I’d love to use differential mode on my scope. Your tutorial talks a lot about ground and where to reference it. I take all of that into account when I’m taking my voltage measurements.
If you do what everyone else does, then we can expect their results. I’m going to have to invest in differential probes to do this. My scope doesn’t seem to want to cooperate even when I do what the users manual says to do.
Differential probes will isolate and protect the ground so that it doesn’t send -100V into my scope and into my mains.
If you do what everyone else does, then we can expect their results. I’m going to have to invest in differential probes to do this. My scope doesn’t seem to want to cooperate even when I do what the users manual says to do.
Differential probes will isolate and protect the ground so that it doesn’t send -100V into my scope and into my mains.
Drive it hard (full power and with a load). Do the output FETs heat up, excessively?
I don't really see problems with the drive but the rest is heating while the amp is driving hard.
I don't think I've worked on one of these but the drive for most amps like this have drivers that run so hot that they damage the board (some more than others).
What were the original output FETs? What are you using now?
In differential mode, there is no risk of driving voltage back to the supply. Differential probes can be expensive. All you need is two matched probes ($5 each). Find a forum that covers your scope and ask them about the settings.
I don't really see problems with the drive but the rest is heating while the amp is driving hard.
I don't think I've worked on one of these but the drive for most amps like this have drivers that run so hot that they damage the board (some more than others).
What were the original output FETs? What are you using now?
In differential mode, there is no risk of driving voltage back to the supply. Differential probes can be expensive. All you need is two matched probes ($5 each). Find a forum that covers your scope and ask them about the settings.
I have never worked on US Amps. This one is my first.
The outputs were IRFP264N’s and the drivers were all the same as what was in it (mpsa56’s coupled with mpsa42’s). I replaced them with IRFP4229’s with the same gate resistor values.
The power supply drive circuit was A1266’s with IRF1010E’s. So I replaced them with BD140’s with IRF3205’s since they are not yet obsolete.
The last person who worked on this amp butchered it badly. I was able to get it working enough where I contacted you for help as I was going nuts trying to figure it all out.
I now have to order parts to replace some of the stuff I’ve subbed (same parts just size differences).
And yes, I will go with the forum idea first. I’ve always wanted to see what the true scope patterns look like but wasn’t able to figure out my scope settings enough to do it. When you were describing it I was like yes I remember that from the tutorial - ground references are not always DC 0V ground where the battery supplies its power. I just never heard it called differential measurement according to the scope. Now that I know what you are referring to I’m going to be investigating how I can get that all done.
When I inject audio, that drive pattern simply starts moving back and forth quickly indicating that it’s duty cycling.
The outputs were IRFP264N’s and the drivers were all the same as what was in it (mpsa56’s coupled with mpsa42’s). I replaced them with IRFP4229’s with the same gate resistor values.
The power supply drive circuit was A1266’s with IRF1010E’s. So I replaced them with BD140’s with IRF3205’s since they are not yet obsolete.
The last person who worked on this amp butchered it badly. I was able to get it working enough where I contacted you for help as I was going nuts trying to figure it all out.
I now have to order parts to replace some of the stuff I’ve subbed (same parts just size differences).
And yes, I will go with the forum idea first. I’ve always wanted to see what the true scope patterns look like but wasn’t able to figure out my scope settings enough to do it. When you were describing it I was like yes I remember that from the tutorial - ground references are not always DC 0V ground where the battery supplies its power. I just never heard it called differential measurement according to the scope. Now that I know what you are referring to I’m going to be investigating how I can get that all done.
When I inject audio, that drive pattern simply starts moving back and forth quickly indicating that it’s duty cycling.
If the outputs run hotter than expected, you may have to adjust the current through the CCS. If they run cool at high power, leave it as is.
So I got the rest of the parts and installed them into this amp.
I clamped the board back to the case and powered it up. I did a low power test with a 4 ohm 500W dummy load. The amp worked alright, nothing got too outrageously hot.
