Orion 2150SX burnt resistor ???'s

[Chamberman]

I bought this amp used and the only story I got on it was that it had a burnt resistor. I have not torn into this amp yet, I plan to pull the board this weekend and begin troubleshooting it. I figured I would show you Orion experts a pic of the resistor that burnt on this amp and see if any of you have any theories on what I should be looking at when I get it apart. Also does anyone know for sure what value resistor this was before it roasted? I found an older pic of the amp internals and it looks like it was probably a 100 ohm resistor but the color codes were not that clear in the pic I saw.

Overall the interior of the amp looks pretty good. It does not look like anyone was messing around inside of the amp which I hope works to my advantage. There is a brown discolored spot on the clamping bar near the rear of the amp that has me concerned, I put an arrow pointing to it in the full amp picture. The burnt resistor has a red circle around it in the full amp pic.

Fullamp1.jpg

Burntresistor1a.jpg

Burntresistor2.JPG

[Chamberman]

I had a chance to spend a little time this evening with the meter checking a few components.
Unfortunately as I feared the rear power supply section where the brown mark is located appears to have a severe problem. The rear most component in that bank measures 2 ohms from pins 1 -2 and 9 ohms from pins 1 - 3. All of the other devices in the rear power supply section on that same side measure 22 ohms from pins 1 - 3. The power supply components on the opposite bank all measure significantly higher (115 ohms iirc) and similarly from pins 1 -3. So it looks like I'll be replacing at least a few power supply components.

[Perry Babin]

That's a 100 ohm resistor.

You likely also have shorted output transistors (or possibly shorted driver transistors).

To remove the clamps, use the following method. Remove all screws that hold the board to the sink before doing this.
http://www.bcae1.com/temp/orionampclamps.swf

[Chamberman]

Well I am going to put this 2150SX on the back burner for now. I have an HCCA 225 Digital Reference that will not power up and I plan to use it in one of my vehicles once its working so I am going to concentrate on that for now. I'll start a new thread for that problem.

[Chamberman]

Good news on this amplifier and a few questions. I decided to start working on this amp again a few weeks back and there ended up being a number of bad parts, 1 - MPSU 07 & 57, 3- 2N6488's, 1 - 2N6491, 4 - NDP7060's, the FEN 16 diode and an MPSA 56 in the power supply section. The past threads on this 2150SX amp were an invaluable source of info for me. I ended up substituting some of the blown parts based on those past threads, MJE 15030's & 15031's replaced the MPSU drivers that were blown and IRF3205's for the blown NDP7060's.

I powered it up for the first time this evening and did not expect it to work right off the bat, but I was pleasantly surprised when it powered right up and sounded great.

Only one side of the power supply section had the blown 7060's so I replaced that whole bank. I did not touch the other side because they measured fine and I'm wondering now if I should just bite the bullet and replace the other side even though its working. Anyone have any insight on this?

Many of the electrolytic caps were bulging so I went ahead and replaced ALL of the electrolytic caps except for the 10uF ones. I also replaced the 2N5639's in the muting circuit because they seem to be a weak point based on past threads. I also replaced all of the MPSA 56/06's in the power supply section (the 4 on the end of the board).

One other question I have, the DC offset is 30 mV on one channel and 40 mV on the other. This seems high to me, is there anything I should check or replace to address this offset or should I just live with it?

[Perry Babin]

Did you measure the offset directly across the speaker wires/terminals?

40mv is a bit high but certainly won't cause any problems.

If you want to try to reduce it, you could try replacing the op-amp that drives the power amplifier. The op-amp will generally have a 2.7 ohm resistor in series with the output terminal (pin 1 or pin 7). If you want to get it as low as possible, install a socket in that location and buy 10 op-amps and find the one with the lowest offset.

Again, 40mv is a bit higher than many techs find acceptable but it doesn't indicate that there is a problem.

[Chamberman]

Yes the DC offset was measured directly at the speaker temrinals. I measured it both with the speakers connected and with the wires disconnected and it measured the same. I just wanted to make sure that the level of DC offset was not a precursor of some component failing. The last thing I want to happen is to have a bunch of my new components go up in smoke.