Phoenix Gold RX600.5 Will not power on

[rockrink]

Draws about 1.5 amps, unloaded. No lights come on whatsoever.

TL494C chip DC voltages
1. 0
2. .56
3. .07
4. 0
5. .01
6. 0
7. 0
8. 1.9
9. .05
10. .05
11. 1.9
12. 1.2
13. .575
14. .575
15. .575
16. 0

I pulled the data sheet on the 494 and was expected 5 V on pin 14 and 12 V on pin 12. This amp was built in December 2013 according to the tag. The only PG diagram I can find online is a different unit altogether from 1995.

Thanks for any help or suggestions.

 

[Perry Babin]

Where did you find the diagram online?

PG amps typically supply pin 12 with voltage through a PNP transistor that has it's emitter directly connected to B+. Find that transistor.

 

[rockrink]

Hey Perry,

I found the diagram at Electrotanya.

I traced back from pin 12, through one resistor and one diode to a 649A transistor. It's emitter traces directly to the DC input. However, when I measured its voltages, the emitter only had 4.3 volts. I then verified at the DC input that something is loading down the 12 volt input to 4.3 volts. Current draw is about 1.5 A.

 

[Perry Babin]

What's the voltage on the base of that transistor?

Is the transistor heating significantly?

 

[rockrink]

Base was 3.7 volts.

Not hot at all. Comfortable to the touch. Reading room temperature with the digital laser thermometer.

 

[Perry Babin]

It's defective. There may be a reason that it failed but it has failed.

 

[rockrink]

Ok. I'll order a replacement. Meanwhile, what else should I check?

 

[Perry Babin]

Do you have any transistors in stock?

 

[rockrink]

I have some, but they are mostly small signal transistors.

Oddly, here is what the tester showed out of circuit for that transistor.

 

[Perry Babin]

You can jump the transistor location from collector to emitter to allow further testing. Be careful when applying power in case there is a shorted component being fed by that transistor.

I don't trust those testers. The BE junction is essentially a diode and more than about a volt difference means that that junction is damaged.

 

[rockrink]

Will do.

I should clarify -- emitter voltage was 4.3, base was 3.7.

 

[Perry Babin]

The amp is starving for voltage. Why isn't it getting 12v on the B+ terminal?

 

[rockrink]

I am not sure. I had a fully charged, known good battery connected and was getting the same voltage reading on B+. Battery read in excess of 12 volts unconnected and then B+ read 4.3 volts (approx.) connected.

I do not know enough about these circuits to isolate the cause, but it feels like something is definitely loading down the input voltage. Obviously, it's not a dead short.

Is there a circuit I could study somewhere that is representative of this type of amplifier power distribution? To be honest, I don't even know which all components are fed B+ directly in these amps. I think I am spoiled because in my other electronics work, I rely heavily on schematics and diagrams.

I appreciate all of your help. Obviously, I am hungry for information.

 

[Perry Babin]

Install a 10 amp fuse in the B+ line of the amp. If it drags the voltage down without blowing the fuse, the battery is bad, assuming that you're measuring the battery voltage directly across battery terminals.

I don't know specifically what information you're looking for but page 20 of my bcae1.com site is a good start.

 

[rockrink]

Thank you.