Hello everyone,
I've got a Nakamichi PA-5A II amp that is have some difficulties. If I had to boil it down, my main question is: is this fixable? Or more accurately, is this fixable without putting hundreds of dollars into it?
I got the amp in 2013 as part of a craigslist deal, so I'm not sure what sort of abuse it was put through to cause the problems. It never worked, but I knew that when I got it. It was used to drive some Polk SDA SRS 2.3s, which is what I was buying - the amp was thrown in on the deal. The speakers work great, so whatever caused the damage to the amp apparently didn't damage the speakers. Just from what I remember, I plugged it in and the light on the front would always come on, but would get random behavior from the unit. Maybe 1 in 5 times I would get the left channel to work. I decided to add it to the project pile and it's sat there until now.
I finally got around to opening up the top and found problems with the right channel PCB - two scorch marks on/near resistors, and a possible capacitor (C104R, the green one) with a top that is beginning to expand (but not clearly bubbled, I'm focusing on the scorch marks for right now). One of the resisitors appears to have melted part of the plug for the right channel RCA input where it connects to the PCB (it is immediately below it). I tried to unplug the input wire from the pcb and can't budge it - looks like it's melted into place.
The resistor with less damage around it is R109, and I can't get a good enough look at the other resistor to see which one it is. I think the label is in the burned section. Looking at the left channel, the resistor in the identical spot is R102L (making the burned one R102R). I can't find the schematics for the PA-5A mk2 anywhere online to check anything there. I can find the service manuals for the 5 mk 1, and both the 7 mk1 and 7 mk2, but I'm not sure how much crosses over between the different versions. The only meter I currently have is a digital multimeter, so no variable power supply, function generator, nor oscilloscope. I think the R109 is reading around 76 ohms (the color bands look faded so I'm not sure what it's supposed to be without the manual) and the more damaged one is too difficult to get to without disassembling the amp more.
So far I've only done a visual diagnosis, which is a terrible way to go about it, but what do you think would be the most likely thing to have caused this issue? I've worked on some small electronics before (recently replaced some capacitors on a mid-2000s Dell monitor to revive it), but never an amplifier. I find it weird that a resistor near an input is showing damage, yet a lot of what is downstream of that looks fine. Would having someone overpower an RCA input blow a resistor like this? Is it likely that whatever blew the resistor caused a whole string of stuff downstream to be damaged, just not to the level of being visual (there is very light browning on the PCB on components below the burned section)?
I'd also hate to replace a few components and have them immediately fry due to PCB damage, is this damage to the PCB too extensive to even attempt a fix with or should I just consider it a lost cause? Do I need to take it apart and look at the back of the board to truly know? If it's toast, is there a way to get a new board from somewhere? My quick googling doesn't show a good place for replacement parts (my goto for individual components is DigiKey). I know you can get boards printed from places, but I don't have the schematics for the PA-5AII to get an exact copy made.
Is there a way to completely and safely disable the right channel and use the amp for a single channel? Would be an interesting way to power a center or a subwoofer. At this point I'm really just trying to get a handle on my options and I hate to just throw in the towel and scrap the thing.
I guess the final option would be to get it to a repair shop and let them worry about it, but I always like to attempt to fix things myself to save some money and learn a bit in the process. I'm in the Kansas City area, and I'm not even sure if there is a good shop around town that deals with Nakamichi. I'm always willing to just wait it out till I have the money for a shop to take a look at it, but if it looks like they'd have to essentially rebuild the amp from scratch, I'd be more inclined to put that money into just replacing it with a known working model.
Any help would be appreciated. Thanks a ton!
I've got a Nakamichi PA-5A II amp that is have some difficulties. If I had to boil it down, my main question is: is this fixable? Or more accurately, is this fixable without putting hundreds of dollars into it?
