Fender Stage 112 se trouble

[mikeaaron_1999]

This is a Fender Stage 112 se (1992)

Ok, so a buddy of mine asks if I can take a look at his amp because it's making a buzzing noise. I bring it in, plug in my guitar, it sounds ok. I crank up the volume to about 3 and it sounds good but low and high notes give a crackle and pop when played. After about five minutes of playing with tones and volume and striking notes on the guitar, it starts to make a rythemic pop, pop, pop, and it didn't change when I disconnected the guitar or turned all pots to "0". So I pulled it appart to take a look. Someone has been in here before. The first obvious things I found were CR28 had been replaced at one time and also Q2. However, when they were replaced, the master solderer used too much heat, burned the board and thus, removed the pads from the solder runs. So he/she brilliantly soldered a "jumper" between one leg on the top of the board @ CR28 to one leg on the bottom of the board @ Q2. I was glad to see the jumper was at least insulated. Because this was something goofy, I checked CR28, found it was "OL" one direction with continuity the other as a diode should be. No problem. So I cleaned up the solder on both sides of the board associated with both components, re routed the "jumper" to be completly on the bottom of the board, put it all back together again and turned it on. Instantly, I got that pop, pop, pop, pop. Nothing plugged into it, all controls at "0". I manipulated the controls, plugged an instrument into it, and nothing changed. The pop, pop, pop continued. I unplugged the instrument, turned it off, pulled it all appart again and just the control portion sitting on the bench plugged it back in, turned it on. The pop, pop, pop, was still there, so I went for my DVM but before I could probe any test points the fuse blew and the amp went dead. Sooooo, now what???? Any help would be greatly appreciated! I have also looked at all the solder joints to all the other components on the board. they are all attached and soldered, not cracked or broken. Also, replacing the fuse does no good. I've blown three now and can not check power levels at IC's without the board being powered. So I'm stuck.

[52 Bill]

Let me see if I get this right. You pulled the chassis out on your bench or did you pull the pc board out of the chassis for testing?

Blown fuses on these amps usually mean a shorted output transistor. Check the TIP147 and TIP142 outputs for shorts. If Q2 has been replaced, I'd guess that one or more of the TIP147s have been changed as well.

Until you can get the amp up and running again you really can't deal with the original popping problem. Go you have a light bulb limiter?

[mikeaaron_1999]

I pulled the pc board out of the chassis. When it is installed in the chassis or if it is out, it is blowing fuses. I will check the TIP 147 and 142 for shorts. The only components I can tell are not original are CR28 and Q2 as they look like newer components and the obvious over heating on the board around each component. I do not have a light bulb limiter. Heard someone mention it a time or two and I can probably search the net for a quick build. At any rate, I'll check the possible shorts and go from there. Thanks 52 Bill!

[52 Bill]

If you ran the amp with the outputs not bolted down to the heatsink and chassis, then definitely check them for shorts. Some of these amps will cook the output transistors even at idle unless they are bolted down. I've had it happen once, you learn that lesson really quick. Fender had issued a tech note warning about this.

The light bulb limiter just puts an incandescent light bulb in series with the ac line of the amp. If the amp is drawing too much current, the bulb lights up and limits the current to the amp, saving the fuse and often a bunch of parts. It will allow you to take voltage readings even with a shorted part in the amp to help find out where the problem is.

[mikeaaron_1999]

Ok, it appears that Q3 has been replaced at some time because the data on the package is different than all the others. So here are the readings I had when checking them. I checked the legs from left to right one at a time to ground. Q3 Left leg: 1.435k Ohms Center: 0.7 Right: 0.6. Q4 Same as Q3. Q5 Same as Q3 and Q4. Q6 Left: 2.058k Center: 0.7 Right: 4.39. Q7 Left: 2.057 Center: 0.7 Right 0.7. Q8 Left: 5.0 (seemed to keep rising but settled around 5.0k Ohms) Center: 0.7 Right: 4.8 (and seemed to rise also) So it definatly looks like I'll be picking up some 142's and 147's to start off with. Thanks again for the help! I'll get me a limiter also before I dive back into this thing. I always kept the heatsink connected so not to burn them up. And talk about excessive heatsink compound on them. Wow!

[52 Bill]

To test the transistors use either a diode test setting or ohms. The leads on the transistors are from left to right: base-collector-emitter. Put one lead on the base and then read the value to the other two legs. Then reverse your leads and test again. Just like a diode, there will be a low reading in one direction and a high or no reading in the other direction.

