Dynaco ST-400 Amo - about to blow or what??

Archived from groups: rec.audio.tech (More info?)

In article <1111990347.830347.115580@o13g2000cwo.googlegroups.com>,
<kafer77@ameritech.net> wrote:

>I just purchased an old Dynaco ST-400. It does work (and send what
>seems to be a non-distorted signal to each speaker. However, I need
>help interpreting what it does upon powerup. When power is applied
>there is a brief "flash" of light in the clear plastic "window" between
>the two front mounted fuses. The light does not stay on. At about the
>same time there is a noticeable "clunk", and I assume this in a relay
>kicking in. (When I turn the amp off, I also hear a "clunk". Thus, I
>assume the noise is a power relay kicking in and out.)
>
>What I really care about is the brief "flash" of light seen through the
>clear section between the fuses. Is this OK? Should this light stay
>on? What is the flash?

If I recall correctly (it's been a decade since I sold my ST-416, big
brother of the SD-400) the light you are seeing is an indicator lamp,
which reports the activity of the "Dynaguard" circuit. The relay you
are hearing click, is a speaker-protection relay.

This circuit is designed to keep the speakers disconnected from the
amplifier output circuitry if there is any significant amount of DC
voltage present at the amplifier outputs. Such DC output can occur
during power-up or power-down as the result of voltage transients in
the supply circuitry, or as the result of circuitry faults in the amp
or in the "upstream" components such as the preamp.

Again, if I recall correctly, it is entirely normal for the Dynaguard
indicator light to flash briefly when the amp is first powered on. The
output-protection relay should close with an audible click at the same
instant that the light goes off, plus or minus a bit.

I think that what you are seeing, and hearing, is quite normal and
healthy for this amp.

The one thing I can think of to mention, is that the speaker relay in
these amps can be prone to dirty-contacts problems after some years of
use... I don't think Dynaco used a relay with self-wiping contacts.
If you hear distortion, it may be necessary to open up the amp
(carefully... there's a lot of power stored in the supply caps) and
clean the relay contacts with a burnisher.

--
Dave Platt <dplatt@radagast.org> AE6EO
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