While waiting for parts in the 600.4 I decided to dig into this.
500x, pc-3084c.
Supposedly no sound on 2 channels but they all work fine. Maybe just the fact that it was in 4 channel mode? Rcas aren’t broke.
Idled at 2amps which seemed high. It only has one pair of output fets per channel. Turned all bias pots ccw. Channel 1 seemed to have a much lower setting then the others.
Channels 2-3-4 adjusted as expected. Channel 1 behaves as some have noted here before… voltage across source stays at zero then suddenly jumps at the pot position you’d expect it to be left at. Idle current jumps 30ma and source res voltage goes to about 3-4mv. Turning ccw causes it to go up to about 7mv before it starts coming back down.
Found this thread. Yep that channel is oscillating even after returning to full ccw and will only stop after resetting remote. No input. Once it starts oscillating and you turn pot ccw frequency drops.
Jumping front/back rca shields doesn’t make difference in oscillating but it does drop idle current about 200ma after reset with pots fully ccw
I want to try the mpsa42/92 Perry suggested first but in case that doesn’t work out …. What are the feedback cap designations for this board to experiment with? I couldn’t find a 3084 schematic and the 3085 board looks different enough that I probably shouldn’t use as a reference
500x, pc-3084c.
Supposedly no sound on 2 channels but they all work fine. Maybe just the fact that it was in 4 channel mode? Rcas aren’t broke.
Idled at 2amps which seemed high. It only has one pair of output fets per channel. Turned all bias pots ccw. Channel 1 seemed to have a much lower setting then the others.
Channels 2-3-4 adjusted as expected. Channel 1 behaves as some have noted here before… voltage across source stays at zero then suddenly jumps at the pot position you’d expect it to be left at. Idle current jumps 30ma and source res voltage goes to about 3-4mv. Turning ccw causes it to go up to about 7mv before it starts coming back down.
Found this thread. Yep that channel is oscillating even after returning to full ccw and will only stop after resetting remote. No input. Once it starts oscillating and you turn pot ccw frequency drops.
Jumping front/back rca shields doesn’t make difference in oscillating but it does drop idle current about 200ma after reset with pots fully ccw
I want to try the mpsa42/92 Perry suggested first but in case that doesn’t work out …. What are the feedback cap designations for this board to experiment with? I couldn’t find a 3084 schematic and the 3085 board looks different enough that I probably shouldn’t use as a reference
The designations are the same. See attached.
Try the caps. They seem to work without any other mods.
Try the caps. They seem to work without any other mods.
Same board used for a 200a4, 400a4, and 500x? Talk about recycling.
I’d rather mess with through holes than tiny 0805 packages. I’ll order both at the same time but… are caps the better solution?
I’d rather mess with through holes than tiny 0805 packages. I’ll order both at the same time but… are caps the better solution?
Better? I don't know since the transistors haven't been tried and fully tested (not just powering up and setting the bias).
I've seen different boards in the same models. That's one reason that the PC# is more important than the model for Rockford amps.
Why do 0805 parts cause you problems?
I've seen different boards in the same models. That's one reason that the PC# is more important than the model for Rockford amps.
Why do 0805 parts cause you problems?
More like fear … zero smt experience. I’ve been doing through hole and wire before I could walk but never any smt. I’m sure I can manage… may not be pretty but with some tweezers and a magnifying glass I’m sure I can get by.
I’ll be happy to contribute to the testing effort and get any measurements etc.
I’ll be happy to contribute to the testing effort and get any measurements etc.
For 0805s (or similar), apply new solder to both ends. Heat one end for about 1-2 seconds then quickly move the iron to the other terminal and slide the resistor into a vacant area adjacent to the resistor. Tweezers can work but aren't needed.
If there is no vacant area, mask off an area with Kapton tape and slide the resistor onto the tape.
To replace... Remove all solder from pads and add a bit to one pad. Heat the solder and slide the resistor into position. Solder the other terminal.
0805s aren't small. If you need a magnifying glass, look into a pair of drugstore 'cheaters'. Try them in the store to see what diopter you need for really close-up viewing. Walmart has them for cheap (3 pairs for $7?). You can stack them for occasions where you need to see closer than the glasses can do on their own.
If there is no vacant area, mask off an area with Kapton tape and slide the resistor onto the tape.
To replace... Remove all solder from pads and add a bit to one pad. Heat the solder and slide the resistor into position. Solder the other terminal.