I got the amp in 2013 as part of a craigslist deal, so I'm not sure what sort of abuse it was put through to cause the problems. It never worked, but I knew that when I got it. It was used to drive some Polk SDA SRS 2.3s, which is what I was buying - the amp was thrown in on the deal. The speakers work great, so whatever caused the damage to the amp apparently didn't damage the speakers. Just from what I remember, I plugged it in and the light on the front would always come on, but would get random behavior from the unit. Maybe 1 in 5 times I would get the left channel to work. I decided to add it to the project pile and it's sat there until now.
I finally got around to opening up the top and found problems with the right channel PCB - two scorch marks on/near resistors, and a possible capacitor (C104R, the green one) with a top that is beginning to expand (but not clearly bubbled, I'm focusing on the scorch marks for right now). One of the resisitors appears to have melted part of the plug for the right channel RCA input where it connects to the PCB (it is immediately below it). I tried to unplug the input wire from the pcb and can't budge it - looks like it's melted into place.
The resistor with less damage around it is R109, and I can't get a good enough look at the other resistor to see which one it is. I think the label is in the burned section. Looking at the left channel, the resistor in the identical spot is R102L (making the burned one R102R). I can't find the schematics for the PA-5A mk2 anywhere online to check anything there. I can find the service manuals for the 5 mk 1, and both the 7 mk1 and 7 mk2, but I'm not sure how much crosses over between the different versions. The only meter I currently have is a digital multimeter, so no variable power supply, function generator, nor oscilloscope. I think the R109 is reading around 76 ohms (the color bands look faded so I'm not sure what it's supposed to be without the manual) and the more damaged one is too difficult to get to without disassembling the amp more.
So far I've only done a visual diagnosis, which is a terrible way to go about it, but what do you think would be the most likely thing to have caused this issue? I've worked on some small electronics before (recently replaced some capacitors on a mid-2000s Dell monitor to revive it), but never an amplifier. I find it weird that a resistor near an input is showing damage, yet a lot of what is downstream of that looks fine. Would having someone overpower an RCA input blow a resistor like this? Is it likely that whatever blew the resistor caused a whole string of stuff downstream to be damaged, just not to the level of being visual (there is very light browning on the PCB on components below the burned section)?
I'd also hate to replace a few components and have them immediately fry due to PCB damage, is this damage to the PCB too extensive to even attempt a fix with or should I just consider it a lost cause? Do I need to take it apart and look at the back of the board to truly know? If it's toast, is there a way to get a new board from somewhere? My quick googling doesn't show a good place for replacement parts (my goto for individual components is DigiKey). I know you can get boards printed from places, but I don't have the schematics for the PA-5AII to get an exact copy made.
Is there a way to completely and safely disable the right channel and use the amp for a single channel? Would be an interesting way to power a center or a subwoofer. At this point I'm really just trying to get a handle on my options and I hate to just throw in the towel and scrap the thing.
I guess the final option would be to get it to a repair shop and let them worry about it, but I always like to attempt to fix things myself to save some money and learn a bit in the process. I'm in the Kansas City area, and I'm not even sure if there is a good shop around town that deals with Nakamichi. I'm always willing to just wait it out till I have the money for a shop to take a look at it, but if it looks like they'd have to essentially rebuild the amp from scratch, I'd be more inclined to put that money into just replacing it with a known working model.
Any help would be appreciated. Thanks a ton!
If it was me, I'd be tempted to reuse the power supply and chassis and build something like a slightly modified BA3 to accept the higher rail voltages in your power supply.
Most of the expense of the amp is in the chassis and power supply so you're not giving up on much by rebuilding a new amp.
Most of the expense of the amp is in the chassis and power supply so you're not giving up on much by rebuilding a new amp.
I'm no tech....but C104 definitely isn't original. May not be a bad idea to check value and polarity. And it looks like there may be some heat damage under the k240 ( pair of 2sk170's, right? ). The resistor in the upper left corner is completely toast.
You're dead in the water without a schematic, really need the service manual.
