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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Newby needs stereo amp repair help
I have a Yamaha A-720 amp that plays but has poor freq response in the
bass region. One channel is worse than the other by a little bit. I have voltmeters, an AF generator and a 'scope but what I need is something I can read about basic troubleshooting and repair. I don't see anything that appears to have been tinkered with except maybe the output transistors have been replaced. There are 2 pairs of capacitors on the main section that have something that looks like a mastic attaching them to the chassis. Is this someting that was done for mechanical stability or is that "mastic" electrolytic dielectric? Any help appreciated as this is a nice amp when working. |
#2
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Newby needs stereo amp repair help
powerdoc wrote:
I have a Yamaha A-720 amp that plays but has poor freq response in the bass region. One channel is worse than the other by a little bit. I have voltmeters, an AF generator and a 'scope but what I need is something I can read about basic troubleshooting and repair. I don't see anything that appears to have been tinkered with except maybe the output transistors have been replaced. There are 2 pairs of capacitors on the main section that have something that looks like a mastic attaching them to the chassis. Is this someting that was done for mechanical stability or is that "mastic" electrolytic dielectric? Any help appreciated as this is a nice amp when working. It's hard to say. The "goo" I've seen tends to be matte, while the glue tends to be shiny, if that's any help. Goo vs. Glue -- that's kinda catchy. -- The e-mail address in our reply-to line is reversed in an attempt to minimize spam. Our true address is of the form . |
#3
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Newby needs stereo amp repair help
The "goo" is just this side of a brick color and is matte. I don't
think I've ever seen something like this in a piece of elec. equip. before. Is it common to do this to stabilize components? |
#4
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Newby needs stereo amp repair help
powerdoc wrote:
I have a Yamaha A-720 amp that plays but has poor freq response in the bass region. One channel is worse than the other by a little bit. I have voltmeters, an AF generator and a 'scope but what I need is something I can read about basic troubleshooting and repair. I don't see anything that appears to have been tinkered with except maybe the output transistors have been replaced. There are 2 pairs of capacitors on the main section that have something that looks like a mastic attaching them to the chassis. Is this someting that was done for mechanical stability or is that "mastic" electrolytic dielectric? Any help appreciated as this is a nice amp when working. Amplifiers these days seem to be based on power op-amp ics. I don't know if there are 2 independant ics both operating in bridge mode for each channel or one quad ic package. The feedback circuit network of each amplifier does contain capacitors which cause the response of the amplifier to roll off (reduce) towards the high fequency end to prevent possible instability. A book stocked by Maplin Electronics (www.maplin.co.uk) Troubleshooting & Repairing Consumer Electronics without a Sechimatic. (stock/order code NQ62S) may give a general guide. Okay Bobscar |
#5
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Newby needs stereo amp repair help
"powerdoc" wrote in message oups.com... I have a Yamaha A-720 amp that plays but has poor freq response in the bass region. One channel is worse than the other by a little bit. I have voltmeters, an AF generator and a 'scope but what I need is something I can read about basic troubleshooting and repair. I don't see anything that appears to have been tinkered with except maybe the output transistors have been replaced. There are 2 pairs of capacitors on the main section that have something that looks like a mastic attaching them to the chassis. Is this someting that was done for mechanical stability or is that "mastic" electrolytic dielectric? Any help appreciated as this is a nice amp when working. Some amps use capacitive coupling to the speakers ... I have no idea if this is so in your case. If it is so, they should be replaced. |
#6
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Newby needs stereo amp repair help
Is anyone familiar enough with Yamaha amps to know if the factory did in fact secure some of the larger caps with some mastic or is this goop dielelectric from the capacitor? |
#7
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Newby needs stereo amp repair help
"Charles Schuler" wrote in message . .. "powerdoc" wrote in message oups.com... I have a Yamaha A-720 amp that plays but has poor freq response in the bass region. One channel is worse than the other by a little bit. I have voltmeters, an AF generator and a 'scope but what I need is something I can read about basic troubleshooting and repair. I don't see anything that appears to have been tinkered with except maybe the output transistors have been replaced. There are 2 pairs of capacitors on the main section that have something that looks like a mastic attaching them to the chassis. Is this someting that was done for mechanical stability or is that "mastic" electrolytic dielectric? Any help appreciated as this is a nice amp when working. Some amps use capacitive coupling to the speakers ... I have no idea if this is so in your case. If it is so, they should be replaced. Seconded, but I'm pretty sure that these Yammies are DC coupled output, but my first suspicion would still be caps. Look particularly for any smallish electrolytics that are sited near to any obviously hot looking components. The only proper way to assess the condition of an electrolytic capacitor, is with an ESR meter. A capacitance meter or a multimeter will not tell you the full story. If you can lay hands on a schematic for it, suspect any inter stage coupling caps. The four caps that you are referring to are the main smoothers for the power supply. Trust me, if they were leaking their internal vitals, you'd know about it ! The reddy-brown goo is usually just an adhesive - often originally hot-melt. It's purpose is just to add mechanical stability, as you suspect. Often, it starts off life a completely different colour, but changes to these muddy colours over the years. It can actually become slightly conductive also, and is known for causing problems in higher voltage circuits, such as may be found in TV sets or monitors. If this gunk is spreading across any uninsulated wire links in the board, might be worth clearing it off, just in case. You can always replace it with some fresh hot-melt. Be careful where you're probing when it's on. Output transistors behave splendidly as fuses .... Arfa |
#8
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Newby needs stereo amp repair help
Yes, the caps were glued down in many units from this era in the Yamaha
line. The stuff dries out and once the solvents are gone it apparently absorbs moisture from the air and corodes leads and jumpers. Very common to see these old amps with brown goo and green leads in proximity to it. Clean it all off and check the caps because at this age they may be bad anyway. Leonard "powerdoc" wrote in message oups.com... Is anyone familiar enough with Yamaha amps to know if the factory did in fact secure some of the larger caps with some mastic or is this goop dielelectric from the capacitor? |
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