fender stage lead 212 troubleshooting

[Capt_Dunzell]

You guys have the patience of a saint to be helping me through this. 

J111 is in the circuit

I have 17mv across R79 and R81


The reverb control has no effect.

The hum goes away when I plug in to the power amp input jack.

P5-5 47v
P6-1 -47v
P7-2  18mv


I pulled J111 out and plugged the ipod into the power amp input and the went away and I have a clean tone. The hum is also gone with J111 out.

No sound goes through the normal input at this point.

[Enzo]

When you had the 15v rails missing, not much can work.  By repairing the 15v rails, other existing problems are now able to reveal themselves.   It is quite common to repair a power supply problem only to find something else needing repair too.

You just verified the power amp works.

I am going to guess the volume controls only affected the hum when the TL604 jumpers were in place, and not with them out?

Did we establish that you get no signal passing AND you get hum on BOTH channels?  (selected one at a time by jumper)

If so that really points to the first stage.

I don't know if we did this already, but plug a cord into the regular input, then measure for DC voltage on the free end of the cord.    There should be none.   hat is a cheap way to check for shorted D1, D2, wwhich if bad can inject ripple hum into the signal path and squash your real signal.

[Capt_Dunzell]

Ladies and Gentlemen we have a working amplifier!

J111 is back in the circuit, playing through an ipod both channels are now working and no hum!

There was another bad op-amp even though it wasnt pulling the 15v rails down. and a bad connection in P2 that I discovered by bumping it...I eliminated P2 after being unsuccessful at cleaning the plug ends and ran wires directly to the board.

still have to switch channels with a jumper as the replacement for the tl604 isnt here yet.

So, the two on board pots, bias, and offset, do they require any additional adjustment?

[Capt_Dunzell]

I am still running on the limiting lamp, I should be able to up the bulb wattage to double check everything and then pull the lamp out and give it a go?

[Capt_Dunzell]

Pulled the limiting lamp....eveything is working great...no hum, clean tone....trying a guitar in place of the ipod next.

[Capt_Dunzell]

no limiting lamp, guitar for the input, everything is working....ran for 30 minutes and all is holding.

[J M Fahey]

Ladies and Gentlemen we have a working amplifier!

:tu:

no limiting lamp, guitar for the input, everything is working....ran for 30 minutes and all is holding.

<3)

Cool, way to go.

Now, *who* was wanting to throw the towel just a few posts back ?  :lmao:

[lurkalot]

Ladies and Gentlemen we have a working amplifier!

Excellent news, well done Capt_Dunzell.  :dbtu: 

[Roly]

Ladies and Gentlemen we have a working amplifier!

Meh. {shrug}   8|


But no, seriously, very well done Capt, that's great to hear, and good onya for sticking with it.   :dbtu:

Okay, so you've still got one IC to go, but you are effectively out of the woods now.

Should you adjust the bias/idle current?  Generally I leave that alone unless I have good reason.  Run the amp on idle keeping an occasional finger on the heatsink.  It should get a bit warm, just enough so you know it's on, but not hot, and hold that same "warm" for an hour or two; if so, leave it alone.  If it starts to get "hot" while idle then you should try turning the bias current down a bit.


This has been one of the really nasty ones.  It thankfully isn't often that you get an amp that has suffered some gross insult like having the mains connected to an input or hit by lightning, but when you do the result looks something like this one.

It is uncommon for more than one part to fail at a time, and when that happens you can generally see how, for example, a shorted output led to a shorted output transistor led to burnt emitter power resistors and perhaps a dead rectifier, maybe even a melted power transformer, but while such cascade failures do happen, they are (thankfully) not all that common.

I hope that you can now see that all the components you replaced at first didn't actually come near the real problem, and had the potential to actually introduce new faults.  My policy is to do as little as possible until I have a handle on what is going on/has happened, then try to move with "surgical precision"; not only finding and double checking that a part is faulty, but trying to understand why, what might have been the cause of the failure, because (particularly with DC coupled solid-state output stages) some obviously dead component such as a blown fuse or shorted output transistor, may only be a symptom of the real basic cause (e.g. wires twisted together inside the plug of a speaker lead - and I've had this one several times).

In this case I'll surmise that somebody connected the output of another amp to this one, let's just guess mistakenly to the Preamp Output.  Not getting the desired result they turned up the first amp and hit the guitar hard.  Even a 50 watt amp is capable of around 35 volts peak, and this would have caused the final preamp IC to fail, thereby placing the excessive voltage on both the + & - 15 volt rails, leading rapidly to other IC's failing and finally the +/-15 volt zeners.

Once the -ve rail had failed the power amp would not unmute due to J111 staying on.

