Greetings DIYers,
I think I have an interesting problem that would be fairly easy for some of you to figure out.
Recently, I got the DIY bug with my 19 year old Hafler XL-280 and decided to replace the electrolytics with the exception of the big 7800uF jobs. As usual, even after much reading, I rushed into the job and replaced the caps with Nichicon/XICON caps. Powered the unit up with no VARIAC etc. and - all was well. It actually sounded pretty much like I had always remembered. For the record, one side always ran a little hotter and occasionally this amp sounded a bit harsh to my ears.
Anyhow, I was listening to it one day and the right side just sort of fizzled. It did not go totally out, but volume was reduced and it was a bit distorted. I switched off and let the unit cool, and upon powering up again the right side was definitely lacking in bass. The output sounded clear and the mids and highs seemed all there, but no bass.
I came to the forum and found this:
The Xl-280, starting from the speaker terminal, has a 47k resistor in series to the negative input of the differential pair. In parallel with this resistor is a 3.3k resistor is inseries with a 10pf capacitor to compensate the feedback loop.
From the differential pair input there is a 270 ohm resistor in series with two parallel high frequency compensation capacitors in parallel to ground. One is 330pf fixed, and the other 120pf variable. That 120pf capacitor is the excilinear (null) adjustment.
There is also a 2.2k resistor in series with a 1000uf NP electrolytic in parallel with a 0.1uf poly film capacitor to ground to set the gain and low frequency cut off.
Someone suggested that the 1000uF or 2.2k R might have blown, so...
I decided that if I was taking the thing apart again, I would do it right.
After quite a bit more reading, I purchased Panasonic FC caps, new PS diodes, and some other high quality caps for the driver board. I got a Variac and made a current limiting extension chord with a lightbulb socket. I splurged for a BK810C precision cap tester and cherry picked 2 47pF polystyrenes for C1 which had been micas (propylenes in the schematic), I was rollin'.
This was pretty much what I wound up with
For the right (bad) side, I tested the 1000uF, the 2.2K resistor and various other parts - found nothing wrong except the 47 Ohm carbon comps at R34,35 had nearly burst into flame judging by the melted areas underneath them. Nevertheless, they tested 47.1 and 56.3 Ohm - I replaced them with matched 47.1 Ohm 1/2W flameproof metal-films.
Tested the big main caps since I was there, they were all 8000+ uF - don't know about ESR or leakage.
Assemebled all my good stuff with a few experiments like XICON 1431 caps and an Orange Drop at C4.
Powered her up and she looked good, here she is. - this is the 'bad' side.
This is the good side
So everything looked good, and I hooked up the Maggies.
Right side sounds crystal clear but a little low on gain and definitely lacking the low bass.
MY QUESTION IS:
Given the schematic and parts list below, what is it! This is driving me nuts. I think all rare things (like FETS) are Ok given the clarity of sound and the good voltage and bias readings. What in the gain and low frequency cutoff circuit could do this. The ONE cap that I did not test was C13, lemme guess, that's probably it - damn Murphys Law.
BTW in my rebuild, I found a blown electrolytic bypass cap (C15) on the GOOD side, funny.
XL280 Schematic
XL280 Power Supply schematic
XL280 Driver Parts Layout/Voltages
XL280 Parts List
I would really love to hear any stories from people that have messed with a XL280, this amp has great sentimental value for me and I would love to hear it sing again.
I think I have an interesting problem that would be fairly easy for some of you to figure out.
Recently, I got the DIY bug with my 19 year old Hafler XL-280 and decided to replace the electrolytics with the exception of the big 7800uF jobs. As usual, even after much reading, I rushed into the job and replaced the caps with Nichicon/XICON caps. Powered the unit up with no VARIAC etc. and - all was well. It actually sounded pretty much like I had always remembered. For the record, one side always ran a little hotter and occasionally this amp sounded a bit harsh to my ears.
