Blue Jinn

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?



« Last Edit: July 09, 2019, 05:19:25 PM by Blue Jinn »
not my thing. I am here just to unbalance the equation --pucho812

 http://www.bluejinn.org


Gus

Re: MXL V69ME mods revisited. Unity gain tube stage ---> FET?
« Reply #1 on: July 13, 2019, 08:16:33 AM »
I have two I bought years ago
I made  post here about them
I have not done anything to them
IMO the capsule to tube wire length is to long the tube should have the pins closer to the capsule.
The solid state section in the ones I have looks like a Schoeps circuit even with the JFET first stage,they have a potting compound over parts of the circuit. I did not spend time tracing the solid state section of the circuit.


Thing I might do with them

Sell them

use the bodies

Install EQ network between the tube and JFET to adjust for the capsule used. IMO this is an obvious thing to do.

Install a U67 or cathode biased gain stage with a transformer out like the Brauner VM1 because of the B+ used as a cardiod only microphone and add a plate to ground LPF before the transformer you use. You might need to change the heater circuit depending on what you do.

Remove the tube and go direct to the JFET I would trace the solid stage section to make sure this will work. You might want to change pin 2and and 3 because the tube stage inverts.

OR just use them like they are

Blue Jinn

Re: MXL V69ME mods revisited. Unity gain tube stage ---> FET?
« Reply #2 on: July 30, 2019, 03:38:38 PM »
Thanks. This one isn't working, so I need to do something,

What do you think of the overall concept though? I don't know what the design goals were, and why the capsule leads are so long, but it seems like I could keep the overall concept I wrote above?

-Modify the PSU for 12vdc out - I haven't opened it up yet, but the a schematic I got for it, says the transformer puts out 13v, so with a full wave bridge I think there should be just enough there to drop in a 7812 in place of hte 7809 that's in there
-Keeping the unity gain tube stage and rewire for 12v heater supply
-Keep the stock 32mm capsule
-Adding a capacitor plate to ground to tame the high end
-Simplify the FET stage and use the same 12v from the heater supply

I think I'd also just re-do the tube stage instead of trying to tweak the board that's in there.

appreciate your thoughts.
« Last Edit: July 30, 2019, 03:46:12 PM by Blue Jinn »
not my thing. I am here just to unbalance the equation --pucho812

 http://www.bluejinn.org

Gus

Re: MXL V69ME mods revisited. Unity gain tube stage ---> FET?
« Reply #3 on: July 31, 2019, 07:36:35 AM »
Have you tried a different 12ax7 type tube?

First I would fix it.

Then I would try a low pass filter between stages

Gus

Re: MXL V69ME mods revisited. Unity gain tube stage ---> FET?
« Reply #4 on: August 03, 2019, 08:16:03 AM »
I took a quick look at one I have.
The heater has 9VDC regulator in the PS with two dropping resistors in the microphone to get the heater close to 6VDC. This is nice it causes a slow start by the added resistance

6VDC/300mA = 20ohms both filaments in parallel
The are two resistors in series 2.9ohm and 6.2ohm for 9.1ohm total
300mA x9.1 ohms = 2.73VDC

The PS has Zener regulation for the B+ and a microphone PCB has Zener regulation from the B+with a 1N4756 (47VDC) it looks like that is for the solid state section.
I have not traced the circuit any further because one board has a epoxy over a ceramic based sub assembly.

The 47VDC is suspiciously close to 48VDC phantom. So I wonder if the solid state part is a Schoeps like circuit with the phantom power resistors on the PCB
« Last Edit: August 03, 2019, 08:28:05 AM by Gus »


 

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