Hi all,
I've got a Marantz Pm400 on my bench, nice amplifier. It works, one channel playing fine, the other has weird noise which almost gets dangerous to speakers - it's something like a cracking/almost a bit like fireworks, sometimes gets lower, then suddenly comes back again, never stops. Still, that channel plays music with normal volume, too. No overheating, it's not a hum or buzz (it's volume independent), it's not the power supply (as said, other channel is fine). Changed most electrolytics in the defective channel, no difference. Decided to start isolating the problem, so I disconnected the power amp section from the pre amp section and grounded it's input. As expected, still the same noise problem in the defective channel, while other good channel now dead silent, as it should be. Grabbed my oscilloscope and started to trace the noise and also compare with the other good channel at some strategical points.
Below I add an excerpt of the schematic of the defective channel (power amp part), where I marked green dots on places where there was no noise present and red dots, where it was present. The noise looks like some random "spikes" around some millivolts, quite intense, of varying frequency and amplitude, it's not white noise or something. It's more a crackling.
From what I understand, after doing some research, this will probably be caused by some transistor in one of the stages of the power amp which is failing and developed this noise, right? If it was a resistor or a capacitor, would be more like white noise and more consistent, lower amplitude, no?
Also, doesn't seem to be a broken solder joint or intermittent "short circuit", as I tapped around that section with a wooden tool quite thoroughly and nothing really changed.
Haven't done the freeze spray trick yet.
Now I'm reaching out to you guys, as I don't understand very well the design of the output stage, it's a little complex, I know the first stage is the diff amp and I know what the last stage does, obviously, but can't identify well the rest (don't know where the VAS is, for example), so not sure if maybe with some help understanding the circuit better, finding the culprit would be easier?
As you can see from the red dots, I'm pretty sure the noise appears somewhere after the stage of Q707 and Q709, as it is not present at the base of Q705 nor Q707, but is present at base of Q713 and Q711. It's also not present at the common point for differential stage (at Zener), but it is present directly at emitter of inverting diff amp transistor and its base, which is where the feedback arrives from the output power transistors (which obviously carries the noise), which is also why it is present at the emitters of QN01 and QN03. Am I thinking correctly here? Or could the noise go the other way round, could it be the output transistors and the feedback connection injects the noise signal in the whole circuit over inverting input of diff amp stage?
What would be best to do next? Please, some opinions on this.
a) Apply freeze spray (I only have a non-invertible compressed air can, I hope it does the job) to the transistors which are suspects, to see if something changes
b) Disconnect R723 and 725 (output transistor resistors), so that no feedback goes to inverting input of diff amp and see if that changes noise? Or would that make the amp instable or other problems during this brief testing?
c) Disconnect R709 (resistor to base of inverting diff amp transistor) Q703, so that feedback doesn't get there and start isolating this way one transistor after the other, working from left to right, interrupting signal temporarily to test?
d) is it possible to measure (in terms of voltage, ohms or current) a transistor which has developed this kind of noise problems? or, as it is still partially working (it stil puts the music through), measurements would all be normal?
e) lastly, should I simply start substituting transistors in pairs, if necessary, starting with Q709 and Q707, then QN01 and 03, then Q703, hoping to eliminate the problem by luck, so not to loose much time in tracing it? this is a paid task, but not per hour, obviously.
Thanks in advance. And sorry for low quality of schematic, it's the only service manual I can find.
I've got a Marantz Pm400 on my bench, nice amplifier. It works, one channel playing fine, the other has weird noise which almost gets dangerous to speakers - it's something like a cracking/almost a bit like fireworks, sometimes gets lower, then suddenly comes back again, never stops. Still, that channel plays music with normal volume, too. No overheating, it's not a hum or buzz (it's volume independent), it's not the power supply (as said, other channel is fine). Changed most electrolytics in the defective channel, no difference. Decided to start isolating the problem, so I disconnected the power amp section from the pre amp section and grounded it's input. As expected, still the same noise problem in the defective channel, while other good channel now dead silent, as it should be. Grabbed my oscilloscope and started to trace the noise and also compare with the other good channel at some strategical points.