In a commercial repair shop this kind of cascade failure might be BER - Beyond Economic Repair, and I think you can now see why.  In any case it's the sort of repair you come to dread because even after you have found and replaced all the obviously dead silicon there may remain a working but damaged op-amp just waiting to flip out and take your reputation as a repairer with it.  So don't be too surprised if in a few days, weeks or months it suddenly rolls on its back and plays dead again.

BUT - by now you and this amp should be old adversaries, you should know the innards like few guitarists ever get to know their amps, and it should contain little or no fear for you now - the next time it plays up you will have it up on the bench with its guts hanging out in no time, and the bit between your teeth, taking no nosense.  :trouble

I also hope that this whole thread stands as an object lesson in how to move step by step, stage by stage, through fault-finding, about isolating what seems to be wrong and proving it, bringing that part up, then moving on to the next problem.  And this amp is atypical in having so many faults at once; it's more normal to find just a single failed op-amp or power supply zener through old age or excessive heat, not a whole raft of dead stuff.  I haven't been keeping score, but you must have quite a pile of dead stuff on your bench by now.

As I said, a particularly nasty repair, and I think you are justified in feeling rather proud of yourself for having stuck it to the end.  Enjoy.  :dbtu:

[Capt_Dunzell]

Ran on an idle for a few hours this morning, heatsinks warm, not hot.

The peavy module arrived, they had two different ones, I got the less expensive one, but It doesnt seem like it should work, the pins dont make sense when I look at it.

When it is plugged in it turns channel two on and off, but when channel two is off it doesnt activate channel one.

In looking closer at the board there is no connection on pin 6 which is what should go for channel one..maybe I should have gotten the other module.

[J M Fahey]

I think the unconnected pin is #3 or #7 , definitely not #6 (don't know which way it should be pointing).
Try to plug the module the other way.

[Capt_Dunzell]

That may be, but it wont plug in to the socket in the correct orientation to make it pin 3 or pin 7.

Pin 7 is channel 2 though, so maybe it must be pin three.

[DrGonz78]

Looking at the datasheet pin 1 is ground...

Pin2 is marked as "A" on the data sheet, it ties over to the base of Q1 J175 and I think you should look at the transistor there to makes sure it is sound. Perhaps is was Zapped out of the circuit too!

Pin3 and Pin4 are tied together which is S1 and S2 tied to Pin 6 and Pin7 S1/S2. This is the switch.

So that leaves out pin5 and 8 as voltage carriers.

Looking at the picture pin1 and pin2 on the chip PV replacement (refer to picture) look like these ones to me, but not quite sure of this as a fact. Just a hunch after looking at it and studying it's layout. So, one channel is shorting to ground maybe due to the Q1 malfunction. To me it would seem that sure the pin out could be different on the replacement it is for a 604 chip but not DPST. Maybe this is only for SPST type switch and needs some modifications? Not sure I know exactly what I am saying here... Maybe even typing something silly will cause someone more knowledgeable to explain what could be going wrong here...?

Also, the more expensive other module you saw on that site is discontinued and really was the kit version to build what you have now. So, that is not reason why this won't work.



http://www.ti.com/lit/ds/slas042/slas042.pdf

Edit: Too funny a double posting... Also, you said on the main board there was nothing tracing anything to pin6? Explain which pins are what according to the picture... Also, show a link to the other module you thought it could have been. I might even have DPST thing wrong and it is as Juan said a "SPDT" type circuit needed. These things always confuse me at first... Or rather as Juan really put it... "Two independently driven SPST switches" Now that the solution here...

[J M Fahey]

Are you using a Fender made or a Peavey replacement module?
Because I can be quite sure that Peavey won't issue anything non-functional *but* maybe they use the TL604 in a different way than Fender did.
So the module not necessarily works somewhere else.
Say one of them *always* uses them as an SPDT, both halves driven from the same pin, and only 3 "switch legs" needed, while the other uses them as 2 independently driven SPST switches (4 legs needed).
2 mutually incompatible modules might be built , emulating either one or the other behaviour.
A *full* TL604 emulation can of course be made ... but it will take much more than 3 Fets and a couple resistors and diodes.
Let me think about it, *maybe* I can cook up some solution ... but don't hold your breath

EDIT: Oooops!! , simulposting !!
But anyway DrGonz and I are hinting to the same problem: the replacement module meant for a Peavey won't necessarily work there.
PS: why don't you lift the module circuit and post it here?

[DrGonz78]

My final thought is that 3 FETS will not get the job done here on this replacement circuit. However, I have a question as to whether J175 Q1 plays into the that role of making the logic of the switch come to life?