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Crate GX-140CH Footswitch/LED Mystery

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  • Crate GX-140CH Footswitch/LED Mystery

    So I picked up a Crate GX-140CH REALLY cheap because it wouldn't change channels. Only the overdrive channel worked. When I got it home and had a chance to really test it out I found:

    The front panel channel switch showed the correct lit red LED when set to the overdrive channel and sounded fine, but when set to the clean channel via the front panel switch only silence and the green LED was dark. With the foot switch it was same.

    The over drive channel has two "drive" settings. They switched fine on the front panel and via the foot switch.

    The chorus was "stuck" on regardless of the front panel or foot switch setting.

    I found the schematics (attached) and I figured out that all the switching went through the foot switch jacks regardless of if you used the foot switch on front panel or the foot switch. Note that the foot switch defeats the front panel switches. I gave the foot switch jacks a good cleaning and the chorus now worked correctly with the front panel and foot switch!

    Since the clean channel was still MIA I tore it apart. I found that I was not the first person in it. Someone had messed with Q16, which appears to be the channel switching BJT as well as other seemingly random bad looking solder joints. There was nothing left of the base PCB pad of Q16. It is a MPS-A55 so I don't know if it is original or a replacement, but who ever did the "work" really butchered it, even by my standards (). I jumpered around the bad pad and clean up a few other areas that appeared to have been messed with and found another couple of bad pads/junctions and jumpered around them. I reflowed and added a little more solder on a few other iffy looking spots as well as all of the jacks, pots and switches on the preamp PCB. I never removed the power amp PCB since the issue seemed to be with the preamp board since all of the switching appears to be handed there with the exception of the traces which run to the foot switch jacks. The preamp and power amp board are connected via a multi-connector cable.

    I put it back together and tested. Viola! The clean channel works and all of the front switches work as they should. I tried the foot switch and the channels switch as they should, but I noticed that the red LED only come on VERY dim, if at all, when the overdrive channel is selected with the foot switch. The LED on the foot switch lights correctly and the green panel LED lights when the clean channel is on. Hmm. So I test the foot switch with a Crate Blue Voodoo and it works fine with that amp. I try another foot switch I have laying around without an LED and the front panel LED lights act correctly. If the TRS foot switch cable is plugged into the amp but not plugged into the foot switch itself the drive channel is activated and the red LED on the front panel light.

    I'm kind of stumped. Am I losing voltage to the front panel LED somewhere when the foot switch is activated? Any ideas?

    53101H2_.pdf
    443XXC3.pdf

  • #2
    Are you positive it is the exact footswitch for that amp? It sounds like maybe it's designed for a f/s that does not have lights in it. So maybe now 2 lights are having to share the voltage that is meant to run one light?
    On clean is there a light on both the footswitch and the amp that are both correct brightness?
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


    Comment


    • #3
      Maybe you are correct about the mode of foot switch.

      The model of foot switch called out in the manual is Crate CFP-1, -2 or -3 (-1 = 1 button TS jack, -2 = 2 button TRS jack, -3 = 3 button with 1 TRS jack and 1 TS jack). The foot switch I have is the Crate CFS-2. They both have LEDs and as far as I have been able figure out via Google is that the CFS is the molded plastic version of the CFP, although maybe there is more to it. I've attached the schematics for the CFP from Crate's support site (there is none listed for the CFS) as a well as the manual for the CFS, but it looks like the manual is kind of generic for their foot switches and there isn't a separate one for the CFP. It's really kind of confusing what, if any, functional differences might be between the CFS and CFP.

      CFS_OM.pdf
      CFP Footswitch Schematics.pdf

      When clean is chosen on the foot switch the panel is one but foot switch LED is out. When using the same foot switch to turn the chorus on the foot switch and panel LED are on with the correct brightness. I also tested this with the Blue Voodoo and in this case the panel and foot switch LEDs are on with the correct brightness when the overdrive channel is selected.

      Comment


      • #4
        I checked with Crate (LOUD Tech) and they told me the only difference between pedals is cosmetic and that the pedal I have should work correctly with this amp.

        I'm going to take some measurements later, comparing the channel switching and chorus on/off circuits since those circuits appear very similar. I understand of the circuit around Q16 somewhat, but not totally. I somehow feel the LED issue has something to do with the zener (D29).

        Comment


        • #5
          Agree, check the voltage across D29 with footswitch in clean and OD conditions. Also check that resistor connected to the panel led. (R48)
          Originally posted by Enzo
          I have a sign in my shop that says, "Never think up reasons not to check something."