Below I add an excerpt of the schematic of the defective channel (power amp part), where I marked green dots on places where there was no noise present and red dots, where it was present. The noise looks like some random "spikes" around some millivolts, quite intense, of varying frequency and amplitude, it's not white noise or something. It's more a crackling.
From what I understand, after doing some research, this will probably be caused by some transistor in one of the stages of the power amp which is failing and developed this noise, right? If it was a resistor or a capacitor, would be more like white noise and more consistent, lower amplitude, no?
Also, doesn't seem to be a broken solder joint or intermittent "short circuit", as I tapped around that section with a wooden tool quite thoroughly and nothing really changed.
Haven't done the freeze spray trick yet.
Now I'm reaching out to you guys, as I don't understand very well the design of the output stage, it's a little complex, I know the first stage is the diff amp and I know what the last stage does, obviously, but can't identify well the rest (don't know where the VAS is, for example), so not sure if maybe with some help understanding the circuit better, finding the culprit would be easier?
As you can see from the red dots, I'm pretty sure the noise appears somewhere after the stage of Q707 and Q709, as it is not present at the base of Q705 nor Q707, but is present at base of Q713 and Q711. It's also not present at the common point for differential stage (at Zener), but it is present directly at emitter of inverting diff amp transistor and its base, which is where the feedback arrives from the output power transistors (which obviously carries the noise), which is also why it is present at the emitters of QN01 and QN03. Am I thinking correctly here? Or could the noise go the other way round, could it be the output transistors and the feedback connection injects the noise signal in the whole circuit over inverting input of diff amp stage?
What would be best to do next? Please, some opinions on this.
a) Apply freeze spray (I only have a non-invertible compressed air can, I hope it does the job) to the transistors which are suspects, to see if something changes
b) Disconnect R723 and 725 (output transistor resistors), so that no feedback goes to inverting input of diff amp and see if that changes noise? Or would that make the amp instable or other problems during this brief testing?
c) Disconnect R709 (resistor to base of inverting diff amp transistor) Q703, so that feedback doesn't get there and start isolating this way one transistor after the other, working from left to right, interrupting signal temporarily to test?
d) is it possible to measure (in terms of voltage, ohms or current) a transistor which has developed this kind of noise problems? or, as it is still partially working (it stil puts the music through), measurements would all be normal?
e) lastly, should I simply start substituting transistors in pairs, if necessary, starting with Q709 and Q707, then QN01 and 03, then Q703, hoping to eliminate the problem by luck, so not to loose much time in tracing it? this is a paid task, but not per hour, obviously.
Thanks in advance. And sorry for low quality of schematic, it's the only service manual I can find.
Attachments
I would suggest You power it with a 100W lamp in series in order not to make further damage and give it a good cleaning.
Is there a speaker out switch or relay ? Check.
Have You checked the pots R717 / 718 ? Move them 1 or 2 degrees to each side monitoring output DC.
Check output drivers isolation between chassis. Bad or punctured mica.
but could be a broken or corroded leg. Check each component leg and wires.
Is there a speaker out switch or relay ? Check.
Have You checked the pots R717 / 718 ? Move them 1 or 2 degrees to each side monitoring output DC.
Check output drivers isolation between chassis. Bad or punctured mica.
Thanks for your suggestions. I haven't used the light bulb technique here as the amp seems stable, and the other channel is playing fine. But I might consider it. Yes, it could be a corroded or broken leg, sure. But as I tapped on all the transistors, this should have caused some reaction? Also, music is not "intermittent" on that channel, it's the noise that varies, in intensity and frequency of incidence (not frequency of tone, it's almost always like pops and cracks, "fire crackers"). I didn't touch the pots, as there's no significant level of constant anormal DC output even in the defective channel. But I will check.
The idea that there might be isolation problems to the heatsink is a good one, thanks, I'll have to check that.