0805s aren't small. If you need a magnifying glass, look into a pair of drugstore 'cheaters'. Try them in the store to see what diopter you need for really close-up viewing. Walmart has them for cheap (3 pairs for $7?). You can stack them for occasions where you need to see closer than the glasses can do on their own.
Well relative to 1/4w resistors or to92 0805 is small. I thought 0805 was the smallest smt package… I just looked up a few smt practice boards on Amazon. Wholly crap 0402!!! A wrongly timed sneeze and they are gone for good.
Good call on the kapton tape… that will make things easier and safer for the surrounding parts. Here’s hoping the transistors work so it’s a non-issue.
Good call on the kapton tape… that will make things easier and safer for the surrounding parts. Here’s hoping the transistors work so it’s a non-issue.
So stock caps and mpsa42/92 made no difference. Oscillation and the abrupt change in current was still there. Mpsa42/92 and 7pf caps worked. I think I’m going to change back to mpsa06/56 on that channel and put the 7pf caps everywhere just to have the channels match.
Go back to the original transistors. There's no need to modify it more than absolutely necessary.
So all Cx13 and Cx15 caps 7pf, ch1 transistors back to mpsa06/56 - ch1 oscillates but not as bad. When turning the pot back part way it stops whereas before with stock caps the only way to get it to stop is cycling remote.
So - is preference to increase cap value or keep 7pf caps with mpsa42/92 on all channels?
Do the transistors increase chance of limiting bandwidth like the caps do? If not then it seems like 7pf caps + mpsa42/92 is the solution
So - is preference to increase cap value or keep 7pf caps with mpsa42/92 on all channels?
Do the transistors increase chance of limiting bandwidth like the caps do? If not then it seems like 7pf caps + mpsa42/92 is the solution
If the 42/92 transistors didn't make a big difference, they're not likely part of the solution. You can go to the next higher value in the caps, if necessary.
Before doing that, confirm that the resistor and Zobel components across the speaker output terminals are within tolerance.
Before doing that, confirm that the resistor and Zobel components across the speaker output terminals are within tolerance.
Ok 10pf caps and the bias is happy.
Perry I can’t find a previous thread of yours I recall reading. When scoping output of this amp on the rear channels there is some noise. Amp is running in 2 channel mode all pass. If I move rear switch to low pass it goes away. Or recalling your thread and jumper front to back rca shields it goes away. I think you said to just permanently install a jumper… or was there more to it? Schematic shows they both go to “I” but that doesn’t appear to be the case.
Roughly 1k between front and rear rca shields to battery ground and 100ohms between each other.
Perry I can’t find a previous thread of yours I recall reading. When scoping output of this amp on the rear channels there is some noise. Amp is running in 2 channel mode all pass. If I move rear switch to low pass it goes away. Or recalling your thread and jumper front to back rca shields it goes away. I think you said to just permanently install a jumper… or was there more to it? Schematic shows they both go to “I” but that doesn’t appear to be the case.
Roughly 1k between front and rear rca shields to battery ground and 100ohms between each other.
Is the noise audible?
Are you going to be using the two rear channels alone?
On some of the diagrams of similar amps, they show I and J as the two grounds. See attached.
Are you going to be using the two rear channels alone?
On some of the diagrams of similar amps, they show I and J as the two grounds. See attached.
R100 in that schematic appears to be the same as R16 on the board but I can’t find R16 in the 3084 schematic.
No, the noise is not audible. Not sure about using the rear channels separately. Wouldn’t they be tied together anyway if the same head unit was feeding all 4 channels ?
No, the noise is not audible. Not sure about using the rear channels separately. Wouldn’t they be tied together anyway if the same head unit was feeding all 4 channels ?
Yes, if the head unit had 4 channels or if you used y-cables.
Virtually no diagram will ever be perfect. The ones that are for the exact revision for amps that are 100% robotically assembled, may be but maybe not even then.
It's nice to see that you've read other threads. Many times it's good because I'll forget things that I haven't had to think about for 10 years or more.
Virtually no diagram will ever be perfect. The ones that are for the exact revision for amps that are 100% robotically assembled, may be but maybe not even then.
It's nice to see that you've read other threads. Many times it's good because I'll forget things that I haven't had to think about for 10 years or more.
Just thinking out loud. In two channel mode that resistor essentially is in series with the input to the rear channels. That explains why, when shorted, the input levels jump up and consequently the output. Was scratching my head for a while trying to figure out why current jumped up when shorting them.