          Comment


          • #6
            I just got a chance to take a few measurements:

            Front Button

            Clean
            D29 = 1.7mV
            R48 = 0V

            OD
            D29 = 2.1V
            R48 = 114mV

            Foot Switch

            Clean
            D29 = 8.6mV
            R48 = 0V

            OD
            D29 = 1.7V
            R48 = 0V

            For comparison I measured the zener (D34) on the chorus switching as well as R92:

            Front Button

            chorus off
            D34 = 7.0mV
            R92 = 15.7V

            chorus on
            D34 = 2.8V
            R92 = 14.1V

            Foot Switch

            chorus off
            D34 = 5.3mV
            R92 = 15.7V

            chorus on
            D34 = 1.7V
            R92 = 14.1V

            I then measured the voltage (to ground) at the clean and OD LEDs when they in their respective ON state:

            Clean
            front switch = 14.3V
            foot switch = 14.3V
            OD
            front switch = 117mV <--- how is this even on?
            foot switch = 0mv

            I'm not sure how to test the zener, but for kicks I set the DMM to ohms and measured both directions on D29 (OD) and D34 (chorus). I didn't get any really stable measurements on either but they both acted the same.

            The best I can figure from my testing is that it looks like the voltage to the OD LED, even when it works using the front panel switch, is low.

            My next step is to pull the board again and see if all the traces/solder joints in the OD LED circuit area are good.

            Comment


            • #7
              If you don't have a replacement zener to try, and you don't find any solder problems, seeing as you are pulling the board anyway, try swapping D29 and D34.
              Originally posted by Enzo
              I have a sign in my shop that says, "Never think up reasons not to check something."


              Comment


              • #8
                Happy New Year! It's 3:35am here.

                It does not appear to be the zener. I swapped D29 and D34 as suggested and nothing changed. I looked around and couldn't find any thing obvious like a bad joint or trace. I reflowed a few just in case but no change.

                I'm going to have to study the area of the circuit around Q17 and Q20 to figure out what is going on there. It seems like the circuit around Q16 is okay and those two areas seem to be the most likely to affect the LED, at least to me.

                Comment


                • #9
                  I looked around the components and traces of the PCB that are in the areas/circuits of the schematic that appear (to me at least) to be related to the channel switching. Those being around Q16 and around Q17 and Q20, both shown on page two of file 53101H2_ attached in my first post. I couldn't see anything obvious and measured all of the resistors and caps in those areas and they measured within spec. I tested for bad solder joints and traces without any luck.

                  I measured the test points called out on the schematic in those areas, at least as best as I could figure trying to find the appropriate component junctions. Some values were way out of whack so I'm guessing I was testing the wrong point. I don't have any way of sending a sine-wave signal, but I don't believe it matters for the areas I was testing. Here are my results with the "correct" values in parentheses:

                  Using front panel switch:

                  TP 15
                  Condition "A" = -12.8VDC (-10VDC)
                  Condition "B" = -12.8VDC (-10VDC)
                  Condition "C" = -31.9VDC (0VDC)

                  TP 16
                  Condition "A" = 9.7VDC (10VDC)
                  Condition "B" = 0VDC (0VDC)
                  Condition "C" = 10.0VDC (10VDC)

                  TP 17
                  Condition "A" = 0.5VDC (10VDC)
                  Condition "B" = 14.1VDC (10VDC)
                  Condition "C" = 14.1VDC (0VDC) Gain select 1
                  Condition "C" = 14.1VDC (10VDC) Gain select 2


                  Using the foot switch:

                  TP 15
                  Condition "A" = -12.9VDC (-10VDC)
                  Condition "B" = -12.9VDC (-10VDC)
                  Condition "C" = -32.2VDC (0VDC)

                  TP 16
                  Condition "A" = 10.0VDC (10VDC)
                  Condition "B" = 0VDC (0VDC)
                  Condition "C" = 10.0VDC (10VDC)

                  TP 17
                  Condition "A" = 0.5VDC (10VDC)
                  Condition "B" = 14.1VDC (10VDC)
                  Condition "C" = 14.1VDC (0VDC) Gain select 1
                  Condition "C" = 14.1VDC (10VDC) Gain select 2

                  I bolded and underlined the values that seemed most out of whack, but there are a few where the voltages should be 10VDC and I was measuring 14VDC.

                  There was VERY little difference between the front panel switch and foot switch tested values.

                  The only things I can think of doing at this point is:
                  - pulling the 1N94s and testing each one
                  - swapping zener D31 with D29 or D34, since swapping D29 with D34 made no difference
                  - replacing Q16, Q17 and Q20

                  I would rather figure out the problem than shotgunning in hopes of getting lucky.

                  I think I understand what's going on around Q16, but not so much with the Q17/Q20 area. It looks to me that the Q16 area just does the LED switching and the Q17/Q20 area is the actual channel and gain changing. Of course I could be completely wrong. Can anyone confirm or correct my interpretation? Is there another area related to the switching that I am missing?

                  Any thoughts would be appreciated! Thanks!

                  On the bright side I got to use my brand new Christmas presents, a Hakko F888D and Soldapult.

