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Fender Frontman 212 Intermittent Weak Distorted Output

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  • #16
    Originally posted by bobloblaws View Post

    I'm running alligator clip test leads from speaker out to either my test speaker or dummy load, both + and -. I'm not sure what you mean that it must not get grounded. The - lug is already grounded anyway, no? So whether I connect my negative speaker terminal to the - lug on the amp or to the chassis is effectively the same thing I would think.
    No, the speaker (-) lug is not grounded. It connects through current sense resistors R114 & R115, sort of like a NFB type thing.
    If you connect any of your (grounded) test equipment to the speaker (-) lug, you short out that feedback element and can cause all kinds of test induced weirdness. So you need a fully floating load or speaker, and your test equipment grounded to chassis.

    Not saying it has anything to do with the problem, but may lead you to look for other ghost problems that are test induced. (possibly items like in post #12)
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


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    • #17
      Originally posted by g1 View Post
      No, the speaker (-) lug is not grounded. It connects through current sense resistors R114 & R115, sort of like a NFB type thing.
      If you connect any of your (grounded) test equipment to the speaker (-) lug, you short out that feedback element and can cause all kinds of test induced weirdness. So you need a fully floating load or speaker, and your test equipment grounded to chassis.

      Not saying it has anything to do with the problem, but may lead you to look for other ghost problems that are test induced. (possibly items like in post #12)
      OIC, as it happens I have not been grounding any test equipment to the speaker out '-' lug, both DMM and scope are connected to the chassis and only speaker or dummy load connected to the lug.

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      • #18
        Ok.
        You can also use isopropyl on a swab as a quick acting 'freeze', it just warms again much quicker than using freeze spray.
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


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        • #19
          Originally posted by g1 View Post
          Ok.
          You can also use isopropyl on a swab as a quick acting 'freeze', it just warms again much quicker than using freeze spray.
          Oh, OK, thanks for that tip.

          I completely bypassed the FX return area and breadboarded R62 and R64 to ground just in case. It took about two minutes for it to go rogue, long before the heatsink got hot. Q14, Q15, Q18, Q19, Q20, and Q21 are attached to the heatsink. I did notice Q12 and Q13 getting fairly warm, I don't know if that is expected, it is a bigger transistor than the TO-92 form factor. I'm gonna let everything cool down and see what happens to those two on power up and possibly try the alcohol hack.

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          • #20
            Or a can of that spray off your keyboard duster, like they sell to computer users.
            Education is what you're left with after you have forgotten what you have learned.

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            • #21
              Originally posted by Enzo View Post
              Or a can of that spray off your keyboard duster, like they sell to computer users.
              Yeah, I always have that around, never thought to try that before. That freeze spray is damn expensive.

              At this point I can pretty confidently say it's not heat related. When I fired it up again from room temperature it took much longer this time to flake out on me and when it did I tried the alcohol trick for the heck of it but no effect. So I've ruled out heat, loose parts, FX loop, plus all those parts I shotgunned mentioned at the beginning. I keep saying I'll probably end up going to town re-soldering everything and frankly that's all I can think of other than wait for it to lock into the bad state and do some more scoping.

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              • #22
                Originally posted by bobloblaws View Post
                Yeah, I always have that around, never thought to try that before. That freeze spray is damn expensive.

                At this point I can pretty confidently say it's not heat related. When I fired it up again from room temperature it took much longer this time to flake out on me and when it did I tried the alcohol trick for the heck of it but no effect. So I've ruled out heat, loose parts, FX loop, plus all those parts I shotgunned mentioned at the beginning. I keep saying I'll probably end up going to town re-soldering everything and frankly that's all I can think of other than wait for it to lock into the bad state and do some more scoping.
                I'll throw you a WAG. Are you also monitoring your input voltage? As Nevetslab discovered last month or so , just a small variance in the input voltage caused a drumseat shaker to display some rather odd symptoms. If that is a similar issue for you a variac could help suss out the intermittent.

                nosaj
                soldering stuff that's broken, breaking stuff that works, Yeah!

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                • #23
                  Originally posted by nosaj View Post
                  I'll throw you a WAG. Are you also monitoring your input voltage? As Nevetslab discovered last month or so , just a small variance in the input voltage caused a drumseat shaker to display some rather odd symptoms. If that is a similar issue for you a variac could help suss out the intermittent.

                  nosaj
                  Input voltage, do you mean the 120 AC mains voltage? Or post PT?

                  I do have a variac with a meter on it.

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                  • #24
                    The freeze spray is not exclusively for thermal issues. It will also 'shock' the part, often enough to make it show intermittent issues. So while it does not seem the fault is temp. related, the freeze technique is still of value.
                    Several here have solved problems with these amps by replacing the differential pair Q9 & Q10. Some have seen it enough that they just replace them right off. I appreciate that you want to actually find the problem before just blindly replacing parts. But you have also pointed to the same area with what you found probing around D21 to trigger/cure the problem. So you might want to concentrate on the circuit in that area with the canned air. (upside down is supposed to work better when using air instead of freeze spray)
                    Originally posted by Enzo
                    I have a sign in my shop that says, "Never think up reasons not to check something."


                    Comment


                    • #25
                      Originally posted by bobloblaws View Post
                      Input voltage, do you mean the 120 AC mains voltage? Or post PT?

                      I do have a variac with a meter on it.
                      Mains voltage.

                      nosaj
                      soldering stuff that's broken, breaking stuff that works, Yeah!

