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Post by Deleted on Oct 17, 2010 6:38:29 GMT
Tom At the attached link is a layout diagram of the PCB. Perhaps you can show us your exact connection points ? A good photo of the PCB would be even better. rockgrotto.proboards.com/index.cgi?board=chitchat&action=display&thread=3622&page=1I am not surprised at the USB noise problem, as a friend had rather poor performance from his laptop using USB to the Benchmark DAC. At the next listening session he used a new PC with SPDIF instead. Altronics sells a SC USB power injector kit, which could be modified to supply better +5V power from perhaps a linear regulator, or even a SMPS if a choke was fitted at the input to the onboard regulator? Alex
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Post by whittie on Oct 17, 2010 6:52:13 GMT
Alex, I followed that diagram when I built the board. I attached the Earth point to the underneath of the PCB at the 'Earth Point'. The potentiometer is not connected to chassis earth because I can't get any solder to stick to the metal parts of the ALPS pot. I have uploaded some Hi-Res pics of the PCB underneath and on top views here: members.iinet.net.au/~johnwhitfield/SCHA%20Pics/Since then I have redone the PSU PCB as per the SCHA Tweaks thread and added the Toshiba transistors to the amp PCB. One thing I'm not sure of relevancy is that I did hit one of the 100nf capacitors with the soldering iron and melted a small edge section where one of the input pins are. I don't think this effected the capacitor itsself as the good bits are deeper into the shell, but I have ordered some more anyways just in case.
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Post by Deleted on Oct 17, 2010 7:01:24 GMT
Alex, I followed that diagram when I built the board. I attached the Earth point to the underneath of the PCB at the 'Earth Point'. The potentiometer is not connected to chassis earth because I can't get any solder to stick to the metal parts of the ALPS pot. I have uploaded some Hi-Res pics of the PCB underneath and on top views here: members.iinet.net.au/~johnwhitfield/SCHA%20Pics/Since then I have redone the PSU PCB as per the SCHA Tweaks thread and added the Toshiba transistors to the amp PCB. One thing I'm not sure of relevancy is that I did hit one of the 100nf capacitors with the soldering iron and melted a small edge section where one of the input pins are. I don't think this effected the capacitor itsself as the good bits are deeper into the shell, but I have ordered some more anyways just in case. Tom You need to earth the case of the Alps potentiometer. I use recovered earth ring connectors from reclaimed gold RCA sockets. I snip throgh one side to spread it out a little more, then slip it over the shaft after soldering an earth lead to the connector. You should be able to fashion something similar ? Alex
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Post by whittie on Oct 17, 2010 7:55:48 GMT
Alex, I'll try that, I do have some suitable material lying around.
One thing tho, when I couldn't get anything to stick I just held some wire against the pot and the ground and it made zero difference, so I'm not sure that is the cause of the hum. Tom.
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Post by Deleted on Oct 17, 2010 8:03:01 GMT
Alex, I'll try that, I do have some suitable material lying around. One thing tho, when I couldn't get anything to stick I just held some wire against the pot and the ground and it made zero difference, so I'm not sure that is the cause of the hum. Tom. Hi Tom Please check fig.5 in the link to the Tweaks Thread. SC says it is necessary. Regards Alex
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Post by whittie on Oct 17, 2010 8:24:56 GMT
Hey Alex,
Yes I saw in the paperwork that came with the amp that is in the instructions to earth the chassis of the pot.
So, I did that and I still have the hum in the right speaker. It is only the right speaker tho...
Tom.
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Post by Deleted on Oct 17, 2010 20:42:19 GMT
Hey Alex, Yes I saw in the paperwork that came with the amp that is in the instructions to earth the chassis of the pot. So, I did that and I still have the hum in the right speaker. It is only the right speaker tho... Tom. Tom Did you check with your DMM, that after the earth was fitted, that there is a low ohms reading from the shaft of the pot to mains earth ? Alex
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Post by whittie on Oct 18, 2010 0:26:52 GMT
Hey Alex,
I didn't check from the shaft itsself, but from the body of the pot it is 0.4 Ohms.
Back when I first built this kit I had an LED not light up on the right habd side of the board (Closest to the headphone jacks, closest to the power in from the psu) and then I reversed the polarity of the power from the PSU and fried an op-amp, is there any chance that this could have caused some damage to the right channel to give this hum?
Tom.
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Post by Deleted on Oct 18, 2010 0:35:08 GMT
Hey Alex, I didn't check from the shaft itsself, but from the body of the pot it is 0.4 Ohms. Back when I first built this kit I had an LED not light up on the right habd side of the board (Closest to the headphone jacks, closest to the power in from the psu) and then I reversed the polarity of the power from the PSU and fried an op-amp, is there any chance that this could have caused some damage to the right channel to give this hum? Tom. Tom I can't say for sure, but after something like reversed polarity, I would have done complete voltage comparisons between channels,using the quiet channel as the reference, and I would have replaced all electrolytics to be on the safe side. Damage isn't always noticed immediately. You would have verified that DC Out was O.K. but did you also check the voltage drops across all 4 Output stage emitter resistors ? Alex
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Post by whittie on Oct 18, 2010 0:41:43 GMT
I would have done complete voltage comparisons between channels,using the quiet channel as the reference, and I would have replaced all electrolytics to be on the safe side. Damage isn't always noticed immediately. You would have verified that DC Out was O.K. but did you also check the voltage drops across all 4 Output stage emitter resistors ? Alex Hey Alex, What exactly do you mean by 'electrlytics'? I did not check the voltage across the output stage emitter resistors, as again I'm not sure what they are. I have since the polarity put in a new opamp, new transistors (Toshiba spec), modified the psu and a new pot. However, apart from the transistors I have not done any checks on the amp pcb. Using perhaps the circuit picture, can you direct me with what to check first to try and track down a fault? Tom.