The idea that there might be isolation problems to the heatsink is a good one, thanks, I'll have to check that.
Not likely a bad semiconductor. Use freeze spray to further localize the noise.
More likely a bad connection. Do check the pot.
More likely a bad connection. Do check the pot.
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Knock gently to localize the site (pencil).
Possible poor contact in relays and switches, poor soldering. Defective parts.
Possible poor contact in relays and switches, poor soldering. Defective parts.
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I'd suggest plastic chopstick rather than pencil - one day you'll be working on high voltage stuff and forget pencil lead is a conductor of electricity...
Definitely sounds like a bad connection, dry solder joint, damaged component leg, hairline crack in PCB, that sort of thing. Its a crackle, that's all you need to know to immediately say "bad connection".
Definitely sounds like a bad connection, dry solder joint, damaged component leg, hairline crack in PCB, that sort of thing. Its a crackle, that's all you need to know to immediately say "bad connection".
Fixed one of these a few years ago with a noisy/crackly channel, was a transistor on the tone control board from memory,
now you have to find it 🙁
swapping each to92s from right to left should uncover which one it is....
regards
james
now you have to find it 🙁
swapping each to92s from right to left should uncover which one it is....
regards
james
Thanks for all the suggestions. In a couple of days I'll have time to do some more tests. I'll definitely check the bias pots and transistors for bad connections/cracks on PCB traces, etc.
I'm pretty sure it's not related to switches or relays, as I've isolated the circuits and clearly identified the noise on oscilloscope, it's coming from within the power amp section, more precisely the points I referred to already.
I also believe it will probably be some transistor that got noisy. On another forum, I received the hint that it might very well be the diff amp pair right at the input of the power amp circuit. I might start there swapping transistors if the other tests lead nowhere.
I'm pretty sure it's not related to switches or relays, as I've isolated the circuits and clearly identified the noise on oscilloscope, it's coming from within the power amp section, more precisely the points I referred to already.
I also believe it will probably be some transistor that got noisy. On another forum, I received the hint that it might very well be the diff amp pair right at the input of the power amp circuit. I might start there swapping transistors if the other tests lead nowhere.
Hi ppl,
problem is now diagnosed: in fact a noisy transistor in diff pair, just as suspected. It took me quite some time, though, as (weird enough) there was only one defective transistor, Q701.
Here's how I diagnosed:
in the first measurements I did, no noise was visible at its collector. But I hadn't measured its base. So, at the time, as I was measuring noise at the base of Q703, I (wrongly) deducted the defective transistor would be Q703 and exchanged it with the one from the other good channel. It didn't solve the problem, obviously. The noise only got there coming from the feedback loop. It was only when I decided to short the base of Q701 to ground (before, I had always shorted in front of the 470ohm resistor, now I shorted behind it) that I noticed that the noise went away, so I took the short out and hook my probe onto the base of the transistor (note that resistor was floating free, then there's 33k to GND), and there was the noise! I assumed it wouldn't come from the resistors, would be really unlikely, as there not of the old carbon composite kind, so I decided to swap Q701 and Q702, and bingo! Now the noise went to the right channel instead of the left! It's Q701. It has to be said, btw, that none of the four transistors of the differential pairs of the two channels are the ones indicated in the schematic, I don't know why. They are all the (infamous) 2SA750, known to become noisy (it's on the list in that one really useful sticky thread here on this forum). I can't understand why Maratnz would have decided to go for these instead of the 2sa1127 originally indicated in schematic. I guess this could be the result of some old repair done on this amp? I mean, some solder points look like they are not from the same time as others. Maybe the amp had some major failure once and someone decided to substitute all 4 diff pair transistors with A750, which was a really bad choice, if that's true...
So, I now have to look for the best equivalent transistor and order, say, 20 of them, so that I can match them? I've never done this before, how can this be done easily? I don't have a hfe meter. I'll search about this on the Internet, but any hint is welcome.
problem is now diagnosed: in fact a noisy transistor in diff pair, just as suspected. It took me quite some time, though, as (weird enough) there was only one defective transistor, Q701.
