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Author Topic: fender stage lead 212 troubleshooting  (Read 46788 times)

Roly

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Re: fender stage lead 212 troubleshooting
« Reply #30 on: November 10, 2012, 10:09:33 PM »
Humm, the positive side zener way low, and the series resistor on the negative side way high.

5-6volts on the +ve zener sure ain't right.  The question is if it's the zener itself or something downstream drawing too much current.

The "500 ohms" of R84 ain't possible if it's still intact, so it looks like that's effectively open and needs replacing.  This by itself would explain the failure of the JFET to unmute the power amp.

Isolate the +/-15V supplies from their loads and get them healthy.  If the voltage across D19 remains at 5-6 volts with all load disconnected you need a new 15V zener too, but be sure to check the two 150 ohm 5 watt resistors that bring the +/- rails in for correct value as they may have had a good cook.

{I'm curious about what has happened here because it appears that both +ve and -ve supplies have failed, and it is not at all normal for two supplies to fail like this without a common cause.}
If you say theory and practice don't agree you haven't applied enough theory.

Capt_Dunzell

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Re: fender stage lead 212 troubleshooting
« Reply #31 on: November 11, 2012, 04:16:21 PM »
I replaced both of the 150ohm 1 watt resistors but they did test okay.
I also replaced R-84.
I dont have any zeners to use to replace the ones on the board, I will need to pick some up, however they do test good.
Probably a dumb question but how do I remove the load from the + and - 15 supply?

Roly

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Re: fender stage lead 212 troubleshooting
« Reply #32 on: November 11, 2012, 09:59:19 PM »
I was hoping that there might be a cable/connector you could just pull, but if not then you will have to lift one end of each of the 10 ohm resistors, R83 and R84, and check that you are getting the correct 15 volts across each zener.

If so the zeners are good and there is excessive load somewhere down the line; if not one (or both) of the zener voltage is off meaning you need a new 15V zener(s) of the same or larger wattage.  So check your zener voltages with the two 10 ohm resistors open before buying new ones.
If you say theory and practice don't agree you haven't applied enough theory.

Capt_Dunzell

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Re: fender stage lead 212 troubleshooting
« Reply #33 on: November 11, 2012, 10:58:46 PM »
Ugh,

After replacing those parts I have:

R61 -1.5
R64 41.3

d19 -.5
d21 -.5

R85 +19
R86 -19

The 680 mfd caps +16 on one side, -16 on the other.

R84 heats up and cant be touched.n I followed the traces on the board out and can visibly see an issue.

I am going to pick up replacement zeners. This is really confusing to me.

Capt_Dunzell

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Re: fender stage lead 212 troubleshooting
« Reply #34 on: November 11, 2012, 11:11:31 PM »
I meant I cant visibly see an issue, apparently I cannot spell tonight.

I have ordered the zeners.

Roly

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Re: fender stage lead 212 troubleshooting
« Reply #35 on: November 12, 2012, 10:41:26 AM »
V @ R61 looks like we still don't have a -15V rail happening.

Can we forget about R64 please; it's on the +55V main supply rail which has nothing to do with the +/-15V supplies.  The "+15V" annotation applies ONLY to the arrow to the right, not the rail to the left.

No voltage (0.5V) across either zener D19 or D21.

The 680uF caps are on the +24V supply side.

R84 getting hot is very significant.  It's only 10 ohms so a bucketload of current must be going down there somewhere.  Are you certain you mean R84 10 ohms and not R86 150 ohms getting too hot to touch?

AGAIN - lift one end of R83 and R84 (both 10 ohms) to isolate the supplies from their load and see if that restores your +/-15V zener voltages (D19 & D21).  Fitting new zeners won't help if the lack of voltage (and obviously excessive current) is due to an external short - in fact they may still be healthy.  Lift the resistors, measure the voltage across each zener and post.
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

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Re: fender stage lead 212 troubleshooting
« Reply #36 on: November 12, 2012, 06:34:43 PM »
Capt, with a hand meter you can "check" diodes on the diode test function.   That tests them as simple diodes and at about 1 volt.   What your meter will not do is test a zener for proper zener voltage.  A 15v zener that has failed down to 5 volts will still act like a diode on testing.  But that is only part of the test.

Capt_Dunzell

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Re: fender stage lead 212 troubleshooting
« Reply #37 on: November 12, 2012, 09:19:15 PM »
Maybe its time I throw the towel in on this one.

I pulled on end of R83 and R84, no change on the zeners, I actually could not get a stable reading without them, the reading bounced all over.

Is there a way that I can test them since it sounds like the diode check on my meter wont do this properly?

I have ordered replacements but wont have them for a few more days.

I miss posted the hot resistor, it is R85 and R86 that are getting very hot.


Enzo

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Re: fender stage lead 212 troubleshooting
« Reply #38 on: November 13, 2012, 12:19:08 AM »
A zener has to check as a regular diode first, so it conducts only in one direction and is not shorted.  but it also must break down in reverse at its zener voltage.  The only way to test that part is to apply voltage to it.  Your circuit does that.

