Re: stereo amp cleaning
- From: "Mark D. Zacharias" <spammenot@xxxxxxxxxxxx>
- Date: Sat, 02 Jun 2007 01:53:18 GMT
g wrote:
On May 30, 8:39 pm, "Mark D. Zacharias" <spamme...@xxxxxxxxxxxx>
wrote:
Dave wrote:
"Mark D. Zacharias" <spamme...@xxxxxxxxxxxx> wrote in message
news:Q%d7i.6286$C96.4135@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Dave wrote:
The bias diode is essentially two diodes in series. (Sometimes
three, actually). In this case two, looks like. A diode check
function of a DMM would be OK for a static check, and if it became
necessary you could use the DMM to help match a replacement pair
of diodes to replace it with. However, using regular diodes might
not work as well in terms of correctly responding to heat changes.
The proof is in the operation, and in this case that means the
actual voltage drop across the device in-circuit, while theampis
running. Bias diodes opening up is uncommon but it does happen,
and when it does, catastrophe is the result.
In your case the resistors may have burned because of a momentary
short at the speaker wires, etc although this would ordinarily have
destroyed the output transistors as well.
If the bias diode is two diodes in series, why does the parts list
call it a varistor? It's a funky-looking thing that looks like one
of those connectors you put at the end of a wire to connect it to a
screw. Don't think I'd likely find another one. It's physically
screwed to the heatsink the outputs use.
Has technology evolved so that a momentary crossing of speaker wires
could not destroy expensive output transistors? You see all sorts
amps with "protection" circuits in them nowadays and lots ofamp
module IC's which boast both thermal and overcurrent resettable
protection...
I measured D405 vs D406 in circuit with no power applied and the
value, although useless quantitatively, was the same for both
channels. I'll check it in operation when I get all the resistors
replaced (and Q417).
Bonus for me: the one transistor used in thatampwhich I've got in
stock is 2SC945, which matches Q417 the bias transistor. I like BD
Enterprises which you directed me to, they seem to stock all sorts
of transistors nobody else does, or their search facility
immediately suggests an alternative if they don't have the original.
Dave
I was going by the schematic, and had not seen the physical
mounting. Of course the best choice to replace the "diode" would be
to order it from Harmon. I would be comfortable using diodes out of
my tool box if necessary though, using the diode check function of a
DMM to help match the voltage drop of the original (from the other
channel, that is).
As I state in another post, output transistors cannot be fully
protected from shorts without also protecting them from the music.
Mark Z.
If the amp is beefy enough, protection circuits will NOT affect the
music.
greg
Not necessarily affecting the sound, just triggering and shutting the amp
down when the over-current condition is detected. It would only affect the
sound if the amp driving signal is reduced or "soft-clipped" to deal with
the overload, an approach I have never been much impressed with as a
technician. The amps seem to blow up all the same, often along with the
transistors etc used to perform the limiting (old Marantz amps for example).
Mark Z.
.
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