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Vox Escort Lead 50 repair/tone problems

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  • #16
    Ok, digging around my amp I can't see a op amp near the input (I can't see one at all, actually), and now I bring up the escort lead 30 schematic I'm wondering if the preamp is closer to what's in that... My input resistors are certainly not 470k though, they are 100k on both high and low inputs.. however they DO both go from tip to ground of their respective jack sockets as per the Escort Lead 30 schematic. It looks like I have a 10n cap on the Low and 470n cap on the High rather than both 100n like on the escort lead 30 schem.

    Still, its 'High and low' on mine, not 'normal and bright', so I'm guessing the Lead 30 design gets the sonic difference in channels later with those two inputs going to the base of 2 different transistors with their own respective 'normal and bright' pots. Mine just has a 'master volume' and 'volume' (which works like 'gain') which effect both channels equally, so I'm guessing those two different caps on mine are what gives the difference in sound between the high and low channels. Both those caps are chicklets, so TMU not likely to be bad but I certainly could change them...

    Worth trying 470k resistors on the inputs? Or would 100k with this design be fine and so I should look further into the signal path?

    Dammit I wish I could find a schematic for this amp!

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    Last edited by OwenM; 06-13-2019, 11:06 PM.

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    • #17
      470K would probably be better but it depends on the rest of the input circuit. Can you post some close-up photos of the circuit board , both component side and foil side?

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      • #18
        Anyway, if the sound improves with the buffer, the input impedance of the amp is suboptimal. What's wrong with using the buffer? You might even build in a simple input buffer.
        - Own Opinions Only -

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        • #19
          Originally posted by dmeek View Post
          470K would probably be better but it depends on the rest of the input circuit. Can you post some close-up photos of the circuit board , both component side and foil side?
          I will do, I'll get the PCB out tomorrow and see where it's going after the input!

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          • #20
            Originally posted by Helmholtz View Post
            Anyway, if the sound improves with the buffer, the input impedance of the amp is suboptimal. What's wrong with using the buffer? You might even build in a simple input buffer.

            The buffer I used was feeding it from an aux output of a desk (chastise me if that isn't the same thing, I thought as it was a way of isolating the PU from the circuit it would work like a buffer). Definitely into the idea of building a buffer into the amp though!

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            • #21
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              Here's some shots, the wires and 100k resistor on the 'low' pot are off on these photos.The 'high' tip goes to the 10n chicklet and the 'low' tip goes to the 470n chicklet.

              The 'high' channel seems to go through the 10n chicklet followed by the .022 green cap, the 'low' through a 470n chicklet then a 22k (I think?) resistor... then they meet...

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              • #22
                Can you take one more photo of the foil side? Its too overexposed on the left. Also a shot of the whole component side.

                I see the 4136 quad op-amp there which suggests it's more like the Bass 50


                Here's the input section. It's getting late here, I'll work up a mod tomorrow,

                https://music-electronics-forum.com/...1&d=1560479296
                Attached Files
                Last edited by dmeek; 06-14-2019, 02:29 AM.

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                • #23
                  I am assuming that the circuit board picture was before you re-flowed any questionable solder joints.

                  Edit: it's sometimes tough to tell in photos but some of those joints look like they need a touch up.
                  Last edited by DrGonz78; 06-14-2019, 09:19 AM.
                  When the going gets weird... The weird turn pro!

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                  • #24
                    Originally posted by OwenM View Post
                    The buffer I used was feeding it from an aux output of a desk (chastise me if that isn't the same thing, I thought as it was a way of isolating the PU from the circuit it would work like a buffer). Definitely into the idea of building a buffer into the amp though!
                    You can use any pedal with a buffered bypass, like most Ibanez and Boss effects.
                    - Own Opinions Only -

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                    • #25
                      Vox really LOVED inverting input Op Amps, it´s all the SuprTwin uses: input preamp - reverb drive - reverb recovery - Baxandall tone control



                      by the same token, here´s the AC30SS:



                      either it´s an obsession or the Designer read only up to chapter 2 on the Op Amp Design book and didn´t go any further.

