DC at speaker terminal help

[frankfurts]

OK done some testing and I've found.

+3.3V
+HI
+9
Are all good.
-HI and -9 are missing
Can't find yes points for +VL yet.

[frankfurts]

OK VL is not there as its for a foot pedal. My model isn't compatible with one.

[frankfurts]

OK the problem is with the diode I replaced I think I destroyed the solder pad a little any tips on how to repair it?

[frankfurts]

I definitely have something else that's shorted. I didn't realize the diode I replaced was on a dual layer part of the PCB so I fixed the connection on the top and as soon as my bridge rectifier was intacted I powered it up ( with the bul limiter) and straight away burnt out the two diodes on the negative side of the bridge.

[g1]

  If it's not triple layer pcb, consider yourself lucky. 

Attached is the very extensive service manual for the III:

[frankfurts]

  If it's not triple layer pcb, consider yourself lucky. 

Attached is the very extensive service manual for the III:

Haha it may well be. I failed to notice the 2 layers. The must be a short on the negative somewhere.

Probably my only concern when I pulled it apart was the way the power amp bolts to the chassis. And I think it's probably what's causing tho problem now I think of it.

The top tab is also connected to pin 3 of the amp which is the negative supply. However there it is booked to the chassis. I did notice the bolt has a plastic washer. Is this supposed to go inside the whole in the tab to insulate it from the chassis? In just using it as a regular washer.

[phatt]

YEEEEP datz ya problem,,, No *Insulation washer* yikes. Which means you have created a dead short.
And before you power on again do a continuity test and make darn sure the washer is actually insulating.
Also depending on the type of washer you may need some thermal paste. (the white grease stuff)
Phil

[frankfurts]

Well the whole amp is mounted onto a sort of insulation tape so when I get a chance tomorrow. I'll replace the diodes and fix the washer. Then I'll probably be back to my original problem.

[g1]

 In that spider 3 manual, look at page 4 of the pdf.
It shows how the output IC is mounted.  If it has a grey silpad like that (no rips or damage) you don't need heatsink grease.  The silpad insulates the back of the IC from the chassis.
  Now the tab of the output IC needs to be insulated from the mounting screw.  This is what the plastic shoulder washer does.  The narrower part of the shoulder washer fits into the hole in the tab of the output IC.
After the shoulder washer, there should be a flat washer, split washer, and nut.
After it is all mounted, check that there is no low resistance between tab of output IC and chassis.

[frankfurts]

Rectifier and Poweramp all fixed now. I can listen to music from my phone through the mp3 input perfect but when I plugged a guitar in I get little or no sound.

Sometimes by playing around with the pots etc I can get it to work for a bit, however much quieter than it should be. Also it doesn't seem to be 1 consistent setting that makes it work.

I measured the pots and they are all 10K and adjustment is working.

[g1]

  Have a real good look at the solder connections on the pots and jacks.  Maybe just resolder them anyway.

[frankfurts]

I checked all the pot connection continuity and they are all sweet.

I think I may have found a problem with the input jack. I'll start browsing the prints you guys sent me to work it out.

Are input and output jacks wired the same way? I done some tests comparing the two and on both I get a circuit across each pair of connections with no jack in.

However when I plug the guitar in I get open circuit across the pairs on the output jack but on the input I get a circuit of 4k on one 300k on another and open circuit on the third. I also have a circuit between the top and bottom pairs on input jack only.

Sounds suss to me. Like I said I'll look the prints up and confirm but I figured you guys would probably be able to tell me straight up.

Thanks for helping,  I really appreciate it.

[frankfurts]

So I found my problem.  My 1M grounding resistor (R1) is short circuited. It's a tiny little black one that I can't even read the writing on it let alone replace it.

Anyone know of an easier position for me to put another resistor as looking at it I very much doubt I can replace this.

[Roly]

They got it in there, therefore you can get it out.

I think I may have found a problem with the input jack.

...

I found my problem.  My 1M grounding resistor (R1) is short circuited.

Now hold on a minute here.

I assume we are talking about R1/1Meg across the input socket?

A) you are aware that this socket has a shorting contact in parallel with R1, and that if an open lead is not plugged in to the socket the input is shorted?


B) Shorted 1Meg resistor is quite unusual in itself, and cause for a few moment pause.  Is it burned up?  If so, how much voltage would it take coming in the input to fry a resistor of this wattage like this?

If this is not a mistake but a real failure, then it has profound implications for the components surrounding, not least the first op-amp.

But this now seems to be working okay now you say?

My intuition is that this diagnosis needs to be revisited in more detail.   8|


If this socket solders directly onto the PCB, have you carefully gone over its solder connections with a lens and a bright light?  With modern gear this is a primary fault zone.

[frankfurts]

Ok cheers didn't realise about the shorting resistor I only read about the 1meg grounding one so that makes sense I'll go over the solder and might just touch it up anyway.