Re: Newby needs stereo amp repair help


"Charles Schuler" <charleschuler@xxxxxxxxxxx> wrote in message
news:d6OdnelHUvbQuFXenZ2dnUVZ_sKdnZ2d@xxxxxxxxxxxxxx
>
> "powerdoc" <dkingloff@xxxxxxxxxxxxxx> wrote in message
> news:1137175153.194751.266300@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
>>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


.

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