Anyhow, I was listening to it one day and the right side just sort of fizzled. It did not go totally out, but volume was reduced and it was a bit distorted. I switched off and let the unit cool, and upon powering up again the right side was definitely lacking in bass. The output sounded clear and the mids and highs seemed all there, but no bass.
I came to the forum and found this:
The Xl-280, starting from the speaker terminal, has a 47k resistor in series to the negative input of the differential pair. In parallel with this resistor is a 3.3k resistor is inseries with a 10pf capacitor to compensate the feedback loop.
From the differential pair input there is a 270 ohm resistor in series with two parallel high frequency compensation capacitors in parallel to ground. One is 330pf fixed, and the other 120pf variable. That 120pf capacitor is the excilinear (null) adjustment.
There is also a 2.2k resistor in series with a 1000uf NP electrolytic in parallel with a 0.1uf poly film capacitor to ground to set the gain and low frequency cut off.
Someone suggested that the 1000uF or 2.2k R might have blown, so...
I decided that if I was taking the thing apart again, I would do it right.
After quite a bit more reading, I purchased Panasonic FC caps, new PS diodes, and some other high quality caps for the driver board. I got a Variac and made a current limiting extension chord with a lightbulb socket. I splurged for a BK810C precision cap tester and cherry picked 2 47pF polystyrenes for C1 which had been micas (propylenes in the schematic), I was rollin'.
This was pretty much what I wound up with
For the right (bad) side, I tested the 1000uF, the 2.2K resistor and various other parts - found nothing wrong except the 47 Ohm carbon comps at R34,35 had nearly burst into flame judging by the melted areas underneath them. Nevertheless, they tested 47.1 and 56.3 Ohm - I replaced them with matched 47.1 Ohm 1/2W flameproof metal-films.
Tested the big main caps since I was there, they were all 8000+ uF - don't know about ESR or leakage.
Assemebled all my good stuff with a few experiments like XICON 1431 caps and an Orange Drop at C4.
Powered her up and she looked good, here she is. - this is the 'bad' side.
This is the good side
So everything looked good, and I hooked up the Maggies.
Right side sounds crystal clear but a little low on gain and definitely lacking the low bass.
MY QUESTION IS:
Given the schematic and parts list below, what is it! This is driving me nuts. I think all rare things (like FETS) are Ok given the clarity of sound and the good voltage and bias readings. What in the gain and low frequency cutoff circuit could do this. The ONE cap that I did not test was C13, lemme guess, that's probably it - damn Murphys Law.
BTW in my rebuild, I found a blown electrolytic bypass cap (C15) on the GOOD side, funny.
XL280 Schematic
XL280 Power Supply schematic
XL280 Driver Parts Layout/Voltages
XL280 Parts List
I would really love to hear any stories from people that have messed with a XL280, this amp has great sentimental value for me and I would love to hear it sing again.
haflerfreak:
sorry to hear about your troubles. i've always liked the XL-280, too.
the stupid questions first before the smart guys respond:
0. are you sure the speaker is still good? Maybe switch them to make sure. Don't laugh - I could tell you some funny stories ...
1. did you check the solder connections (especially where you replaced parts)? Do more than visually inspect; wiggle the parts while checking for electrical continuity, especially R26 and C8.
2. i am worried about the burned areas under R34 and R35. i assume you did not see this under the good side? Are C16-C19 OK? Any way you could provide some voltage measurements for the transistors on the bad side (and maybe compare to same on the good side)?
3. Are you able to measure +/- rail currents (lifting the rail fuse makes it easier - BTW, you checked that these fuses are good, right)? I wonder if the output mosfets are even active - the rail fuses don't disable the driver board when they open ...
all for now,
mlloyd1
sorry to hear about your troubles. i've always liked the XL-280, too.
the stupid questions first before the smart guys respond:
0. are you sure the speaker is still good? Maybe switch them to make sure. Don't laugh - I could tell you some funny stories ...
1. did you check the solder connections (especially where you replaced parts)? Do more than visually inspect; wiggle the parts while checking for electrical continuity, especially R26 and C8.