                      An I am only half kidding here.

                      Problem is that inverting Op Amps measure beautifully and in modern times they also simulate beautifully ... if driven by a 600 ohm output generator that is.
                      And they suck when driven by ... oh .... a pesky passive electric Guitar or Bass

                      But who would plug such things there?

                      Remember VOX went through **8** owners, probably about same amount of Factories all over the place (including USA based Vox Thomas, Marshall subcontracts and Korg Japan ) and only revered ones are the original ones from the Dick Denney era: AC4/15/30 .... but the others? ... not so much.

                      Compared to these poor British design ones, the USA Vox Thomas designs are WAY more resonable.

                      Including higher impedance inputs and the power amp preclipper.

                      Just check input preamps in, for example, US VOX Pathfinder V1011

                      https://music-electronics-forum.com/...1&d=1560433139

                      168k or 236k into an emitter follower transistor, not "tube perfect 470k or 1M" but WAY more reasonable than 22k , or even worse 22k in parallel with a "bright" capacitor which lowers input impedance even more and can only be driven from a bench/lab generator.

                      I´m certain that besides the original ones which were sold on quality, sound and stunning looks, to you-know-who .... most others were sold on MOJO - stunning looks - Guitar Gods shoulder rubbing.
                      Juan Manuel Fahey

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                      • #26
                        Originally posted by dmeek View Post
                        Can you take one more photo of the foil side? Its too overexposed on the left. Also a shot of the whole component side...
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                        Here you go, so much easier in daylight The ? pot is a 1 Meg Log, the EQ pots are 47k Lin and the master volume on the power side of the board is a 47k log.

                        Thanks so much for your time! Its really educational for me going through this process as well!

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                        • #27
                          Originally posted by DrGonz78 View Post
                          I am assuming that the circuit board picture was before you re-flowed any questionable solder joints.

                          Edit: it's sometimes tough to tell in photos but some of those joints look like they need a touch up.
                          Yeah I have given it a once over, they aren't as bad as they look in that darker photo!

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                          • #28
                            Here's the mod. First remove all the yellow wires and there's one trace to cut under the IC. If you don't have 68Ks you can use the 100Ks-not much difference.

                            https://music-electronics-forum.com/...1&d=1560521824
                            Attached Files

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                            • #29
                              Is the cabinet cool looking?

                              I'd probably gut it and turn it into a tube amp, doesn't help, just sayin'

                              That's what I did with an early 70's SS Marshall 2x12 combo.
                              Now it's a 1987/2204 flame thrower.



                              EDIT: WOW! They are great looking, it would make a perfect AC15!

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                              • #30
                                Originally posted by dmeek View Post
                                Here's the mod....
                                Thanks so much, I'm so grateful for your time! I need to buy a couple of bits but I'll do it and report back how I get on.

                                Just double checking, the dot next to the old .01 and the .022 / 47k caps and resistor, is that left unconnected? I mean, it looks like it is from the sketch, I'm just idiot-checking myself that its not meant to go to ground as well (not that I can see a reason why it would!).

                                I know almost anything will work, but any preference for what type the 0.01uf cap is?

                                So this is basically taking that first op-amp from the chip and putting it into a non-inverting config.. am I right in saying the only purpose of that op amp is making the master vol work? As the signal is continuing down that 1 meg resistor and the rest of that first op-amp stage is concerned with how much of the signal goes to ground? Don't worry about a thorough breakdown though, I have to get my head around op amps and their uses yet anyway! As a hobbyist I've only really sorted out tube amps till now and the odd transistor that turns up on a newer valve amp board is about as far as I can say I understand with solid state
                                Last edited by OwenM; 06-15-2019, 12:00 AM.

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