You're dead in the water without a schematic, really need the service manual.
pcb traces are big and fat enough to be easily repaired with wire and solder
take plenty of pictures for reference , dismantle pcb from amp , then buzz for further tips
if you need help in proper dismantling , post several decent pictures here
take plenty of pictures for reference , dismantle pcb from amp , then buzz for further tips
if you need help in proper dismantling , post several decent pictures here
I have both a PA5 mk2 and a PA7 mk2.
The schematic from PA7 mk2 will describe the PA5 mk2.
C104 should be a large green electrolytic.
R109 should be 75 ohms.
R102 should be 220 ohms.
Have not yet concluded what caused damage, but I would think one or more of the transistors is shorted.
Certainly fixable. Just find and replace all damaged components. PCB is not a problem to repair.
The schematic from PA7 mk2 will describe the PA5 mk2.
C104 should be a large green electrolytic.
R109 should be 75 ohms.
R102 should be 220 ohms.
Have not yet concluded what caused damage, but I would think one or more of the transistors is shorted.
Certainly fixable. Just find and replace all damaged components. PCB is not a problem to repair.
Assume V+ is 59volts and Q101 shorted Drain to Gate and maybe Q104 also shorted Collector to Emitter. And Input shorted to ground.
Current of 59volts / (75 + 220) = 200mA thru the two resistors
200mA x 220 = 44v across 220 ohm resistor
200mA x 75 = 15v across 75 ohm resistor
44v x .2A = 22 watts absorbed by 220 ohm 1/2 w resistor
15v x .2A = 3 watts absorbed by 75 ohm 1/2 w resistor
Current of 59volts / (75 + 220) = 200mA thru the two resistors
200mA x 220 = 44v across 220 ohm resistor
200mA x 75 = 15v across 75 ohm resistor
44v x .2A = 22 watts absorbed by 220 ohm 1/2 w resistor
15v x .2A = 3 watts absorbed by 75 ohm 1/2 w resistor
75 ohm looks more like a 1 watt resistor.
Repair may have been attempted replacing original 1/2 w 75 ohm with 1 watt 75 ohm.
I also think I see heat damage under Q101.
Repair may have been attempted replacing original 1/2 w 75 ohm with 1 watt 75 ohm.
I also think I see heat damage under Q101.
Can't quit. I'm on a roll. Having fun.
If Q104 shorted, should have destroyed 10v zener diode, ZD11.
Maybe 330pF capacitor, C101 could have been stressed also.
If Q104 shorted, should have destroyed 10v zener diode, ZD11.
Maybe 330pF capacitor, C101 could have been stressed also.
Thanks for all the replies! Haven't been able to check out a whole bunch, but here's what I've seen so far:
I figured my first step, no matter what is toast, is to take the right side board off so I've got it separated from the main unit. I had to cut the RCA cable off the plug (in the upper left corner) due to the plug being melted. Couldn't separate it and figured if I have to replace it anyways, who cares?
Removed all the screws from the PCB and hit another snag - the transistors on the heat sink are stuck on. Makes sense to have them on there tight, but I can't seem to dislodge them. Tried lifting up with the grey pad (cooling sheet?) they're on and still nothing. Nothing in the service manual for the 7II says anything about how to remove/reattach these besides "Loosen screws, remove pcb". Thought about using a screwdriver to apply some shear force, but decided against it for now. Would using a hair dryer to heat things up help loosen them? Don't have a true heat gun so I'm trying to improvise without desoldering everything.
A look straight on, haven't yet unscrewed the connections going toward the center of the unit:
Was able to slightly tilt the board away from the heat sink and take a look at the back of the PCB - from what I can tell the back side looks ok, but it'll take getting the transistors separated before I feel 100% about it. Slight burning looks to have wrapped around where the resistor was close to the edge, but from what I can tell it didn't hurt any traces/solder.
I did a random sample of multimeter tests on resistors/diodes and found some read fine, others didn't find a reading. This was true on the left near the scorch marks, and on the right side well away from anything visible, so it looks like I'll need to check everything one by one to make sure what is/isn't fried (was going to anyways, but this confirms it). All the large teal resistors (I believe rated for multiple watts) connected to the heat-sinked transistors seem to be ok, it's the smaller ones on the board that seem to be having trouble.
