My fender stage 185 amp won't power up

[Enzo]

Hi spin, welcome to the place.

Just a nit pick: please don't use the term "nothing" when taking readings. Zero volts or infinite ohms (an open) are still readings. Be specific, so zero volts, if it is a voltage check. It can be real confusing with resistance. Does nothing mean no resistance as in a dead short or zero ohms? Or does nothing mean an open indication?

[spindarec]

I got zéro volts on q15, q16 and to the base of the output transistor, and yes it´s a voltage check.

[Jazz P Bass]

Quote" I got zéro volts on q15, q16"

What exactly is this supposed to mean?

Remember this: a transistor does not exist alone.
It requires the proper voltages on each & every leg to operate properly.

So if you want to troubleshoot this, list the BCE voltages of Q15.

[spindarec]

Voltages of Q15:
B=0V
C=0V
E=0V

[Enzo]

CP10, CP12 are AC, there is no polarity, the two wires work either way.

If the collector of Q15 has zero volts, do the collectors of output Q17,Q19 also have zero volts? The collector of Q15 is connected directly to the +50v rail. If there is zero volts on it, perhaps your +50 is missing, or at least not getting to all the places it needs to be.

WHile you are looking at CP10/12, look for those two 270 ohm 7 watt power resistors. Each one should have about 50v on one end and about 16v on the other end. One is positive voltages, the other negative voltages. They should get pretty warm while running.

[spindarec]

i think my +50 is missing because Q17 and Q19 have zero volts too.The same thing for the two 270 Ohm 7 watt power resistor, 0V on each sides…I've checked the voltage of CP10 and CP12 and 0 volts too.
I checked the Volts after the light bulb limiter and i have only 0.4 volts, my lamp bulb is eating too much current. It's a Halogen eco 53watts, 220/240Volts.

[Jazz P Bass]

Progress.

Ditch the limiter & with power off, check the resistance to ground of the +50 V rail.(should be low & then rise high as the filter caps charge up.)

Then check the diodes in the bridge rectifier circuit.

If all is well, you should read +50 directly out of the bridge rectifier circuit.

If so, then you have an open trace to the output section.

[spindarec]

Progress.

Check the resistance to ground of the +50 V rail.(should be low & then rise high as the filter caps charge up.)

.

Diodes in the rectifier circuit are Cr16 to Cr19 ?
Résistance so I put the tester in ohm position right ? Then I put the black pin on the chassis but where i should put the probe ?
Thanks

[Jazz P Bass]

Not too sure what you are asking.

If you want to test the diodes, you use the resistance scale of your ohmeter.
The probes go on on either end of each diode.
You are looking for a short. (very low ohms, if the diode is bad)

No power to the amplifier.

When that checks out, set your meter to read Volts DC

The Red probe goes where CR50 & Cr51 meet. (+50Vdc)
The Black probe goes to chassis.

Power up the amp

Wouldn't hurt to also check the -50Vdc rail.
Red probe where CR52 & CR53 meet.

I find it a safe practice to not probe a hot (on) amp.
Hook up a test lead then turn on the amp.

[spindarec]

If I power up the amp without a limiter , fuses will blow immediatly and I don't have 6,3A fuse anymore. I got only 10A or 3,5A fuses.

[52 Bill]

If I power up the amp without a limiter , fuses will blow immediatly and I don't have 6,3A fuse anymore. I got only 10A or 3,5A fuses.

When you power up the amp with the lightbulb limiter, does the lightbulb light up brightly and stay bright? If it does, then there is still a shorted component in the amp that must found.

As long as the lamp lights up and limits the current there will be no voltage for you to read in the circuits.

With the amp unplugged, test the rectifier diodes and output transistors with your meter. Does your meter have a diode test function?

[spindarec]

Yes my meter have a diode test function. I'll check that.
And yes the lightbulb light up brightly and stay bright.

[52 Bill]

You still have a short in the amp. Unplug it and test the diodes and output transistors.

Find diodes CR50, CR51, CR52 and CR53 on the pc board. Test each one with your meter, using the diode test function. If you are lucky, both wire leads from each diode will be out in the open. But I think on that board the diodes may be end mounted with only one lead showing. You may need to be creative in finding connecting points for the hidden leads, like CP10 or CP12.

The fast way to test the output transistors for shorts, is to test diodes CR48 and CR49. They are essentially connected across the Collector/Emitter junctions of the output transistors and will test as shorted if the transistor is shorted.

If you find any component that tests shorted, you will need to remove it and test it again out of circuit.

[Enzo]

Wait, start at the start. CP10 and CP12 are the wires from the power transformer - probably red I guess. Pull them off and park them where they won't touch anything. Now only the primary of the transformer is connected. Now power up. Does the bulb still light bright?

[spindarec]

The lightbulb brights a little ( It´s not shining bright like a diamond ) not as much than when cp10 and cp12 are connected. CP10 and Cp12 are white one wires.
I've tested CR50 to CR53 with my diode meter and it shows:
CR50 :001/001
CR51: 001/001
CR52: 500/1476
CR53: 496/1466
Seems like there's something wrong with diodes, do i have to desolde and test it out of the board ?

Thanx'gain.