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#1
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Please help save my Nad 304 !!
Hi,
I'm hoping someone might be able to help. I stupidly tried to solve an intermittent channel fading/distorting problem on my old beloved Nad 304. I know nothing about circuit boards but thought it may be a dusty pot or dry solder joint. I opened the case (power off!) and sprayed Servisol switch cleaner in the pots. There are four plastic 'screw adjusters' on the board(named vr302 and vr303). I also sprayed these and screwed them open and closed a few times to ensure fluid starts cleaning.I think this has caused the problem. I then left the amp for a couple of days to 'dry out'. I then connected up and switched on. Everything seemed perfect, no more crackling/fading. But after about 1 minute, I noticed that 2 larger components(covered in plastic?), were cooking, and stinking! I think it was caused by adjusting the plastic screw adjusters, whatever they are, and not turning them back to their original position. I quickly turned off, and re-tuned these plastic screws to the position I thought they were in originally. Again I turned on and again, the amp was sounding great.No cooking components. I thought I had solved it, but after a few hours use, 2 fuses blew on the circuit board. Obviously something still isn't quite right. Can anybody tell me what the 4 white plastic screw adjusters are, and how would I ensure the correct positioning of them? Also the plastic covering on the larger components that overheated, is slightly but noticeably 'melted'. What are these and should/can they be replaced? If I took the amp to be repaired, and gave them this information, could they diagnose the problem without long, expensive testing? Sorry about the long and 'novice' message, but I really don't want to lose this amp if possible. Thanks for your time, Len |
#2
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Hi,
I'm hoping someone might be able to help. I stupidly tried to solve an intermittent channel fading/distorting problem on my old beloved Nad 304. I know nothing about circuit boards No comment seems necessary... but thought it may be a dusty pot or dry solder joint. I opened the case (power off!) and sprayed Servisol switch cleaner in the pots. There are four plastic 'screw adjusters' on the board(named vr302 and vr303). I also sprayed these and screwed them open and closed a few times to ensure fluid starts cleaning.I think this has caused the problem. Ummm... more than likely. I then left the amp for a couple of days to 'dry out'. I then connected up and switched on. Everything seemed perfect, no more crackling/fading. But after about 1 minute, I noticed that 2 larger components(covered in plastic?), were cooking, and stinking! I think it was caused by adjusting the plastic screw adjusters, whatever they are, and not turning them back to their original position. I quickly turned off, and re-tuned these plastic screws to the position I thought they were in originally. Again I turned on and again, the amp was sounding great.No cooking components. I thought I had solved it, but after a few hours use, 2 fuses blew on the circuit board. Obviously something still isn't quite right. Can anybody tell me what the 4 white plastic screw adjusters are, and how would I ensure the correct positioning of them? I do not know that specific product. However, from your description, I'd guess that these may very well be the "bias" and "offset" adjustments for the amplifier output stage. The "bias" control adjusts the amount of "bias" or "idle" current which flows through the output transistors when there's no music playing. If the bias current is set too low, then there's an excessive amount of distortion when playing low-level signals. If the bias is set too high, the output transistors dissipate more power than is necessary, can overheat, and can even go into a nasty condition known as "thermal runaway" which can result in letting all of the Magic Blue Smoke out of the transistors. The "offset" pots, if present, are used to balance out small errors or inequalities in the circuit's symmetry, thus ensuring that the average voltage coming out of the output transistors (the "DC offset") is close to zero. If it's misadjusted, DC current can flow through your woofers (which may make result in some audible problems under some circumstances) and there may also be excessive current flow through part of the output stage. Now, my guess is that you probably tweaked things in a way which left the amp with *much* too high an idle current setting - too much bias. You may also have deranged the offset setting, if the amp has one. The excessive bias would have resulted in too much current flowing through the output transistors, overheating them. Basically, by misadjusting the bias, you injected your amp with a massive dose of "speed", and it went meth-freak on you :-) Your amp may have individual transistors, or more likely has a larger "power module" containing both the output transistors, a driver stage, and other components. This module (perhaps one per channel) would probably be fastened to the heatsink. I suspect that this is the "larger components, covered in plastic" that you mention. Also the plastic covering on the larger components that overheated, is slightly but noticeably 'melted'. What are these and should/can they be replaced? They're probably the power module. If there's an SK part number on them, that's likely what they are. If I took the amp to be repaired, and gave them this information, could they diagnose the problem without long, expensive testing? I suspect so. At the very least, the amp will need to have its bias and balance controls readjusted to meet the manufacturer's specifications... and, of