As I don't have a good way to test capacitors, any of them I'm not feeling good about I'll probably just swap out (at least near the burn marks). I can do the test where you remove it from the circuit and check the resistance, but at that point I might as well put a new one in if I find a suitable replacement. Found a similar bi-polar Nichicon capacitor at Mouser to replace C104 (even appears to be green in color like the original). Hate to go to all that work, but to me there's not much difference between replacing 50 components and replacing 80 components. It's better than frying any other repairs I do or having something fail in a few months and I have to dismantle again.
C104 beginning to have a pushed out end:
It's my first time really needing to check out transistors - to check for shorts you just measure the resistance between the legs, correct? Can this be done in circuit or do they need to be removed?
Once again, thanks for all the help! I'll try to keep posts coming of any progress I am able to make.
I figured my first step, no matter what is toast, is to take the right side board off so I've got it separated from the main unit. I had to cut the RCA cable off the plug (in the upper left corner) due to the plug being melted. Couldn't separate it and figured if I have to replace it anyways, who cares?
Removed all the screws from the PCB and hit another snag - the transistors on the heat sink are stuck on. Makes sense to have them on there tight, but I can't seem to dislodge them. Tried lifting up with the grey pad (cooling sheet?) they're on and still nothing. Nothing in the service manual for the 7II says anything about how to remove/reattach these besides "Loosen screws, remove pcb". Thought about using a screwdriver to apply some shear force, but decided against it for now. Would using a hair dryer to heat things up help loosen them? Don't have a true heat gun so I'm trying to improvise without desoldering everything.
A look straight on, haven't yet unscrewed the connections going toward the center of the unit:
An externally hosted image should be here but it was not working when we last tested it.
Was able to slightly tilt the board away from the heat sink and take a look at the back of the PCB - from what I can tell the back side looks ok, but it'll take getting the transistors separated before I feel 100% about it. Slight burning looks to have wrapped around where the resistor was close to the edge, but from what I can tell it didn't hurt any traces/solder.
An externally hosted image should be here but it was not working when we last tested it.
I did a random sample of multimeter tests on resistors/diodes and found some read fine, others didn't find a reading. This was true on the left near the scorch marks, and on the right side well away from anything visible, so it looks like I'll need to check everything one by one to make sure what is/isn't fried (was going to anyways, but this confirms it). All the large teal resistors (I believe rated for multiple watts) connected to the heat-sinked transistors seem to be ok, it's the smaller ones on the board that seem to be having trouble.
As I don't have a good way to test capacitors, any of them I'm not feeling good about I'll probably just swap out (at least near the burn marks). I can do the test where you remove it from the circuit and check the resistance, but at that point I might as well put a new one in if I find a suitable replacement. Found a similar bi-polar Nichicon capacitor at Mouser to replace C104 (even appears to be green in color like the original). Hate to go to all that work, but to me there's not much difference between replacing 50 components and replacing 80 components. It's better than frying any other repairs I do or having something fail in a few months and I have to dismantle again.
C104 beginning to have a pushed out end:
An externally hosted image should be here but it was not working when we last tested it.
It's my first time really needing to check out transistors - to check for shorts you just measure the resistance between the legs, correct? Can this be done in circuit or do they need to be removed?
Once again, thanks for all the help! I'll try to keep posts coming of any progress I am able to make.
you can check transistors in situ , if you have enough experience or if you have specialised transistor tester
however , often fastest way of repair is to pull it out (one by one !! , especially if you don't have schm of amp you're working on) , check with diode/hfe/component test(er) , replace if needed , solder back
if in doubt - replace ......golden rule
however , often fastest way of repair is to pull it out (one by one !! , especially if you don't have schm of amp you're working on) , check with diode/hfe/component test(er) , replace if needed , solder back
if in doubt - replace ......golden rule
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