While comparing the parts at the VT20+ board with the standard application in the data sheet I recovered something very strange. I checked the voltages at all pins of the power amp circuit and found something strange: Pin 1 and 2 had permanently LOW level, which will mute the entire circuit. This LOW signal will than mute the circuit, because it is also passed to pin 1. In the case of internal problems this transistor will go into a conducting state, so that the voltage at pin 2 then switches to LOW. So it needs a pull-up resistor to function properly – the resistor rises the potential to a High state when the internal transistor at this pin is switched off. FAULT at pin 2 is an open connector output. The /SD signal at pin 1 is for shutting down (muting) the circuit. The wiring of the output stage in the VT20+ is basing of the circuit for bridge operation, as it is shown in the data sheet of the TPA3110D2:įigure 1: Standard wiring of a TPA3110D2 in bridge mode There are also some photos at VT20+ user forums and also at that show the inner parts of the VT20+. Make some photos of the wiring so that you can look at while assembling. Remove all buttons of the rotary switches and potentiometers Remove the circuit board from the front panel White connector from Foot switch connector White connector located at the input board Remove yellow + blue (ground) wire off the speaker chassis (7 mm wrench)
It will be a very good idea to look for someone who can help you, looking at the oscilloscope or multimeter while you are trying to pinpoint the pins.ħ long metric screws on the control panel Also you must strictly avoid shorting adjacent pins! So you have to use a magnifying glass to be able to put the probe head on the right pin. The pins of the power amp circuit TPA3110D2 have a spacing of 0,65 mm only. This makes it even hard to check the device.
VOX VT20X SETTINGS SERIES
With beginning of Vox VT20+/VT40+/VT120+ series it became almost impossible to change defect parts because Vox/Korg have changed the layout in the amp section to using SMD.
It turned out that repairing a VT amp is not so easy. So I thought this would be a minor issue, because I already had fixed a problem in a Vox DA5 without problem. The seller said it had worked fine but suddenly stopped to produce sound at the speaker. In 2013 I purchased a used VOX VT20+ guitar amp at ebay for a quite good price (60 Euro). The main amp was permantenly muted due to a wrong controlling signal, possibly caused by wrong board design. No sound at the speaker output, but the VT20+ had sound at the headphone connector.