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Fender 'The Twin' red Knob, power supply circuit voltages off

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  • Fender 'The Twin' red Knob, power supply circuit voltages off

    Hi there,

    I 'fixed' a Fender Twin RedKnob recently, some on the forum may remember, where a solder braid had been used to band together 3 power resistors which had been replaced in place of one. The solder braid had slipped off and conducted where it really shouldn't. I fixed that and replaced a few bad components. Now it works fine on the Low setting, but cuts out at low volume on the high setting.

    On measuring the voltages, especially the different voltages for Hi and low power operation the switch no longer adjusts them to their specified voltage.

    I'll sum up the original fault quickly; The erroneous bridge was between C111 positive and R132, right side of it on the schematic. This essentially put B+ on the lights and switches circuit, the bottom of the power transformer secondary on the schematic. D108 zener, over on the left blew and created a path to ground. I replaced that, replaced r130 that was open, and R306 that was dying.

    After that the amp went from silent to sounding 'almost great' on the low power setting, the high power setting is silent at low volume, can be kicked by increasing volume or gain, but still is only about the same volume as the low power setting.

    On the attached photo of my annotated schematic I've labelled the 3 power lines away from the power circuit X Y and Z (yup I scrubbed out calling them A, B, C as the schematic uses these labels for other voltages later!)

    X is fine

    Y is +236v ish (correct for low power, high power SHOULD be +469v)

    Z is +473v ish (correct for high power, low power should be +236v)

    So something in the switching circuit has gone and not applying the correct effect when the switch is in one or the other position.

    R304 and R305 showed an odd reading in circuit, but pulled they are correct at 220K. C301 and C302 when pulled charge and discharge on a meter's voltage fine.

    I can't see enough what this circuit does to be able to make proper sense of this. So my question is, do I suspect the big caps? As they are crucial for bridging things in the High and Low power circuits operation? It looks to me like they are the ONLY components that can cause such a big voltage shift in the power-switch's operation, as the inductor and stuff on the right (C306, C307 etc) would impact the circuit evenly, on both high and low power if failing. The big caps C301 C302 would have been abused by the shorted B+ in the fault, I think

    Or maybe it's something further up in the (screen? voltages) that those power rails (X, Y, Z) provide...

    I can't see R128, which would be next in line after the failed R130, but figured as it works nicely on low power that is PROBABLY fine. I get correct voltages at A, B and C, on the bottom right of the schema now anyway.

    SO, my focus is basically on, something I missed? Am I looking at the big caps (C301, C302). I wanted to get a little input before I bought those big ole ones as, FWIW, they test OK out of circuit. I know that doesn't say much about their operation at 400v! But really, they are the only thing that I can see, in my fairly basic 'ohms law and not much else' approach that could cause the voltage rails Y and Z to be unaffected by the HIGH/LOW power switch!

    Click image for larger version

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    Fender Red Knob TheTwin Annotated (dragged).pdf

    PS - and I realise it may be a big PS... The PCB traces on the power board are all wrinkled up, maybe that just happens, but I'm wondering if it was from heat of the short circuit, and I might be looking at a mechanical fault. It LOOKS like everything is ok, it hard to look at the bottom of the pcb easily without desoldering all of the transformer pins, but I guess a split somewhere may cause the high/low switch to be ineffective, though I'd need to get my thinking cap on to work out where...
    Last edited by OwenM; 01-27-2020, 10:40 PM.

  • #2
    Something must be mis-wired. Y and Z are supposed to be on either side of the choke, so no way to have that kind of 250V drop across the choke if it is ok.
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


    Comment


    • #3
      Originally posted by g1 View Post
      Something must be mis-wired. Y and Z are supposed to be on either side of the choke, so no way to have that kind of 250V drop across the choke if it is ok.
      Hmmmm, thanks, ok I'll have a look... The choke may very well be toast as well though I don't know how to check it with just a DMM and a cheap ESR meter?

      I'll go back and idiot check my findings... It's also a bit confusing because even if 250V WAS somehow dropping over an almost-open choke I can't see how it could effect the voltage on Y so much in the OPPOSITE direction on the other setting well...
      Last edited by OwenM; 01-28-2020, 01:32 PM.

      Comment


      • #4
        Ok I've gotten somewhere.

        I was measuring the voltages off of what SHOULD have been electrically common points. Now I found the pads for either side of the choke, it just sits at 235v no matter what is selected.

        So Y and Z above are always on 235v, whatever the Hi/Lo switch is set at

        The chokes resistances is about 150ohms ish

        C301- is electrically common to the chokes output-transformer-center-tap side when the switch is on Lo.

        However on Hi, C301+ has about 2MegOhm resistance the chokes output-transformer-center-tap side.

        C301+ connects to R128 and R304 fine, as well as the relevant rectifier diodes.

        So, I think all it can be is the trace between that group of connected components, or the switch itself on 'Hi'...

        EDIT - All the traces seem to be correctly common, when I was fiddling with the switch there was a 'small boing' sound and now it all works and voltages are correct... Think I have an intermittent switch...
        Last edited by OwenM; 01-28-2020, 03:44 PM.

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        • #5
          Here is the complete schematic Red Knob The Twin.pdf

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          • #6
            If switch wiring is correct, then new voltage information indicates a bad switch is quite likely.
            Originally posted by Enzo
            I have a sign in my shop that says, "Never think up reasons not to check something."


            Comment


            • #7
              That was it, thinking about it it probably hasn't been used on high for years so after a gentle clean it seems to make contact correctly every time now.