Here's how I diagnosed:
in the first measurements I did, no noise was visible at its collector. But I hadn't measured its base. So, at the time, as I was measuring noise at the base of Q703, I (wrongly) deducted the defective transistor would be Q703 and exchanged it with the one from the other good channel. It didn't solve the problem, obviously. The noise only got there coming from the feedback loop. It was only when I decided to short the base of Q701 to ground (before, I had always shorted in front of the 470ohm resistor, now I shorted behind it) that I noticed that the noise went away, so I took the short out and hook my probe onto the base of the transistor (note that resistor was floating free, then there's 33k to GND), and there was the noise! I assumed it wouldn't come from the resistors, would be really unlikely, as there not of the old carbon composite kind, so I decided to swap Q701 and Q702, and bingo! Now the noise went to the right channel instead of the left! It's Q701. It has to be said, btw, that none of the four transistors of the differential pairs of the two channels are the ones indicated in the schematic, I don't know why. They are all the (infamous) 2SA750, known to become noisy (it's on the list in that one really useful sticky thread here on this forum). I can't understand why Maratnz would have decided to go for these instead of the 2sa1127 originally indicated in schematic. I guess this could be the result of some old repair done on this amp? I mean, some solder points look like they are not from the same time as others. Maybe the amp had some major failure once and someone decided to substitute all 4 diff pair transistors with A750, which was a really bad choice, if that's true...
So, I now have to look for the best equivalent transistor and order, say, 20 of them, so that I can match them? I've never done this before, how can this be done easily? I don't have a hfe meter. I'll search about this on the Internet, but any hint is welcome.
For what I can find, the 2SA992 seems to be a good equivalent. BC560 could work and is easier to find here, but I suppose it would be risky, as amp seems to have rails at about +-43 V and the BC560 datasheet states maximum VCE at 45 V...
I had a search about hfe matching (unfortunately my DMM doesn't have that function) and seems there are some test rigs to do this, not too easy, but I wonder: if I buy them together, hopefully all from the same lot, won't hfe be sufficiently similar? Or do they HAVE to be hand matched for a diff pair? I've read some varying opinions about this...
I had a search about hfe matching (unfortunately my DMM doesn't have that function) and seems there are some test rigs to do this, not too easy, but I wonder: if I buy them together, hopefully all from the same lot, won't hfe be sufficiently similar? Or do they HAVE to be hand matched for a diff pair? I've read some varying opinions about this...
On another place I had the recommendation to go with 2sa970 instead. So I ordered them and now the repair and the testing are done, it definitely was a defective transistor, to go safe I changed all 2sa750 for the 2sa970 and the result is great, amplifier working fine without any noise! Thanks everyone for helping.
A good visual check is always good first.
Dry joints, unsoldered joints, bad connectors, dodgy pots, bad relay etc.
Bad zeners, old caps.
Dry joints, unsoldered joints, bad connectors, dodgy pots, bad relay etc.
Bad zeners, old caps.
Yes, sure, did all this, but it clearly was the transistor, to make sure I swapped them between channels and the problem went to the other channel 🙂
Unfortunately this unit came back to my bench - not exactly for the same problem (a bad connection in Phono section), BUT: I detected while testing, that the unit still shows random "pops" (almost sounding like gun shots) during operation, only on the right channel. I managed to isolate the problem, again, to the power section. This had the majority of small signal transistors replaced by me, as you can read above, except for the last drivers and power output transistors. Drivers are something like D667 and D666, I've never come across them listed on any place on the net as potentially being problematic, so I don't believe it could be a transistor? Or could it still be one of them? Unless it was one of the output power transistors? Is that even possible? Unfortunately I can't easily see which are mounted (they are below a metal plate which is hard to get off), as many times the service manuals of Marantz don't corrspond to actual transistors installed.