                  Comment


                  • #10
                    I'll try to answer some of your questions about the circuit particulars later, unless someone else cares to jump in.
                    Meanwhile, can you check a few things:
                    Shown on the drawing above/near Q16 are +& - 15V at R99 & R100. What are you measuring there?
                    At cathode end of D29, with front panel red led (D1) on, what is the voltage without footswitch, and then with footswitch led also on.
                    Originally posted by Enzo
                    I have a sign in my shop that says, "Never think up reasons not to check something."


                    Comment


                    • #11
                      Originally posted by stoneattic View Post
                      I think I understand what's going on around Q16, but not so much with the Q17/Q20 area. It looks to me that the Q16 area just does the LED switching and the Q17/Q20 area is the actual channel and gain changing. Of course I could be completely wrong. Can anyone confirm or correct my interpretation? Is there another area related to the switching that I am missing?
                      Q16 is turned on and off by the chan. switch. This runs the D1 & D2 leds, but also the point 'A' (at TP15) which will go to switching and muting Fet's at various points marked 'A' in the pre and power amp.
                      Q17 is turned on and off by the gain select switch. This in turn switches Q20 off and on. The 2 of them control the voltages at 'B' and 'C' (TP16 and TP17). These go to other points B and C in the circuits where there are switching Fets that do the actual gain select switching.

                      As far as I can tell, the only time there are 2 led's running is when the OD channel is selected. So there is no other situation with 2 leds running to compare it to (I don't see any panel led for the chorus).
                      So this may actually be normal for this model. The BV may do it differently.
                      So when both Leds are running with OD channel selected, there is the footswitch led in parallel with D1/R48. R48 results in D1 being dim only when there is also another led in parallel with it.
                      Originally posted by Enzo
                      I have a sign in my shop that says, "Never think up reasons not to check something."


                      Comment


                      • #12
                        I took some quick measurements:

                        The -15V point near R99 is measuring -16.2V regardless of switch position and which switch is used.
                        The +15V point near R100 is measuring +16.2V regardless of switch position and which switch is used.
                        I'm guessing that's close enough?

                        I measure 2.1V at the cathode of D29 when OD is on with front switch (~0V when clean is selected) and 1.7V when OD is on using foot switch (~0V when clean is selected).

                        BTW, there are two LEDs running when the chorus is selected using the foot switch (chorus panel LED and foot switch LED). The panel LED for the chours is in area C2 of page 2 of the preamp schematic.

                        Thanks for taking the time G1, I really appreciate your help!

                        Comment


                        • #13
                          I just took a few other measurements:

                          To confirm that the resistors related to the OD LED are good I got:
                          R48 = 22.6 (22 spec'ed)
                          R100 = 2.18K (2.2K spec'ed)

                          I also measured the voltages in and out of R92, which is where the chorus LED gets it's +15V from and get 15.98V going in and 1.97V out (LED side) when the LED is on via either the front panel or foot switch.

                          So it looks like the foot switch is "losing" ~.4V to the foot switch LED when in the OD channel, but not with the chorus. FWIW, it's the same TR portion of the TRS foot switch for the OD and chorus switching (different jack of course).

                          Comment


                          • #14
                            R92 only powers the amp's led. The footswitch led for the chorus gets is power from R108 (near D34 on drawing), via pin 6 of connector.
                            That's why chorus doesn't have the problem, the 2 leds aren't in parallel like they are for the ch. switching.
                            I really do think it's a design issue and not a failure or fault per se.
                            R100 has to power both Led's for ch. when the footswitch is used. I wouldn't want to change it's value as it may affect the operation of Q16. You could add a resistor like R48 in series with the led in the footswitch. This will make them the same brightness, and if still too dim you could try 10 ohm for both instead of 22's. But with 10's the ch. led on the amp may be too bright when no footswitch used.
                            Originally posted by Enzo
                            I have a sign in my shop that says, "Never think up reasons not to check something."


                            Comment


                            • #15
                              Maybe you are correct and it is just a design flaw. Right now the foot switch gets used between the BV and this amp so I don't want to mod this particular switch, at least for now. I have the parts to build another foot switch (3 switches to do the chorus as well) so I think I will try adding a 10 ohm resister to that switch and bumping R48 down to 10 as you suggested and see if the LED is too bright without the foot switch.

                              I'm at work now without the amp or schematic in front of me, but I'm wondering if there is a way to mod the amp so that the LED sees 20 ohms when the foot switch is connected or disconnected. I'm thinking maybe there is a way to by changing R48 to 10 ohms and wiring another 10 ohm resistor in that is bypassed when the foot switch (with 10 resistor) is connected. If the cable were just TS I could use a TRS jack to switch the new resistor in and out, but short of adding a separate switch I haven't figured out a way.

                              Or maybe add an LED internal to the amp that is also on when the front LED is on via the panel switch and is off when the foot switch is used. Then adjust R100 to get the appropriate current/brightness? Since the LED is on when the switch is open this seems possible, at least in my head without the amp or schematic in front of me.
                              Last edited by stoneattic; 01-05-2017, 07:02 PM.

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