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Fender Stage Lead 2 grounding out

Started by DrewV, December 12, 2013, 08:25:15 PM

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DrewV

Hi All, So, I'm trying to bring this amp back to life. Initially it blew both fuses between the transformer and the rectifier diodes. After replacing the fuses and bringing the voltage up slowly (variac + lightbulb limiter) I was able to get some sound from a line in source. However, when I tried to increase the V the amp started sucking current like there was no tomorrow. All transistors read good on a transistor checker.After some checking I found that R78 was burning up.   I can't figure out what is causing R78 to burn. I replaced a number of carbon comp resistors, the worst out of spec was R78, which was going open at 200 ohms. Every single diode reads good. Lifting the cathode leg of D17 eliminates the short. The electrolytes all check out good. I put a 9vdc charge on the filter caps which came back at about 7.5vdc.
I was going to throw in the towel on this amp, but it just bugs me that I can't figure it out. All insights appreciated. thanks Drewv
Attached is all the info on this amp. A few of the values and/or part numbers are different than what I'm looking at, but nothing major.

Enzo

If R78 is burning up, then chances are real good that Q12 is bad.

If lifting D17 stops the short, I cannot imagine why lifting D18 wouldn;t do the same.   I'd check those four diodes again closely.  Also make sure both halves of the transformer winding that serves them are working.

DrewV

Testing Q12 in circuit it reads good. D17 was the only one I lifted a leg on. Early on I checked both windings coming off the transformer and got a good reading of + 37 / -37v. Tomorrow I'll pull Q12 to test out of circuit as wel as the other three rec diodes. Thanks Enzo I'll get back to you with my results manyana. Drew

Roly

Quote from: DrewVAll transistors read good on a transistor checker.After some checking I found that R78 was burning up.

One of these observations has to be wrong, and I find smoke pretty convincing, so you have at least one dead transistor the transistor checker didn't show up.

See "Testing Semiconductors" on this page.


Disconnect the speaker, or remove fuse F201 to isolate it.

My personal hit pick would be Q201 gone short Collector to Base with the Emitter open.  It is fairly rare for a driver (Q12) to be damaged without one of the main output transistors - not impossible, just less common.  So both Q201 and Q12 have to come out and be tested out of circuit.  Whenever I find a dead output or driver transistor I also pull their partners, in this case Q202 and Q11(?) to be sure that they aren't damaged as well (because they often are).

As Enzo said, I would also carefully check each of the diodes in the bridge rectifier, D15-18.

I'm also one of those people who use a limiting lamp rather than a Variac (or with a Variac) because a Variac alone can still pump a damaging amount of power into the amp.

HTH

p.s. you can't reliably test these transistors in circuit, that is likely where you are being misled.
If you say theory and practice don't agree you haven't applied enough theory.

DrGonz78

Studio lead ii schematic but it's a Stage Lead ii? I guess they must be pretty much the same but here is the schematic anyway.
"A person who never made a mistake never tried anything new." -Albert Einstein

Enzo

Those are different, but then in yours, R78 would be hard pressed to burn up.


I wouldn;t suspect ONLY Q12 bad, but it would be the likely direct current path for the resistor fire.


Be very careful testing transistors in circuit.  For example Q12 and Q203 are in parallel, so you have to pay attention to your meter and which junction it is really testing.   Same with resistors, while in circuit there is often parallel paths that obscure the true value.


The variac must be used with a current meter.  When one sees current START to rise, one turns it back off.