2. i am worried about the burned areas under R34 and R35. i assume you did not see this under the good side? Are C16-C19 OK? Any way you could provide some voltage measurements for the transistors on the bad side (and maybe compare to same on the good side)?
3. Are you able to measure +/- rail currents (lifting the rail fuse makes it easier - BTW, you checked that these fuses are good, right)? I wonder if the output mosfets are even active - the rail fuses don't disable the driver board when they open ...
all for now,
mlloyd1
PSz. said:
if this was a DH200 (maybe DH-220 also), i would offer the same guess, too.
hafler used to put the speaker fuse in the feedback loop and put a resistor in parallel to maintain the dc path, so you could still hear some sound if that fuse opened. however, that's not done anymore according to the schematic for the XL-280.
mlloyd1
Hey mlloyd1, thanks so much for the reply. You asked the right questions. When the right side fizzled, I tried switching inputs and sources to narrow it down. I would absolutely swear that I tried switching the speakers when this all happened.
I must say, I'm a bit embarrassed.
I just switched the old magnepans and I am seriously not sure that I can believe my ears.
I cranked that balance over to R and that left speak just sings.
I have 2 other stupid posts on this site, one that wound up being a blown DMM fuse and another about a jittery bias pot (that I just adjusted little by little with the power off each time), but this takes the cake!
I have to go to work right now (almost late), but I have an old pair of JBLs that I will pull out tonight to verify this and ENJOY my XL280!
Will post listening stories about the caps too.
Thanks again guys.
I must say, I'm a bit embarrassed.
I just switched the old magnepans and I am seriously not sure that I can believe my ears.
I cranked that balance over to R and that left speak just sings.
I have 2 other stupid posts on this site, one that wound up being a blown DMM fuse and another about a jittery bias pot (that I just adjusted little by little with the power off each time), but this takes the cake!
I have to go to work right now (almost late), but I have an old pair of JBLs that I will pull out tonight to verify this and ENJOY my XL280!
Will post listening stories about the caps too.
Thanks again guys.
No, it's the amp
Hey everyone,
Switched over to a pair of old JBLs using brand new wire, it's the amp. I was pretty sure I had verified that it wasn't the speakers before I took stuff apart.
well, at least the Magnepans are Ok and I wasn't THAT boneheaded.
But this leaves us with the original low frequency cutoff problem...
I can assure you that it is not a bad solder connect, nor is it fuses, and, of course, not the speakers.
One of the reasons for my apparent confusion in the diagnosis is that it is somewhat subtle. The soundstage is pure and clear and focused - but the bottom couple octaves of bass are missing from the right channel.
I have checked C8, R26 and most, if not all, of the resistors and diodes. Everything fine.
I am honestly a bit paranoid to take voltage readings off of transistor legs etc. while this thing is powered up, even though I have a voltage chart for the driver board. I am a cautious and ignorant newbie who just wants his amp to go but is too poor to get it analyzed by a shop (not to mention I don't trust anyone these days that claims to be educated).
All I can tell you guys is that R34 and R35 melted pits about half way through the board - my soldering iron doesn't even melt this board!!!!
Hey everyone,
Switched over to a pair of old JBLs using brand new wire, it's the amp. I was pretty sure I had verified that it wasn't the speakers before I took stuff apart.
well, at least the Magnepans are Ok and I wasn't THAT boneheaded.
But this leaves us with the original low frequency cutoff problem...
I can assure you that it is not a bad solder connect, nor is it fuses, and, of course, not the speakers.
One of the reasons for my apparent confusion in the diagnosis is that it is somewhat subtle. The soundstage is pure and clear and focused - but the bottom couple octaves of bass are missing from the right channel.
I have checked C8, R26 and most, if not all, of the resistors and diodes. Everything fine.
I am honestly a bit paranoid to take voltage readings off of transistor legs etc. while this thing is powered up, even though I have a voltage chart for the driver board. I am a cautious and ignorant newbie who just wants his amp to go but is too poor to get it analyzed by a shop (not to mention I don't trust anyone these days that claims to be educated).
