Markbass Micromark 801 No Power

[TomCarlos]

MEF members...

My friend has a Markbass Micromark 801 and it appears to be DOA. There is no power light, no fan, etc.

Step 1 is a visual inspection and I noticed a problem with the wires going to to the fan. Each wire goes to some type of device. I'm not sure if this is merely a connector or some type of fuse. At least one wire on each connector is hanging on by a thread. You can see these wires in the photo. Any ideas on what this might be?

I think it might be hard to find a schematic for this one... I have been looking for a couple days.

Tom

[Enzo]

They look like crimp splices to me. As if someone replaced the fan, and took the new fan wires and crimped them to the clipped off old wires on the board.

All amps are similar basically. The mains comes up the power cord, possibly a removable cord, so an IEC connector, then a fuse or breaker, and a power switch, maybe a thermistor, and to the primary of the power transformer. All those things should be easily identifiable. And easily checked.

Hold the power cord in your hand, look at the plug. Set your meter to ohms, and measure resistance between the two flat blades on the plug. have the amp switch ON. Try this with some amp that you know works, just to see what I mean. Now on the bad amp. A good amp you would be measuring the resistance of the transformer primary through the power cord and switch. Might be 5 ohms, might be 500 ohms, I have no idea. What we don;t want to see is OPEN. If open, find the primary wires from the transformer, measure resistance across those. That skips all the mains wiring. If that checks open, then the transformer may be shot. But if the whole shebang checks open, go back and individually cheack each item: each side of the cord, the switch, the fuse holder and fuse. anaything else in there.

[TomCarlos]

Yup...

I checked the fuse and measure ~11 ohms across the primary windings.

The secondary has the molex type connector with 3 pairs of wires coming out of the Xfrmer. There is also a thick yellow wire coming from the Xfrmer that is not connected to anything (I think it is not used in this amp).

Red to Red - 4.2 ohms
Brown to Brown - 7.2 ohms
Blue to Blue - Open :-(

There is no AC voltage on ANY the secondary windings :-(

Tom

[Enzo]

11 ohms across the primary is a good sign for the transformer. but do you also get the 11 ohms across the power plug prongs? And the same test the other way, when the unit is plugged in and turned on do you see 120vAC across that primary? No point in looking for secondary voltages if we don't have mains on the primary.

[TomCarlos]

Yes... I measure 120vac at the primary. The power switch looks a double pole (above and to the right of the molex connector). In any event, I see 120vac on the terminals feeding the transformer. I'll do some more prodding tonight but I am starting to think the transformer has a problem.

[Randall]

Sounds like an open secondary to me.

[TomCarlos]

Ok... must be divine intervention but the transformer and amp are now working.

I removed the transformer to see what kind of markings it might have. Duh... the BLUE wires are CENTER-TAPS for the secondaries!!!

With the help of an Italian to English converter...

Rosso = Red
Blu = Blue
Marrone = Brown

Red-Blue-Red now measures 2.3 ohms from the Blue to the Reds
Brown-Blue-Brown now measures 3.9 ohms from the other Blue to the Browns.

I then measured the voltages with the Molex unplugged -

Red-Blue-Red now measures 29.7 vac from the Blue to the Reds
Brown-Blue-Brown now measures 16.3 vac from the other Blue to the Browns

I cleaned the pins on the Molex connector and secured it back to the PC board. Voila, the light comes on and the amp is working. Maybe there is a loose wire on the molex or somewhere but that seems to be fixed now.

Anyway... just wanted to close the loop on this.

Tom