Channel 4 of my Alpine MRV-F540 was a little scratchy so Ipulled it from the car and set it aside for a few years. Now I'm trying todiagnose the problem.
I checked the DC offset of each output, and all are < 3mV except for channel 4, which measured about 21 mV. After that I checked thesignals entering and exiting the crossover boards, and all of the channels areabout the same, so I'm assuming the problem is elsewhere.
In checking the voltages on each leg of the outputtransistors it's apparent that channel 4 is different than the other channels,and out of balance. Here's what they look like:
Click this link to see the schematic close up: https://photos.app.goo.gl/pieLZonPvrOAuiHs2
And here's what they measure like. I'm powering this with12VDC to do these tests, so I expect that the 29.17 VDC voltage on thecollector is probably due to that as the service manual uses 14.4VDC fortesting.
Ch4 Ch3 Spec
E -0.0212 0.0019 0.00
C 29.18 29.17 34.6
B 0.425 0.447 0.45
B -0.398 -0.376 -0.38
C 0.978 1.000 1.04
E -1.021 -0.997 -1.04
E -0.0212 0.0016 0.00
C -29.18 -29.17 -35.2
B -0.474 -0.451 -0.45
Is there a typical problem point that would cause these to be out of balance? There's no bias pot that I can find on the schematic or on the board, though I haven't pulled the board out yet to look at the back side. Thanks!
I checked the DC offset of each output, and all are < 3mV except for channel 4, which measured about 21 mV. After that I checked thesignals entering and exiting the crossover boards, and all of the channels areabout the same, so I'm assuming the problem is elsewhere.
In checking the voltages on each leg of the outputtransistors it's apparent that channel 4 is different than the other channels,and out of balance. Here's what they look like:
Click this link to see the schematic close up: https://photos.app.goo.gl/pieLZonPvrOAuiHs2
And here's what they measure like. I'm powering this with12VDC to do these tests, so I expect that the 29.17 VDC voltage on thecollector is probably due to that as the service manual uses 14.4VDC fortesting.
Ch4 Ch3 Spec
E -0.0212 0.0019 0.00
C 29.18 29.17 34.6
B 0.425 0.447 0.45
B -0.398 -0.376 -0.38
C 0.978 1.000 1.04
E -1.021 -0.997 -1.04
E -0.0212 0.0016 0.00
C -29.18 -29.17 -35.2
B -0.474 -0.451 -0.45
Is there a typical problem point that would cause these to be out of balance? There's no bias pot that I can find on the schematic or on the board, though I haven't pulled the board out yet to look at the back side. Thanks!
Attachments
Oops.
These little 2SK2145 SMD JFET guys have 5 leads, and afterdesoldering with wick I was gently prying each of the leads up with an Exactoto make sure they were completely desoldered and it went "Pop!" and flew up and behind my workbench into the pitof eternal peril. All of the SMD stuff on this amp is stuck down to the board with these little redadhesive blobs and they don't want to let go without a fight.
The data sheet for the 2SK2145 makes me feel like we shouldexpect them to fail pretty regularily:
"Using continuously under heavy loads (e.g. theapplication of high temperature/current/voltage and the significant change intemperature, etc.) may cause this product to decrease in the reliabilitysignificantly even if the operating conditions (i.e. operating temperature/current/voltage,etc.) are within the absolute maximum ratings."
The Alpine service manual calls out 2SK2145 - Y GR BL forthese, so I'm assuming that means it can use any of those three versions.Evidently that suffix indicates the "Drain Current" IDSS classification,as follows
Y: 1.2 to 3.0 mA,
GR: 2.6 to 6.5 mA,
BL: 6.0 to 14.0 mA
DigiKey stocks both the Y and the GR versions, should Ifavor the Y version with the lower drain current?
I'll order four of them and swap them all out, as thatwarning on the data sheet makes me wonder if the others aren't far behind.
These little 2SK2145 SMD JFET guys have 5 leads, and afterdesoldering with wick I was gently prying each of the leads up with an Exactoto make sure they were completely desoldered and it went "Pop!" and flew up and behind my workbench into the pitof eternal peril. All of the SMD stuff on this amp is stuck down to the board with these little redadhesive blobs and they don't want to let go without a fight.
The data sheet for the 2SK2145 makes me feel like we shouldexpect them to fail pretty regularily:
"Using continuously under heavy loads (e.g. theapplication of high temperature/current/voltage and the significant change intemperature, etc.) may cause this product to decrease in the reliabilitysignificantly even if the operating conditions (i.e. operating temperature/current/voltage,etc.) are within the absolute maximum ratings."
The Alpine service manual calls out 2SK2145 - Y GR BL forthese, so I'm assuming that means it can use any of those three versions.Evidently that suffix indicates the "Drain Current" IDSS classification,as follows
Y: 1.2 to 3.0 mA,
GR: 2.6 to 6.5 mA,
BL: 6.0 to 14.0 mA
DigiKey stocks both the Y and the GR versions, should Ifavor the Y version with the lower drain current?
I'll order four of them and swap them all out, as thatwarning on the data sheet makes me wonder if the others aren't far behind.
We'll I replaced all four of the little buggers. My soldering isn't as nice as the original, but I only had one bridge that needed fixing and all was well. I went with the GR version of the little FETs, as that was what DigiKey had the most of. I was able to determine that the original ones were the Y version.
After the repairs CH4 is measuring only 4.5 mV of offset, which is way better (it used to be 21 mV). All of the legs of the output transistors are balanced now - yay!
The other channels vary from 0.1 to about 2.8 mV, so Ch4 isn't too far. Maybe some other component was taking too much heat when everything was out of balance. I'll look over the schematic some more to see if I can figure out where that could be coming from just for fun.
I'll give it a full power sound test this weekend. Thanks for your help Perry!
After the repairs CH4 is measuring only 4.5 mV of offset, which is way better (it used to be 21 mV). All of the legs of the output transistors are balanced now - yay!
The other channels vary from 0.1 to about 2.8 mV, so Ch4 isn't too far. Maybe some other component was taking too much heat when everything was out of balance. I'll look over the schematic some more to see if I can figure out where that could be coming from just for fun.
I'll give it a full power sound test this weekend. Thanks for your help Perry!
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.