Studiomaster Powerthouse 300 8.2, 1996

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

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.

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Occam's Raison again

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:

Cheers,

Gareth.

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linkage

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load

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until

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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.