Rockford T20001bd R465

[Dr Zeus]

Ive got this amp on the bench trying to fix it up. Customer said it makes a burning smell. Sure enough, R465 burned up to the point where it measured ~180k. I'm not precisly sure what this resistor's value should be, but it looks like a 0.1 ohm like most Rockford amps use.

Location of this resistor is directly between two large caps just behind the spearker terminals.

Also, would there be anything other than the resistor itself which would have caused it's failure?

 

[Perry Babin]

10 ohms.

Is the capacitor that's in series with the resistor shorted?

 

[Dr Zeus]

The two large ones (Pictured above) are measuring like normal capacitors. The burnt resistor left its mark on both of these.

 

[Dr Zeus]

I put a new 10 ohm resistor into R465. After about 2 minutes of idling while powered on, the new resistor gets raging hot. There is 0vDC across this resistor, and also 0vDC referencing amp's ground on both sides of this resistor. There is however AC voltage referencing amp ground, which starts at 0.1vAC and rises to ~2.6vAC after about 20 seconds of the amp being powered on, and 6.6vAC across this resistor.

 

[Perry Babin]

The inductor may be shorted. Do you see a lot of AC on the scope (no audio input)?

 

[Dr Zeus]

Speaker output has +-5vAC on them. I think you are right because if I twist and tug on the inducer the output clears up; so I need to find the short.

Edit: nope its something else and not the inductor. When I activate SW101 - the Phase 0/180 switch - the new 10Ohm resistor almost instantly started to smoke. That switch is activating something in the pre-amp section. Could it be a failed opAmp?

 

[Dr Zeus]

The opamps to the left of the fan all have only +-6vDC on the power pins referencing amp ground.

Opamp U306 has the following pin voltages:

Pin 1: -4.47
Pin 2: -1.316
Pin 3: -1.757
Pin 4: -5.98
Pin 5: 0.021
Pin 6: 0.002
Pin 7: -0.007
Pin 8: 6.06

 

[Perry Babin]

They operate off of ±6v supplies. It appears that the op-amp is operating properly.

You need to look at the signal through the chain of op-amps to see what changes when you move the switch and the resistor begins to overheat. Email me if you want the schematic diagram.

 

[Dr Zeus]

R465 just litterally popped pretty violently in place. It is now reading ~2M ohms and is likely open. Im hoping I can troubleshoot this amp with it out of circuit.

I believe the 0/180 phase switch caused it to go.

 

[Dr Zeus]

Im now troubleshooting this amp with R465 removed as having it installed is hazardous to my health.

Could you email me the schematic to supertegrs@yahoo.com if you have a moment? OpAmps U106 and U107 have adverse reaction on the output pins to gain adjustment in the form of DC voltage drom from ~0.2vDC at minimum gain to ~-0.4vDC at maximum gain. This is referencing amp RCA shields.

With an input source and no load, if I turn both the gain and the crossover controls up about 1/4 way the amp pops into protection mode.

This amp appears to only produce possitive rail voltage. Where is the best secondary center tap for measuring the opAmps? Ive been using RCA shields or amp general ground.

 

[Perry Babin]

DC offset is a common issue but generally not a problem (especially at low levels). Switching the subsonic filter on helps.

Do you see any oscillation on the audio op-amps up to U107, pin 1?

 

[807tubenut]

I have the yellow/green prototype board that's unpopulated, if you need trace scans or pics let me know.

 

[Dr Zeus]

Its been a few days since I last worked on this amp, and tonight I believe I figured out that the gain control may be shorted internally across two two of the pins.

EDIT: Nevermind. I pulled the gain pot and its fine. Theres a trace underneith tying two pins together.

 

[Perry Babin]

They're shorted by the board. That's normal.

 

[807tubenut]

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