Dead Fender Stage 185-replacing thermisto

[kepeb]

the power amp in jack has previously been replaced with a non fender switching jack socket.
the pre out has physically seen better days. the plastics all broken up but turns out its working ok.

i can try the direct out but have to do that in the morning really. this one IS seriously loud if it did come through.
although. there isn't any (at all) sound from the speakers still.

[kepeb]

well i guess i need a dual opamp.
i'll get hold of one.

would you think it was the opamp that failed or something else that shorted and ruined it?
just thinking maybe if the latter is true, i'll just end up frying another one too.

[Phostenix]

the power amp in jack has previously been replaced with a non fender switching jack socket.
the pre out has physically seen better days. the plastics all broken up but turns out its working ok.

I assume it's a PCB mounted jack. Does it have all the switch contacts? That jack has 2 switches. It should have 9 legs.

[Enzo]

The thermostat parked on top of that output transistor is in series with the mains. If the amp gets too hot, it opens the mains lead. It is not on the schematic page, it is on the board layout page, top center.

[Phostenix]

well i guess i need a dual opamp.
i'll get hold of one.

would you think it was the opamp that failed or something else that shorted and ruined it?
just thinking maybe if the latter is true, i'll just end up frying another one too.

TL072 should be cheap and plentiful, even at the local shop.

I'm more inclined to think that something happened to that opamp as a result of an input signal to the power amp in. Even a good static charge might have damaged it. Opamps that are exposed to input jacks are commonly damaged.

Oh, and get yourself a proper .22 ohm resistor, too.

Once the pre amp is passing audio into the power amp, we can see what's next.

[kepeb]

yea all the legs are there and connected via wires instead of mounted.
that worked fine before tho. i think it might also be why that resin was there some of the strength to hold up the pbc is missing without that i guess, its ok like that tho.

you have a suggestion where i could get the proper 0.22ohm 20w power resistor while i'm getting that opamp?
kev

[Phostenix]

The thermostat parked on top of that output transistor is in series with the mains. If the amp gets too hot, it opens the mains lead. It is not on the schematic page, it is on the board layout page, top center.

Once I got the drawings printed out today I found it there.

Have you got any suggestions on this so far?

[Phostenix]

you have a suggestion where i could get the proper 0.22ohm 20w power resistor while i'm getting that opamp?
kev

I don't. I get most of my stuff from Mouser here, but I figure you are looking for a local source. I don't know what's available to you. Sorry.

[kepeb]

well.
i definitely spent quite a while looking for this value resistor before.
i know its not the best option, but if its the only one... which would be the best route to take if i were to use a combination of different values in the place of r141?

i get how the resistance changes in series parallel but i was never sure about the wattage. could i use 2 11w resistors for instance?

(i would work that as 2x 0.11 in series, or 2x 0.44 in parallel?)

[kepeb]

maybe two of these?
0,47 Ohm, 11W, wirewound :: 0,1 Ohm - 82 Ohm :: Wirewound 11W :: Resistors :: Passive Components :: Electronic Parts :: Banzai Music

[Phostenix]

Is that a local store or do you have to mail order?

[Phostenix]

You're right, it's not easy to find this value. I don't know enough about the distributors you have, but NTE has a 25 watt .22 that would work nicely - NTE 25WD22 like this:
25 Watt Wirewound Resistor 0.22 Ohms | NTE Electronics | 25WD22

It might be easier for you to search that NTE number with the places you shop.

Newark has them, but I don't know if they are in Europe. If I click on international sites on Newark.com, MCM is one of the choices.
NTE ELECTRONICS|25WD22|25W .22 OHM 5% | Newark.com

[Enzo]

0.22 ohm 20 watt is not as common as some other types. I would think that two 0.47 10 watt (or 11 watt), wired in parallel and then installed should be close enough, yes.

You guys seemed to be progressing along, I didn;t want to derail your train of thought.

Sems to me if you have an IC loading down the power supply, replacing that ought to help. And of course the .22 ohm resistor.

generally if some IC is doing that to the power rails, it usually will get very hot. SO touching the top of each chip is often a fast way to locate a bad one. Most op amps do not get hot in normal service, so hot ones are immediately suspect.

And of course as with any system, if the power supply isn't right, the whole thing won;t be right. Always get the power supply right before worrying about other stuff.

[kepeb]

yea, it does seem to be moving forward now.
i put a socket in place of the ic i took out shouldn't matter if it does break another one now i can just switch it out.

the power resistor is still proving to be a problem. i'll keep looking for the best option but am i right in thinking the value is more important at low levels than the wattage?

cos i have that 7w 0.22 in there atm
kev.

[Enzo]

I am too lazy. Use Ohm's Law. Calculate what the maximum power through the speaker load would be. At its impedance, solve for current. That current through the .22 ohms will tell you the maximum dissipation for that resistor. FOr safety, double that amount, and the result would be the minimal wattage for the part.