I decided to check most of the passive components in that channel and all measures fine, also no bad solder, as far as I can detect. No bad connections to speakers, too. No speaker relay mounted in this unit. So, I'm lost. I noticed a carbon composite resistor in the output and temporarily took it out but no improvement. Electrolytic caps have been replaced before by me (no mistakes made, triple checked). Any ideas? Thanks!
I guess I can again start swapping the driver transistors between the channels... Also it's hard to measure/follow the pops with oscilloscope, as they only happen from time to time...
I decided to check most of the passive components in that channel and all measures fine, also no bad solder, as far as I can detect. No bad connections to speakers, too. No speaker relay mounted in this unit. So, I'm lost. I noticed a carbon composite resistor in the output and temporarily took it out but no improvement. Electrolytic caps have been replaced before by me (no mistakes made, triple checked). Any ideas? Thanks!
I guess I can again start swapping the driver transistors between the channels... Also it's hard to measure/follow the pops with oscilloscope, as they only happen from time to time...
No, it doesn't, it's not related to any interference, I've checked that. It's totally random and apparently isn't temperature related either. There may be times as long as 2 hours without any pop and then you may get like three in a row (and some are really strong, so this can't be ignored) right after switching it on. And other times there comes one or two after 45 minutes of listening. When I received the amp in the first place, it had this phenomena on both channels, and much more often than now. At the time, I changed the 2sa750 which are known to cause trouble. But now there are no other known trouble makers left... and it seems the problem is actually getting worse with time. I will have to swap the drivers between channels to be able to exclude them. But if they are not the culprit, I really can't imagine what else. What about the power output transistors? has any one on here ever seen any of them causing this kind of trouble?
I have a Pioneer M1500 which once had the popping noise problem,
I took the diff. transistor 2SA798a off and changed another one and it became normal.
Later I found the legs of the defected transistor has some dirt on it,
so I sprayed and cleaned it, rob the legs clean, then I swap it back to the amp.
Surprise me it came back to normal.
After a few months the popping noise came back again.
This time I cleaned the legs thoroughly and used detergent..
Solder it back and it works fine for years up to now.
Could it be a similar thing to your amp?
I took the diff. transistor 2SA798a off and changed another one and it became normal.
Later I found the legs of the defected transistor has some dirt on it,
so I sprayed and cleaned it, rob the legs clean, then I swap it back to the amp.
Surprise me it came back to normal.
After a few months the popping noise came back again.
This time I cleaned the legs thoroughly and used detergent..
Solder it back and it works fine for years up to now.
Could it be a similar thing to your amp?
Alexchoi: you were spot on! Thanks! Here, it was one of the driver transistors in the problematic channel (a 2SD666 if I'm not mistaken right now), it probably had oxidated legs or a bad solder joint or whatever, because as I was exchanging transistors between good and defective channel (to see if it was a noisy transistor), the problem suddenly disappeared after exchanging these drivers!
It's been working fine since, this was already weeks ago, I haven't had time to reply earlier.
We're always learning - I hadn't come across something like this before. Reminds me of what happened recently with a PM500: it had a weaker/distorted channel and I was thinking transistors but fortunately I went the "check bad solder joints" way first, knocking with plastic tool around the PCB and surprise! when I knocked against speaker protection relay, the sound went better! I unsoldered and opened the relay and it had contacts totally oxidated, black! I used fine sand paper and de-oxit and after this, the amp is working great! (btw, had to do the same treatment to channel selector switch...).
It's been working fine since, this was already weeks ago, I haven't had time to reply earlier.
We're always learning - I hadn't come across something like this before. Reminds me of what happened recently with a PM500: it had a weaker/distorted channel and I was thinking transistors but fortunately I went the "check bad solder joints" way first, knocking with plastic tool around the PCB and surprise! when I knocked against speaker protection relay, the sound went better! I unsoldered and opened the relay and it had contacts totally oxidated, black! I used fine sand paper and de-oxit and after this, the amp is working great! (btw, had to do the same treatment to channel selector switch...).