Tonight I powered it up with a cluster of dummy loads equaling 1 ohm and 2KW. I did another low power test to start. The amp power supply was stone cold but the outputs got more hotter. The outputs temps sat at around 41C and at one point the last bank became hotter by about 8C. Then all of a sudden the outputs blew. No snapping or bang, just quietly. My power supply went into current protection. So after powering the amp down I have short across the outputs and nothing else.
Maybe I should try harder to source outputs that are of the originals?
I clamped the board back to the case and powered it up. I did a low power test with a 4 ohm 500W dummy load. The amp worked alright, nothing got too outrageously hot.
Tonight I powered it up with a cluster of dummy loads equaling 1 ohm and 2KW. I did another low power test to start. The amp power supply was stone cold but the outputs got more hotter. The outputs temps sat at around 41C and at one point the last bank became hotter by about 8C. Then all of a sudden the outputs blew. No snapping or bang, just quietly. My power supply went into current protection. So after powering the amp down I have short across the outputs and nothing else.
Maybe I should try harder to source outputs that are of the originals?
What are you referring to as 'the last bank'?
How much was the highest current it was drawing when testing?
Music or sine wave?
Current rating of the power supply?
How much was the highest current it was drawing when testing?
Music or sine wave?
Current rating of the power supply?
The last bank refers to the last two fets on each side the closest to the voltage regulators. I always speak as if the power supply is on the left and the outputs are laying to the right.
I set my power supply to 14.4V - 20A, but only turned the source up a little bit so that the amp is drawing 10A of the available 20A it can provide. I inputted a sine wave of 60Hz with a CD on a deck connected to the same power source.
I hadn't got to the powermax which can supply 100A yet before it blew.
I set my power supply to 14.4V - 20A, but only turned the source up a little bit so that the amp is drawing 10A of the available 20A it can provide. I inputted a sine wave of 60Hz with a CD on a deck connected to the same power source.
I hadn't got to the powermax which can supply 100A yet before it blew.
I need the circuit board designations. If different from the MD2, give their designations from the MD2 diagram.
This amp has 2 output banks. Each bank has 4 FETs.
What outputs did you use?
From a reputable distributor?
This amp has 2 output banks. Each bank has 4 FETs.
What outputs did you use?
From a reputable distributor?
Sorry Perry
This amp has 2 output banks, each bank has 6 fets.
The first two fets directly fed by the drive circuit in each bank are getting slightly hotter than the rest by about an average of 10deg C. That would be Q48 and Q49 as well as Q60 and Q61. The low side Q60 and Q61 are getting the hottest. This is the same side that has the weird gate pattern.
I bought the output fets from DigiKey. They are the IRFP4229’s. They are very close to the IRFP264N’s. The only thing I didn’t do, is mess with the gate resistor values to try and figure out the weird gate pattern. I assumed that the high side looks alright so the gate resistor values should be acceptable at 47 ohms.
This amp has 2 output banks, each bank has 6 fets.
The first two fets directly fed by the drive circuit in each bank are getting slightly hotter than the rest by about an average of 10deg C. That would be Q48 and Q49 as well as Q60 and Q61. The low side Q60 and Q61 are getting the hottest. This is the same side that has the weird gate pattern.
I bought the output fets from DigiKey. They are the IRFP4229’s. They are very close to the IRFP264N’s. The only thing I didn’t do, is mess with the gate resistor values to try and figure out the weird gate pattern. I assumed that the high side looks alright so the gate resistor values should be acceptable at 47 ohms.
It's possible that the replacements need more drive (higher CCS current) but that shouldn't cause the difference in temperature for 2 of the 6 FETs in the bank. Possible reasons:
Different batch
Gate resistors out of tolerance
Something different in the insulators, heatsink compound applied
Different heatsink geometry/profile
Different batch
Gate resistors out of tolerance
Something different in the insulators, heatsink compound applied
Different heatsink geometry/profile
I removed all the outputs. One per side died and the rest are OK. I powered the amp up and it’s oscillating.