Finally test from the collector to the emitter. This is where you will find most shorts. These are Darlington transistors, so you should get a high/low reading here. Normal bipolar transistors will usually test high in either direction here.

[mikeaaron_1999]

Yes, did all that as well. Looks like three of the 142's are shorted and one of the 147's are shorted. So Q3, Q4, and Q5 bad, Q8 good, Q7 bad, and Q6 good. I'll replace them as soon as I can and go from there. Thanks again! This has been a great help! Hopefully this will restore my fuse blowing problem so I can proceed to the popping and get this amp back to my buddy!

[mikeaaron_1999]

Ok after taking all 142's and 147's out and checking them individually, only one of each were shorted. So I replaced them, put it all back together and now I'm not blowing fuses but I have that pop again and when an instrument is connected, the speaker sounds blown. I tried the speaker with a good amp and it's crystal clear. So I downloaded the schematics and on Monday I will troubleshoot the test points for specified voltages and values. Thanks again for all the help. As always, any help and all tips are appreciated.

[mikeaaron_1999]

Ok, it's been a while, but I've done some more work to this thing. Now I'm in it about $60.00 and wondering if I should go any further. I replaced the blown Darlingtons, I found one of four diodes (CR 13,14,15,16) broken and one shorted out completely. I replaced all four of them. Took wire wound resistors (R 106,107,108,109,110,111) off of pc board, took resistance readings, they were all out of tolerance. Replaced all six. Inserted a new fuse into it, installed the board to the heat sinc and bolted everything down, crossed my fingers, plugged in my guitar, turned it on and it sounded wonderful! No pop/crackle, speaker sounded brand new, I played through it for approximately ten minuets and then it went dead. The fuse had blown again. So I checked the darlingtons... Q5 and Q7 were shorted, AGAIN. Pulled them out, checked all the other resistors and diodes in the circuit, everything else checks good. So my next question is....WHY ARE MY DARLINGTONS FRYING? I had a suspicion that possibly U6 or U7 is bad and causing the transistors to fry. So I purchased six burr brown opa2134's to replace the TL072's and the 4560's. Do I replace them, the fried darlingtons and try again? Or do I throw in the towel, give it back to my buddy eat the $60.00 and wish him good luck? PLEASE HELP!!!!!

[Enzo]

First while I doubt your op amp IC is bad, sure, why not replace it and be sure. However, when your amp is not working and keeps blowing up, why oh why are you stuffing OPA2134s in it instead of what the schematic calls for? The time to modify an amp is AFTER you have it running, not while trying to fix it. This is a good way to introduce new problems while looking for the old.

I find it unlikely your wirewound resistors - R106 et al - are off value. The schematic calls for 0.47 ohm. Most hand meters, DMMs, do not measure low resistance very well, the meter leads and probes often add some fraction of an ohm at the very least. If they are damaged and check at 100 ohms instead of 0.47 or something like that, then certainly replace them. And if Fender stuffed 0.33 ohm instead of 0.47, stay with it. For the most part, I consider those cement resistors to either be open or OK.

You need a metered outlet to plug the amp into, or some way to monitor how much current the amp is drawing. Or set up your ammeter in place of the fuse or otherwise in series. I see a 4A fuse. Your amp may work and sound OK at first, but without monitoring it, you don;t know if the amp is drawing 1A idling along or if it is sitting there drawing 3.99A just waiting to pop.

Look up "light bulb limiter," and make one and use it.

This is where I always use a variac and ammeter. But a bulb thing will do.

Next time, disconnect the speaker load and run the amp without, see if it runs a whole hour, say. And check the speaker wires, at idle, is there any DC offset there?

You replaced diodes. The bias diodes? CR13-16? If so, did you use BYV26D like it calls for?

Q1,Q2 are limiters. Near them are a whole pile of diodes, CR18,19,etc. Check them all. I won't worry about the zeners zening, but check them as diodes.

Note your bias diodes run off +/-16VDC, through a couple 3.3k resistors. Are both those supplies up to snuff?

And we need to scope what this amp is doing. Scope across the speaker leads, scope the output of U7a, scope the TIP bases. We are looking for unheard RF oscillation that can have outputs cooking while you never know.

WHen you are playing, is the heatsink getting HOT? Does it get hot just sitting idle?

