Dead Fender Stage 185-replacing thermisto

[Enzo]

Unless otherwise noted, you should take all voltage readings with your meter black probe connected to the chassis. There are four power transistors on the heat sink. Two of them should have about +50VDC on their casing. the other two should have about zero VDC on theirs. Those two should have about -50vDC on their emitter legs, which connect to the 0.22 ohm 5 watt resistors nearby.

Please refrain from saying things like "No reading" or "nothing" on the meter. Zero volts is still a reading. Nothing or no reading to me means the same thing as not connecting your probes.

[Phostenix]

Kev,

Check the resistance from the output xstrs to ground (power off) using a higher range on your ohm meter. It should be a fairly high value. I just want to do some obvious things first like check for shorts there.

With the power on, check for DC voltage on the speaker leads. You can disconnect the speaker(s) for this test. This should be a small value - less than 50mV DC typically.

With the power on, check those voltages from ground to the output xstr cases. Enzo's right on the voltages (I was guessing). Remember, anything Enzo says always overrules anything I say.

Can you get a .22 20watt resistor easily? How are you replacing that - are you tacking resistors onto existing leads coming up from the PCB or removing the board? Has the board been out of the chassis?

Do you get audio out of the preamp output?

Finally, find R136 on the PCB - 27K resistor - Red, Violet, Orange color bands - and check for audio on BOTH ends of that resistor with your probe. This will tell us whether audio is getting to the power amp.

[kepeb]

hey,

ok i did some checks on the bits you said.
the dc readings are as you said +50vdc on 2 of the transistor cases, and 0vdc on the other two.
i found the two 5watt resistors and got the -50vdc too.

i did the resistance check you asked too, the resistance seems to continually rise on two of them. i got to 200.0 on the 200k setting and it kept going up when i switched to 2m but not as fast rising.

there was no change to the reading when i put it across the speaker out (mv setting)

i'm not sure i can get the exact resistor replacement but there are some that are very close i found online.

i have had the board out of the chassis before, to replace a broken jack but the r141 and 2 others have been replaced on top to existing wires before, so i was just intending to change them this way, i could get the board out if i need to tho.

yea i did get audio at the preamp out. and at the reverb points.
i will check that resistor this afternoon.

so are the voltages a good sign then? this mean the transistors are ok?
kev.

[Phostenix]

Yes, it sounds like the output xstrs are OK. I'm curious about the solder joints on R141. If you're heating it from the top, I'm wondering if the joints on the actual PCB is still good & solid. The pictures have been a great help, BTW. The next thing is to see if the audio is getting to R136.

[kepeb]

i checked r136 there is no audio either side with the amp on.

[kepeb]

from what i can see under the edge of the board it all looks ok on the pcb.
the r141 is right at the edge of the board so i can make some of it out.

do you think i should get the board out?
if it is necessary it will be worth the time it takes to get it out
kev.

[Phostenix]

i checked r136 there is no audio either side with the amp on.

With your music source in?

[Phostenix]

from what i can see under the edge of the board it all looks ok on the pcb.
the r141 is right at the edge of the board so i can make some of it out.

do you think i should get the board out?
if it is necessary it will be worth the time it takes to get it out
kev.

If you can see it & the solder joint is not cracked, hold off for now.

[kepeb]

yea
i double checked.
running a ipod through channel one, theres very little with the amp off.
when the amp is on i got audio from the preamp out section again but not from r136

[kepeb]

just gonna check again with the master vol turned up

[kepeb]

no definitely nothing on r136

[Phostenix]

Can you confirm that if you plug a cable into the preamp out jack that you have audio there? Use your probe & touch it to the tip of the plug on the other end of the cable that you plug in (music source into the input, of course).

Are you familiar enough with 8 pin ICs to know how to count the pin numbers on them?

[Phostenix]

I forget now what your input setup was like. You got a 1/4" plug into the guitar input jack of the amp, right? There's a mute circuit built in to the input jack so that if there's no cable plugged in, it will mute the output (keeps you from getting that big buzz when you plug in or unplug a cable).

[kepeb]

yea down 1-4 then up 5-8 on the right with the notch at the top. right?

[kepeb]

yea, definitely a jack input otherwise theres no sound at the pre-out