All I can tell you guys is that R34 and R35 melted pits about half way through the board - my soldering iron doesn't even melt this board!!!!
To mlloyd1:
Sorry about my sluggish responses, I work all the time.
The polycarbs at C15,C16 tested fine, I have replaced the electrolytics at C17,C18 twice and I have always suspected my earlier attempt using XICON 100uFs and no Variac/lightbulb may have had something to do with R34,35 getting hot. To be honest, I don't know if those resistors got hot with the original 19 year old IC caps or with the new ones I have tried.
You said:
3. Are you able to measure +/- rail currents (lifting the rail fuse makes it easier - BTW, you checked that these fuses are good, right)? I wonder if the output mosfets are even active - the rail fuses don't disable the driver board when they open ...
I thought the bias reading from the fuse clip was the rail current?!? It is a fairly steady 285mA.
BTW another observation -
I have noticed that when I power down with source at low volume and drain off the caps, the 'bad' side goes silent almost right away while the good side plays for 10 seconds or so.
Thanks for any feedback, I hate 'just replacing stuff'...
Sorry about my sluggish responses, I work all the time.
The polycarbs at C15,C16 tested fine, I have replaced the electrolytics at C17,C18 twice and I have always suspected my earlier attempt using XICON 100uFs and no Variac/lightbulb may have had something to do with R34,35 getting hot. To be honest, I don't know if those resistors got hot with the original 19 year old IC caps or with the new ones I have tried.
You said:
3. Are you able to measure +/- rail currents (lifting the rail fuse makes it easier - BTW, you checked that these fuses are good, right)? I wonder if the output mosfets are even active - the rail fuses don't disable the driver board when they open ...
I thought the bias reading from the fuse clip was the rail current?!? It is a fairly steady 285mA.
BTW another observation -
I have noticed that when I power down with source at low volume and drain off the caps, the 'bad' side goes silent almost right away while the good side plays for 10 seconds or so.
Thanks for any feedback, I hate 'just replacing stuff'...
haflerfreak:
don't apologize; between my day job and honey-do lists, i have very little time for play these days, too.
sorry if i wasn't clear. looking at the schematic, measuring current through F1 or F2 yields currents through the output mosfets. somewhere around 300mA is ok (Hafler usually wants about 100mA per mosfet, so 285mA is in the ballpark). i assume you measured both sides?
however, this doesn't help tell us how much current the driver board is using, though. if R34 and R35 have burned areas, something on the driver board is (or used to) draw a lot more current than it should have.
hmmm, i forgot the other stupid question: you did check carefully to make sure the replaced electrolytic caps were installed with correct polarities, right? caps with large voltages across them usually inform you of their unhappiness at being connected incorrectly with a noisy celebration, complete with smelly confetti.
so, what voltage measurements can you do?
mlloyd1
don't apologize; between my day job and honey-do lists, i have very little time for play these days, too.
sorry if i wasn't clear. looking at the schematic, measuring current through F1 or F2 yields currents through the output mosfets. somewhere around 300mA is ok (Hafler usually wants about 100mA per mosfet, so 285mA is in the ballpark). i assume you measured both sides?
however, this doesn't help tell us how much current the driver board is using, though. if R34 and R35 have burned areas, something on the driver board is (or used to) draw a lot more current than it should have.
hmmm, i forgot the other stupid question: you did check carefully to make sure the replaced electrolytic caps were installed with correct polarities, right? caps with large voltages across them usually inform you of their unhappiness at being connected incorrectly with a noisy celebration, complete with smelly confetti.
so, what voltage measurements can you do?
mlloyd1
Oh yeah, I double and triple check electrolytics before I power them up. I have never put one in wrong, and I hope to avoid it in the future.
I actually did not measure the bias from both sides and the B+ 285mA is from my most recent adjustments. Before I played with it, I just assumed it was biased high (over 300mA) because it always ran hotter than the other side and it now idles decidedly cooler than ever before. It worked like that for years...
