Rockford Fosgate RF? - Model unknown sync clips

[Dr Zeus]

I think this is an RF series amp; and I personally had one of these when Rockford failed to sell them back in 1997-ish. its a Rockford 4 channel amp, likely 30w X 4 RMS. I remember it ran my whole system at the time, but this amp was never considered to be anything more than entry level and likely costed only about $150 brand spanking new back then.

Needless to say I have one in for repair, which I believe has bad power fets as it draws too much on REM. Unfortunately I cannt test the fets because the clips are in the way, and I just dont know how I can remove the clips. The whole bottom of the amp is part of the sync and nothing seems to budge.

Any tricks to getting the PCB outa this one? It seems imposible and I'm out of ideas.

 

[Perry Babin]

On these, I think you'll have to use two pairs or needle-nosed pliers. Grab the clip on each side and twist it to rotate the top of the clip counter-clockwise (from the viewpoint in the photos).

 

[Dr Zeus]

Two pliars didnt do it. I ended up using a small iron crow-bar. I'll likely have to use a small hammer to tap the clips back in. In any case I can start troubleshootig and fixing this amp.

 

[Perry Babin]

I've used the pliers to remove this type of clamp before. The following page shows a good way to get them back on. It makes it less likely that you'll hit a cap or resistor when hammering them back into place.
http://www.diyaudio.com/forums/car-...sistor-clamps.html?postid=1317683#post1317683

 

[Dr Zeus]

This amp has both of one out of four channel output transistors shorted so I clipped them off the board (540 and 9540). All Power supply fets (MTP 50n06v) were shorted so I clipped those off the board. I dont know if the TL494 is working right because I get iratic DC voltage from each of the supply fet Pad/Leg 1.

The surface mount TL494 is not looking so good measuring the following:
Pin 1: 0vdc
2: 4.47
3: 0.035
4: 1.325
5: 1.5
6: 3.320
7: 0
8: 10.9
9: 10.4
10: 5.71
11: 11
12: 9.97
13: 4.97
14: 4.97
15: 4.97
16: 0

Pad/Leg 1 of 2/4 the power fets measure 5.87vDC. The other 2 fet pad1s measure 0vDC

Pad/leg 1 of the power fets are going to a circuit series of ~8 surface mount transistors. Of the Punch series amps I've worked with, this amp circuitry looks similar.

 

[Perry Babin]

The IC appears defective due to the voltage on pin 9. Before replacing it, confirm that there are no driver transistors or solder bridges pulling it up.

 

[Dr Zeus]

All four 10ohm gate resistors are out of tollerance, but also before the gate resistors; the series of 8 small transistors are not supplying any voltage to the fet gate with 0vDC.

 

[Dr Zeus]

Pin 9 of the 494 is leading directly into the ~8 transistors near the power fets without stopping anywhere along the way. Its hard to test these little buggers but I'll keep trying.

 

[Perry Babin]

It's likely that one marked 1A (or 1-something) is shorted from collector to emitter. It's likely Q1 or Q4.

P.S. I think this is probably a Rockford 4.6x.

 

[Dr Zeus]

Testing these little ones between collector and emitter, I thought it was Q1 with is a 1G transistor so I carefully removed it. To my surprise, it actually turned out to be Q2 which is reading 33ohms from collector to emitter even off the PCB. Q1 and Q2 collectors and emitters are wired in paralell on the pcb.

Its really tough to read; because the transistor possibly defaced itself when it failed. I only see a 2 on its back, and then a brown/black spot right next to that. The only other transistor with 2 is a 2G at Q3 which is testing alright.

 

[Perry Babin]

That's common when the power supply FETs fail in Rockford amps but would not explain the 10v on pin 9. Double-check Q4.

 

[Dr Zeus]

Man you are the best! Q4 is a 1G and its shorted to 33ohms between center and top right. You think that would cause 10v on pin9?

I dont know what a 1G or 2G is. How can I look these up without having to ask all the time?

So far, this amp needs:

4 power fets 50n06v (I'll likely use IRF3205ZPBF-ND cause I have a big order coming in of them)
2 output fets 540, 9540
4 source resistors (3w, 0.1ohm)
4 Power fet gate resistors SMD 100 (10 ohm)
1G
2G

 

[Perry Babin]

1G and 2G are MMBTA06 and MMBTA56. You can look them up on the following site but it's not always definitive.
The SMD Code Book

Do you still have 10v on pin 9?

Is the 1k resistor R6 within tolerance?

 

[Dr Zeus]

R1 is 1k and reading fine.

Nope; Pin 9 is now reading 1.786vDC

I saved that shortcut to my favorites. Thank you!

 

[Perry Babin]

Pins 9 and 10 should read ~5v.

That was R6, not R1.

 

[Dr Zeus]

Pin 10 is now reading 2.8vDC. It was reading ~5vDC before; removing one of those little transistors must have done something. I'm guessing I'll need to order a new TL494C

 

[Perry Babin]

Post the voltages on the 494 again. Be sure that you don't have any solder bridges on the SMD driver transistor pads.

IC#
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:
Pin 15:
Pin 16:

 

[Dr Zeus]

New readings

IC# TL494C
Pin 1: 0.003
Pin 2: 4.47
Pin 3: 0.060
Pin 4: 1.449
Pin 5: 1.491
Pin 6: 3.298
Pin 7: 0
Pin 8: 11.48
Pin 9: 1.79
Pin 10: 2.88
Pin 11: 11.46
Pin 12: 10.39
Pin 13: 4.93
Pin 14: 4.93
Pin 15: 4.93
Pin 16: 0.000

Old readings:
IC# TL494C
1: 0vdc
2: 4.47
3: 0.035
4: 1.325
5: 1.5
6: 3.320
7: 0
8: 10.9
9: 10.4
10: 5.71
11: 11
12: 9.97
13: 4.97
14: 4.97
15: 4.97
16: 0

 

[Perry Babin]

Connect pin 4 to pin 7. IF both 9 and 10 go to ~5v, the 494 is likely OK. If not, post the voltage on them.

 

[Dr Zeus]

Connected Pin 4 & Pin 7, Pin 9 goes to 3.98vDC and pin 10 goes to 4.68vDC