course, the blown fuses will need to be replaced. There's surely a procedure for this in the service manual. If there was no actual damage to the power modules, then it probably wouldn't require more than 15 minutes to half an hour to do the job. The bad news is that there's some possibility that the overdriving has damaged the power module. One or both of the modules may have shorted out, internally. Or, the damage may be less obvious than that... the transistors may still be partially OK, but may have drifted out of spec and/or become unstable. It's possible that the amp would work for a while, if re-biased properly, but might then "run away" and blow its fuses again. The service shop would probably replace the fuses, re-do the bias/offset adjustments per the manufacturer's spec, and then operate the unit into a "dummy load" at partial and/or full power for an hour or so to make sure that it remains stable. If it doesn't operate properly and reliably after adjustment, the power modules would have to be replaced and the adjustments re-done. -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#3
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Len wrote:
Hi, I'm hoping someone might be able to help. I stupidly tried to solve an intermittent channel fading/distorting problem on my old beloved Nad 304. I know nothing about circuit boards but thought it may be a dusty pot or dry solder joint. I opened the case (power off!) and sprayed Servisol switch cleaner in the pots. Very bad idea. It's not 'pot cleaner' and doing this usually just results in the *needed* lubrication in the pot being 'washed out'. There are four plastic 'screw adjusters' on the board(named vr302 and vr303). I also sprayed these and screwed them open and closed a few times to ensure fluid starts cleaning.I think this has caused the problem. You bet - they are 'presets' that are adjusted at manufacture to set up bias current in the output stage typically. By 'opening and closing them' you ruined the manufacturers setting. I then left the amp for a couple of days to 'dry out'. I then connected up and switched on. Everything seemed perfect, no more crackling/fading. But after about 1 minute, I noticed that 2 larger components(covered in plastic?), were cooking, and stinking! The output transistors. I think it was caused by adjusting the plastic screw adjusters, whatever they are, and not turning them back to their original position. Very likely. I quickly turned off, and re-tuned these plastic screws to the position I thought they were in originally. Again I turned on and again, the amp was sounding great.No cooking components. I thought I had solved it, but after a few hours use, 2 fuses blew on the circuit board. Goes to show you didn't remember right. Obviously something still isn't quite right. Uhuh. Can anybody tell me what the 4 white plastic screw adjusters are, and how would I ensure the correct positioning of them? They are presets that you shouldn't touch, VR means variable resistor. You'll need a service manual and test equipment to get them back in the correct position. Also the plastic covering on the larger components that overheated, is slightly but noticeably 'melted'. What are these and should/can they be replaced? Probably just insulating covers. They can probably be replaced if they are a common type. If I took the amp to be repaired, and gave them this information, could they diagnose the problem without long, expensive testing? Yes. You'll give them a good laugh too no doubt. Sorry about the long and 'novice' message, but I really don't want to lose this amp if possible. You're lucky it didn't fry totally. Your original problem is most likely a bad solder joint that no amount of Servisol would fix. Graham |
#4
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Dave Platt wrote: The bad news is that there's some possibility that the overdriving has damaged the power module. I don't hink that NAD has ever used those nasty modules. I suspect that the plastic covers will be 'diamond shaped' to cover some TO-3 transistors. One or both of the modules may have shorted out, internally. Or, the damage may be less obvious than that... the transistors may still be partially OK, but may have drifted out of spec and/or become unstable. No, that doesn't happen - transistors either fail or are OK after abuse IME. Transistors don't fail 'unstable'. It's possible that the amp would work for a while, if re-biased properly, but might then "run away" and blow its fuses again. I don't think so - see above comment Graham |
#5
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In article ,
Pooh Bear wrote: I don't hink that NAD has ever used those nasty modules. You could be correct there. I suspect that the plastic covers will be 'diamond shaped' to cover some TO-3 transistors. Also quite likely. No, that doesn't happen - transistors either fail or are OK after abuse IME. Transistors don't fail 'unstable'. I was thinking more along the lines of a power output module, which contains driver transistors, power output transistors, bias regulation and temperature-compensation components, and various other passive components as well. Overheating of such a module might have bad effects on the passives (causing resistors to shift values, causing 'lytics to leak), or bad mechanical effects (e.g. damaging the mounting of a thermal-compensation diode, so that its temperature no longer accurately tracks that of the transistor case or heat-sink). It's possible that the amp would work for a while, if re-biased properly, but might then "run away" and blow its fuses again. I don't think so - see above comment Well, any time a component has failed or been seriously overstressed, I think it's still a good idea to make sure that the total system works properly out to the limits of its expected behavior. A quickie "Yeah, it powers up and plays music OK" test might not catch some sorts of residual faults which had been caused by the earlier -- Dave Platt AE6EO Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior I do _not_ wish to receive unsolicited commercial email, and I will boycott any company which has the gall to send me such ads! |
