Dead Fender Stage 185-replacing thermisto

[Lee_S]

New to this forum and per 52 Bill's suggestion. created a new thread from a previous one related to problem/replacement of a Thermistor (TH1) on a Fender Stage 185 AMP. 52 Bill provided some help on how to access the chasis/pwb (noted below). Thanks 52 Bill !!l

Now that I have access to the chasis/pcb, I hoping I can replace the thermistor on the component side of the pcb versus having to take it apart from the chasis. From the schematic, I see there are a few paths that the thermistor connects to which are visible and accessible on the component such as the fuse mount on one end and a jumpered path from where the transformer primary (cable) brings in power on the other end. Has anyone tried/had any luck with soldering the themistor on the component side of the pcb? I don't yet have the part which was recently ordered, and trying to figure the easist way since I would prefer not to remove the pcb from the chasis.

Otherwise if that doesn't work, I would appreciate any help/tips on the easiest way to disconnect the pcb from the chasis. Other than the plastic standoff/screw and power related cables that would need to disconnected, I see 5 bolts from the rear of the chasis for a metal plate which houses/heat sinks several power amp type components.
Thanks again, Lee

From 52 Bill
Lee:
Welcome to the forum.
It would probably be better for you to start a new thread for your amp rather than picking up an old one.

To remove the chassis from your amp, start by unhooking the wires that attach the amp to the speaker. Then remove the two screws on the sides of the case and the four top screws. The chassis will drop down and out the back of the case. The chassis will probably get stuck on the self-adhesive foil shielding that has been applied to the inside of the wooden case, so just try and wiggle it out. Do not remove any of the control knobs or other screws until you have removed the chassis from the case.

[52 Bill]

...I would appreciate any help/tips on the easiest way to disconnect the pcb from the chasis. Other than the plastic standoff/screw and power related cables that would need to disconnected, I see 5 bolts from the rear of the chasis for a metal plate which houses/heat sinks several power amp type components.

Lee:
I'm sure that you can find a way to replace the thermistor without taking out the chassis, but personally I don't like working that way. And when an amp comes in with this sort of repair done, the first thing it makes me of is: "Someone that shouldn't have been in here worked on this before. Now I will have to double and triple check everything before I can be sure of what is going on in the amp."

As I recall, the thermistor is located near an outside edge of the pc board. Can you just loosen some of the screws enough to lift the end of the board that you need to access?

Removing the entire board is time consuming, but it isn't that hard. I do a lot of those repairs, and with experience I can have the board out, the part replaced and the board back in the amp in 15-20 minutes. So if it takes you an hour, it still doesn't seem that bad to me.

If you choose to remove the board, all of the pots and jacks on the front panel need to be un-bolted. There are a number of philips head screws in white plastic standoffs that need to be loosened (but not removed). And yes the five 1/4" hex head heatsink mounting screws on the bottom of the chassis need to be removed as well. You do not need to pull any wires from the board, but you may need to cut a cable tie to free up some of the wires. (Some of these models have certain cables running away from the board and tied to a chassis point.)

Once the board is loose, pull it straight back away from the front panel until the pot shafts clear their mounting holes. Angle up the pot side of the board until it clears the front panel edge. You may have to move the heatsink bar from under the board to get the board far enough back to clear the pot shafts. If you do, make sure that you remember how it was oriented, as it will only align with the board in one direction.

It may also help if you take a few photos of the chassis before you remove anything, so that you can see how things go back together when you're done.

Hope this helps.

[Enzo]

Sigh, we never have time to do it right anymore, but we always seem to have time to do it twice. I am sure you could figure out which parts it ultimately connects to and then cobble the new part in mid air somehow. By the same token if you break a guitar string, you could probably unreel the string, tie the two ends together in a knot and keep using the string.

JUst remove the circuit board and solder it in right.

Yes, there will be 20 screws or however many. BIg screws through the heat sink, smaller ones through the pc board. I don;t recall the innards of that one, but you may have to pull wires off the power switch to allow the board to move. If the controls are on the main board, then all their knobs and shaft nuts have to come off. Jacks? maybe them too. Seems to me ther is usually enough slack in the reverb and speker wires that they can stay connected.

[Lee_S]

52 Bill and Enzo,
Greatly appreciate your assistance on replacing the thermistor!!
Thanks again, Lee

[kepeb]

pro 185

i have a pro 185 thats been sitting around for over a year now.
seems like both you guys have worked on this or the stage 185.
i remember some time back reading the previous post relating to the thermistor and gave up after it was seemingly ok. not that i could find a replacement anyway.

i have changed out 2 parts on this amp and had it running again but each was short lived.
i changed out a broken/shorted input jack which wasnt allowing any sound through. i found this by accident when i banged on top of the amp pleading with it. got the proper fender part, replaced it, all was fine except.

my daughter in a moment of madness reached up and grabbed at a resistor on the end (now known to me as a 0.22ohm 20watt ww power resistor) i replaced this with a 10 watt of the same value before i knew what the difference was.
all was fine until it stopped one night. now i cant figure it out. would this of caused permanent damage?

i checked for continuity the thermistor is intact and so was the resistor next to the speaker out. all the fuses are fine, theres power because the audio signal shows up between the pre amp out and power amp in. from there i'm stuck.

should i repost this new? or just sell my poor 185 as scrap? its gotta be moved, i'd love to be able to have it running again instead of getting rid.

any thoughts on next steps? cheers guys

[Phostenix]

On the schematic I found here:
http://music-electronics-forum.com/t2472/#post64284
the only .22 ohm resistors I see right away are 5 watt (R167, R169, R170, R173). ??? Is that resistor you replaced open when you measure it?

