Ross MTA1200 Power Amp

[The Dude]

Looking for a schematic for this, or info on who to call for one. A Google search gleans only ads for used ones. I have one that won't power up. Checked switches, fuses, outputs, rectifiers, etc. Nothing obvious. Anybody heard of them or have info?

[J M Fahey]

Looks like a generic among generics, with a Ross label printed on, very Korean if old, China/Vietnam if modern.
I have repaired Ross lbelled mixers, small PA boxes and the odd grphic equalizer , al made in Korea and looking Samick made, but usually the lowest bidder is the OEM maker of the day, today it might be anybody.

Unless somebody here is AWSC for them, doubt youŽll get any specific data, although somebody recognizes it under other brand.
What does Ross itself say?.

But even without schematic: have you checked the Power Transformer for open primary?
You might have a dead thermal fuse embedded in the winding.

Post some amp guts.

[The Dude]

I'll try to answer some of those, Juan- and add some info.

What does Ross itself say? I can't find them, so I haven't asked. Are they defunct? If anybody has contact info, I'll gladly call them.

have you checked the Power Transformer for open primary? Yes- it's not open. There are also a couple of "slow on" thermistors that check good. This appears to be a rather primitive SMPS. I do have a current monitoring variac and it does show some minimal current draw when I fire up the amp.

Unverified, but I found a couple of forums where comments indicated that Crest Audio may have made these amps for Ross. I may try to contact Peavey who now owns Crest. It's worth a shot, but I'm not optimistic they will have anything.

And, lastly, here are some gut shots. Maybe someone will recognize it. 2 shots- one is the amp, and one is the power supply. Both are on a single heat sink assembly- one top and one bottom.

[Jazz P Bass]

From what I can gather the output section is Class H (smooth rail switching)

That power supply looks like a 1990's attempt at an SMPS.