When I redid all the caps, I reset bias on both sides to 285mA with a Variac and now both heatsinks seem to warm up almost identically - I assume that is good. There does not seem to be anything inclined to meltdown at the moment, but I haven't been running the amp for very long periods, just enough for testing etc.
Well, I can see we are going to need some numbers to work with, so I will try to get the cover off and take voltage and current readings for various things that I feel safe accessing. Today is my 1 day off, so I might be able to report later tonight hopefully.
Oh yeah, mlloyd1,
I can't quite describe the feeling to you when I suspected my $1000 each Magnepan MG2.6/R might be the culprit and not my $600 XL280!!
It was actually kind of funny. Talk to you later
I actually did not measure the bias from both sides and the B+ 285mA is from my most recent adjustments. Before I played with it, I just assumed it was biased high (over 300mA) because it always ran hotter than the other side and it now idles decidedly cooler than ever before. It worked like that for years...
When I redid all the caps, I reset bias on both sides to 285mA with a Variac and now both heatsinks seem to warm up almost identically - I assume that is good. There does not seem to be anything inclined to meltdown at the moment, but I haven't been running the amp for very long periods, just enough for testing etc.
Well, I can see we are going to need some numbers to work with, so I will try to get the cover off and take voltage and current readings for various things that I feel safe accessing. Today is my 1 day off, so I might be able to report later tonight hopefully.
Oh yeah, mlloyd1,
I can't quite describe the feeling to you when I suspected my $1000 each Magnepan MG2.6/R might be the culprit and not my $600 XL280!!
It was actually kind of funny. Talk to you later
From weird to weirder
Hey everybody,
Have you ever heard the saying, 'A wise man knows that he knows nothing at all'? Well, I don't feel too wise at the moment and I think it is quite possible that I owe PSz an apology.
Like your typical arrogant know-it-all type (Oh, I detest those people!) I all but ignored a comment made by PSz. He had mentioned an XL-280 having a 'soft-blown' fuse that left the amp still playing but not very well. I had visually inspected the fuses and replaced the rail fuses, but knowing my track record, I decided I had nothing to lose before I tore the cover off and started testing voltages and currents.
I had installed 4 amp fuses years ago to protect my Maggies (the fuses can be higher). I removed the fuse on the 'bad' side and installed a brand new 7 amp.
Lo and behold, my problem may be solved - after weeks of sweating it!
At this point, I am afraid to hope as I have already had a couple good and bad false alarms. The reason I don't sound absolutely positive is that the JBL test speakers I have hooked up right now sound like crud compared to my Maggies and I have to hear the Mags before I make the final call.
Still, I can tell you right now that the bass coming from the right channel sounds pretty much 'all there'. Amazing.
I still thought I noticed heavier bass from the left side, but that happens to be the corner speaker and the right is almost in the middle of the room. Those of you familiar with acoustics will understand what a corner can do to your bass.
I switched the speaks on the amp channels again and, sure enough, the bass remained heavy on the left but the right speaker on the left channel now sounded slightly anemic.
Simply amazing. I am happy to post my follies as I believe we learn as much or more from our mistakes as we do from our successes. I hope this thread helps someone someday...
I will post final results later if all is well, and voltages if not 
Hey everybody,
Have you ever heard the saying, 'A wise man knows that he knows nothing at all'? Well, I don't feel too wise at the moment and I think it is quite possible that I owe PSz an apology.
Like your typical arrogant know-it-all type (Oh, I detest those people!) I all but ignored a comment made by PSz. He had mentioned an XL-280 having a 'soft-blown' fuse that left the amp still playing but not very well. I had visually inspected the fuses and replaced the rail fuses, but knowing my track record, I decided I had nothing to lose before I tore the cover off and started testing voltages and currents.
I had installed 4 amp fuses years ago to protect my Maggies (the fuses can be higher). I removed the fuse on the 'bad' side and installed a brand new 7 amp.
Lo and behold, my problem may be solved - after weeks of sweating it!