[Phostenix]

OK, now I see yours is a PRO 185 (time for bed). If it's a similar emitter resistor & it opened up under load, the transistor associated with it would effectively turn off & the other power transistors on that side of the power rail would have to take load without it. It's possible that they failed. Have you checked the output transistors?

[Phostenix]

You say that there's audio between the preamp & power amp, so it obviously powers up without blowing the line fuse(s). Are there any other fuses in the power amp? Have you measured for DC voltage on the speaker output? Have you measured the DC voltage on the power amp rails?

[kepeb]

no i didnt test the transistors or the rail voltage.
to be honest i have limited knowledge when it comes to this, i was afraid of doing damage to the amp or to me if i touch the wrong thing. i have only really worked on effects and stuff thats relatively small voltages.

i really appreciate the fast detailed response tho, i dont often have a lot of luck requesting help on forums

so to test the rail (dc?) voltage for the power out i put the common to the chassis and measure where/ what kind of voltage am i looking for?
there arent any other fuses in there. theres 3 total and they're all still good.

i'm not sure which transistor would be related to it, it seems like theres just this one 20w power resistor to ground before the speakers?

i could do pictures, would that help or just confuse it?

thankyou for helping
kev.

[Phostenix]

Do you have a correct schematic for the Pro 185? Yes, pictures of the insides would be helpful. What test tools do you have? Ideally, you should have some test leads with mini clips on the end (instead of alligator clips). If you don't, the cheapest solution is probably these adapters that Radio Shack sells:
Mini Test Clip Adapters - RadioShack.com

You want to be able to clip onto components on the board with the power off, power up the amp & get the readings, and then power off the amp to move the clips to the next component. That keeps you from having to worry about slipping and shorting something & it keeps your hands out of the voltages. I'm a little anal about pulling the power cord from the wall when I re-position or solder so that I can take a quick glance at the plug just to be sure.

[kepeb]

thats a great idea cheers.
we used to have Tandy over here which i think was like your radio shack.
the only thing now is a catalogue based thing or ebay. i will look for and buy some of those for the future.

until then i have:
some shielded multicoloured croc clips, solder iron, solder gun, 1 x analogue and 1x digital multimeter both with pointy probes and a few starter kits old parts and spares from different projects over the last couple years.

i think the digital mm is the better of the two but even that was only equal to about 15 - $20 i guess.

all i could get was the same schematic you linked to previously. i think i downloaded from the same place about a year ago.

[kepeb]

this is the pre out power in section (effects loop)


this is that side of the board


here is the left of the board with the main power out bit


and here is close up on the fuse bit nex to the power out

[Phostenix]

I've sent an email to Fender to see if I can get the schematic.

What you're calling power out in pics 3 & 4 - does that go to the speakers?

Can you take a pic of the entire chassis & another that includes all 4 output transistors and the power amp section?

Is R141 the resistor you replaced? It looks to me like a 2.2 ohm, not a .22 ohm. You said you checked that with an ohmeter?

Where is it that you've been able to check that the audio is OK? Did you plug into another amp from the Send jack?

With the power OFF, check the resistance between chassis ground & the top of the metal cans of the output transistors. It should look like a short initially & then climb up. You're measuring across the power supply caps doing that, so as they charge the resistance will go up. After about a minute, what does the resistance read? Do that on both end power transistors (you can check all 4, but the end ones should give us both rails).

If you can clip your ground lead of your DMM onto the chassis ground, you could get the DC voltage readings on those same power transistors with the unit powered up. Set the range to something over 50 volts DC & touch the tip of the other probe to the top of the transistor cans. Check all 4 - 2 should be postive voltage & 2 should be negative. Do this with one hand & keep the other hand away from everything - stick it behind you if you like. This isn't the scariest amp in the world, but it's a good habit to get into....

[kepeb]

hey,
ok, so i got the pictures i think i got in what you said...
the bit i was referring to before above r141 in the pic is where the speaker connection goes out (black and white wires)

r141 was the one i replaced. that is actually the 3rd or forth on i've had in there. the original was replaced before i owned it but i have an old picture of that one.

the last one that was in there WAS 0.22 ohm but i took it out when the amp stopped again thinking it had blown because it was half the originals wattage.
i put the one in there recently just to see what was going on shortly before i posted my first message.

at the same time i used a simple audio probe with a mini speaker attached and went along probing some of the components to see where i could get audio from. i setup a walkman on repeat to play through the input channel one. then touched around. with the power off there wasn't much.
with it on you could get audio at the reverb connection points too.

the only things i tested with the mm were continuity in speakers and thier cables, fuses and reverb connectors just in case.
i also checked the resistances of the big resistors to see that they wern't just open or shorted.

atm the r141 reads 2.5ohm, about right i guess.
i cant get anything from the tops of the transistors like you said... i was touching the top of the can and the chassis?
kev.

[Enzo]

It certainly does look like you have a 2.2 ohm 10w in the amp instead of the 0.22 ohm that should be there. That resistor is important. it is in the speaker return leg to ground. If the resistor is open or if the traces to it are damaged, then you get no sound. And the extra 2 ohms you added are not helping.

All the 185 series amps use the same schematic