Rockford fosgate AMP

[windozeuser]

I'm working on a RF amp, the power supply is working fine I replaced all the power supply mosfets and drivers, and it idles with 1.7amps of current and doesn't go into protect mode anymore.

The output mosfets were bad but not shorted, I removed them completely from the amp and it still worked fine idling with 1.7amps of current. The output drivers I replaced them all, and then turned the amp on and two small capacitors blew....

I checked the drivers nothing heated up, and I didn't mix anything up. Did the caps blow because the drivers were bad before and replaced them and they were trying to drive the output mosfets that aren't installed?


The two caps that blew are in the power supply though, and when I tested the amp when they blew, I only had two ground screws installed on the board. The board has like 20 ground screws to the chassis, and my friend said that could have blew the caps.

The one cap that blew is tied to ground right off of B+ 12 volts, the cap is rated for 25 volts how could it blow with a bad ground? Could it be from the switching powersupply?


The other cap is in the power supply too tied to ground , but I dont have that on the schematic to show you

What is this cap for?


Thanks

 

[cowboybob]

...What is this cap for?


Thanks

B+ leveling.

Check all the PS grounds for continuity.

Is the 1.7A idling current normal?

<EDIT> PS. Why is this amp called an RF amp? All the Rockford amps appear to be audio amps (I know nothing about the brand Rockford amps).

 

[windozeuser]

Ah thank you, it is an Audio amp, RF was a dangerous obscure acronym for "RockFord Fosgate" sorry about that. Its a model 20001BD amp, ah I think the 1.7amps is normal from what I can find

I only had one PS ground screw in when the two caps blew up, but the motherboard for the amp takes like 20 ground screws. I'm just wondering how only using one ground screw can blow the caps up? Is it because a large current flows or voltage spikes in the switch power supply grounds, like a ground loop type situation?

What exactly is B+ leveling?
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Thank you!

 

[cowboybob]

Ah thank you, it is an Audio amp, RF was a dangerous obscure acronym for "RockFord Fosgate" sorry about that. Its a model 20001BD amp, ah I think the 1.7amps is normal from what I can find

I only had one PS ground screw in when the two caps blew up, but the motherboard for the amp takes like 20 ground screws. I'm just wondering how only using one ground screw can blow the caps up? Is it because a large current flows or voltage spikes in the switch power supply grounds, like a ground loop type situation?

What exactly is B+ leveling?
|

Thank you!

Most times I'm a big fan of NOT putting back tie-down screws when I expect to have to "go back in" to a device for further (or additional) repairs - it just saves time.

But in this case, not knowing what screws are involved in the overall situation of the grounding traces on the PCB, I'd recommend replacing them all before testing the board. That is, unless you can positively identify the ground traces with screw holes, in which case you need only to replace those screws.

B+ leveling is just another term for "smoothing" the DC, or removing any AC ripple (in this case, from the alternator) from the DC line feeding the amp and possibly finding its way into the audio amp circuits.

Since you seem to have a problem with the amp output circuit, an AC ripple would be the least of your current problem(s). Just leave the cap in question (C61) out of the circuit and see what happens (do not apply an input signal at this time - just get the idling condition stabilized).

BTW, I'm assuming that the B+ (whatever DC value that is) listed on the schematic is now the correct level since the PS repair?

 

[windozeuser]

Yeah the B+ is the correct level thanks for your help I'm learning a lot! ,

Heres the other capacitor that blew. C74, This cap does the same thing is it for noise?

I'm still not sure why if you dont have some ground connected the caps would blow? What causes that? Does it create current or an over voltage situation for these caps?

Thank you for your help!

Would it also be "Safe" to operate the amp without this cap for testing purposes?

 

[windozeuser]

C74 would be a coupling capacitor right? How the hell can these things explode with lack of ground cant find no answer lol

 

[cowboybob]

C74 would be a coupling capacitor right? How the hell can these things explode with lack of ground cant find no answer lol

Understand that a grounding system isn't just limited to those caps (C74 is also a filter cap, not a coupler). Again I suggest that you do a continuity check of ALL the grounding points of the PCB and chassis ground.