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Post by Deleted on Oct 18, 2010 1:20:53 GMT
Hi Tom The electrolytic capacitors that I am referring to are the 100uF 16V near the LEDs,and the 470uF (or what they are now) directly above and below the I.C. There may not be any damage, but new ones aren't very expensive. The output stage emitter resistors are the 4 x 4.7 ohm resistors near the 4 heatsinks, so they are easy to access for measuring. Alex
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Post by whittie on Oct 18, 2010 11:16:30 GMT
Heya Alex,
Well I can finally report success!
I went over all the components you mentioned along with triple checking the board for any loose solder joints after having an LED drop in an out trying to measure the voltage across it. I found about 3 dry solder joints going ocer the board with a microscope.
However, that was not actually the cause of the problem. Being lazy I didn't disconnect the pcb's whilst doing the job and while trying to plug headphones in without electrocuting myself whilst the amp was on the buzz got worse and better as the pcb was moving around. Turns out the right channel input cables were just too close to the mains power switch. I rerouted the cables and the switch and, VIOLA, problem solved.
Thanks for all the assistance, it was a great help and motivation to keep trying to solve this persistent issue.
Tom.
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Post by Deleted on Oct 18, 2010 11:24:43 GMT
Hi Tomm Glad to hear that you have now solved your problem, as well as possibly preventing a few further down the track. Incidentally, twisted wires are O.K. for short distance input cable runs, but ideally,( going by that photo) you should have used screened cable. Alex
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Post by whittie on Nov 16, 2010 13:21:59 GMT
Hey All, Alex specifically, hopefully you'll get an email notification about this, but please, anyone that can help I just have ANOTHER issue or two to go over. To recap: I made my first SCHA, LED didn't light, reversed the polarity from the PSU and smoked an op-amp, fixed the bad solder on the LED, put the polarity and all was good. Since then I've built a second amp as well, also talked about in this thread. Well, the second one I built a little better physically, so I took that to work (We have mandated electrical equipment testing so I figured the better built amp would be less of an issue/HSE risk if the issue ever came up. New amp passed all tests with flying coulours ) and brought the original SCHA home. I have ordered a couple of JLH boards and so thought "What the heck, all-out build it is". So, I've since installed a Blue Velvet pot and just last week an Audio-GD Moon module. Now, what I've noticed since the install is a fuzzy buzzing to a lot of songs. It seems to just be like a distortion/buzz of the mid-high notes. It doesn't happen with all recordings, but most. Thinking it was an earth issue, I re-did all the earths, no dice. I then dropped the original burr-brown on-amp back in and the same fuzz is there, just not as prominent. The amp is fed from the same source as my stereo amp and it has NO fuzz, so I think it is amp related. My question then, after the background, is could it be because all that time back I reversed the polarity from the PSU and fried the LMXXXX op-amp? All I did post the frying was replace the op-amp. Now, I know you told me I should replace all the capacitors, but before I go ahead and order them, is there anything else that I should be looking at that I might be missing? I thought it was the Moon module that was causing the issue, but like I said, once I noticed the fuzz I could hear it, albeit MUCH softer, even with a different, new, opamp. I tried running in the Moon module for 5 days straight and it made no difference, so I'm more and more confident that it is an amp issue, but the only thing I can think of is the polarity issue, way back at the beginning. Your thoughts would be greatly appreciated! Tom.
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Post by Deleted on Nov 16, 2010 20:46:15 GMT
Tom That one is a bit hard to diagnose remotely. It could also be due to wiring layout.However, I would definitely replace those electrolytic capacitors as previously discussed. They are also there for stability reasons, not just filtering. Alex
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Post by whittie on Jan 4, 2011 7:01:03 GMT
Alex, Success! When I was ordering the JLH bits I ordered all new caps to replace the ones on the board, including one I also managed to burn one at some stage.... I also noticed I had the 2SA and 2SC transistors the wrong way around..... I don't think that would help either! Either way, back together and listening and the hiss is gone I also put the JLH together and the transformation is quite something else! In the tweaks thread you mention that some of the board mods are not noticeable until the JLH is installed and WOW is the JLH a nice mod! Tom.
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Post by Deleted on Jan 4, 2011 8:03:05 GMT
Hi Tom Good to hear that you now have it firing on all cylinders. Just remember that the JLH needs at least 48 hours on it for it to fully stabilise. Regards Alex
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