My fender stage 185 amp won't power up

[spindarec]

Hi,

First of all, thanks for all the precious informations you could be able to give me.
I'm french so, sorry for my english ;o)
So, I bought a fender stage 185 amp there's 2 weeks ago, since yesterday it was working fine but this morning i power it up and play some chords on my guitar, the sound was not really clear and a bit far. I moved the jack in the plug and the sound came back as clear as usual but 5 minutes later the problem happened again. i decided to power off the amp and restarting it few minutes later. At this moment nothing happened, it would not power up and i heard a "pop" inside the cab. I removed the amp part and saw on the circuit that the XF1 fuse was blown, i replace it by a new and power it up again but it would not and the XF3 fuse blow.
Now i replaced all the fuses of the circuit and every time I try to power on the amp, XF1 or XF2 and XF3 cracks !!!
I don't know if there's a relation but the switch for the reverb on my footswitch doesn't work, the reverbs works manually with knobs but not on my pedal. The two channels clean and boost are okay but the center switch for the reverb is not working.
There is a component C57 a little bit burned and the wires from the spring reverb are a bit sticky and i don't know if the soldering is fine.
There is another component on the second picture with a kind of white chalk powder on it, is it okay, mayo have to change it ?

thanks for your advices.

Guiom

[gearhead63]

First off, do you have a volt/ohm meter and do you know how to use it? Second, Can you follow a schematic diagram of the circuit.
R141, the component with white chalk powder, is a power resistor.You need to check it, with the amp unplugged, with an ohm meter to see if it is open.
It looks ok but you won't know till you check it, and it by itself shouldn't be a cause for a blown fuse.
You need a light bulb limiter (there is a thread here on making one) so you don't blow any more fuses.
The component C57 doesn't look right. We need to get a schematic posted here so we are all looking at the same thing.
Also, have you tried disconnecting the speaker and turning it on.

Here is a schematic I found in another thread here:
Fender 185 Series revF.pdf

I am not sure if this is the exact schematic for your amp. You will need to verify it.
Also, here is a link to the other thread on the Stage 185 not powering on. May have useful info, maybe.
http://music-electronics-forum.com/t2472/
There are others here who have experience with this amp and I'm sure can give more info than I can right now.

[spindarec]

Thanks for your answer, yes i have volt/ohm meter and i know how to use it. for the schematic diagram it will be harder but ill do my best.
And yes i tried with the speaker and spring reverb unplugged, same thing.

[Jazz P Bass]

There are only a few things that can blow the fuses.

#1 on the list is a blown output section that drives the speaker.
So disconnect the speaker until you get it sorted out.

Set your meter to read Volts dc & try to measure the speaker leads that come out of the amp.

If there is a voltage there, higher than 1 volt dc, your output section has an problem.

[spindarec]

Thanks but i can not test the output section because all the fuses blows when i turn the amp on.

[Jazz P Bass]

In that case you will have to remove the chassis & check the output transistors for a shorted condition.

Q17/ 19 & Q18/ 20 should read high resistance Base to Collector & Collector to Emitter.
Base to Emitter will read low because of in circuit resistors.
If there is any doubt you can remove the transistors for further testing.

There are other items that can fail short & blow fuses but the output is the first thing that I would check.

Also, look up 'Lamp Limiter' & build one.
It will save the amp from more damage when testing it in a failed state.

[gearhead63]

That is why you need to build a lightbulb limiter. I usually use like a 40watt bulb to start, it will cut the voltage being supplied to the amp from 120 to about 60, then I move up to a 60 watt if I think I've got it figured out. You can also solder a 100-150 ohm resistor (5 watt or so ) across the fuse and put it in. The whole idea is that it limits how much current can flow into the amp and keeps you from blowing fuses and burning other components.
Also as mentioned, disconnect the speaker and don't hook it up until you figure out what is wrong. And check the speaker with your ohm meter to be sure the voice coil isn't shorted.

Here's a link to making a light bulb limiter:
Light Bulb Limiter

[spindarec]

Thanks for your answers. I'll make this lightbulb and then check the output transistors like you mentionned.
Another thing, I've removed the main circuit board from the chassis and look at the soldering points of each components. On of them is almost unsoldered, one transistor. Could it be him the culprit ? Thanks.

[gearhead63]

I doubt if a bad solder joint on a transistor is gonna blow the fuse. Get your lighbulb limiter put together, check across the speaker wires for DC voltage
and post what you find. One step at a time.

[52 Bill]

I doubt if a bad solder joint on a transistor is gonna blow the fuse.

It really depends on what transistor he is talking about.

Test all 4 rectifier diodes CR50-CR53 for shorts and all power transistors for shorts.

The large chalky part is a power resistor. The chalk is the filler around the wirewound resistance element. It started out as a solid filler, but from repeated heat/cool cycles it has turned chalky. Make certain that it has not come loose from the board, but as long as it remains solid, it does not need to be replaced.

[Jazz P Bass]

Another thing, I've removed the main circuit board from the chassis and look at the soldering points of each components. On of them is almost unsoldered, one transistor. Could it be him the culprit ? Thanks.

Please tell what transistor this is.

The bad solder joint may be the 'cause' of the failure (by rerouting currents to unintended spots), and the failure itself may have damaged more components.

[spindarec]

I'm not at home for the moment, i'll answer to you when i'll be back home.
I live in France and the voltage in our country is 220v and not 120v, is it important for the light bulb limiter ?

[spindarec]

the bad solder joint is on Q15 transistor.

[spindarec]

a piece of Q21 is broken too. look at the picture.

[Jazz P Bass]

Q21 appears to be a mute JFet.
Antithump at turn on.

Q15 is the 'driver' transistor for the positive output rail.