At this point, I am afraid to hope as I have already had a couple good and bad false alarms. The reason I don't sound absolutely positive is that the JBL test speakers I have hooked up right now sound like crud compared to my Maggies and I have to hear the Mags before I make the final call.
Still, I can tell you right now that the bass coming from the right channel sounds pretty much 'all there'. Amazing.
I still thought I noticed heavier bass from the left side, but that happens to be the corner speaker and the right is almost in the middle of the room. Those of you familiar with acoustics will understand what a corner can do to your bass.
I switched the speaks on the amp channels again and, sure enough, the bass remained heavy on the left but the right speaker on the left channel now sounded slightly anemic.
Simply amazing. I am happy to post my follies as I believe we learn as much or more from our mistakes as we do from our successes. I hope this thread helps someone someday...
I will post final results later if all is well
, and voltages if not
I think I finally know
Hey mlloyd,
You don't need to wonder about that fuse, I have spent hours with the 2 sets of speakers and all new fuses. I have switched the speaker wires many times, and after all the listening and testing............
It's the speaker, I think I'm positive now.
On a hopeful note, I have to assume that the fault is probably in the crossover network and if I'm not mistaken Magnepan is still in business, so that means parts/info are probably available.
I think it is probably in the crossover because the bass panel functions without any flapping or buzzing (like a broken wire on the panel) and when my system went down the very first time, it was with a kind of fizzling/buzz out, like a cap (or inductor coil?) fizzling.
Oh well, looks like I will be talking to the folks that hang out on the 'loudspeaker' forum.
I'm sorry about all my waffling, this has had me baffled, I guess I just didn't want to believe it was the speaker, I doubt I will be able to afford to repair it
I would like to thank you guys once more for the help with the amp, even though it didn't 'fix' it, I learned some valuable amp lessons here -
on to crossover dissection.
Hey mlloyd,
You don't need to wonder about that fuse, I have spent hours with the 2 sets of speakers and all new fuses. I have switched the speaker wires many times, and after all the listening and testing............
It's the speaker, I think I'm positive now.
On a hopeful note, I have to assume that the fault is probably in the crossover network and if I'm not mistaken Magnepan is still in business, so that means parts/info are probably available.
I think it is probably in the crossover because the bass panel functions without any flapping or buzzing (like a broken wire on the panel) and when my system went down the very first time, it was with a kind of fizzling/buzz out, like a cap (or inductor coil?) fizzling.
Oh well, looks like I will be talking to the folks that hang out on the 'loudspeaker' forum.
I'm sorry about all my waffling, this has had me baffled, I guess I just didn't want to believe it was the speaker, I doubt I will be able to afford to repair it
I would like to thank you guys once more for the help with the amp, even though it didn't 'fix' it, I learned some valuable amp lessons here -
on to crossover dissection.
haflerfreak:
yeah, many times you get a good news/bad news story when trouble shooting.
good luck though - maybe it will be something easy and not too much dinero$$$
but, it seems you still have something going on with one channel of the amp - those resistors didn't get so warm for nothing.
you really don't want to fix the maggies and damage them again right away ...
mlloyd1
yeah, many times you get a good news/bad news story when trouble shooting.
good luck though - maybe it will be something easy and not too much dinero$$$
but, it seems you still have something going on with one channel of the amp - those resistors didn't get so warm for nothing.
you really don't want to fix the maggies and damage them again right away ...
mlloyd1
Yes, I agree about the resistors, that did make me a bit nervous to find that. Like I said, somehow those carbon comps actually survived and tested at 47 and 56 Ohms, so hopefully they did some protecting?
I do think this amp was possibly running at a very high bias for a short time because of cap changes/tinkering, would this cause those resistors to heat up under load? That was also the same time my trouble started.
At one time this side had an intermittently loose input wire (The wire that connects to #2 at the top of the driver board). I also think the main ground at terminal 8 almost gave at one point. All of these things have been corrected.
-- Did I mention that this amp was bought 'used' from a HiFi shop and appears to maybe have been a kit model.
I think it could even be possible that the resistors melted the board way back in 1989 under the previous owner.
