Blue Jinn
Well-known member
Hi,
This is the second one I've picked up with problems. (But a pretty good deal) This one has very little signal and mostly a wooshing sound, which I've read is fairly common fault.
The one I have has a resistor on one of the boards that has overheated and discolored, which I've seen photos of on other forums also.
No one seems to have a schematic. This guy has a partial schematic of the tube stage: http://www.mark-o-matic.com/v69-mod.html. There's another board with a FET and two darlington transistors, as well as a mystery daughter board. Other threads here confirm there is a unity gain tube stage, followed by a solid state stage. The layout in the mic is a little different than e.g. the HST-11, so gutting it completely might be a little challenging.
I don't think I can repair it w/o a schematic, so thought why not:
1. Keep the unity gain tube stage, (and maybe just simplify it on perfboard)
2. Replace the solid state stage with a conventional Schoepsish circuit
3. Modify the PSU for 12vdc for the tube heater and the FET stage, I have a reverse engineered schematic for the PSU, it looks like the transformer puts out ~13VAC so with a full wave bridge I am assuming I could replace the 7809 regulator with a 7812.
I'd have to hand wire everything on perf board, and there's not a whole lot of real estate, but it seems fairly straight forward to follow the "Alice mic" schematic for the FET stage (and just modify that somewhat for getting 12v directly rather than from phantom) and the tube stage seems relatively simple.
Thoughts?
This is the second one I've picked up with problems. (But a pretty good deal) This one has very little signal and mostly a wooshing sound, which I've read is fairly common fault.
The one I have has a resistor on one of the boards that has overheated and discolored, which I've seen photos of on other forums also.
No one seems to have a schematic. This guy has a partial schematic of the tube stage: http://www.mark-o-matic.com/v69-mod.html. There's another board with a FET and two darlington transistors, as well as a mystery daughter board. Other threads here confirm there is a unity gain tube stage, followed by a solid state stage. The layout in the mic is a little different than e.g. the HST-11, so gutting it completely might be a little challenging.
I don't think I can repair it w/o a schematic, so thought why not:
1. Keep the unity gain tube stage, (and maybe just simplify it on perfboard)
2. Replace the solid state stage with a conventional Schoepsish circuit
3. Modify the PSU for 12vdc for the tube heater and the FET stage, I have a reverse engineered schematic for the PSU, it looks like the transformer puts out ~13VAC so with a full wave bridge I am assuming I could replace the 7809 regulator with a 7812.
I'd have to hand wire everything on perf board, and there's not a whole lot of real estate, but it seems fairly straight forward to follow the "Alice mic" schematic for the FET stage (and just modify that somewhat for getting 12v directly rather than from phantom) and the tube stage seems relatively simple.
Thoughts?