In article <
68e28f5e-a05a-44f4...@googlegroups.com>,
pf...@aol.com <
peterw...@gmail.com> wrote:
>I have had this beast for about 10 years now, and it has developed a specific symptom - the right channel will cut out entirely - silent-same-as-no-signal on occasion. No pop, no prior acting out, no distortion.
>Otherwise, no issues.
>
>On the output board is a DPST >NC< 24V reed relay. Meaning (to me)
>that when it goes into protection-mode, the relay gets energized and
>output is cut. The schematic (HI-FI Engine) seems to bear this out.
No, it looks to me as if the circuit works the other way around. The
relay is designed to short the output signal to ground *until* the
relay is energized. During normal listening, the relay is energized,
the contacts are open, and the relay "vanishes" from the audio path.
Take a look at how the relay is driven/controlled, from an auxiliary
output on the power supply board.
>Picture of board here - down the page.
http://members.quicknet.nl/gerard.slikker/hkcitation17.htm
>
>There seem to be a number of versions of this relay. Additional part-numbers are: 200-002-7610 and 328-46-B
>
>Questions: Anyone else with this symptom in this device? I would hate to replace the relay only to find that something else is going on and the system is doing its job.
Don't have one myself. Here are my thoughts, for what little they may
be worth.
Since the relay is a DPST single-coil, and only one of the two
channels is affected, it seems unlikely that the relay drive is
bad... if it were, both contacts would drop to NC simultaneously and
both output signals would be shorted to ground. So, if it's a fault
in the relay, it must be in the contacts for that one channel -
somehow they're sticking closed. I'd be more likely to suspect a
reed-relay fault where the contacts _don't_ close properly (due
to e.g. lack of contact self-wiping, or the use of a standard reed
relay which requires a wetting current when you'd really want a dry
relay) and that doesn't seem to be the case here.
It _could_ be a case where you've got inadequate current drive to the
relay, and for some reason this affects only one of the two reeds, but
that seems less likely.
I think the first thing I'd suggest, for practical trouble-shooting,
would be to catch the problem when it exists, and then trace the
signal through the output board... see if you've got audio at J1/3 and
then follow it through the output driver and see where it disappears.
>And, may I substitute any decent 24V DPST relay? I have lots of room to play around.
It doesn't look to me as if the protection circuit is at all
sophisticated... the relay is driven from a simple time-delay circuit
which is driven by the +24 regulated supply. The relay driver is an
MPSA13, which is good for 500 mA continuous. The relay itself is just
a short-to-ground NC and the contacts aren't in the normal audio
signal path at all. So, it doesn't look very critical to me... any
small DPST-NC or DPDT 24-volt DC relay would probably serve (just
remember to keep the diode across the coil!)
>b) I have recapped the power-supply, and the output voltages are as
>they should be.
Other possible faults would be a bad 'lytic in the signal path (I've
occasionally seen them go entirely open), or bad switch contacts (input
selector, tape-monitor, etc.), or a broken solder joint that's
undergoing thermal cycling. The "tap the boards with a chopstick"
technique might help pin down the location of the fault.
I think a signal-tracing exercise would be a good idea, before you
replace anything.