Everything seems fine at the moment with the amp, bias has been reset to 285mA and both sides get evenly warm. The sinks have only gotten 'hot' once since I got her running again, and that was after 4 hours of listening.
Unless there is something up with my acoustics and/or lousy hearing, the amp seemed to sound fine with my pair of backup JBLs, but not with my Magnepans.
Would checking voltages be the only way to really verify that that side is totally Ok? I don't seem to be finding much out from listening evaluations (although I did mention the heatsinks seem to warm up slowly and evenly).
I do think this amp was possibly running at a very high bias for a short time because of cap changes/tinkering, would this cause those resistors to heat up under load? That was also the same time my trouble started.
At one time this side had an intermittently loose input wire (The wire that connects to #2 at the top of the driver board). I also think the main ground at terminal 8 almost gave at one point. All of these things have been corrected.
-- Did I mention that this amp was bought 'used' from a HiFi shop and appears to maybe have been a kit model.
I think it could even be possible that the resistors melted the board way back in 1989 under the previous owner.
Everything seems fine at the moment with the amp, bias has been reset to 285mA and both sides get evenly warm. The sinks have only gotten 'hot' once since I got her running again, and that was after 4 hours of listening.
Unless there is something up with my acoustics and/or lousy hearing, the amp seemed to sound fine with my pair of backup JBLs, but not with my Magnepans.
Would checking voltages be the only way to really verify that that side is totally Ok? I don't seem to be finding much out from listening evaluations (although I did mention the heatsinks seem to warm up slowly and evenly).
Hafler XL280
HaflerFreak:
Thanks for the info with your tweaks and challenges - solved!
I am a newbee, long time ago bought a kit and assembled them --went OK. Now time to change those PS caps.
I'd like to change the PS caps from 7800 uf to 8400 uF each; do I need to reset the bias or do nothing.
Any precaution before powering it up.
Thanks for the help.
HaflerFreak:
Thanks for the info with your tweaks and challenges - solved!
I am a newbee, long time ago bought a kit and assembled them --went OK. Now time to change those PS caps.
I'd like to change the PS caps from 7800 uf to 8400 uF each; do I need to reset the bias or do nothing.
Any precaution before powering it up.
Thanks for the help.
Back to the drawing board
Ok, I hesitate to make this post because there are undoubtedly some people out there that think I am completely mad by now (and maybe I am) - but I am back to testing my XL280.
For those that don't want to read my clueless threads, here is the deal:
I was listening to my XL280 one day and the right side fizzled on a piano note. I turned everything off, let it cool and then started checking for problems. Ever since the 'fizzle', that channel has near perfect mid-bass/highs, with no distortion, but absolutely no deep bass. My guess is very little or nothing below 60Hz or so. At one point, I thought it was in the crossover of my right speaker, but I am back to the amp after much testing.
So I thought it was a low frequency cutoff issue, maybe a cap died somewhere. I took the amp apart, measured PS voltages (ok), and proceeded to check parts out of circuit.
Absolutely not one thing tested bad.
I tested nearly all of the resisitors in circuit, only a couple wouldn't read in-circuit. All resistors test OK.
Tested all of the caps except for 4 or 5 of the micas - all OK.
Replaced C8, the 1000uF BiPolar in the LFC (but the old one was OK).
Checked C9, the .1 poly film in the LFC - tested .0985.
Checked R26 in the LFC - 2216 Ohms (spec is 2210).
Anything I bothered to check tested fine. The only anomaly that I have discovered is 2 pits melted halfway into the board underneath R34 and R35 - but the resistors tested OK at 47.1 and 56.3 Ohms.
Checked the zeners and 1N4148's in circuit and these appeared OK (maybe not?).
Replaced all electrolytics throught the amp with Panasonic FCs and a Panasonic BP for each 1000uF cap.
Powered her up slow and checked bias - reset to 285mA but I must say the bias reading was none too stable on either channel, I suspect maybe my cheap DMM?
