Rockford Fosgate 400.4 hums

Dr Zeus

Member
2010-10-17 5:12 am
This amp seems to be in fair condition for the most part. Its a Rochford Punch 400.4, light grey in color built with what looks like a cast/aluminum heat sync.

IMG_20101101_162156.jpg


This amp has two notable problems.

1 - one out of the 4 channels was playing very dirty and I found a transistor way out of tollerance - almost shorted with 50ohms from pin1 to pin 3 - so I clipped it off the PCB. The amp now powers on and is a little more stable for finding the remaining faults.

2 - The power supply seems to be supplying what I believe is osilation voltage throughout the rest of the amp. I'm not sure if it is normal, but heres a couple of pictures of the power fets across the OScope:

Scope shield at amp ground

Pin2: Scope set to 10v/div
IMG_20101101_183708.jpg


Pin3: Scope set to 10v/div
IMG_20101101_183606.jpg


All power fets test good. I tried removing 1/2 of them and then the other 1/2 and the problem still shows which means it likely is not the fets themselves. The image from Pin3 represents itself throughout the rest of the amp, even at speaker terminal outputs.

This is the second most powerfull amp I've had a chance to work with and I have to say I'd like to see it restored. This one is going to be donated to my Brother-in-law's audio project.
 
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Dr Zeus

Member
2010-10-17 5:12 am
Scope is connected to amp's ground for all these images (0 ohms).

I'm sorry, I was incorrect with a previously posted image. This image represents what is coming out of PIN1 of the fets:

IMG_20101101_183606.jpg


And here is what is coming out of the speaker terminals. Setting the crossover cards to low-pass does not change the noise.

IMG_20101102_185411.jpg
 

Dr Zeus

Member
2010-10-17 5:12 am
I still think you may be on to something ground related. Resistance between amp ground and RCA shields:

Front RCAs : 1.5k Ohms
Rear RCAs : 960 Ohms

Whats also interesting is there is a summation/out RCA. From the rear RCA input shields the sum-out measures 0 Ohms, but from the front RCA shields the summation measures 560 ohms
 

Dr Zeus

Member
2010-10-17 5:12 am
I'm not sure if I can do that with this frequency generator. Do you mean just attached a fused link from the output RCA shield to battery ground? I dont see any ground outputs/inputs on this generator.

If you plug both into front or both into rear, are either clean?
All the speaker outputs are laced with noise. The rear L+ terminal is the worst referencing ground. The rear Right channel is blown; one output transistor measured 50 ohms from pin1 to pin3 so I clipped it off the board but even still it's output has noise. This was problem #1 with this amp.

Also, seems I've made another mistake. Looking at these amps upside down for so long...
In post #9 i said this:

I still think you may be on to something ground related. Resistance between amp ground and RCA shields:

Front RCAs : 1.5k Ohms
Rear RCAs : 960 Ohms

Whats also interesting is there is a summation/out RCA. From the rear RCA input shields the sum-out measures 0 Ohms, but from the front RCA shields the summation measures 560 ohms

Its actually more like this:
Front RCAs : 960 Ohms
Rear RCAs : 1.5k ohms
 
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Well... I don't see anything out of the ordinary. Some of these 4 channel amps had a lot of noise but it wasn't audible.

If there are no open grounds (non-bridging speaker wires and the heatsink have 0 ohms continuity to the ground terminal, it's probably normal.

Have you tried touching the scope ground clip to the non-bridging speaker terminal for the channel being tested to see if it cleaned it up?

Does not using the ground clip make a difference?
 

Dr Zeus

Member
2010-10-17 5:12 am
If there are no open grounds (non-bridging speaker wires and the heatsink have 0 ohms continuity to the ground terminal, it's probably normal.

I do hope you mean the bridged terminals and not the non-bridged. I measure .2ohm from ground to bridged terminals, and 9k ohms from non-bridged to ground terminal.

The heat sync on this amp does not appear to be grounded anywhere to the PCB. Its open from amp ground to heat sync. and almost everything else I touch. When I power the amp I see 30m ohms from sync to just about any terminal.

If I dont use the scope'd ground clip then I get a lot of garbage on the display. The scope has a ground terminal - if I use that instead of the probe's shield I get the same results as the probe's shield.

My test speakers sound -ok-, my PPI amps are a lot cleaner, as are the older DSM rockfords I have. If its OK then maybe this amp was just not one of the better ones ever built.
 
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On most rockford amps, the heatsink is grounded via one or more of the screws that go through the board. Sometimes, the traces (difficult to see sometimes) are burned.

For the two photos (audio and power supply waveforms), I'm assuming that the scope vertical amp settings were the same for both of them. If not, what were the settings?

The non-bridging terminals are directly connected to the amp's ground terminal.
 

Dr Zeus

Member
2010-10-17 5:12 am
THis is definately getting wierd. I must be in upside-down land or something. lol

I just checked top and bottom of the PCB and the screw holes dont have any traces connecting the PCB to the heat sync, nor do the screws have any contact surfaces. Nothing open or burned.

The photos are of different scope settings. When measuring the speaker terminal outputs I set the scope to .2v/div, and it reads up about .5v of noise on the scale. With measuring the 1st or second leg of the fets I set the scope to 10v/div. The 3rd leg of the fets measured at .2v/div have a similar but slightly cleaner noise to the speaker terminal outputs.

You say non-bridged but the DVM must be playing april fools on us. Check this 30 second video:

VID_20101102_212003.mp4 video by unclemeat2010 - Photobucket
 
Please don't assume anything. If you don't understand, ask.

The non-bridging terminals are the terminals that are not used when the amp is connected in bridged configuration to the speakers.

If you're not going to use identical settings and the settings aren't obvious, you need to state what they are. Having the scope set to a lower vertical amp setting was misleading/confusing. If you set the scope to 10v/div, and drive the output to ~15v, the noise will be much less significant.

You can ground the heatsink easily. Scrape the solder mask from the copper near one of the screws near the ground terminal (after confirming that the copper is directly connected to the ground terminal) and solder either a wire loop or a ring terminal to the board. That should further eliminate the noise.
 

Dr Zeus

Member
2010-10-17 5:12 am
I admit its my fault. Thats like 3 times in this thread. I'm sorry. i keep trying to remind myself that this is a learning experience and I still havnt much clue how these things really work - but I trying my best at fixing them.

I ran a lead from the amp's ground to the sync and it doesnt change the noise.

Maybe this amp is really OK and I'm just hearing things or something, or maybe there is just something wrong with it. I want to say the power supply may not be in its best shape but I have no way to tell. its reallyhard to see any noise when the scope is set to 10v/div, and I know at .2v/div it can pick up all sorts of things.

The powersupply in this amp seems to be emmitting some kind of frequency; whether its rf or just some kind of line voltage I am just unable to determine.

If I connect the scope's shield clip directly to the end of the probe the scope's line is straight and perfectly clean.

IMG_20101102_222441.jpg


If I drag the looped probe near this amp's transformer coil or other power supply circuits I get this image with the scope set to .2v/div. For comparison I tried the same test with a Punch 60 and that was not emmitting anything like this punch 400.

IMG_20101102_220009.jpg