Q40, Q41, and Q53 are all running hot still. Q41 runs the hottest at 105C and the others are at about 85C. Is that because of the amps output frequency being 68KHz?
Also, the drive signal has cleaned up a lot since removing the fets. If I am to change the gate resistor value, what would I change it to? Right now they’re 47ohms. I’m wondering if I should try 22ohms. At this point I’m going to replace all the outputs, gates, and diodes once again.
I did not originally replace the big 3W resistors (R149, R150, R151), but should I? They don’t look cooked and while in operation they don’t heat up. The voltage across them all looked normal.
I did check my batches for all the output fets and they are all the same. Also, this amp came with kapton tape that was all slashed up. Instead I put silpad strips with thermal paste both under the silpad and under the fets. I’m starting to think I had a gate resistor or two that might have been drifting while being driven hard?
Q40, Q41, and Q53 are all running hot still. Q41 runs the hottest at 105C and the others are at about 85C. Is that because of the amps output frequency being 68KHz?
Also, the drive signal has cleaned up a lot since removing the fets. If I am to change the gate resistor value, what would I change it to? Right now they’re 47ohms. I’m wondering if I should try 22ohms. At this point I’m going to replace all the outputs, gates, and diodes once again.
I did not originally replace the big 3W resistors (R149, R150, R151), but should I? They don’t look cooked and while in operation they don’t heat up. The voltage across them all looked normal.
I did check my batches for all the output fets and they are all the same. Also, this amp came with kapton tape that was all slashed up. Instead I put silpad strips with thermal paste both under the silpad and under the fets. I’m starting to think I had a gate resistor or two that might have been drifting while being driven hard?
68k is relatively low and may drop when the amp is driven but that's something you'd need to confirm.
In some of these amps, the drivers run hot. I don't know how hot they run in this amp. Is the board darkened around the hottest of the transistors?
The 3w resistors are in direct parallel so unless your amp is different, I don't think they are causing any heating problems (unless, possibly if they are FAR out of tolerance).
The gate resistor value is a difficult decision, especially if it's being used to create dead time.
Do the outputs run hot at idle, off of the heatsink
In some of these amps, the drivers run hot. I don't know how hot they run in this amp. Is the board darkened around the hottest of the transistors?
The 3w resistors are in direct parallel so unless your amp is different, I don't think they are causing any heating problems (unless, possibly if they are FAR out of tolerance).
The gate resistor value is a difficult decision, especially if it's being used to create dead time.
Do the outputs run hot at idle, off of the heatsink
The drivers are not causing the board to darken where they sit. When you look at the drive waveforms at those transistors, the duty cycle looks like it’s pretty well minimum. The rest of the output drive circuit seemed to be running relatively cool while the amp was being pushed.
I checked across all the gate resistors while the amp was working and it was all relatively even at 0.230V.
I’m going to order new fets. I don’t stock IRFP4229’s as I don’t usually use them. It’ll be a couple of business days before I will get new ones.
When I tested the amp out of the case the outputs warmed up but for some amps that’s normal so I didn’t suspect anything at that point. I wanted to clamp it to the case to make sure I didn’t burn anything while testing. The power supply fets never got hot while in the case or out. They stay stone cold always.
To blow one output per side does baffle me though. I’m going to order 22 ohm, 10 ohm, and 5 ohm resistors. These resistors are quite small in comparison to what I stock for through hole stuff.
I checked across all the gate resistors while the amp was working and it was all relatively even at 0.230V.
I’m going to order new fets. I don’t stock IRFP4229’s as I don’t usually use them. It’ll be a couple of business days before I will get new ones.
When I tested the amp out of the case the outputs warmed up but for some amps that’s normal so I didn’t suspect anything at that point. I wanted to clamp it to the case to make sure I didn’t burn anything while testing. The power supply fets never got hot while in the case or out. They stay stone cold always.