                      Comment


                      • #26
                        Originally posted by bobloblaws View Post
                        Unfortunately I don't think I have freeze spray at the moment. I'll order some. However, my feeling is it is not heat related. I have touch tested the transistors in the broken state (as well as I could mangae) and didn't notice anything different than the normal state. Also, the length of time the amp has been on doesn't seem to be a factor. For example, I've returned to it after the amp having been off for a couple of hours and turned it to find it "broken".

                        Anyway, I'm quite perplexed. I started tracing backstream to see if I could figure out at what point the waveform started going bad, in other words, hoping I could find a point where the signal was not distorted in either state. I ended up backing up into the preamp, which is crazy because my test signal is being injected straight into the FX return. For example, right now I am scoping pin 7 of U5. Generally there is no signal there as you would expect, but occasionally it will show a beautiful sine wave and I can actually hear the test signal despite being hooked up to a dummy load. So right now it is flipping between those two states and the DMM reading I am monitoring on the power amp 0V rail changes from about 14mV when there is no signal at pin 7 and 2mV when the waveform appears. I have no idea how signal is showing up at these points in the preamp since the connection from C39 is open when the signal lead is plugged into the FX return jack. In any case, checked the +16V and -16V supplies and they are both at specified voltages in either state.

                        Now, a little more explanation is necessary. The above is what I've observed since I tried to temporarily "fix" the issue by touching something metallic to D21 about an hour ago. Prior to that I've was messing with it most of the afternoon and the situation was more like I was describing last night. In that case the signal at pin 7 and other points in that general area was pretty reliably present and the OV rail hanging around 15mV or so, and when it went rogue the 0V rail would go to -900mV or -400 mV and the perfect sine wave would change to something wierd as in this pic (which is actually scoped on pin 1 of U5). Other than basically re-soldering every solder joint on the PCB and hoping for the best I'm at a loss.

                        [ATTACH=CONFIG]56461[/ATTACH]

                        EDIT: I was a little off track in the first paragraph of this post. I just switched my scope probe from U5 to the amp output and discovered that in the scenario where the 0V rail shows 15mV and there is no signal at pin 7 of U5, there is actually no signal at the output. So that is actually abnormal. When it flips over to the other state where the 0V rail is at 2mV (effectively zero I presume) there is signal at the output and it is somewhat audible even without a speaker driver. This is obviously the working state, and I had it backwards above. In any case, it is behaving differently than it did for most of yesterday and today. At the moment the 0V rail is not periodically going negative which is the original issue I was encountering. Having said that, I'm going to try plugging back into the input again instead of the FX return to see if that makes any difference. But I'm still mystified why I'm even getting signal on U5 with the preamp supposedly bypassed.
                        The issues are usually not “heat” related in the normal troubleshooting sense. I suspect there are problems with the junctions or lead contact attachment points within the transistors. I suspect a bad run of them since I’ve seen this problem many times. The freeze minutely expands and contracts the structures within the transistors. Be aware that if you freeze a diff pair most will destabilize as the Hfe for each individual transistor will creep differently and you may get a little noise. But a loud pop or a crackle usually means a leaky transistor. Honestly these particular amps are FREQUENTLY a bucket of worms and very time consuming with intermittent problems. But.. they make a great cheap cab and speakers to build a tube amp into. I find them on CL non working for $35 all of the time.

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                        • #27
                          A plea:

                          When I was a kid learning, the mains were referred to as "the AC line". It wasn't until I started online stuff I encountered "mains" as the word for the AC grid. Easy to see how AC line and line input (signal) can be confusing. I switched over to mains in my own writing.

                          Fast forward to today, and when I saw this:
                          Are you also monitoring your input voltage?
                          I thought it referred to the input signal.

                          I urge us all to try to use consistent terms. Stick with mains for mains, or at least "wall outlet voltage" for the 120vAC/240vAC. Input to me always means a signal input.
                          Education is what you're left with after you have forgotten what you have learned.

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                          • #28
                            Good points Enzo. I think mains is a British usage- at least that's where I first saw it- but it has become more common over here which is good as it quite unambiguous.

                            Now, how can I stop guitarists asking me to replace the 'input' jack on their guitars? I never understood that one.

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                            • #29
                              I put this away for a couple of weeks, today I went and resoldered virtually every solder joint on the whole board. No change, no surprise.

                              The only thing I can think to do at this point is try shotgunning some new parts again. The question is, which parts are more likely to have some possibility that they are related to the issue at hand. As I mentioned earlier I had already verified Q16, Q17, Q18, Q19, Q20, Q21, C55, C66, and D39 (note that only Q20 and Q21 have been permanently changed. I have replacements for Q18 and Q19 but when it still malfunctioned with the new ones I just put the originals back in and saved the new ones. The other components I merely tested out of circuit with DMM. I've also tested Q9 and Q10. I am suspicious of Q14 and Q15 as they are a relatively small package transistor surrounded by the heat sink for Q18, Q19, Q20, and Q21 (see photo). Another one I'm curious about is the thermistor RT2 which is also incorporated into the heatsink (not visible in the photo though). What is the purpose of that thermistor?


                              Call me crazy, but I'd really like to get this thing working again. The suggestion to pitch the PCB and use the cab and speakers as a platform for a tube amp is interesting, but I actually like the sound of this and I like the idea of having at least one nice solid state amp around.







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                              • #30
                                I never understood that one.
                                Or this one: "I get no sound from the input."
                                Education is what you're left with after you have forgotten what you have learned.

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