Acoustic Control 450 Base Amp - Check Up / Tune Up Low Output

[Enzo]

When I see an amp make a good signal unloaded but can't drive a load, it tells me the output cannot provide the current. A bad output is possible, but usually I look for an open circuit. Open resistor, cracked solder joint, cracked circuit copper trace.

And g1 makes a point, is your scope ground clipped to the speaker negative terminal? Should NOT be.

[TomCarlos]

When I see an amp make a good signal unloaded but can't drive a load, it tells me the output cannot provide the current. A bad output is possible, but usually I look for an open circuit. Open resistor, cracked solder joint, cracked circuit copper trace.

And g1 makes a point, is your scope ground clipped to the speaker negative terminal? Should NOT be.

The negative lead of the scope is connected to the negative terminal of the Power Supply cap.

I will get in there yet again and take a careful look at the components and traces surrounding the output transistors.

In the meantime, I happen to have six MJ15003 transistors. If I need to pull the existing STC transistors for any reason, I think I will replace them.

[TomCarlos]

After dickering with this amp for a while, my friend and I have decided to pull the Power Amp board out and go through it slowly and methodically. Being a used amp, I have no idea what has been done to it before. I'm not sure if there is a significant difference between the 170045 vs 170051 boards (schematic in first post). But I will assume the amp worked at one point in time. So we'll be looking for bad components, traces, etc.

It looks like the Reverse Ground switch is going bad. We're leaning towards simplifying the wiring in the power supply. If I am reading the schematic correctly, for 120V input, the transformer is using the Black and White leads for the primary. Everything else will get disconnected. We will also remove the board for the Line / Hi low switching. Death caps will get yanked too.

I'm not sure why have the 7A slow blow fuse. Any reason why it should stay?

We're also planning to replace the output transistors. I assume if we install the MJ15003, we'll check the symmetry pot but shouldn't expect or need to change any other components?

Thanks for the final hints.

[Enzo]

Tom Tom Tom, that 7A fuse is there for the same reason AMpeg used similar. They put a 5A fuse in the holder - and if it blows, the owner can replace it. The 7A is inside, so when that owner stuffs a 30A fuse in the holder, the amp is still protected by the 7A.

The difference between the 170045 and 170051 is right on the schematic. See the VIEW A and VIEW B just above the main bridge rectifier. Says 170045 only, the 170051 is wired like the main schematic itself. Note the VIEW areas on the main schematic have dotted lines around the circuit in question. Also look at the small chart that says TABULATION in the same area, it tells different resistor values depending upon which board.

If you are yanking the death cap, then the ground switch doesn't matter.

Why do you want to tear out the mains wiring? The amp works doesn't it? I mean it powers up and does everything the mains should do?

This is a common split primary power transformer, there are TWO primaries. For 120v they are parallel, for 240 they are in series. That is what the 120/240 switch does. If you only use the black and white wires, then you are losing half the current capability of the transformer. The BLK/W and BLACK/RED are wired in parallel with it. I'd surely just leave that voltage switch alone.

And the high low switch was for when the voltage from the wall was low - often the case on poorly wired stages. Flip the switch down and it boosted the power supply voltage a few volts.

One lesson I have learned over and over in my years is that just because I don't understand something, that doesn't mean it doesn't belong there. yes, I have learned that the hard way.

New transistors will certainly require new bias adjustment. (The VIEW A part)

[TomCarlos]

Thanks Enzo.... points well taken. Now that I take a closer look at the wiring on the transformer, your comments make sense.

I will try to study the circuit carefully. The long term concern is finding an STC replacement if one of those transistors should fail. So I will need to learn how to properly bias the transistors (unless we are talking about setting the symmetry control).

[TomCarlos]

In time, I will get back to this thread. I yanked the board out for careful inspection. I found that R423 was a 10 ohm resistor, when in fact it should be a 1 ohm. So I will need to check each and every component on this board.