fender stage lead 212 troubleshooting

[Capt_Dunzell]

Should I order up some of the IC's in case they are needed?

[Roly]

R83 and R84 disconnected
Zeners are +1.5v and -.07v
R85 and R86 are cool

Okay, that seems to confirm that both zeners are indeed cactus.   :tu:

Now with R83 and R84 still disconnected I want you to measure the +/-15V rails on the preamp side, i.e. across C49 and C51 (100uF 16V) to determine if there is any voltage coming back from elsewhere (that shouldn't be). Both of these should be a zero, but it's just possible you may get a surprise and find a voltage where there should be none.

If there is, stop and post results (or you may kill the new zeners - I want to eliminate the possibility that the 24 volts supplies are somehow finding their way back to the 15 volt rails).

Do you order replacement IC's now?  I don't see any IC type numbers on the circuit but most common dual op-amps are only a dollar or so.  My normal method is to first find the fault, THEN take Berlin, however (only if there is no voltage coming back, as above)...

Restore R83 and R84 and run for a couple of minutes, while testing the IC's to see if you can find one that is  noticeably warmer than the others to the touch.  If you find one such, remove it and see if that restores the +/-15V supply rails.  If so, order a new one.

Exceptions: IC4 and IC6 - do not remove, but post results.

Since they are not marked on the circuit please list the type numbers of the IC's used.

[Capt_Dunzell]

The 15v zeners arrived, got a pack of 100.

Fitted a pair in, have +15 and -15 as it should be with R83 and R84 disconnected.

C49 and C51 both have -.6v on them.

IC1, 2, 3, 5, and 7 are all MC4558CP

IC4 is a CD4007UBE

IC6 is a TL604CP

I went ahead and reconnected R83 and R84 and could not detect any of the IC's getting warm at all.

[J M Fahey]

You always leave us with a little suspense
"I went ahead and reconnected R83 and R84 and could not detect any of the IC's getting warm at all." .......... *and?* .............
*do* you or *don't* you still have +/-15V, with op amps connected?
You are not saying so

[Capt_Dunzell]

Sorry, results are the same for the 15 volt supply with R83 and R84 connected.

The zener voltage dropped to +3.2, and -.5 again.

removed one leg of R83 and R84 and the zener voltage was restored to +15 and -15.

I have not removed any of the op amps at this point because none of them felt warm.

[Capt_Dunzell]

I went ahead and ordered some 4558's so I will have them on hand. If I dont need them I will have them for something else in the future.

Again I truly appreciate all of the help on this thing, I am completely lost on it without the assistance.

:dbtu:

[Enzo]

OK, so at least you have 15v rails back.  But R83,84 cause it to collapse.

Probably OK, but verify they are still 10 ohms.   And also verify R85,86 are still 150 ohms.   If they have failed way high, they could still feed the zeners alone but collapse under the normal circuit current load.

SOMETHING is loading down the 15v supplies.  ICs are the first suspects, but they ought to get warm/hot if they are doing this.  But other stuff connects to those rails.

On the schematic, lower left, see preamp out jack?  Right above it are D9, D11.  Shorted?  Also by input jack, see D1, D2.  SHorted?

I tend to think neither supply is shunting to ground, since they both are dragged down.  I tend to think then that they are shunting together.  But never overlook any possibilities.

[Roly]

The 15v zeners arrived, got a pack of 100.

That's just about a lifetime supply; and with the 4558's you're going to be set for some fuzzbox builds or the like.   


I'm glad that JM and Enzo have jumped in at this point coz I'm starting to run out of ideas.  I was worried that the 24 volt supplies to the reverb driver were finding their way back to the 15 volt rails, but the (near) zero voltage with the 15 volts supplies disconnected seems to rule that out.

{Curious point, the 10 ohm resistors are getting hot and the 150 ohm infeeds aren't.}

I agree with Enzo that this looks like a 15 volt rail-to-rail short ('tho not a dead short from the small voltage difference).

Capt - you said earlier that you had changed a number of electrolytic caps.  Did you change any on the preamp board/area, and if so could you please look at them very carefully to be certain you get them back in the right polarity?  Also check your soldering for any tiny tails that might be shorting.

[Capt_Dunzell]

I replaced all of the electrolytics, C49, C51, C25, C47, C48, C52, C53, C54, C55 polarity is correct on the board.

I have gone over the board for hours on several occasions with a bright light and magnifier and havent found any solder crosses.

With D1, D2, D9, and D11, pulled they test good on my meter. With a standard diode is this an okay test? Or is it not very valuable like the zeners?  They are 1n4448's. I dont have any of these on hand some 1N1419's that look similar in specs.

Is it worth pulling some ICs and pull some voltage tests even though they didnt get warm?

R83 and R84 and brand new, 1/2 watt and 10 ohms. 
R85 and R86 are also new, 1 watt flame proofs as that is what I had here, right at 150ohms.

I dont know if I should have done this or not but with D1, D2, D9, and D11 removed I get +3.2 and-.5 on the zeners still, would this point to the IC's even when they dont get warm?

