Phoenix Gold Phorum

Amp was setup running two front speakers and the rear speakers were subwoofers.
One of the subwoofer channels only fired outward on a bass hit and was dead on the inverse of the bass hit wave. Took a lot to actually see it but you could hear it was less powerful.
Speaker swap determines its the channel not the speaker.

I opened the amp case after getting the wiring schematics. Turned the power on with a battery charger to see that 2 of the LEDs on the circuit board not lit up.

This is where I need help. How do I figure out which if any transistor is failed? Do these LEDs give any hints? I can send the wiring diagram and provide which LEDs do not light up. They are D104 and D102 on the schematics.

I have a multimeter and soldering skills but no idea on reading the schematics or how to test these components even after watching a bunch of YouTube repair videos. Please help.

Thank you!!

Hello and welcome on the forum!
is the Green LED light up?

No thermal or overload issues.

I suspect that the issue might have been caused by the crossover pots as they seemed to be corroded up. So I'll have to take those apart and clean them up carefully too.

First of all there are other forum members who have many, many more knowledge about these amps than me. I'm also in the learning curve but anyway I will try to help you before the other members chim in.

Are you the first owner of this amp or not?
The 2 red LED's are part of the bias circuit. You have no bias current. Without bias current the output transistors (SA1943/ 2SC5200) are not working.
The first thing you need to cheque is if there are resistors that are burnt or open with the beep function of your multimeter.

Cleaning the crossover pots is ok (remove dust) but don't use cleaning products on these. Turn them up and down a few times. If this doen't help you need to replace them.
good luck, and let us know the progress of your repair.

Are there specific resistors I should be targeting? Ones in and around the LED's that are off?
With a continuity test, a good resistor WILL beep and bad one will NOT beep, correct? And I am looking for onces that do NOT beep to flag as bad.

Second owner of amp. But have owned the amp since 2002.
I have NEVER touched the bias POTS (if you are wondering).

start with the 10 0.51Ohm 1W resistors. If these are ok start with everything that is connected wit the 2 LED's. You said that the 2 front channels are working so it has to be in the 2 rear channels. Sart with the resistors and then the diode's. If you have a magnifying glass use it. If a resistor is discolored the chance is high it's open.
For resistors: if there is no beep it's open. But te be sure if you find one that is not beeping, do also the resistance test. The marked colors on the resistor gives you the value.
For diodes: you can find many Utube video's that show how to do this, just type in "diode test". For testing resistors and knowing the value of the resistor there are also many tutorials on Utube. If you did not touched the bias pots this could (in my opinion) not be the problem.

I won't be able to get to this until the weekend. Just letting you know I'm not disappearing.
Thanks.

It's getting pretty common for the current source LEDs to fail in these amps. The very first thing I'd do is check to be sure they haven't failed. After that, start going through the fusible resistors in that channel. The fusible resistors look a bit different than the regular counterparts. They also have an extra stripe that will not make sense. The fusible resistors will open with no outward sign that they have failed. If you find one open, it must be replaced with a flameproof/fusible part. It's pretty rare for the amp to power up with failed drivers or outputs. It can happen, but it's unusual. Transistors typically fail shorted and that will normally trip the overcurrent protection. Sometimes they will fail open or just leaky v and the amp will continue to run, but it's not common.

Good luck,
Jason

BTW, if the LEDs have failed, do not buy just random 3mm LEDs. You need to get LEDs with a Vf (forward voltage) of 1.2-1.4V. look at the datasheet before you buy. Most random LEDs in eBay or Amazon are not going to be characterized or come with a valid datasheet. You will be better if ordering from Mouser or Digikey so you know exactly what you are getting.

You can also remove the LEDs and populate the two unpopulated series diode locations for each LED. If you go that route, the diodes to use are 1n4148.

Later,
Jason

great info man thanks for sharing.