Dead Fender Stage 185-replacing thermisto

[Phostenix]

will i need a new 1w 10ohm for r174 then? is that broken?

Yes. It sounds like it's open. If you take the board out, you'll be able to see what's going on with those 2 parts. I'm guessing the coil has a bad solder joint. It could be open, but you could see that by looking at it - is the wire broken? The coil should read as a short with an ohmmeter. The 10 ohm resistor will read "shorted" in circuit normally because it's in parallel with the coil.

[Phostenix]

That 7 watt .22 will be OK for testing, as long as you don't get crazy.

If there is heatsink compound to the chassis, then yes, you need to redo that.

[Phostenix]

You know, just as a test, can you clip in a jumper (croc clips) between the side of R174 that HAS audio & CP2. I think CP2 is one of the speaker wires in your amp. CP1 should be connected to the other speaker wire and that .22 ohm resistor we've spent so much time talking about. Clarify that you are connecting the output of the amp (R174) with the speaker wire that is NOT tied to R141 or you will short the output of amp to ground (through R141). That would be very bad.

Once you've got R174 jumpered to the speaker wire, you should have a working amp. This will be fine for testing. Then you will know if you need anything else.

[Phostenix]

I've got a better idea for the last test. With the power off, measure the resistance between the side of R174 that has NO audio & ground to make sure it isn't shorted to ground. If it is not shorted, just clip your croc clip lead across R174 to jumper it for the test. If that gets you audio to the speakers, you know R174 (and the coil in parallel with it) is open.

[Enzo]

R174 is only 10 ohms to start with. With a coil across it, it should measure a dead short. I can imagine the coil breaking off at one end and that leaving poor little R174 to conduct all the power to the speaker and burn itself out. SO either R174 measures about zero ohms from end to end, or the coil is broken.

kepeb, when you say R174 has "no reading," do you mean it shows no resistance as in zero ohms? or do you mean it measures the same as holding your meter probes in the air? That second one is indeed open.

[kepeb]

hey,
yea across the resistor r174 its like holding the probes in the air. so open i guess.

the side with audio to ground gives no reading

the side without audio reads 5.0 ohms. just thought i'd check this was right before i do that test

[kepeb]

AWESOME! i have sound!

i jumpered over the resistor and its got sound out.
so does that mean the coil is ok?

i just need the two resistors then.

[Phostenix]

Success!

Since the coil & R174 are in parallel, jumpering one also jumpers over the other, too.

I read here recently where someone's amp had that coil (most amps these days have the same coil & resistor at the output) break loose - the solder joint(s) cracked, leaving the resistor to take the current of the load and it failed. This sounded like it might be your scenario since the audio was on one side of the pair, but not on the other.

Do a visual inspection of the coil. Do you see any breaks? Is it loose at either end if you wiggle it? The only way to know for sure is to pull the board & check the other side.

Plug a guitar into the amp with R174 jumpered and play a little. Make sure it sounds normal on both channels. Test the controls & the loop, too. As long as you don't play it fully cranked, it will be OK. Do give it a couple of loud bursts, though, to make sure it can deliver some punch without doing anything strange. I just want to make sure there's nothing else going on before you order parts.

[Enzo]

Pull up on each end of the coil to see if its wire is broken off at one end. Also look closely at its solder. The coil is literally just a piece of wire, so it is either broken somewhere or not soldered. And replace the resistor with a good one as it needs to be there across the coil.

[kepeb]

well. it took a while to figure it out but the coil is broken as you suggested.
it all looked fine and the solder was ok underneath but it was cracked through a leg and i didnt see it.
can i get replacements for this?
do i need a specific type?

i was about to go ahead and get what i need from banzai music. they seem pretty good.

[kepeb]

http://www.banzaimusic.com/Royal-Ohm-10-ohm-1W.html

is that the right resistor also?
there are 3 or 4 types and they dont really look the same.
this is 10ohm 1watt tho.

theres the other resistors on there too. i cant see any coils tho.

[kepeb]

yep.
i'm stumped at coils.
i'm definirtely not sure how or why they even work.
i have found a couple that are the same value 2.5uf but they look different and the information i have found leads me to believe i have to be quite accurate in amount of turns and wiregauge or thickness here.
is this important?

RADIO RF CHOKE COIL 2.5 uH 200 MA HAM RADIO (x2 pcs.) on eBay (end time 05-Nov-09 22:15:07 GMT)

or

2 Coil Choke Inductor 2.2uh 1.8A 1H64C2201 ref 3060 on eBay (end time 18-Nov-09 14:54:48 GMT)

any final thoughts?
the rest of my bits should be on the way tomorrow

[Phostenix]

I wonder if you would be better off contacting a Fender dealer in your area & asking them if they can get these parts - ask them about the .22 ohm resistor, too.

I'm not sure that either of those coils would work in this application. Try checking with speaker builders/repairers, too.

[kepeb]

i'll try that then.
are these coil measurements pretty specific then?

[Enzo]

The coil of wire is just a coil of wire. The inductance is determined by the dimension of the coil, the numnber of turns, etc. But it is not critical. The coil wire is broken off at one end, probably where it goes through the board, right?

Remove the coil, take the broken end, and unwind one turn there enough to form the lead to fit back through the hole in the board. Scrape the enamel off the wire end so it will take solder. Reinstall the coil. AMp should now work.