Dead Fender Stage 185-replacing thermisto

[kepeb]

cool cheers.
so thats pin 6 of all the opamps? i'll check now
is there a safe way or good way to check for heat from them, how hot is too hot?

[kepeb]

all the opamps have 0.0 and 0.02v on pin 6

[52 Bill]

Originally Posted by kepeb

cool cheers.
so thats pin 6 of all the opamps? i'll check now
is there a safe way or good way to check for heat from them, how hot is too hot?

Dual opamps have outputs on pins 1 and 7.

For the heat test, I touch the top with my little finger. Be careful, sometimes bad ones will get really hot. Most will feel cool to the touch, some will feel warmer. It's just one way to help find a bad one on a board full of chips. The warmer ones get voltages checked first, as that will be the real test.

[kepeb]

ok cheers
i'll check.

i also just checked the zener diodes cr55 cr54 that had 2.0 instead of 16v outs with the diode setting and got readings both ways.

and on the ohm setting
in forward bias theres about 600 ohm and in reverse theres 800.
is that nornal for zener diodes?
kev.

[kepeb]

theres about 1volt on u9 pin 7 all the others are about 0.01-2v
...

[kepeb]

sorry that wasnt clear 0.01 - 0.02

[Phostenix]

Originally Posted by kepeb

and on the ohm setting
in forward bias theres about 600 ohm and in reverse theres 800.
is that nornal for zener diodes?
kev.

The zeners have the capacitors in parallel with them, so you won't get an accurate reading without removing them (or at least one end of them).

[Phostenix]

Originally Posted by kepeb

theres about 1volt on u9 pin 7 all the others are about 0.01-2v
...

I'm still leaning towards U9. The other opamps are still functioning with the lower voltages or you wouldn't have audio out of the Preamp Out, but it stops there at U9. I'd remove it & see if the voltage comes back up to +-16. How much experience do you have de-soldering components? I'm guessing you don't have a solder sucker, but do you at least have de-soldering braid? It's easy to overheat parts with the braid, so you need to be careful. Give ICs a chance to cool down between legs as you desolder them.

[kepeb]

well i never took an ic out before. i got a sucker but no braid.
i may put a socket in there if i got to replace it.
is this ic expensive/available?

would i be able to use a cr&ppy 741 to test it afterwards?

[kepeb]

the only real problem i can see is that ic is on the fx loop bit of the board which is seperate and for some reason is held in place with some kind of white resin...
how do i remove it from that?

[Phostenix]

If you remove the effects level knob & the nuts that hold that pot & the 2 jacks, etc. from the front, does the PCB have any give? Usually that is silicone sealer type stuff and will peel away or you can cut it with a razor blade. If the solder side is covered in goo, it will certainly make de-soldering more difficult....

[Phostenix]

Originally Posted by kepeb

well i never took an ic out before. i got a sucker but no braid.
i may put a socket in there if i got to replace it.
is this ic expensive/available?

would i be able to use a cr&ppy 741 to test it afterwards?

Solder sucker is great. Braid is nice with ICs to get the last remnants sometimes.

A 741 is a single opamp, so it won't work as a replacement here. TL072, RC4558, RC4559, NE5532, etc. would be acceptable replacements.

[kepeb]

its pretty damn solid... i did wonder about that before
is there any way to bypass the effects loop off board and skip that section or is that particular opamp just on that bit of board by coincidence?

[kepeb]

ok. i'll make a note of the opamps and go ebaying maybe.
although...
i did discover when i finally removed the bit of board, the pre out jack is wrecked.
it fell to bits. i'm wondering if that was shorted somehow inside.

[kepeb]

well it doesnt look like that makes a difference
should i go ahead and remove it then?

[Phostenix]

So, you got the board out? What condition is the Power Amp Input jack in?

Is the trace side of the PCB clear of goop that you can desolder U9? I say remove it & let's see if it makes a difference.

[kepeb]

k. i went ahead any way.
looks like i git 16v on pin 8 without the chip in!

[kepeb]

wow. well that feels like progress.. all the other opamps check out at 16v on pins 4 +8 too

[Phostenix]

Sweet!! I don't feel like all that time we spend yesterday probing signals was a complete waste of time now.

What's the condition of those in & out jacks?

[Phostenix]

If it's possible to keep that sub board safely away from the chassis, I'd love to know if injecting an audio signal onto R135 now will produce output from the speakers.

[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]

Originally Posted by 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 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]

Originally Posted by 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.

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]

Originally Posted by 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.

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

Have you got any suggestions on this so far?

[Phostenix]

Originally Posted by kepeb

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.