[mikeaaron_1999]

Ok, I will hold off to install the 2134's. Of the six cement resistors, two showed 0.0 resistance one showed complete open and the other three showed somewhere in the vicinity of 1.01k ohms to 0.97ohms so I just replaced all six with exact part numbers that were on the old ones. The replacements also reflected the values from the schematic exactly. 0.47ohms 5W 10%. I have a variac I will use, it is a 0-140VAC 5A .7KVA. I will also use the ammeter in place of the fuse. I will have to purchase a new TIP147 and if the supplier does not have any on hand, I'll have to wait for them to come in. It may be a week or two when they come in so I will not be able to apply power until next week some time or the week after at the latest. Yes, I did replace the bias diodes because as I stated, one was broken completly in half at the glass body part of the packaging and one was completly shorted both ways. I could not get BYV26D anywhere, but I did purchase NTE-575's which are identical in every way except the package. After this last time of blown darlingtons, I pulled the bias diodes out to check them and they were 100%. All diodes near Q1 and Q2 (CR18, 19 etc) all check out good. 0.5 one way, OL the other except for the zeners, they read 0.7 one way and OL the other. So they are all good. I also checked the 3.3k resistors, they are all reading 3.3k as advertised. So, until I get the new TIP147 in, I will have to wait to apply power and use the scope to check for DC at the speaker wires and U7a and TIP bases. When I do get the 147 in, and ready to apply power, how should I set the variac up? Start low and go up? or is there a particular voltage I should keep it at? Before everything went bad this last time, while setting idle, the heat sinc bolted to chasis, chasis out of cab, power on, the amp was 100% quiet. no feed back, no hum, no click or poping. Heat sinc was warm to the touch, not hot or uncomfortable. That is why I decided after about twenty minutes of no problems to plug the instrument in. I plugged just a plain jain fender strat into it, with all controls on guitar at mid, P.U. switch for neck P.U. and amp controls as follows: (normal channel, mid shift off) vol- 1, treb- 3, mid-5, bass-7, (drive channel, select off) gain- 1, contour- 5, treb- 5, bass- 5, reverb- 0. Like I said before, I plugged instrument into input #1 no foot switch, nothing else, and played this way for about ten minutes. It sounded good, no problems and without any warning, the amp went dead. I disconnected the power cord immediatly, disconnected the guitar and the very next thing I did was touch the heat sinc. It was again, warm to the touch, not hot and not uncomfortable at all, just warm. I felt all along the sinc from left to right and even felt the TIP's, they were all warm and not hot. That is what threw me the most because last time there were blue flames shooting out of two of the TIP's (not the same ones this time) and smoke like you wouldn't believe. The sinc was very hot then and could not be touched for at least five minutes. At any rate, this time, the sinc was just warm. So I looked at the fuse, saw it was blown and then started to check the TIP's with the meter to find Q5 and Q7 shorted collector to emmiter again. That is when I posted on here the last time. So again, thank you for your help I really appreciate it. I'll get the 147, install it, and go from there. In the mean time, anything else you want me to check before I put power to it?

[Enzo]

Here is the thing with the variac,it isn't about the voltage, nio particular voltage is targeted. Well other than the 120v for normal operation.

If something is shorted, it blows teh fuse, which means excess current. Now I plug the unit under repair into it and cautiously advance voltage from zero. I watch the ammeter. A little rise is normal as the caps charge. But if I see the current starting to ramp up while the voltage is still low, then I turn off. No point in applying any higher voltage if it wants to draw a couple amp at say 15 volts.

BYV26D is in stock at Mouser - stock number 625-BYV26DGP-E3/73 22 cents each. 800v 1A fast recovery

OK on the cement resistors then, open or way out of whack is bad. I just don't worry when my 0.33 measures 0.5.

You can remove ALL the TIP from the circuit and power up to test the low voltage parts if you like.


Test signal doesn't matter a lot. I do always test amps with a guitar, but while working on them, I just use a line out from the little stereo receiver on my shelf. The music is a full range thing, so it is easy to hear if something isn't right. And that way I have no guitar tying up my hands or hanging from my neck while I work.


And some advice I like to share, hopefully I haven't already said it to you. Don't assume that problems are always bad parts. The connections between the parts are equally important. For example, I have been concerned over your bias diodes. If one opens, then your TIPs will ALL want to turn on full at the same time. Even if all those diodes are good, a cracked copper trace connecting them together can be just as damaging as an open diode. Not saying that is what is now wrong, but a burnt out, cracked or damaged copper trace can cause havoc.