To blow one output per side does baffle me though. I’m going to order 22 ohm, 10 ohm, and 5 ohm resistors. These resistors are quite small in comparison to what I stock for through hole stuff.
It's VERY common for only one output per phase to fail When one fails, the amp tries to correct by driving the other phase as hard as possible and if the protection circuit fails to work as it should, the weakest output in the hard-driven phase will fail. This generally causes the amp to blow a fuse or the power supply.
Perry,
I reached out to US Amps to see what outputs these guys are using. To my surprise they got back to me and told me that the IRFP264N's were never used in this amp. The correct output fets are IRFP360 or FQA24N50.
I replaced all the gate resistors and diodes before fitting new IRFP360's. After fitting the new fets, I have a strange new symptom that I've never seen before. While watching the current draw on low power (20A or less), I start at around 15A or so, and then after about a minute or so the amp will slowly draw less and less current to idle current. Then after about 2 mins or so, the current draw will slowly increase back to the 15A where I started.
Am I crazy or something? The outputs are warming up nice and even to about 40C while its doing this. I'm using a limited power supply @ 14.4V with no more than 20A. I'm using a deck which is grounded to the same power source as the amp is. I'm using a burned CD with the test tones I got from your tutorial. I have the amp wired to 1OHM across 2KW worth of dummy loads. The drive circuit appears to be warming up to no more than 75C and seems to be happy there.
I found a video on YouTube where a fellow shows all the drive circuits to the power supply and outputs and mine all match (even the weird drive on the low side). The frequencies match as well. His video briefly mentions some kind of feedback resistor in the preamp area causing the amp to not output as much power - he doesn't detail which resistor this would be however. The only feedback resistor that I had to replace was the one the previous repairer removed from the output section back to the preamp section. I put in 100K resistor in there as the diagram that I'm using specified.
I reached out to US Amps to see what outputs these guys are using. To my surprise they got back to me and told me that the IRFP264N's were never used in this amp. The correct output fets are IRFP360 or FQA24N50.
I replaced all the gate resistors and diodes before fitting new IRFP360's. After fitting the new fets, I have a strange new symptom that I've never seen before. While watching the current draw on low power (20A or less), I start at around 15A or so, and then after about a minute or so the amp will slowly draw less and less current to idle current. Then after about 2 mins or so, the current draw will slowly increase back to the 15A where I started.
Am I crazy or something? The outputs are warming up nice and even to about 40C while its doing this. I'm using a limited power supply @ 14.4V with no more than 20A. I'm using a deck which is grounded to the same power source as the amp is. I'm using a burned CD with the test tones I got from your tutorial. I have the amp wired to 1OHM across 2KW worth of dummy loads. The drive circuit appears to be warming up to no more than 75C and seems to be happy there.
I found a video on YouTube where a fellow shows all the drive circuits to the power supply and outputs and mine all match (even the weird drive on the low side). The frequencies match as well. His video briefly mentions some kind of feedback resistor in the preamp area causing the amp to not output as much power - he doesn't detail which resistor this would be however. The only feedback resistor that I had to replace was the one the previous repairer removed from the output section back to the preamp section. I put in 100K resistor in there as the diagram that I'm using specified.
Did they say IRFP360 or IRFP360LC?
Did you try adjusting the CCS current by changing the value of R168 (MD2 designation)?
Did you try adjusting the CCS current by changing the value of R168 (MD2 designation)?
I did not mess with R168 - I left it alone. They replied with IRFP360PBF fets.
By changing the R168 value, what can I expect at the outputs? The drive looks alright and the frequencies are alright. It’s just for some reason the amp is throttling its current from high to low, high to low.
By changing the R168 value, what can I expect at the outputs? The drive looks alright and the frequencies are alright. It’s just for some reason the amp is throttling its current from high to low, high to low.
Increasing the value should reduce the idle current. The difference should not need to be more than about 200 ohms. You can test with a series-connected (in series with the 1k resistor) pot or by inserting various resistors in series with the 1k until you find the right value.