Newby needs stereo amp repair help

[powerdoc]

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.

 

[CJT]

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.

 

[powerdoc]

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?

 

[Bobscar]

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

 

[Charles Schuler]

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.

 

[powerdoc]

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?

 

[Arfa Daily]

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

 

[Leonard Caillouet]

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