Power Acoustik lfa4-840, no RCAs protect mode

[RJLatherow]

Hey All-

Got this amp a few days ago. Did a 'proof of concept' install (temporary wiring) and it was working fine, did the actual install last night and now I get protection mode forever.

Its a Power Acoustik lfa4-840. 12.9v between B+ and ground, and 12.8 at remote. Protection light, so I started disconnecting things one at a time, but it never goes off.

Odd but possibly unrelated note - the protection light is bright until I remove the last RCA cable, then it gets dimmer. Also, I can remove the ground and the amp will still have ground through the RCA. Odd, maybe normal.

I have electronic experience, but not with amplifiers and such. Does anyone have a schematic? Can you point me to where the output transistors would be?

Stupid question, but how do I get the board out of the case? It looks like it slides in, but with all the transitors clipped to the heat sink of the case it looks impossible. At the same time, I can barely get to half of the transistors to measure them-

Thanks!
-rj

 

[Perry Babin]

Never remove the ground before all other wires are disconnected. For some amps, that will damage the head unit.

I don't have the schematic diagram.

Read the basic amp repair page (link below in sig line).

Click HERE for clip removal.

 

[RJLatherow]

Gotcha - Removing the ground was an accident - I had already removed B+ and ground, but the remote was still live and the protection light was lit, presumably with ground from the RCAs.

Thank you for the clip advice - have them off, board removed, and have reread the amp repair page. Broke out the Fluke and started measuring. I was hoping for a simple 'output transistors blown' scenario, but I don't think I'm going to be that lucky.

I have 4 groups of C4466 and A1693. All pin combos read very high resistance. Only odd duck is with the A1693 showing a climbing resistance between base and collector starting around 80K Ohms. It is consistant and still rather high, so I am assuming it is fine-

I'm starting to check the power transistors now - that seems like the correct procedure, right?

Thanks for your sagely wisdom, all!

 

[RJLatherow]

Trying to double check my testing - forgive my ignorance, I've looked at the datasheets, but still can't 100% figure out what kind of transistors C4466 and A1693 are. I am finding references to them being MOSFETs as well as BJTs. Not sure which testing procedures to use-

Thanks-
-rj

 

[RJLatherow]

Do I need to remove the components to test them?

 

[RJLatherow]

One last thing - Most of the tutorials are showing the DMM indicating OL. My Fluke 77 is showing .9-3 M ohms unless I turn off auto ranging. That sound right?

 

[Perry Babin]

When doing initial tests with the transistors in the board, you'll look for very low readings. If they're all reading near 1M ohms, they're not shorted. If they were shorted, they'd read approximately the same as when you touch the probes together.

The OL readings are for transistors out of the board. When in the board, other components can skew the readings.

 

[RJLatherow]

Just for kicks, I pulled out the banks of IRFZ48n and tested (FETs, right?), then pulled out the C4466s and tested (as BJT). I assume that the C4466 are my output transistors and IRFZ48s the power ones.

Everything is testing good so far - do I just start pulling out rectifiers and such and testing them??

-rj

 

[RJLatherow]

Removed all output transistors, nothing plugged in but power, still goes into protect mode.

Checked the pins of the 494:

Pin 1: 0
Pin 2: 5.02
Pin 3: 4.89
Pin 4: .189
Pin 5: 1.54
Pin 6: 3.55
Pin 7: 0
Pin 8: 13.68
Pin 9: .001
Pin 10: 0
Pin 11: 13.67
Pin 12: 12.27
Pin 13: 5.03
Pin 14: 5.02
Pin 15: 5.03
Pin 16: .86

Seems in line with others....

Any ideas as to next steps?

 

[RJLatherow]

Put it back together, and lo and behold green light comes on. Put it in the car, and it goes from red to green in a regular cycle, with a nice whump every time it does. Now I'm just confused. Unless I solved some sort of cold solder joint issue, I didn't do anything to fix it....

 

[mjurban]

Your output transistors are 2SC4466 and 2SA1693. They are complementary to each other. The "2S" prefix is often omitted on device markings. They are definitely BJTs, per the data sheets, which are available on line.

 

[RJLatherow]

Back on the bench - my bench power was a little low on current, which gave a wimpy green. Now that I have that fixed, I get the mostly red/occasional green cycle on the bench as well.

 

[RJLatherow]

I looked up the datasheets for them, but couldn't translate Silicon NPN Triple Diffused Planar Transistor with charts into BJT. Luckily there are lots of parts listing that have a nice concise summary that mentions BJT. 😀. I'm learning -

 

[RJLatherow]

false alarm - I made a microscopic solder bridge reinstalling my output transformers. Now it is back to all red, protection mode all the time. 494 is the same as before, except pin 16 is showing 8-12 volts

 

[Perry Babin]

If it thumped, it likely means that you have a defective channel. Connect the meter across each channel and apply remote voltage. Does one channel produce more DC voltage at startup than the others?