DrewV

OK, the stage and studio amps are basically the same. There are id numbers on the main board which match those on the schematic.
Note that if all multipins are disconnected from the main  circuit board, current is still finding a way to ground.
I removed D's 15 16 17 18 from the circuit. All tested good on a dvm diode setting. I remove Q12 fron the circuit and it tested good on both the dvm and trans checker .
The output transistors Q''s 201 202 203 are chassis mounted. Q201 is a TIP29B.  It checked out good on both my dvm diode setting and my trusty transistor checker. ( I bought it at Radio Shack in 1983 ) I get a reading of 5.1Mohms between the B -C, but only with the positive probe on the base. Reversing the probes gives me a reading of OL.
Q202 & Q203 ( MJ15003 ) likewise tested good w/both meters. All three OT's connect to the board via multipin connectors, so removing from circuit was easy!.
  A few days ago when I still had noise coming out of the speaker I noticed that touching the base of Q8 with a meter probe could make the sound come and go with a pop like noise. It's an RCA 1C04 pnp. A few minutes ago I retested it in the circuit with both meters. The transistor checker says its good. The diode check with the dvm says about .500v both ways between the emitter and the base. Can the circuit be causing this reading? B-C reads like a good diode.

Enzo

Let's use plug numbers.  When you say you removed ALL multipin connectors, that sounds to me like it includes the transformer wires.  SO just what is connected and not connected for your tests?

APparently the output transistors are mounted off the board, on a heatsink, yes?   Are the insulating mica or silicone wafers under the transistors?  So are the bodies of the transistors insulated from chassis?

"Current is still finding a way to ground."  Well maybe.  If your bias circuit were to open - Q201 gets disconnected - then both sides of the output would turn on at once.   Massive current would flow from +37v to -37v, but not to ground.   So what exactly does find a way to ground mean to you?


It is appealing to look for bad parts, but the circuit is parts connected together, and a broken or missing connection is just as deadly to an amp as a shorted transistor.


I would expect probing Q8 would make some noise.

Roly

Quote from: DrewVI get a reading of 5.1Mohms between the B -C, but only with the positive probe on the base. Reversing the probes gives me a reading of OL.

You should use the diode test range and it should look like this;



About 500mV forward (+ve on Base) and open circuit in reverse.  C-E should be open both ways.

Quote from: DrewVThe diode check with the dvm says about .500v both ways between the emitter and the base. Can the circuit be causing this reading? B-C reads like a good diode.

If this is in-circuit I can only repeat what I said earlier

Quote from: Rolyyou can't reliably test these transistors in circuit, that is likely where you are being misled.

It's a pain, but you have to disconnect at least two of the three legs to get reliable readings.
If you say theory and practice don't agree you haven't applied enough theory.

DrewV

Yeah, I thought as much. I'll melt some solder and get back. Thanks Roly

DrewV

 Q8 tests good out of the circuit.  Where to  next?  dv

DrewV

Ok Enzo here's what I've got.
"Let's use plug numbers.  When you say you removed ALL multipin connectors, that sounds to me like it includes the transformer wires." That would be everything that can be unplugged from the board except P8

"SO just what is connected and not connected for your tests?"
I tested with everything connected. Then I tested with the output transistors disconnected. Then I tested with everything except the transformer disconnected.  At one point I tested the red leads from the transformer.



"APparently the output transistors are mounted off the board, on a heatsink, yes?"  Yes, with Q's201/202/203 connected to the board  (via P5 and P6), I get Mohm readings on all legs of all three transistors. ( to the chassis) With the transistors disconnected from the board all reading are OL. Q201 (TIP29B) gives a 0ohm reading from the mounting base heatsink to the chassis but OL on all three pins to chassis ( collector not in contact w/ mounting base).
"Are the insulating mica or silicone wafers under the transistors?" Yes. 
"So are the bodies of the transistors insulated from chassis?" Yes

"Current is still finding a way to ground.   So what exactly does find a way to ground mean to you?"  I'm going from points on the circuit to the chassis.

"It is appealing to look for bad parts, but the circuit is parts connected together, and a broken or missing connection is just as deadly to an amp as a shorted transistor." I've inspected the board ,touching up suspicious looking solder joints. Can't see or smell anything else ( besides R 78) burning. C45 was OL so I replaced it.
Raising the line to about 2 VAC the LBL begins to glow brightly.
Thanks Drew

DrewV

I'm at a dead end with this amp. I hate to throw in the towel, bbut as Kenny R says, " you got to know when to hold 'em, know when to fold 'em".  Thanks  guys for the excellent info, I just don't have the right tools for this baby. Hope your new year is "Bitchen"!! DrewV

DrewV