Studiomaster Powerthouse 300 8.2, 1996

[N_Cook]

I'm being spun the story that this was put into storage and on powering up
again , just the mixer section was working.
So the following is hypothetical but is it a possible scenario?
Does not say anywhere that this amp can be bridged, but also does not say it
can't be bridged so perhaps a green light to the owner to run both channels
together.
Anyway blown fuses and some shorted TO3 in both channels. One 16 amp rated
relay had an obvious brown patch inside its casing, opposite the contacts,
but still worked at test levels when the active was replaced. The good
looking one did not click over but its coil ok and audio output up to the
one contact . Both replaced as due to overheating, the plastic linkage from
contact carrier back to flip lever are melted/deformed. The brown stained
one has a serious pitting , well reverse of a pit, sticking out of a
contact.
Now if due to dirt or initial pitting , then poor contact and then making
and breaking current under serious load could that in combination with
speaker insductance create spikes of a few hundred volts to knock out active
in both channels ?
If the bad relay was passing less amps than it should, then excess load on
the other relay and so both overheat.

If DC passed to the speakers , from blown TO3, due to a short in speaker
lead say, then I doubt the situation would last long enough to cause the
relays to overheat from assumed good state previously

 

[N_Cook]

If DC passed to the speaker, there is a good chance the contacts arced when
the coil was shut off, keeping the connection to the speaker rather than
breaking it. They might bring you a dead speaker later.

But more likely due to Numpty error.
Some Numpty did something stupid to one side and blew it up, then they did
the same stupid thing to the other side.
Like plugging an external power amps outputs into the mixer's power outputs.

Or possibly one side blew ages ago and they'd been running it mono until the
other side blew.



Gareth.

Occam's Raison again

 

[N_Cook]

Whilst we're on the subject, it is well worth noting what I said about the
speaker relay not always breaking the contact because it arcs. Few of these
relays' normally open contacts are grounded, so you may like to modify the
ones you see that are not, as I know you particularly like this sort of
thing.

Info on this (actually courtesy of Phil Allison) can be found here:

http://sound.westhost.com/project33.htm



Cheers,


Gareth.

2 larger 30 amp rated relays are now wired in there, I will leave at that as
far as mods are concerned. Exploring the originals it looks as though even
when new and good there is only just sufficient closing force and movement
to close the contacts. Then with perhaps "normal" heating of 16 amps passing
through, then the plastic mount for the contact carriers may deform slightly
under the on-action and enlarge the gaps. So progressively worse.

 

[N_Cook]

Just got back to testing/finishing this one

Someone previously had replaced one of the 1/4 inch speaker sockets, very
pro looking soldering and heatshrink , but on the wrong pins on closer
inspection.
Connected to tip and ring rather than tip and sleeve of a stereo socket,
ground to tip. So worked fine with mono plugs , presumably no one noticed
antiphase sound. Then someone presumably tried it in bridge mode.