If your missing other ground(s) to the board, then there's ample opportunity for other component/sub-circuits to be incorrectly grounded and cause problems that can cascade into additional problems, which is what your experiencing, best I can tell.

I know it's a pain to return the PCB back to its original, fully grounded condition before testing, but to do otherwise (as has already been done) will likely be a disaster.

 

[windozeuser]

Thank for all your input and help,

I will definitely check all grounds and have all the ground screws installed now.

Please forgive me, but I still can't wrap my head around how the lack of ground caused the caps to puff up in smoke? Dont electrolytic caps only blow if they have over voltage or reverse polarity? Does the lack of ground cause the voltage on the caps to float very high? I know the Audio amp has a switching power supply of around 100 volts on the rails, but with a lack of ground maybe that caused the rails or ground potential to "float" the caps are rated for 25 volts since the ground should only be 12 volts, but a lack of ground was floating?

Is this a right assumption or am I completely off? haha

 

[cowboybob]

Thank for all your input and help,

I will definitely check all grounds and have all the ground screws installed now.

Please forgive me, but I still can't wrap my head around how the lack of ground caused the caps to puff up in smoke? Dont electrolytic caps only blow if they have over voltage or reverse polarity? Does the lack of ground cause the voltage on the caps to float very high? I know the Audio amp has a switching power supply of around 100 volts on the rails, but with a lack of ground maybe that caused the rails or ground potential to "float" the caps are rated for 25 volts since the ground should only be 12 volts, but a lack of ground was floating?

Is this a right assumption or am I completely off? haha

No, a floating capacitor is in little danger of being damaged. In this case, the caps in question were, no doubt, grounded.

But the lack of a ground elsewhere in the circuit, or a situation where there is a "poor" ground (one presenting more [infinite] resistance and the other less continuity than what is called for in the circuit design) will alter the basic biasing and current flow of other circuit(s).

A poor (or no) ground, not apparently related to the cap(s) by merely looking at the PCB, will cause a myriad of problems.

Keep in mind that restoring the ground screws is an excellent idea. BUT keep in mind that, without them being in place previously, additional damge may have occured to the amp beyond the obvious. A domino effect, if you will.

And it's not out of the realm of possibilities that the caps that blew were "weakened". Once replaced, and everything ground tight, the amp may well be just fine.

 

[windozeuser]

Thank you,

yeah this is a nightmare, but nothing else on the board got hot so I'm hoping nothing else got damaged

Is there any war stores or books on anything related to this? This is very interesting,

 

[cowboybob]

Thank you,

yeah this is a nightmare, but nothing else on the board got hot so I'm hoping nothing else got damaged

Is there any war stores or books on anything related to this? This is very interesting,

(My emphasis.)

Well, windozeuser, you asked.

Electronics is, IMHO, nothing more nor less that the manipulation of electricity to acheive some desired end. Pure and simple, right?

As you may be realizing, though, that there's nothing simple in its execution.

This forum has a huge crew of people with vast training, experience and an ocean of war stories that they draw on in their attempts to answer the questions posed here.

I think that, to a person, they would respond to your query with an equally huge array of answers.

I can really only respond with one: go to a library (or the internet) and find anything related to "basic electronics" (I'm assuming a generalized understanding of electricity on your part). And then just start reading.

It's the only real way to learn what you have now come to find "very interesting" (as we, here, did at some time in our past) .

We'll be here when you need us.

CBB

 

[windozeuser]

I am happy to announce the amp is playing and working fine now! Thanks for your help

 

[cowboybob]

Outstanding!

 

[unclejed613]

i'll give you a hint about why you aren't getting any "war stories" about fixing these amps.... Rockford Fosgate's warranty repair policy is to have the units sent to them for repair, so even if you send it to a "factory authorized" service center, they end up sending it to Rockford. you may have noticed some of the transistors are soldered to a ceramic substrate that is then mounted to the heatsink. this is not something that most technicians want to mess with (replacing a TO-220 transistor wave soldered to a ceramic plate is far from easy). Rockford does not supply repair parts.


that said, however, most car amps have ground points that must ALL have screws tying them to ground for the unit to operate properly.