Denon DRA-345R stereo amp pops when switching input

[Chris Jeris]

This may be a fairly standard fix, but I didn't find this
problem in the s.c.r faq or via Google. I would be grateful
for any advice.

I have a Denon DRA-345R stereo receiver/amplifier, hooked up
to a pair of Bose 31 speakers. Both are about ten years old
and have seen hard wear - little abuse (one of the speakers
has been dropped once) but they have moved house 10-12 times,
lived in very dusty environments, been left on 24/7 for weeks.

The problem I have now is that, whenever I push one of the
input select buttons (CD, tuner, video), I hear a loud pop
from whichever speaker is connected to the right-channel output.
The amp has two sets of speaker outs and you hear the pop on
both the "A" right and "B" right outs. The loudness of the
pop seems to be independent of the setting of volume or balance
controls, but becomes quieter if you push several input selects
in quick succession, and louder if you wait several seconds or
minutes. (Something charging, then shorting?) There is a pop
at power-off which is louder than the pop from pushing an
input select button. Presumably there would be one at power-on,
but the relay hasn't kicked in yet. If you turn off a set of
speakers by pushing the speaker-select button on the front panel,
you don't hear a pop (or any music, unfortunately).

The afflicted channel plays music well enough (from my CD
player), but if the amp has been on for an hour or so, the
resulting pop when it's turned off is loud enough to make me
fear damage to the speaker.

I haven't done any troubleshooting other than switching
speakers around to verify that it's the choice of out and not
the speaker that matters. I can solder neatly (have built a
number of circuits from kits) but have no real electronics
knowledge or troubleshooting experience, so I would be grateful
for some direction as to where I should look for causes, and
(in particular) anything I should avoid doing or touching
so as not to cause further damage.

Thanks in advance for any help you can offer,
Chris Jeris

 

[Mark D. Zacharias]

This is a DC problem. Maybe an offset from the amp section, but it sounds
more like a bad input select IC (if any) or maybe a bad muting transistor.
I especially like the muting transistors. See if maybe it has a couple
2sc2878 transistors and measure the voltage at the collectors. There should
be none, but sometimes they get a bit leaky. A reverse bias to the base,
used to "un-mute" leaks through to the collector, and there's a 'pop' heard.

Mark Z.