Posted by
mnemonic on
URL: http://guitar-fx-layouts.238.s1.nabble.com/Ampeg-VH140C-tp30301p50856.html
Been through my pedal on my biennial-ish troubleshooting session, I've been on a 90's death metal kick so I figured I'd try to get this working right. There are a few other discrepancies to clear up that I didn't catch the first time.
Aion now have a kit with an easy to read schematic so thats a good comparator to use. only difference I see between Aion and the stock vh140c factory schematic is the electrolytics are the other way around on the distortion stages (ground end to ground on Aion vs positive to ground on factory schematic). Maybe an error on the factory schematic? Factory schematic errors certainly aren't unheard of.
Link:
https://aionfx.com/app/files/docs/vh_drive_channel_documentation.pdfThe Aion schematic also includes a bit more of the circuit from the factory schematic after the volume control, where the hand-drawn schematic and vero layout stop at the volume control.
Apart from parts missing and noted in the previous post, I see (designators from factory schematic) C35 and C36 2.2u coupling caps missing. The Aion schematic uses a 1uf as these two caps are in series to add to 1.1uf (and i think Q5 between those caps is part of the channel switching therefore omitted).
R59 - 270k to ground from + input of IC3 (IC3A pin 3 on factory schem, pin 5 of IC3 on Anders vero layout)
C39 - 47n coupling cap between IC3b and IC3a on factory schem.
Ive added all those missing parts and also corrected some errors I made in 2020 during my last 'corrections' (1) i had used a 2.2u by accident rather than 3.3u for C23, and also I put the negative lead to ground this time, and (2) I had used two .22n in parallel (for .44n) rather than a single .22n for C24, and I'm not sure why I did that.
Ran into a solidstate power amp and v30 cab (marshall valvestate 8100 FX return), sounds better than it did, closer to 'right' but still muffled/dark on top. I'm not sure if that is right.
Comparing the hand-drawn schematic above to the Aion schematic and factory schematic, the bass control (pot and the 4.7n cap C42) are drawn weird on the hand-drawn schematic, not sure if its correct, I'm having trouble wrapping my head around it.
later after all of the above I noticed that the hand-drawn schematic and vero layout is missing (from the aion schematic) C9 / 1n capacitor to ground after 6.8k resistor, and then the R12 / 4.7k resistor that follows it, all just before the first clipping stage. The hand-drawn schematic does have the 6.8k though. That 6.8k / 1nf form a low pass at 23khz so probably not really super important. The factory schematic has it but its around some transistor for channel switching.
I decided to lift the 4.7nf cap to ground just before the mid control (C32 on factory schematic), the 10k resistor before it with the 4.7nf form a low pass at 3,386hz which is a bit drastic just after distortion. With that lifted I get a bunch more treble response and now it sounds 'right' to me. I dont know if my pedal is still losing high end somewhere it shouldn't, and I'm just compensating with the removal of that filter, but it just sounds way too muffled with that filter there, especially compared to the clips on youtube of all the various vh140c preamp clones.