Hello All,
I have a 500/5 that had a bad channel because one of the bias transistors (Q202 and Q203) shorted out. I replaced both with IRF540's that I sourced from ebay. I made sure not to use IRF540N or IRF540Z parts.
When I hooked the amp back up in the car, several transistors (IRFZ44R) burned up, unfortunately. Google tells me these are N-channel MOSFETs, so I'm guessing they're output transistors.
Can anyone tell me what I might have overlooked? Any idea why those transistors burned out? I don't see any obvious shorts from the board(s) to the chassis. The speakers all read 4-6 ohms. I did run a brief test at home, but I only tested two channels at a time into 8 ohm speakers.
I'd rather learn how to fix this than send it in for repair, so any advice would be greatly appreciated. I'm ok with ordering parts and replacing components, but I'd like to avoid having to do it over and over again.
Thanks in advance,
Wayne.
I have a 500/5 that had a bad channel because one of the bias transistors (Q202 and Q203) shorted out. I replaced both with IRF540's that I sourced from ebay. I made sure not to use IRF540N or IRF540Z parts.
When I hooked the amp back up in the car, several transistors (IRFZ44R) burned up, unfortunately. Google tells me these are N-channel MOSFETs, so I'm guessing they're output transistors.
Can anyone tell me what I might have overlooked? Any idea why those transistors burned out? I don't see any obvious shorts from the board(s) to the chassis. The speakers all read 4-6 ohms. I did run a brief test at home, but I only tested two channels at a time into 8 ohm speakers.
I'd rather learn how to fix this than send it in for repair, so any advice would be greatly appreciated. I'm ok with ordering parts and replacing components, but I'd like to avoid having to do it over and over again.
Thanks in advance,
Wayne.
Q201 is the bias transistor for Q202 and Q203.
The 540s are the output transistors.
The Z44s are the power supply transistors. You'll need to replace all of the power supply transistors and probably all of their gate resistors in the power supply that failed (there are multiple power supplies). Which of the Z44s failed?
Do NOT buy semiconductors from eBay. There are too many fakes and too few legitimate sellers. Buy from a reputable distributor like Digi-Key or Mouser.
Replace the 540s that you purchased from eBay with those from one of the distributors I listed.
You may have had the bias set too high in the channel that you replaced the parts in.
How long did the amp work in the vehicle after you reinstalled it?
What were you using for a power supply on the work bench?
The 540s are the output transistors.
The Z44s are the power supply transistors. You'll need to replace all of the power supply transistors and probably all of their gate resistors in the power supply that failed (there are multiple power supplies). Which of the Z44s failed?
Do NOT buy semiconductors from eBay. There are too many fakes and too few legitimate sellers. Buy from a reputable distributor like Digi-Key or Mouser.
Replace the 540s that you purchased from eBay with those from one of the distributors I listed.
You may have had the bias set too high in the channel that you replaced the parts in.
How long did the amp work in the vehicle after you reinstalled it?
What were you using for a power supply on the work bench?
Thanks for clarifying, that makes more sense. I couldn't imagine why the bias transistors need to be so hefty.
Q904 and 906 completely burned out. The ones around them (Q903, Q908, Q907) are covered in oily soot. As are the nearby components on the motherboard.
Ok, lesson learned, will do.
Not sure how that could have happened, unless it was the act of installing the ebay 540's that did it?
I started with a 0-30V, 0-3A current limiting power supply I built as student long ago. It was set at 12V of course. When it seemed to be unable to supply enough current to the amp, I switched to a computer power supply which I had converted for bench use. The amp seemed to run fine with that - no funny smells. I only ran it for a few minutes though, so maybe not long enough for problems to develop.
In the car, I connected the front speakers first, tested them for about 15 seconds, then proceeded to the rear speakers. That's when a fuse blew. Figuring that it was an old fuse, I replaced it. Instead of the fuse blowing, the amp started to smoke. Guess I should have taken the hint and stopped when the stopping was good.
Thanks again for taking an interest, and for the good advice.
When you get the amp repaired and reinstalled, use nothing larger than a 20 amp fuse and confirm that the amp is working perfectly at low to moderate volume before going back to the normal fuse. The fuse that's recommended offers essentially no protection for the amp. This applies to many of the amplifiers on the market.
