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Fender Stage 112 se blowing fuses and transistors.

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  • #16
    I've been trying measuring voltages across the C-E positions of the amp without the transistors in, I'm getting all sorts of voltages depending on how long the amp's been on for. Are the filter caps a common source of issue? The fact than I can always play the amp for a while before it pops suggests to me that charge is building up somewhere. Could the caps be dodgy even if the test OK with a DMM capacitance test?

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    • #17
      Originally posted by gilmo789 View Post
      I've been trying measuring voltages across the C-E positions of the amp without the transistors in, I'm getting all sorts of voltages depending on how long the amp's been on for. Are the filter caps a common source of issue? The fact than I can always play the amp for a while before it pops suggests to me that charge is building up somewhere. Could the caps be dodgy even if the test OK with a DMM capacitance test?
      With the repeated failures of the output xstrs, and the fact that the audio output signal from this power amp circuit is passing thru the two power supply caps C52/C53 via C.T. to the speaker terminal, and the return current passing thru the five 0.22 ohm/1W resistors in parallel to ground, replacing the two filter caps could be the solution. The DMM capacitance test is done at very low current, not revealing what's happening under high current. Are all five resistors R83 thru R87 good? Measuring them, as the nominal value would be 44 milliohms, you need to be able to resolve this low reading, so nulling out the meter's test lead resistance is required. Lifting each from the circuit might be needed to verify them, if your meter doesn't have the resolution...but at least the reading you do get, if each produces the same, then it's a good indicator all are ok. I've never been a big fan of this type of amplifier circuit, as passing high audio current thru electrolytic caps isn't anywhere near the linearity of wire as we have in conventional circuits, current flowing thru the output bus to the speaker (and thru the RL circuit portion of the Zobel network).
      Logic is an organized way of going wrong with confidence

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      • #18
        Hi,
        thanks for the response. Quickly testing R83-7 in the circuit shows them to each be 0.3Ω on my DMM, so I'm gona guess they're good.
        Re Filter caps: with the LBL on for about an hour, with no transistors in the circuit, the caps build up to about 43V, which drops off steadily, when the amp is turned on.

        According to other random postings on the internet, R82, in parallel with the inductor at the output, can cause problems. I noticed that the solder was a bit cracked here. Could that be responsible for blowing the transistors?

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        • #19
          I wouldn't think so regarding R82. That part sets a limit in the HF reactance of L1, while in the audio range, the large wire forming the 2.5uH coil is essentially a short across the resistor. I would desolder and resolder the resistor...and maybe remove, scrape the oxidation off the resistor leads first so good solder joints are made.

          Did you mean to say 'which drops off steadily when the amp is turned off'? With the power xstrs out of circuit, we're just seeing the quiescent condition of the caps that charge up to their nominal +/- 43V potential (dependent upon AC mains voltage). Still thinking the supply caps could be the problem. I'd have to go back and re-read all the posts...were your power xstr failures initially always being Q5 & Q6? And, with those two parts left out, no failures? If so, makes me wonder about the PCB traces, as well as the emitter resistors R108 & R111 being faulty.

          In looking at your out-of-circuit testing of the xstrs, Q7 was C-E short, while Q6 measured OL for the C-B junction. That's also not good, unless it was a reading fault.

          As had been suggested earlier, have you run the amp without Q5 & Q6 installed (or with their emitter and base resistors lifted to take them out of circuit)?
          Last edited by nevetslab; 06-26-2020, 10:29 PM.
          Logic is an organized way of going wrong with confidence

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          • #20
            Yes, always Q5 & 6, when they were out Q7 went. It's kind hard to troubleshoot because it basically works perfectly with the LBL.

            As had been suggested earlier, have you run the amp without Q5 & Q6 installed (or with their emitter and base resistors lifted to take them out of circuit)?
            I ran the amp without the associated emitter resistors. When I did I damaged Q7 (and 6 apparently).

            I think the traces are OK because when the xstrs blow the faults seem common, i.e. i need to remove them to work out which one failled

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            • #21
              Originally posted by gilmo789 View Post
              Yes, always Q5 & 6, when they were out Q7 went. It's kind hard to troubleshoot because it basically works perfectly with the LBL.



              I ran the amp without the associated emitter resistors. When I did I damaged Q7 (and 6 apparently).

              I think the traces are OK because when the xstrs blow the faults seem common, i.e. i need to remove them to work out which one failled
              I would have lifted both the base resistors as well as the emitter resistors to fully isolate them from the output circuit. I've had issues before leaving the base connected, while having open emitter resistor.
              Logic is an organized way of going wrong with confidence

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              • #22
                I'm running out of transistor... Is there anything to be gained by running the amp with just 1 pair. Q3 and 8?

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                • #23
                  Originally posted by gilmo789 View Post
                  I'm running out of transistor... Is there anything to be gained by running the amp with just 1 pair. Q3 and 8?
                  I will do that sometimes, when I'm trying to get a DC level issue sorted out. It's not sufficient to handle 4 or even 8 ohm levels at full output swing, but higher load impedance or open circuit, it will allow for that.

                  While you probably don't have replacement power supply filter caps, if you do have a square wave generator that can swing 20V P-P, you can measure ESR of the capacitors. I had presented a procedure based on an article I found on the internet a few years ago, and use it to evaluate suspect electrolytic caps. Let me see if I can find that thread I had posted and add that link.