Now that I am back to trying to fix this amp, I plan on doing what I have said I would do all along - check my voltages on the bad side. I will post as soon as I have more info.
If anyone at all has a clue what could take away my lowest bass but leave everything else fine in this circuit, I am all ears.
Here is the schematic again:
XL280 Schematic
I am determined at this point to fix this thing or destroy it trying!
TO ricoh and Stormrider:
Stormrider is absolutely right about reforming and bias. I was very happy to see your post, ricoh, because I will eventually replace my big power caps if I get this thing going again...
I have investigated and it appears that:
The original 75V 7800uF caps are 45 to 47ish mm wide (1-3/4"), 108 mm long (4-1/4") with 22 mm spacing of the terminals.
The closest I have found on Mouser are 75V 10000uF that are 2" wide and 4-1/4" tall with 20mm terminal spacing. I would LOVE to know what mods are necessary to fit new caps in the stock XL280 setup. I suspect you would at least need to pull out a Dremel and grind the holes to accomodate the 20mm as opposed to the 22mm centers. Then, remount/replace the base brackets to accomodate the 2" wide caps?
Let's hear it XL280 owners!! How did you get new PS caps?
Ok, I hesitate to make this post because there are undoubtedly some people out there that think I am completely mad by now (and maybe I am) - but I am back to testing my XL280.
For those that don't want to read my clueless threads, here is the deal:
I was listening to my XL280 one day and the right side fizzled on a piano note. I turned everything off, let it cool and then started checking for problems. Ever since the 'fizzle', that channel has near perfect mid-bass/highs, with no distortion, but absolutely no deep bass. My guess is very little or nothing below 60Hz or so. At one point, I thought it was in the crossover of my right speaker, but I am back to the amp after much testing.
So I thought it was a low frequency cutoff issue, maybe a cap died somewhere. I took the amp apart, measured PS voltages (ok), and proceeded to check parts out of circuit.
Absolutely not one thing tested bad.
I tested nearly all of the resisitors in circuit, only a couple wouldn't read in-circuit. All resistors test OK.
Tested all of the caps except for 4 or 5 of the micas - all OK.
Replaced C8, the 1000uF BiPolar in the LFC (but the old one was OK).
Checked C9, the .1 poly film in the LFC - tested .0985.
Checked R26 in the LFC - 2216 Ohms (spec is 2210).
Anything I bothered to check tested fine. The only anomaly that I have discovered is 2 pits melted halfway into the board underneath R34 and R35 - but the resistors tested OK at 47.1 and 56.3 Ohms.
Checked the zeners and 1N4148's in circuit and these appeared OK (maybe not?).
Replaced all electrolytics throught the amp with Panasonic FCs and a Panasonic BP for each 1000uF cap.
Powered her up slow and checked bias - reset to 285mA but I must say the bias reading was none too stable on either channel, I suspect maybe my cheap DMM?
Now that I am back to trying to fix this amp, I plan on doing what I have said I would do all along - check my voltages on the bad side. I will post as soon as I have more info.
If anyone at all has a clue what could take away my lowest bass but leave everything else fine in this circuit, I am all ears.
Here is the schematic again:
XL280 Schematic
I am determined at this point to fix this thing or destroy it trying!
TO ricoh and Stormrider:
Stormrider is absolutely right about reforming and bias. I was very happy to see your post, ricoh, because I will eventually replace my big power caps if I get this thing going again...
I have investigated and it appears that:
The original 75V 7800uF caps are 45 to 47ish mm wide (1-3/4"), 108 mm long (4-1/4") with 22 mm spacing of the terminals.
The closest I have found on Mouser are 75V 10000uF that are 2" wide and 4-1/4" tall with 20mm terminal spacing. I would LOVE to know what mods are necessary to fit new caps in the stock XL280 setup. I suspect you would at least need to pull out a Dremel and grind the holes to accomodate the 20mm as opposed to the 22mm centers. Then, remount/replace the base brackets to accomodate the 2" wide caps?
Let's hear it XL280 owners!! How did you get new PS caps?
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