#6
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Thanks Dave and Graham,
That's a big help.I can now take it to an engineer and actually give him the names of the troublesome components. That should help. A couple more questions if you don't mind: I always thought 'Switch cleaner' was the safest to use for dirty pots etc, could you give me a name of a dedicated 'pot cleaner'? Also, what was actually frying when the output transistors gave off the foul stench and smoke? There are actually 4, 2 smaller ones, and 2 larger ones.It was the smaller ones that cooked.Would it have been the transparent 'glue' at the base, or the coating? Thanks again Len "Pooh Bear" wrote in message ... Len wrote: Hi, I'm hoping someone might be able to help. I stupidly tried to solve an intermittent channel fading/distorting problem on my old beloved Nad 304. I know nothing about circuit boards but thought it may be a dusty pot or dry solder joint. I opened the case (power off!) and sprayed Servisol switch cleaner in the pots. Very bad idea. It's not 'pot cleaner' and doing this usually just results in the *needed* lubrication in the pot being 'washed out'. There are four plastic 'screw adjusters' on the board(named vr302 and vr303). I also sprayed these and screwed them open and closed a few times to ensure fluid starts cleaning.I think this has caused the problem. You bet - they are 'presets' that are adjusted at manufacture to set up bias current in the output stage typically. By 'opening and closing them' you ruined the manufacturers setting. I then left the amp for a couple of days to 'dry out'. I then connected up and switched on. Everything seemed perfect, no more crackling/fading. But after about 1 minute, I noticed that 2 larger components(covered in plastic?), were cooking, and stinking! The output transistors. I think it was caused by adjusting the plastic screw adjusters, whatever they are, and not turning them back to their original position. Very likely. I quickly turned off, and re-tuned these plastic screws to the position I thought they were in originally. Again I turned on and again, the amp was sounding great.No cooking components. I thought I had solved it, but after a few hours use, 2 fuses blew on the circuit board. Goes to show you didn't remember right. Obviously something still isn't quite right. Uhuh. Can anybody tell me what the 4 white plastic screw adjusters are, and how would I ensure the correct positioning of them? They are presets that you shouldn't touch, VR means variable resistor. You'll need a service manual and test equipment to get them back in the correct position. Also the plastic covering on the larger components that overheated, is slightly but noticeably 'melted'. What are these and should/can they be replaced? Probably just insulating covers. They can probably be replaced if they are a common type. If I took the amp to be repaired, and gave them this information, could they diagnose the problem without long, expensive testing? Yes. You'll give them a good laugh too no doubt. Sorry about the long and 'novice' message, but I really don't want to lose this amp if possible. You're lucky it didn't fry totally. Your original problem is most likely a bad solder joint that no amount of Servisol would fix. Graham |
#7
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"Len" wrote in message ... Thanks Dave and Graham, That's a big help.I can now take it to an engineer and actually give him the names of the troublesome components. That should help. A couple more questions if you don't mind: I always thought 'Switch cleaner' was the safest to use for dirty pots etc, could you give me a name of a dedicated 'pot cleaner'? Also, what was actually frying when the output transistors gave off the foul stench and smoke? There are actually 4, 2 smaller ones, and 2 larger ones.It was the smaller ones that cooked.Would it have been the transparent 'glue' at the base, or the coating? Thanks again Len That would be the plastic packaging that encases the transistor. The transistor was getting so hot that the case it's in was becoming crispy critters. |
#8
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Len wrote: Thanks Dave and Graham, That's a big help.I can now take it to an engineer and actually give him the names of the troublesome components. That should help. A couple more questions if you don't mind: I always thought 'Switch cleaner' was the safest to use for dirty pots etc, could you give me a name of a dedicated 'pot cleaner'? There is no reliable 'pot cleaner'. Any solvent based product will remove the important lubrication grease. This also true in many switches. If you really knew what you were doing, you could disassemble the pot and clean the 'track' and 'wiper' individually - do you have watchmaker-like skills ? If the track has been damaged due to years of wear, this won't help though. Also, what was actually frying when the output transistors gave off the foul stench and smoke? There are actually 4, 2 smaller ones, and 2 larger ones.It was the smaller ones that cooked.Would it have been the transparent 'glue' at the base, or the coating? Without knowing the exact details, my best guess is that you overheated the plastic insulating covers on the power transistors. I don't know what 'glue' you mean - but glue would smell nasty when hot too. Graham |
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