[Roly]

Okay, good.

Testing zeners is a special case, and I think we have now covered that by replacement and testing them in-circuit; with R83 and R84 lifted we get +/-15V on the zeners.

Standard diode test for standard diodes; open circuit one way (meter +ve to cathode, banded end, overrange "1___") and between "500"mV and "700"mV the other (on diode test range).  If they test okay then they don't need to be replaced.

Now unless anybody has a better suggestion (guyz?), I'd start to pull IC's.

The short could be just about anywhere (maybe not even an IC) but I'd nominate IC2 first because one of its outputs goes to an externally accessible point, the Preamp Out socket, via R201 only 680 ohms.  The object of this resistor is to protect the IC from an external short on this socket, but it would provide little protection from somebody accidentally applying an excessive voltage to it, say from another amp output (and since the amp is 2nd hand we have to allow for all sorts of stupidity/accidents in its history).

The next I would try would be IC7 (because half is the reverb driver which is also connected to the +/-24V rails).

During this process I would not re-insert any of the IC's you remove, but put them to one side individually in little plastic bags or whatever, with a note of where each one came from.  Once a dead one is located (by the +/-15V rails coming to life on the preamp board) then that one can be replaced with a new one, and the others returned to their previous positions one-by-one, re-testing at each replacement in case more than one is faulty.  I would not pull IC4 or IC6 yet, but concentrate on the op-amps for the moment.

I hope you have a solder-sucker because de-soldering IC's is a lot easier with one (protip: running a little fresh solder onto a joint before trying to de-solder it re-fluxes the solder and makes it flow better when you suck or knock it off).

I have to warn that it is possible that you will pull all the IC's and still have a short, but we will cross that short circuit if/when we come to that point.

[DrGonz78]

Sorry to interrupt into this great thread... I am really enjoying reading along following all of what is being looked at on the schematic.

I have a question that even the OP might have wondered as well... Can we take voltage tests off of pins 4 and 8 on all the op amp chips? I mean sure we can test voltage there to check but why would that not prove to be useful numbers?

I am following the idea of IC2 being lifted first and then checking IC7. I think something is telling us that +/-24V rails could have something tied into the problem too. Maybe looking at Q2 and Q3 closer(later on down the line of course) and D8 is in there too. IC7 is giving me that gut feeling even more now. Also, have we confirmed what is measured on the 24v rails? Excuse me if I missed something there, but just had to ask. Good luck on this amp and I bet you nail it soon!

[Roly]

You're not interrupting.

I have access to a millivoltmeter and yes, I would be probing the chip pins topside to see if I could get a handle on where all that current is actually going.  This could be worth trying with an ordinary DMM but I'm not all that hopeful.

No, I don't think we have confirmed the +/-24 volts supply, and that is well worth looking at at this stage, thanks for the suggestion.

I have had a niggling worry about that reverb driver for some time, and I hope your comment is confirmation and not reflecting my concern.

Something that still bothers me is that it seems that both 15 volt zeners failed, something that is in itself uncommon, and would not be caused simply by an op-amp shorting the 15 volt supplies together - there has to be some other cause.  (scratches head)

Capt - Check the +/-24 volt rails and post results.  Pulling and testing the reverb driver transistors Q2 and Q3 won't get the amp going, but if one or both turn out to be shorted it may help to explain why, and what else has failed (and you can also leave these out for the time being).

Your observations are helpful @DrGonz78, thankyou.  :dbtu:

[Capt_Dunzell]

On the 24v supply if I am measuring in the right spot (C52, R85 and C53, R86) I have +16 and -16. Strange as I thought earlier on I had +19 and -19?

Q-2 and Q3 are good.

IC1 pin4 -1.8v  pin8 +2.3
IC2 pin4 -1.8v  pin8 +2.3
IC3 pin4 -1.8v  pin8 +2.3
IC4 pin7 -1.8v  pin14  0
IC5 pin4 -1.8v  pin8 +2.3
IC6 pin4 -1.4v  pin8 +2.3
IC7 pin4 -1.8v  pin8 +2.3

I am going to start pulling IC's now.

[Capt_Dunzell]

I removed all of the IC's and installed sockets for them.

I can only get +15v on pin 8 of 1,2,3,5,7 if IC1 and IC2 are not in the sockets, this drops pin4 to -.06v.

R85 and R86 still heat up.

I rearranged a couple of the IC's to check if they gave similar readings in different sockets, IC1 is definately bad as it tanks the voltage readings in any socket.  IC2 installed in any other socket gives the same +15v on pin8 and -.06 on pin 4 no matter what socket it is in, so it seams IC1 has failed.

[phatt]

If I'm reading this right,, You now have all chips socketed then with all opamps removed you should have + 15 VDC on pin 8 of all sockets and - 15VDC on pin 4 of all sockets. (except of course for IC4)
Now just keep inserting each IC until those voltages drop at which point you likely found the culprit.

Well that is how I would approach it.
You will get there it just takes time,,Phil.