Rockford Power 650 Mosfet

[seankane]

Hello everyone.

I recieved a Power 650 in today for repair.

The amp is acting wierd. It powers up fine and produces sound.

It has a pop when turning the volume from zero to sound on and off.

Also The rear channels work fine, but to get the front channels to work right you have to partially unplug the rca's from the rear channels.

If you unplug the rear channel rca's totally or just enough to where the shield is not touching the front channels work great. If you plug the rca's for the rear channels in all the way the front channels get very quiet and you can hear a frequency noise through the speakers.

Also the amp while trying set bias climbs immediately upon moving the pot at all.

Also wondering what the 5th pot is to the left of the bias pots??

Any help would be great.

 

[Perry Babin]

Measure the DC on the input signal lines where they come into the board. Is there any significant DC there?

Do you see a shift in DC as you go from 0 to 1?

The other pot is the threshold for the thermal control for the fan.

Do you mean if you move it from fully CCW or just at some point in the rotation?

 

[seankane]

Measure the DC on the input signal lines where they come into the board. Is there any significant DC there?

Do you see a shift in DC as you go from 0 to 1?

The other pot is the threshold for the thermal control for the fan.

Do you mean if you move it from fully CCW or just at some point in the rotation?

There is no significant DC on the input pads the highest reading was 0.050 vdc.

I see a tiny shift when going from 0-1 volume but only a couple of numbers.

The bias pots are fully counter clockwise and when you move any of them clockwise at all the amp immediately starts drawing current.

How do I set the threshold hold for the thermal fan?

 

[Perry Babin]

Have the output transistors been replaced?

To set the fan, I connect the fan and turn it until the fan starts, then back it off just a bit.

 

[seankane]

I do not know the history of the amp, I suppose they could have been replaced. If so what resistor for the pot do I need to change and what value?

Any ideas in why the front channels only perform properly when the rear is unplugged or only partially plugged in to where the shield is not touching?

 

[Perry Babin]

If there is no DC on the signal lines or on the shields, I don't know why it would pop. Are you using y-cables to feed all 4 channels at the same time?

I think you'd have to reduce the value of the 470 ohm resistor near the pot.

 

[seankane]

Not using Y cables just feeding it from a Rockford head unit using sub output to rear and front output to front.

WHat would be causing the front to be so quiet when the rears are plugged in? This is very strange to me. Maybe something is wrong with the rear section?

 

[seankane]

What issues could a bad input selector switch cause?

With speakers hooked up but no signal touching the RCA sheilds with my fingers makes god awful loud humming and wierd noises through the speakers.

When adjusting the threshold for the fan the higher you turn the pot the faster the fan goes. Once the pot gets to half it just barely starts turning. How high do I go or know what rpm the fan should be at?

 

[Perry Babin]

The fan setting is a personal preference. Some people may want to allow a higher operating temperature as a tradeoff for lower noise. I set them as I described. You'll have to decide what works best for you.

The TO-92 transistor that stands up against the heatsink must be tight against the heatsink with compound applied between the transistor and the sink. It's the temperature sensor for the fan. If you touch your soldering iron to it, you'll see the fan speed increase rapidly.

A bad selector would likely cause one or more channels to be dead or intermittent.

What resistance do you read between the RCA shield and the main amp ground?

 

[seankane]

Do you want me to measure with the black probe on amp main ground and red probe where the rca grounds solder to the board or at the end of the cable itself on the shield?

 

[Perry Babin]

The shields and the connection on the boards should read the same so it shouldn't matter which one you use.

 

[seankane]

They both read the same at the board 85.6 M OHMS

One set reads differently out at the end where it plugs in, and the other set reads the same as at the board.

 

[Perry Babin]

From the schematic diagram, it appears that you should have an 82 ohm resistor between the shield ground and the primary (main) ground. If you have another 650, confirm because it's difficult to read the value on the diagram.

All shield grounds also appear to be directly connected on the diagram. Confirm that also if you have another amp there. If one reads differently at the shield than at the board, the shields are broken in the cable.

 

[seankane]

Yes I have 4 other Power 650's so I will confirm. The rca's were new but does not mean they are not bad. I Will replace them again.

If the rca's had a broken shield inside could this cause the problem I am experiencing?

 

[Perry Babin]

Open shields can cause all sorts of problems but the specific type of problem varies greatly from one amp design to another.

 

[seankane]

Ok Perry replaced the new set of rca's with another new set and the problem seems to be fixed. I cant run the amp hard till tomorrow but no more popping and the channels are working fine regardless of the rca's being plugged in front and rear.

I dont know why the new set was bad and the open shield really caused alot of wierd problems with the amp.

SO far everything seems ok let you know after I set the fan and run it hard.

 

[seankane]

Ok Perry I set the fan threshold and ran the amp hard till the fan kicked on and the fan speed increased as the amp got warmer so I am assuming that it is set correctly.

SO far everything seems to be working A ok no problems. I ran the amp for about 30 min at half volume with no issues.

Thanks again.