                  Found the link: https://music-electronics-forum.com/...sr-quest/page2

                  This procedure begins at post #27
                  Logic is an organized way of going wrong with confidence

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                  • #24
                    I might be able to steal one from work. So I would test the 2 filter caps and if one has a significantly higher ESR than the other, that one is likely shot. Can I tell anything if they both measure the same?

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                    • #25
                      Put transistors back into 3 and 8 positions and ran amp through LBL, voltages seem OK. Made a video of some of the exciting noises.

                      the lamp is my LBL,
                      0.12: you hear a very loud pop, scared the s(*t out of me. Nothing seems damaged though
                      0.37: turn up volume on dirty chanel just a bit, low hum (this was actually because the reverb was up a bit while the leads were unplugged)
                      0.39: turn up gain and you hear what sounds like a grid hum, this happens even when the reverb is all the way down

                      Red light dirty
                      green light clean

                      I plugged in a guitar because the amp is muted with nothing plugged in, with no guitar on the other end of the leads, the gain effect is worse.



                      Even with only 2 transistors I'm getting good volume on the dirty channel but the clean channel is now very quiet, volume increases up to the '2' position and then basically levels off even if I turn the knob all the way up. I've read reports elsewhere of this in combination with popping fuses transistors.

                      No luck testing the filter caps yet. I don't think they would explain the quiet on clean / loud on dirty though

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                      • #26
                        When I first watched/listened to your YouTube post, I heard what you call the pop at 0.12 seconds in, but then re-watching that, I didn't hear that in the playback. Not sure what that would have been, but share your fright when things are disassembled and powered up. I go thru the same freakout when it happens to me too. Now, when the PCB is powered up like this, all of the front panel controls are no longer grounded to the chassis, so that can contribute to hum. Disconnected reverb, if cables are present and connected always contribute hum.

                        I gather that gooseneck lamp is your Light Bulb Limiter indicator.

                        Back to the testing of the capacitors. On Page 2 of the ESR Quest link, there's a link to Measuring Capacitance, which was the basis of my subsequent testing. It allows measuring both Capacitance and ESR with substantial squarewave current flowing thru the cap under test. In my subsequent threads posted, I displayed several caps with both higher-than-normal ESR, as well as some who were shorted, when I got to IN-Circuit testing. As you can see in my set-up photos, a scope is needed, along with the squarewave generator. Following theat Measuring Capacitance link in my post #27, it shows the basic set-up with the square wave generator providing 20V P-P across the capacitor under test. Once the cap is connected via test leads, you have to increase the scope's sensitivity so you can see the full p-p waveform, which if the leads are disconnected, you don't see the waveform without decreasing the scope's vertical sensitivity

                        You can measure both the capacitance with a fair degree of accuracy, as well as the ESR. I'd expect your 3300uF caps to have an ESR @ 1kHz to be around 0.2 ohms or less. If either were nearly shorted, you'd be blowing fuses all the time, so that's not the case. But, if instead, one or both are reading several ohms or more, then you'd want to replace them.

                        Granted, these caps may NOT be the problem, since it has appeared to be related mostly to Q5/Q6. When I'm looking at PCB foil patterns, I use a surgical headlight along with surgical loupes, as seen in my MEF Avitar. Point is, you need high magnification and bright light on the circuit traces. I'll also flex the PCB in the process to reveal fractures that might NOT be visible. I'm still more suspicious of the PCB than of the caps, but, as this has been elusive, if you have a scope and generator to perform the test, it's a valid procedure that comes in handy as the years come.
                        Logic is an organized way of going wrong with confidence

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                        • #27
                          Thanks for all your help so far,

                          Some more things I've noticed:
                          1: There appears to be a hot spot beside two of the diodes in the main power rectifier, Is this normal? thi is without LBL
                          The hottest components seem to be CR41 and 42, and R114 and 115, part of the 16V supply. getting hot to touch.

                          Click image for larger version  Name:	50052636287_493a372c85_w.jpg Views:	0 Size:	32.0 KB ID:	908393Click image for larger version  Name:	50052637727_8ffa50b8a5_w.jpg Views:	0 Size:	48.7 KB ID:	908394Click image for larger version  Name:	50051812958_d0d7597588_w.jpg Views:	0 Size:	40.3 KB ID:	908395


                          2: On the clean channel, I had to replace the bass pot because the shaft was broken. I found one that fit and also had a D shaft. Its the correct value, 15A50k, and it measures correctly. However it seems to be functioning like another volume knob. This is the source of the quiet clean channel btw. If I turn both the bass and volume up I can get good volume. Don't want to push it though.
                          Would this happen if the terminals were arranged differently?
                          Last edited by gilmo789; 06-28-2020, 11:24 AM.

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                          • #28
                            The hot spots around the 5W zeners and resistors are normal. They run hot, that's why they are higher power rating components.
                            Not sure what you mean about pot with terminals arranged differently, they are always the same. Double check your work and that none of the pads/traces are cracked at the pot.

                            When you are testing with less than all the output transistors (without bulb), it can not handle the same load impedance, so don't run at the minimum load, or at any significant power, you will just blow output transistors.
                            Once you get them all installed again, measure the DC mV (at idle) across each of the output transistors emitter resistor. Post results.
                            "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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