you do the output transistor is likely shorted.
If you have 1 or 2 that your not sure about post your results
as follows
1-2:
1-3:
2-3:
You may have shorted outputs causing the power supply fets to fail.
If you have a multimeter set it to ohms.
Place black probe on leg 1 of the output transistor place red probe on middle leg then place black probe on leg 1 and red probe on leg 3 then black probe on middle leg (leg 2) and red probe on leg 3.
You should read nothing near 0 ohms. If you do the output transistor is likely shorted.
If you have 1 or 2 that your not sure about post your results
as follows
1-2:
1-3:
2-3:
Again... Thanks Mike. You must be really good with children. I appreciate the step by step instructions.
Also... Thanks Mr. Babin. I kept seeing references to your links but I couldn't find them. I didn't realize your signature was clickable until now. I will do a lot more reading before asking to be spoon fed from this point forward.
Also... Thanks Mr. Babin. I kept seeing references to your links but I couldn't find them. I didn't realize your signature was clickable until now. I will do a lot more reading before asking to be spoon fed from this point forward.
Last edited:
Work got in the way of my testing, darn it.
Anyway... Pulled all the output transistors and tested them using the diode function test from Perry Babin's link. There are a two banks of transistors, one on each side of the amp. Each bank has three N-Channel FETs (IRF640) and four P-Channel FETs (IRF9640). It looks like this:
N N N P P P P
N N N P P P P
The red "N" designated above is an IRF640 that is shorted out. I read somewhere that if one is shorted out in a bank, you should swap out the whole bank. Does that mean the (NNNPPPP) or (NNNNNN)? All the other transistors read almost identical with each other only passing an in tolerance reading in the correct direction. (Yes, I swapped leads between the P-Channel and N-Channel tests.)
So, which transistors should I swap out? I can get 8 9640's for $20, and 6 640's for $17 from Digikey. Am I better off swapping all of them, or is it safe to go with the one 640, or? Also, with the shorted transistor is there anything else I should be looking for on the output side? I checked the resistors around the removed transistors and everything appears okay in that regard.
Thanks.
Anyway... Pulled all the output transistors and tested them using the diode function test from Perry Babin's link. There are a two banks of transistors, one on each side of the amp. Each bank has three N-Channel FETs (IRF640) and four P-Channel FETs (IRF9640). It looks like this:
N N N P P P P
N N N P P P P
The red "N" designated above is an IRF640 that is shorted out. I read somewhere that if one is shorted out in a bank, you should swap out the whole bank. Does that mean the (NNNPPPP) or (NNNNNN)? All the other transistors read almost identical with each other only passing an in tolerance reading in the correct direction. (Yes, I swapped leads between the P-Channel and N-Channel tests.)
So, which transistors should I swap out? I can get 8 9640's for $20, and 6 640's for $17 from Digikey. Am I better off swapping all of them, or is it safe to go with the one 640, or? Also, with the shorted transistor is there anything else I should be looking for on the output side? I checked the resistors around the removed transistors and everything appears okay in that regard.
Thanks.
For now, reinstall all of the good FETs, clamp everything down,insert a small fuse (10-15 amp) in the B+ line and power it up. Does it work with the missing/shorted FET out of the circuit?
When you find all of the faults, you'll replace all of the 640s and 9640s in that bank. The other side can remain as is or you can replace them if you want to.
You can't simply replace the single shorted FETs. At the very least you'd have to replace the three parallel 640s.
When you find all of the faults, you'll replace all of the 640s and 9640s in that bank. The other side can remain as is or you can replace them if you want to.
You can't simply replace the single shorted FETs. At the very least you'd have to replace the three parallel 640s.
Okay, that's the answer I was looking for. I didn't know if I could damage something powering up with the missing FET. I'll get on it after work tomorrow.
One last question on the 640 FETs. On Digikey they list a IRF640PBF and a IRF640NPBF. The one with the "N" designator lists as a "HEXFET" series and 150W vs. 125W power handling. The "N" version is a buck less at $1.88. Is there a reason I wouldn't want to use this one? It appears better but only 2/3 the price. What am I missing?
One last question on the 640 FETs. On Digikey they list a IRF640PBF and a IRF640NPBF. The one with the "N" designator lists as a "HEXFET" series and 150W vs. 125W power handling. The "N" version is a buck less at $1.88. Is there a reason I wouldn't want to use this one? It appears better but only 2/3 the price. What am I missing?
I guess I'm showing my ignorance. My 640's only have the following info on them:
IRF640
I(diode sign)R 444G
78 C6
So, I thought the PBF and NPBF were just "in house" codes for different manufacturers and options. I guess they don't interchange? I thought a IRF640 was an IRF640.
Sorry about the questions.... I thought I was so smart after reading your links. So guessing by your previous answer, my transistor would need to say IRF640N to use the N version?
Honest... This is the last question till this baby is pumping out some tunes!
THANKS!
IRF640
I(diode sign)R 444G
78 C6
So, I thought the PBF and NPBF were just "in house" codes for different manufacturers and options. I guess they don't interchange? I thought a IRF640 was an IRF640.
Sorry about the questions.... I thought I was so smart after reading your links.
So guessing by your previous answer, my transistor would need to say IRF640N to use the N version?Honest... This is the last question till this baby is pumping out some tunes!
THANKS!
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