If I'm not mistaken, the FETs that failed are for the sub and rear channels. The FETs you replaced are for the front channels. One should not have affected the other unless both supplies failed.
Go back and check every transistor on the heatsink. Which ones have failed (you don't have to list the 900 series FETs that you already found)?
If I'm not mistaken, the FETs that failed are for the sub and rear channels. The FETs you replaced are for the front channels. One should not have affected the other unless both supplies failed.
Go back and check every transistor on the heatsink. Which ones have failed (you don't have to list the 900 series FETs that you already found)?
Ok, I put my multimeter in continuity mode and a probe on each of the outer legs while the transistors are still on the board. Of the transistors along the heatsink, Q302 is the only one that appears to be shorted out.
Not sure if this is the correct way to do things, please let me know if there's a better way.
Thanks again for your interest and rapid replies!
Cheers,
Wayne.
Yes, the other Z44s aren't shorted out. I even checked the little guys, Q409, 421, 420, 411, 419, 407, 404.
I had a look on Digikey and Mouser for Z44Rs and 540s. Of the two, it seems Digikey is most willing to deal in small quantities.
Finding a replacement for the 540s is not straightforward. The original HEXFET part has been discontinued by IRF, and other manufactures have created variations of it. I'm going to have a closer look at the datasheets, but can you tell me which parameters I should try hardest to match up?
Thanks again,
Wayne.
I've been trying to figure out why Q904 and 906 burned out. They're under the same spring clamp, so perhaps I cracked them. What do you think, does that failure mode sound plausible?
I found some IRF540's at a local retailer I've used before, Active Tech (active123.com). They're made by Vishay/Siliconix, and I feel pretty good about them. Unfortunately they didn't have the Z44Rs (just Z44N, which is lower-spec'd). So I'm putting together an order for three Z44Rs to replace Q904, 906, and 908. Do you think I should replace 907, 905 and 909 as well? I'm not sure if they're on the same power supply as the others.
Also, I'm leaning towards replacing the thermal interface material on the heatsink. I've seen threads here where you recommended Kapton MT for D-class amps, though Kapton tape will do for A/B amps. Since the 500/5 is A/B and D for the sub-woofer, which way should I go? I couldn't find Kapton MT tape online, what do you recommend?
Thanks again,
Wayne.
I found some IRF540's at a local retailer I've used before, Active Tech (active123.com). They're made by Vishay/Siliconix, and I feel pretty good about them. Unfortunately they didn't have the Z44Rs (just Z44N, which is lower-spec'd). So I'm putting together an order for three Z44Rs to replace Q904, 906, and 908. Do you think I should replace 907, 905 and 909 as well? I'm not sure if they're on the same power supply as the others.
Also, I'm leaning towards replacing the thermal interface material on the heatsink. I've seen threads here where you recommended Kapton MT for D-class amps, though Kapton tape will do for A/B amps. Since the 500/5 is A/B and D for the sub-woofer, which way should I go? I couldn't find Kapton MT tape online, what do you recommend?
Thanks again,
Wayne.
You need to replace all 6 FETs in the supply that failed.
It's virtually impossible to crack the casing on power semiconductors. The spring clips are far too weak to break the transistors.
The mica they're using is perfectly fine. Clean it up with solvent. Apply new heatsink compound to both sides of the insulator and reassemble the amp.
The tape is more suitable for class D amps but I've used a LOT of it in all types of amps. The MT is probably a bit better thermal conductor so I tend to use it instead of the tape in amplifiers that have transistors that have to dissipate a lot of heat (non class D amps). It works perfectly fine in class D amps.
I doubt that you'll see any difference between the N and the R but it is generally better to go with the original parts.
Have you tried future electronics? They're in Canada.
It's virtually impossible to crack the casing on power semiconductors. The spring clips are far too weak to break the transistors.
The mica they're using is perfectly fine. Clean it up with solvent. Apply new heatsink compound to both sides of the insulator and reassemble the amp.
The tape is more suitable for class D amps but I've used a LOT of it in all types of amps. The MT is probably a bit better thermal conductor so I tend to use it instead of the tape in amplifiers that have transistors that have to dissipate a lot of heat (non class D amps). It works perfectly fine in class D amps.
I doubt that you'll see any difference between the N and the R but it is generally better to go with the original parts.
Have you tried future electronics? They're in Canada.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.