Hello all...
This is part of a longer story which I won't get into now. Suffice it
to say that I've wanted to get my hands on a failed Sherwood RX-4105
or RX-4109 stereo receiver to see just what it is that kills
them...abuse, misuse, weak parts, bad engineering/quality control or
something else. I've never had any luck coming into a truly broken
one, but someone recently gave me an RX-5502 that would just shut down
right after power on. I've been very happy with all of the RX-4105 and
4109 units I own.
The RX-5502 is a so-called multi-zone receiver. That is to say it can
support up to eight connected pairs of speakers, with four of the
pairs playing a different ("room 2") source if that is desired. It has
two complete stereo amplifiers in place, each one claimed to have an
output power of 100 watts per channel. (Obviously they're dreaming if
they think that this receiver is ever going to output 400 total watts
of power without catching fire, but...) This example was manufactured
sometime in 2008.
As found, this set would indeed power on for a few seconds, and shut
down with a blinking standby LED. I started checking things out. In
this set, the amplifier board is separate from the main board, so this
was not terribly hard to do. Every power device tested good with a
simple ohmmeter check, and nothing looked burnt or distressed on the
amp board. This doesn't look like a case of a failed power transistor
to me.
I'm working without service literature or even a schematic as Sherwood
would not provide them, but there is printing on the board that
identifies what each conductor in the ribbon cable going to the amp
board is used for. This set has a "test mode", and unlike similar
models, the "test mode" allows the power to stay on indefinitely while
the display test is running. Testing for voltages is a lot nicer
without having to constantly turn the set back on again! Voltages are
what I'd expect for B+ and B-, but a twelve volt input to the board is
hovering around a few hundred millivolts at most. That could do it!
Removing the amplifier board from the system and running without it
was probably risky, but it seemed like a worthwhile thing to do. With
the amp board removed, there was still no voltage from the +12 volt
connection. It still hovered around 300mV with the set on.
Interestingly, every now and then, a good power up was possible with
the amp board out, and the set would come out of protection.
Okay...where is the +12 volt supply generated?
Over in the power supply section there are a few linear voltage
regulators--two heatsinked 7812s and one freestanding 7912. One 7812
and the 7912 are doing their jobs, but the other 7812 is cold to the
touch and does not seem to be doing anything. (In fact, it was putting
out 300mV when I later checked it.)
Replacing the failed 7812 with an LM340 solved the problem. The set
immediately came back to life with the amp board in place, and it
plays. It appears, based on simple observation, that one 7812 is
powering the coils leading up to the speaker selection/protection
relays and the other is powering the amp board itself. What other
loads might be powered by these regulators has not been determined.
While the set is working, I don't like the temperature at which the
new regulator is running. Within ten minutes, its heatsink is on the
verge of being too hot to touch for more than a few seconds. There is
evidence on the circuit board that these regulators have always run
very hot. I've been in touch with Sherwood America, who said "the
regulator may become too hot to touch and possibly fail". I strongly
suspect there are bad capacitors on the amp and main boards, which
will need to be replaced and may be stressing the regulator. Yet
Sherwood seems to be saying that the extremely hot operation is
*normal* here. (However, it should be said that there is something of
a language barrier with the folks I've been communicating with.)
I could install a fan above or larger heatsink on the regulator and
I'm not above doing it if that is just the way things will be. I
suspect that would force the regulator to operate more reliably.
What I really want to know, though, is whether or not a drop-in
replacement with more current delivering capability than the LM340 or
78xx series exists. I've looked halfheartedly over the years but never
found anything. I could always build a more capable regulator board
and hack it in there, but I don't really feel like doing that. A fan
would be easier and faster.
I'd also like to know if anyone has had an RX-5502 on their repair
bench, and if they could comment on just how hot its regulators were
running. Any thoughts would be very much appreciated!
