"Gareth Magennis"
"Phil Allison"
"Gareth Magennis"

Mark, I have seen lots of these TDA powered speakers, mostly studio
monitors, with the "conductive glue" problem.

At the risk of repeating myself, have a good look at the PCB,
particularly between the pins of the TDA ICs. If there is any kind of
chemical coating, yellow or black, then this really is very likely to be
your problem.
It starts off as component adhesive, but over time melts and runs
everywhere, eventually becoming brittle and conductive, causing erratic
and illogical behaviour.

** The yellow glue problem is at least 30 years old.

Roland " Cube " series amps had problems with it and they were made in
Japan.

VCRs had problems it in spades as do many SMPS too.

Class D amps used in high end line array boxes made by dB Technologies
have it splashed about.

The stuff does NOT melt and move around, it simply goes dark with heat
and time and becomes conductive and corrosive to copper and tin.

Long as it has no contact with metal, it hardly matters.

Now, the colossally stupid Chinese spread it about on PCBs like jam on
toast.


I assumed that because it was SO spread around on PCB's where there was no
need for it (on the pins of TDA IC's for example) that it must have
migrated there.
It beggars belief that anyone would do such a thing.

** In early Roland Cube amps, the reverb drive IC was a SIL power amp -
with yellow glue all over the pins to stop them from breaking under
vibration.

One Alesis powered monitor speaker has the stuff all over a toroidal choke
in the SMPS, to hold it firmly to the PCB. That choke gets hot and the glue
eats right through the enamel - so you have to rewind the damn thing
completely.


The stuff I see in modern equipment is usually black and goes brittle.
Other stuff I have seen, which may be older (?), was more yellow/brown.


** The same glue is pale yellow when fresh, turns light brown with time and
heat and finally black like burnt toast.

This looks like the stuff.

http://www.alibaba.com/product-gs/48...tardant.ht ml

Comes in a convenient sauce bottle.



.... Phil