If you do not have 15v zeners on hand, and canot find them locally, one trick might be to move the good one to the place of the ??? one.

But really, 15v zeners are about as common a part as there is, so if you have about any local electronic repair shops - guitar amp repair, TV repair, stereo repair, CB radio repair, and I bet some others,  They are bound to have them and ought to be willing to sell you a couple.   I sell parts to anyone who walks in and needs something.  I buy 15v zeners 100 at a time.

Roly

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Re: fender stage lead 212 troubleshooting
« Reply #39 on: November 13, 2012, 08:26:59 AM »
{You have the power amp working in good health, and now you're going to throw the towel in?  Over the preamp zener supply?}


With the outputs isolated, i.e. R83 and R84 open, and infeed resistors R85 and R86 getting hot it's a fair bet that your zeners are faulty; however you previously reported 0.5V across the zeners, now you say "I pulled one end of R83 and R84, no change on the zeners, I actually could not get a stable reading without them, the reading bounced all over."  I find this "no change" + "bounced all over" confusing and unexpected.  Typically when a zener goes down it stays down, and this sounds more like a multimeter problem, dirty contact in the range switch or something.

In any case opening the outputs did not result in a nice stable +/-15V, and the hot resistors suggest the zeners are shorted (and this is one way of testing them, using the circuit they are in).  Again. it's a bit odd for both of them to go down at the same time, so I can't help wondering what external thing may have caused that.

Howsoever, fit new zeners, leaving the outputs open, and establish that you have steady +/-15V supplies, then we move on from there.

{Just BTW, what country are you in?  I'm in Australia (and wondering about the time difference)}
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

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Re: fender stage lead 212 troubleshooting
« Reply #40 on: November 13, 2012, 01:51:24 PM »
Looks like he's on GMT-5 so East Canada or USA, no further West than Indiana.
2 hours from here, almost all around the World from you ;)


Capt_Dunzell

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Re: fender stage lead 212 troubleshooting
« Reply #41 on: November 13, 2012, 08:39:26 PM »
I am in Vancouver, Washington.

I dont have any local suppliers, Radio Shack is the only one in town and they dont have much to speak of anymore. They did have some 12v zeners. The 15v Zeners should be here tomorrow or Thursday.

Probably shouldnt have but I popped the 12v zeners in because I couldnt stand not knowing. With one end of R83 and R84 pulled I got +12v and -12v on the Zeners, R85 and R86 did not heat up. I resinstalled the ends of R83 and R84 and +12v and -12v goes away on the Zeners and R85 and R86 heat back up.

Pulling the 12v zeners and will put the 15v in as soon as I get them.


J M Fahey

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Re: fender stage lead 212 troubleshooting
« Reply #42 on: November 13, 2012, 11:08:22 PM »
Don't worry, what you did is fine, you restored functionality to that part of the circuit.
So it looks like there was some Zener problem after all.
What does "goes away" mean exactly?
0V? Some other value ?
Whenever possible try to use numbers to describe things. :)
Thanks.

Roly

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Re: fender stage lead 212 troubleshooting
« Reply #43 on: November 14, 2012, 05:52:59 AM »
Now that's looking good, but I would just like to clarify something; did you check the old zener voltages with R83 and R84 ends lifted?  Can you confirm that you were getting little or no voltage across the old zeners under those conditions?

When you say the voltage across the zeners "goes away" when you restore R83 and R84 suggests that you have something like an op-amp shorting across the rails somewhere downstream in the preamp.


In trying to make sense of a fault we always try and understand the situation that led to the failure.  Why I'm interested in the state of the old zeners is because having multiple faults, two dead zeners, and now what looks like a shorted component somewhere in the preamp, suggests that something pretty gross has happened/been done to this amp.  This does happen, but it's pretty rare - what we almost always find is that a single component failure is responsible (even 'tho other components may have been damaged as a result).

In this case the single failure we are now looking at, an op-amp or something shorting the +15V and -15V together, could explain everything so far, but not the zeners being shorted; that's a worry.  So your answers to the question above is important because they could mean the difference between hunting a simple fault, and something a bit more complicated.

Therefore it could be worth backtracking a bit, reinstalling the old zeners, lifting the R83 and R84 ends, and checking if you get +/-15V, or something else, just to clarify the situation.

I see on the circuit that there is a connector P2 which carries the +/-15V to part of the preamp.  You should locate that and  disconnect it at this stage.

Once we get +/-15V confirmed at the power supply, what we will then be doing is trying to bring the preamp up bit by bit (as far as that is possible) until we find the part that is shorting the supplies.  Are the IC's on the preamp in sockets, or soldered in?
If you say theory and practice don't agree you haven't applied enough theory.

Capt_Dunzell

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Re: fender stage lead 212 troubleshooting
« Reply #44 on: November 14, 2012, 08:11:38 PM »
The IC's are soldered in, I have sockets I can install if needed.

reinstalled the old Zeners.

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

R83 and R84 connected
Zeners are +3.2 and -.5
R85 and R86 get hot