              HOWEVER, things are never easy it seems, now I notice that I can't switch it to channel one, which ever setting the channel-select pull pot is on. Additionally the clean channel volume control also controls the volume of the drive channel I am stuck on... Looks like the B+ hitting that switching circuit fried more than just the stuff in the power circuit.

              It roughly 350v at R132, which is working, but I'm going to have to start looking around that area and see what I can see... Any tips? It suppose S7A/B may be shorted to ground. As far as I can see that switch would override the foot switch control, does that sound right?

              Comment


              • #8
                I would look at all the input jacks. There are complex things going on with the parallel input jacks. If all the switches inside those input jacks aren't working right, you could end up with both channels working at the same time.
                See the owners manual about how the input jacks work, give them all a shot of cleaner, and see if that helps.
                Attached Files
                Originally posted by Enzo
                I have a sign in my shop that says, "Never think up reasons not to check something."


                Comment


                • #9
                  Originally posted by g1 View Post
                  I would look at all the input jacks. There are complex things going on with the parallel input jacks. If all the switches inside those input jacks aren't working right, you could end up with both channels working at the same time.
                  See the owners manual about how the input jacks work, give them all a shot of cleaner, and see if that helps.
                  Great thanks! I'll have a look this evening

                  Comment


                  • #10
                    Ok, so I had a good gentle clean and still all the same...

                    I removed the control panel PCB to try and give them a more thorough clean and found they are quite unusual (to me at least) fully enclosed jacks that I don't really want to try breaking into unless I have too...

                    Click image for larger version

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                    So I've started poking around and testing as much as I can. First thing though, in my stupidity I used a little 6v battery pack to test if the LED's worked, without a resistor, and I think I've blown them, I didn't even think as I've been breadboarding stuff with newer, bigger, LED"s sometimes taking the full voltage and its been ok. But anyway, I tested each, they flickered and now one is open. One drops .2v still when forward biased. Am I right in thinking just normal mini red LED's will work ok? The schematic doesn't seem to suggest anything more specific... EDIT; Those are D1 and D4

                    I've attached a photo of what I've tested so far.. green circle is tested ok in circuit, green dot is something was odd, but tested ok out of circuit. It may not be that legible so I don't expect anyone to pour over it

                    The main things I found -

                    IC101 pins 1 and 2, the lower op amp output. I get channel 1 +0.07Vdc (should be +2.8Vdc), channel 2 0Vdc (should be +0.59v)

                    left of R132 I get 17Vac (should be 30Vac)

                    C303 on the 'D' power line shows a resistance of 300 Ohms across it to ground, in circuit, but looks ok out of circuit (I only have a DMM and very budget ESR meter). This may be normal but I can't see what other paths to ground could be accumulating to give such a low reading when in the circuit. It would have been in the path of the original short circuit so am wondering if I should just replace c303 and c304 as a matter of course.

                    The box, bottom left of the power section says D should be +27Vdc with no input, at the moment I get +4Vdc at the 'D going to other areas of the amp on the schematic' point. On the other side of R306 I get the expected 72Vdc, and R306 is new and tests ok. This really made me think C303 is allowing a low resistance path to ground when under operation voltage...

                    Other than that I'll just keep testing stuff on the control panel PCB I just removed, which is mainly pre amp stuff (seems fine) but has a few of the (far-left on schematic) switching stuff too.. I appreciate that the problem may still be with the input jacks and I might just have to find a way to open them up or try and replace... Once I fix my LED blunder...

                    Any help would be gratefully received!! I do have a scope so if something could be looked at with that I'm all ears.

                    Click image for larger version

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                    Ignore the orange lines, they were my early attempts at trying to highlight stuff in the switching circuit..

                    Fender Red Knob TheTwin Annotated (dragged).pdf
                    Last edited by OwenM; 01-31-2020, 10:43 PM.

                    Comment


                    • #11
                      Standard red Led's that you have will be fine. Disconnect the bad ones so they don't mess up your resistance readings (the one that reads .2V on diode check is low resistance).
                      Supply node 'D' powers the IC's, so IC101 won't give proper readings till it has supply voltage D.
                      Lift one end of D108 and check it. If it measures correct check again resistance from 'D' to ground. If still 300ohms, desolder pin3 of IC101. See if D to ground still measures 300ohms.
                      Originally posted by Enzo
                      I have a sign in my shop that says, "Never think up reasons not to check something."


                      Comment


                      • #12
                        Originally posted by g1 View Post
                        Standard red Led's that you have will be fine. Disconnect the bad ones so they don't mess up your resistance readings (the one that reads .2V on diode check is low resistance).
                        Supply node 'D' powers the IC's, so IC101 won't give proper readings till it has supply voltage D.
                        Lift one end of D108 and check it. If it measures correct check again resistance from 'D' to ground. If still 300ohms, desolder pin3 of IC101. See if D to ground still measures 300ohms.
                        Thanks as ever, g1,

                        well well, D108 tests good, D to Ground is 300 ohms with IC pin 3 connected, once disconnected D to ground is open! Reconnect and back to 300 Ohms

                        Soooo, shorted IC?

                        Comment


                        • #13
                          Yes, IC is bad.
                          Originally posted by Enzo
                          I have a sign in my shop that says, "Never think up reasons not to check something."


                          Comment


                          • #14
                            Originally posted by g1 View Post
                            Yes, IC is bad.
                            Cheers, came today, installed along with IC socket and switches fine now. The voltages still don't tally up here and there but only by a bit, and it's working great so I think I'm going to call that done for now!

                            Thanks again for the help

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                            • #15
                              Question: should channel 2 come in immediately when i step on the footswitch? I noticed there is a millisecond delay when i step on the footswitch to change from clean to overdrive.

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