[mikeaaron_1999]

Ok, You are a tremendous help Enzo! I sure appreciate your time with this. I normally work on black hawk helicopters and not much electronic, more electric generating and avionic stuff. So a lot of this type of testing and so forth is new to me. I mean I understand circuits and components and such, but theory on this sort of stuff is all new! So seriously, thanks a ton! So, from here, I will order the correct bias diodes from mouser, try and get a 147 in as quick as I can, in the meantime though, I will leave the TIP's out and check the low voltage stuff. You have not mentioned you advice about the copper traces, that is a very good piece of knowledge to have. With that said, there has been someone in the past poking and prodding around in there before and there are some pads that were "burnt off" and now missing. Who ever did this in the past soldered jumper wires to complete the circuit where it had been destroyed. And I will have to post tomorrow which exact ones, but I believe there was one from CR28 to either Q1 or Q2 repaired this way and another place where a jumper wire was inserted due to the solder joint "pad" missing. I will inspect the diode traces and see how they look. While I've taken these out and replaced them and taken them out and installed them again, I didn't notice any damage there, but I'll inspect further. So, with the other places that were damaged and repaired with jumper wires, is there a way to restore the copper traces and make them 100%? And if I find one on the diodes that is cracked or damaged, can I repair that or is it a lost cause? Like I said before, I haven't really done much of this circuit board stuff but I enjoy it and would love to progress in this area. Any advice if I do find one damaged or cracked? Again, thank you so much for your help thus far! It really is a life saver. I want to give this guy back his amp in working condition, but I keep running into these problems I don't fully understand and it's frustrating. I'm sure you know about this! I kind of feel bad for him too because it was a used amp he bought for $100.00 thinking it was just old but worked fine. Then all these problems arise in the mean time and I don't want to stick him with another $100.00 or more in repair parts either. I'm not charging any labor but never the less, at $6.00 per Darlington, times nine now, and the cement resistors at $3.99/2, and $10.00 for the NTE 575's. etc...etc.... I'm just afraid to make it not worth it and stick him for the dough. So far, I've paid for everything out of pocket and the wife isn't too happy to begin with. But any way, enough about that. I should have came here before I even opened this thing up. Only after the first death of this thing did I realize I needed all the help I could get on it! So one more time, thank you so much. I'll keep you posted on findings and when I fire it up again. Take care man!

[Enzo]

When copper pads are destroyed or damaged, then adding wires to complete the circuit works fine. One must be careful to not overlook some branch off a trace. A trace might go from A to B, but a branch takes off somewhere along it to point C. If it is gone, then a wire from A to B completes that part, but C is left hanging. Usually on single sided boards this is easier to spot. On two sided boards, you might see a trace A to B on one side, and miss teh trace continuing from B to C on the other side.

If this is just one rpair you are doing, then a few extra dollars won't much matter. But if you plan to do ongoing repair work, you need to learn where to shop for parts. Local shops may be convenient, but they tend to be like shopping for groceries at the 7-11 store, expensive. NTE stuff always costs a lot more than the parts they replace. COmpare the 22 cent diodes from Mouser to what you had to pay. Granted there is no shipping cost at local stores, so if you order one transistor from Mouser, you can expect $7 shipping addded on. So it pays to make up a list of parts needed.

For example, Mouser stock
5W cement resistors - 49 cents each.
TIP147 - $1.65-$2.08 each

Mouser is usually the first place I look, but many guys like Digikey, so I sometimes use them, and sometimes Allied Electronics. And of course there are other suppliers as well.

[mikeaaron_1999]

Back again, for a moment. With out the TIP 142's and TIP 147's installed, I tested all the DC test points from the schematics I downloaded at blueguitar dot org. For test points 17 through 25, it is only checking DC. tp 17 to tp 18 were good. tp 19 is CR50 @ C41 supposed to have -15VDC: I got 0.2VDC. tp 20 and tp 21 were good. tp 22 is R113 supposed to have +45VDC with respect to C.T. I had +47.8VDC. tp 23 is R113 supposed to have -45VDC with respect to C.T. I had -42.8VDC. tp 24 is CR47 @ C61 supposed to have +0.6VDC Clean channel and +0.6VDC Drive channel. I had +0.02VDC clean and +0.05VDC drive. And finally, tp 25 is U6B pin 6 supposed to have -0.6VDC reverb off and -0.6VDC reverb on. I had 0.0VDC reverb off and -1.11VDC reverb on. Now for the AC test points, I did go through them with the TIP's out, but there were so many that didn't match what was supposed to be there and some that did so I just figured it was because the TIP's were not in there. I will hopefully have the TIP 147 I need to install and I can once again power up with the variac and test the AC stuff. But based on what I've found on the DC stuff, is there anything that would be causing what I've found? Any components to swap out or check? Thanks again for any help! I appreciate it!