fender stage lead 212 troubleshooting

[Roly]

{crossed with @phatt)

At the moment it's looking like the amp has had an excessive overvoltage coming in via the Preamp Out socket, and this has caused the failure of IC2, IC1, and both zeners.

Presuming that you are still using the limiting lamp your reading of only 16 volts on the +/-24 volt rails is to be expected.


Okay, to be clear; if all IC's are out of their sockets you can now get +ve 15 volts on all pins 8 and -ve 15 volts on all pins 4, is that correct?

If so, you can start re-inserting the op-amps, replacing the two dead, IC1 and IC2, with new ones.  Do this one-by-one, being careful to insert them in the correct orientation, and check after each one that you still have the +/-15 volt supplies.

If all goes well you can then apply a signal to the input and see if it gets through to the output.  If it doesn't then use your CRO to trace the signal from the input forward to find where it stops.

If you are getting signal output then you can try refitting the reverb driver transistors Q2 and Q3 and retest.

[Capt_Dunzell]

So I have two bad op-amps, and have also found that IC6 is bad, I lose the -15v if it is on the board.

With IC6, IC1, and IC2 out I have +15v and -15v as it should be and R85 and R86 stay cool.

Its a TL604CP, not able to find much on this except for bulk order or ebay from China....I skimmed some posts on the search that looks like this chip is an issue regularly.

I am hoping that my new op-amps show up tomorrow.

[Roly]

I'm feeling quite sure now with the growing pile of dead silicon that this amp has had a serious jolt of excessive voltage into one of its external sockets at some point.

Data sheet and cross reference for the TL604 attached.

[Capt_Dunzell]

I can add one more to the list. I reconnected P2 and tested for voltages, IC10, another op-amp, is dead as well.

Glad I ordered plenty of them.

Peavy has a replacment circuit that plugs into the TL604CP socket since it looks like this IC is out of production, has anyone tried it?  Its cheaper than buying the IC and I wouldnt have to wait for it to ship in from China. http://shop.magicparts.com/PV99598160

[Roly]

{That cross reference turned out to be singularly useless when I looked at it - teach me to take it at face value  xP }

@Capt_Dunzell - the TL604 is a CMOS switch, effectively a solid-state relay for switching signals, and if it's out of production and somebody is making a substitute then you are in luck; yes, go for it (as you don't seem to have much option, and it looks reasonably priced).

For the time being you can simply use a wire link to activate whichever channel you wish, or even put in a small SPDT channel selection switch during testing until the substitute board arrives.

Yet another dead op-amp?  Hooo boy, this one certainly has had a serious dose of electric cancer go through it, maybe even the tail end of a lightning strike or similar, however you do seem to be grinding it down into submission now - you have the main amp and power supplies back up, and the preamps now looks like just a matter of persisting.

I don't suppose we will ever know exactly what happened to it, but I think this is one I won't forget for a while.  When you finally pass the finish line and can plug a guitar into it and make a loud noise I think you will be well justified shouting yourself a libation and a little victory dance.

[DrGonz78]

Makes you wonder what happened to that amp!! LOL    :trouble

Good job troubleshooting ya guys!! I look forward to the end result of a guitar passing signal through the amp soon.

[Enzo]

Yes Peavey makes a workaround for the non-available TL604.  They have been making this thing for a long time.  Yes, it works.

[Capt_Dunzell]

Well no delivery of my replacement op-amps today....so much for any more progress tonight. 

Thanks for letting me know that the Peavy part works Enzo, its on order.  :dbtu:

[Capt_Dunzell]

Op-amps are here and installed....I have +15v and -15v as it should be on every one.   :tu:

The replacement for the TL604 wont be here for another few days.

That said, if I jumper for the channel switch temporarily, which pins would I use?

It looks to me like I can use pins 3 and 4 jumpered to either pin 6 or pin 7 if I understand the schematic correctly.

Can someone confirm this for me so that I dont break something?  :dbtu:

[Roly]

Capt_Dunzell - pins 3 and 4 jumpered to either pin 6 or pin 7   :dbtu:

3 and 4 appear to be the pole or output, while 6 (Channel 1, clean) and 7 (channel 2, effected) are the inputs selected.

+/-15 volts everywhere it's supposed to be?  Great, more real progress.   :dbtu:

[Capt_Dunzell]

Okay, jumpered and started up on channel two with the limiting lamp.....bad hum and no sound through the input...did I move too fast?

[Enzo]

Um, I am old, senile, and forgetful.   Clearly you have replaced a bad op amp or two and solved the loading on your 15v rails.  Great news.

Did we actually determine that the TL604 was actually defective?  Or are we just replacing it along with all the other ICs?  I mean if you stick it in the socket where it was, does it load the rails back down?   If it works, it works.  And if it is dead, then you can jumper and wait for replacements.

Now you have a loud hum, and the amp will not pass signal, right?   Pull your jumpers out of the TL604 socket.   That will cut the circuit more or less in half.  Hum remain or go away?

Assuming nothing, plug something into the power amp in jack to disconnect the whole preamp.  Hum go away?

Turn reverb dowwn, any difference?  For that matter, shake the amp to crash the reverb springs, does that come out the speaker?

The key to solving any problem is to isolate the problem.


If pulling your jumpers stops the hum, then it is coming from before the 604 spot.  You had jumpered to channel 2?  What happens if you jumper to channel 1 instead?  And whatever channel hums, do ANY of the controls affect that hum in ANY way?

[Roly]

...also...

This hum could be due to running via the limiting lamp on reduced voltage.  I think you said the 24 volts supplies were down to only 16 volts which leaves the 15 volt zeners with almost no headroom to regulate.

I would try moving up to the next highest wattage of limiting lamp.

Check that the output half rail is still very close to ground.

Measure the "24 volt" rails and post measurement.

Has the increase in supply voltage reduced the amount of hum?

Can you now inject a signal into the input and trace it along the signal chain using your CRO?

[Capt_Dunzell]

The TL604 is definately bad as I loose -15v when it in in the circuit.

I inserted a higher wattage bulb, was on a 40w, now a 60w, hum still present on both channels, and with jumpers in or out of the TL604 socket.

The amp is not passing a signal, no noise from the reverb tank, the hum increases on channel 1 and channel 2 with the volume and control knobs.

Q9 is heating up, warm, not hot.

I tried running my ipod into the power amp input, there is still sound as before, however now its not clean like it was before when I tried this. Did fixing the 15v supply change something with this?

I will try some signal tracing later this evening.

[Roly]

The sensitivity of the hum to the front-end controls is a good sign.   :tu:

Disconnect the speaker and check the idle voltage coming out of the power amp is still very close to nothing (i.e. that the output stage is still rebalancing the half-rail); if it is, reconnect speaker.

You should also check the voltage across the emitter power resistors R79 and R81 0.22r to work out the idle current through the output pair.  This should be somewhere around 20-40mA, that is you should be getting;

E = I * R

0.04 * 0.22 = 0.0088, say no more than about 10mV.

If it is *considerably* more than this then you will need to note the current position of the bias pot R75, then carefully readjust it for about this idle current ('tho I don't expect you will need to do this).  If it is around this value or less, leave it alone.

Important question; is the FET J111 in or out of circuit at the moment?  (this is the only way I can see that the 15 volt supplies could have any effect on the output stage).

If J111 is out, does plugging your iPod into Main Amp In get rid of the hum?

For the moment turn the reverb control down to minimum (does that make any change?), strap IC pin4 to pin6 so we select the clean channel, Ch1, and we will concentrate on trying to get that up first, then move on to the more complex channel 2.