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I was given one of these modeling amps in a 'makes no sound' condition. I do not have a schematic for this amp. The best I could find was a service manual for a VT80+. Close...but different. Particularly in the output stage.
A new 12AX7, jumpering around the headphone jack (contacts broken & stuck in bypass) and a few connectors cleaned allowed sound through the speaker, but the board itself has sustained physical damage, causing the Preset , Manual & Tuner functions not to work. I was able to access a manual mode by using the 'Bank' function buttons. What sound it produced was pretty good but the process to get there was cumbersome.
So...I opted to order a replacement board from Korg. Installed same & all functions work properly. Except that there is a very evident 120hz buzz that appears to be associated with the output section of the amp. This is evident w/o anything plugged into the amp input. The headphone jack cuts out the speaker & all is clear & clean at that point (thru headphones). The only control pot that affects this buzz is the Power Level (0-30w). No other control has any influence on the signal at all. A spectral analyzer shows a peak right at 120hz, with harmonics at 240 & 360. Noise level increases as power level is increased.
I put the original, damaged, board back in & tested again. It does not have any buzz. Slight hiss, but no buzz. That leads me to believe it to be the replacement board's issue.
So...since I seem to have grabbed the last available replacement PWC in the US from Korg, it looks like a 4-8 week wait while they make another in Vietnam. Or I can try to troubleshoot this as it sits. I've rolled a number of 12AX7's through the socket to no avail. Some very questionable solder joints & more copper showing on the component side than I'm used to seeing. But once I start any re-work, I've accepted ownership. Anyone have any experience with these units or have any suggestions that might help me decide what to do with this? Right now RMA-ing it & getting a replacement sounds logical, but I'm not so certain that the next will be any better.
Regards,
Archie
A new 12AX7, jumpering around the headphone jack (contacts broken & stuck in bypass) and a few connectors cleaned allowed sound through the speaker, but the board itself has sustained physical damage, causing the Preset , Manual & Tuner functions not to work. I was able to access a manual mode by using the 'Bank' function buttons. What sound it produced was pretty good but the process to get there was cumbersome.
So...I opted to order a replacement board from Korg. Installed same & all functions work properly. Except that there is a very evident 120hz buzz that appears to be associated with the output section of the amp. This is evident w/o anything plugged into the amp input. The headphone jack cuts out the speaker & all is clear & clean at that point (thru headphones). The only control pot that affects this buzz is the Power Level (0-30w). No other control has any influence on the signal at all. A spectral analyzer shows a peak right at 120hz, with harmonics at 240 & 360. Noise level increases as power level is increased.
I put the original, damaged, board back in & tested again. It does not have any buzz. Slight hiss, but no buzz. That leads me to believe it to be the replacement board's issue.
So...since I seem to have grabbed the last available replacement PWC in the US from Korg, it looks like a 4-8 week wait while they make another in Vietnam. Or I can try to troubleshoot this as it sits. I've rolled a number of 12AX7's through the socket to no avail. Some very questionable solder joints & more copper showing on the component side than I'm used to seeing. But once I start any re-work, I've accepted ownership. Anyone have any experience with these units or have any suggestions that might help me decide what to do with this? Right now RMA-ing it & getting a replacement sounds logical, but I'm not so certain that the next will be any better.
Regards,
Archie
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