jsteiger

Here's a pic of the  Litz output wires.

Everything on the doa sockets tests over 200k except-
"+V" and "C" @ 3.6k
"C" and "O" @ 52.5K
"+V" and "O" @ 57k

If I change the jumper to "output cap bypass"  everything over 200k except-
"C" and "O" @ 0.00
"+V" and "C" @ 3.6K 
"+V" and "O"@ 3.7k
With the output cap(s) bypassed, you should have around 9Ω. You may need to adjust your DMM's range to confirm. This is all correct.
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Thanks, Jeff and Mike, for your replies. So everything measured fine and I inserted the VP312DI in my rack with an extension jig. I have a Trident DecaDent, but for my initial tests I wanted to use my Fredenstein Bento. I thought if something went wrong, I’d rather blow the cheaper of the two racks. ;) I fired up the preamp, everything worked as expected, and it sounded amazing! The DI on my bass alone was killer. The only issue I had was the high squeaking noise when I engaged phantom power. However, I know that this is an issue with my Fredenstein, I’ve had that before with one of Volker’s preamps. (Actually that’s why I also bought a Trident chassis).

I then tried a different slot that is further away from the Fredenstein’s PSU to see if that changes the squeaking. Same extension jig, same setup. A few seconds after I engaged phantom power, I saw some smoke rising and R10 burnt. Bugger. I highly suspect that this is due to the Fredenstein’s weird 48V rail. But is there anything else I should doublecheck when I replace R10 and fire the VP312DI up next time? Or do I maybe need a resistor with .5 W, not just .25 W?

If this should matter, I have the version with a stepped gain switch, the gar2520 and the FET-ZCON1 plug-in.

Thanks a lot,
Claudio

I was able test the DI and it works with post 2622 engaged and not engaged. The gain and output attenuators both work so I assume  this rules out input and output transformers. I touched up all solder joints and checked for bridges but mics still don't work.
« Last Edit: January 04, 2019, 11:17:07 PM by mrmike186 »

jsteiger

Thanks, Jeff and Mike, for your replies. So everything measured fine and I inserted the VP312DI in my rack with an extension jig. I have a Trident DecaDent, but for my initial tests I wanted to use my Fredenstein Bento. I thought if something went wrong, I’d rather blow the cheaper of the two racks. ;) I fired up the preamp, everything worked as expected, and it sounded amazing! The DI on my bass alone was killer. The only issue I had was the high squeaking noise when I engaged phantom power. However, I know that this is an issue with my Fredenstein, I’ve had that before with one of Volker’s preamps. (Actually that’s why I also bought a Trident chassis).

I then tried a different slot that is further away from the Fredenstein’s PSU to see if that changes the squeaking. Same extension jig, same setup. A few seconds after I engaged phantom power, I saw some smoke rising and R10 burnt. Bugger. I highly suspect that this is due to the Fredenstein’s weird 48V rail. But is there anything else I should doublecheck when I replace R10 and fire the VP312DI up next time? Or do I maybe need a resistor with .5 W, not just .25 W?

If this should matter, I have the version with a stepped gain switch, the gar2520 and the FET-ZCON1 plug-in.

Thanks a lot,
Claudio
R10 the 200Ω resistor? This will have nothing to do with phantom power. My first thing would be to look at the gar2520 (if it was built from a kit). I have honestly never heard of smoking that R in any of my pre's so not sure what would have caused that except for an opamp problem.
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jsteiger

I was able test the DI and it works with post 2622 engaged and not engaged. The gain and output attenuators both work so I assume  this rules out input and output transformers. I touched up all solder joints and checked for bridges but mics still don't work.
The pre should switch from Mic to DI when a 1/4" plug is inserted. FYI, the DI jack is auto-switching so you can only use Mic mode with NO 1/4" plug inserted.

Otherwise, I would look at the plug in relay board for the Mic/DI switching. Its likely stuck in DI mode which means an error on that PCB.
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R10 the 200Ω resistor? This will have nothing to do with phantom power. My first thing would be to look at the gar2520 (if it was built from a kit). I have honestly never heard of smoking that R in any of my pre's so not sure what would have caused that except for an opamp problem.
Hey Jeff,

I did some more tests and strangely enough now the preamp works without any problems (except for the Bento  noise). Not sure what went wrong when I saw the smoke. Possibly I had the extension jig inserted incorrectly and was causing a short? Anyway, would you recommend I still replace R10 or can I risk running the preamp as it is now?

Best regards,
Claudio

jsteiger

Hey Jeff,

I did some more tests and strangely enough now the preamp works without any problems (except for the Bento  noise). Not sure what went wrong when I saw the smoke. Possibly I had the extension jig inserted incorrectly and was causing a short? Anyway, would you recommend I still replace R10 or can I risk running the preamp as it is now?

Best regards,
Claudio
Its likely OK but if its darkened, I think I would replace it since the rest of the preamp is fresh and new.
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The pre should switch from Mic to DI when a 1/4" plug is inserted. FYI, the DI jack is auto-switching so you can only use Mic mode with NO 1/4" plug inserted.

Otherwise, I would look at the plug in relay board for the Mic/DI switching. Its likely stuck in DI mode which means an error on that PCB.


Should I be looking at the relay PCB behind the input transformer? I assume the one behind the 1/4 jack would not cause this.

This may be a silly question but when the relay swithces and makes a click sound,  does it come from the actual unit or though the speakers/headphones? 

Sorry for all the questions but lastly is there any way to bypass the relay to test the rest of the preamp? 

Thanks for all your help.

jsteiger



Should I be looking at the relay PCB behind the input transformer? I assume the one behind the 1/4 jack would not cause this.

This may be a silly question but when the relay swithces and makes a click sound,  does it come from the actual unit or though the speakers/headphones? 

Sorry for all the questions but lastly is there any way to bypass the relay to test the rest of the preamp? 

Thanks for all your help.
The clicking is mechanical from the relay itself, not audible in the output. Check the schematic to see which one is for Mic/DI. I can't recall and would have to look.
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Ricardus

I have two of these on the horizon for a next possible build.
Audio mastering for hire..


I ended up replacing both relay PCB's  and one  of the toggle switches I damaged while troubleshooting  so now I have a functioning  312DI......well almost anyway.  The DI doesn't work while in post 2622 mode but it does while not engaged and the mic input works and sounds great.  I hear the click on the relay when I insert a 1/4 inch plug and when I toggle the 2622 switch so I'll look at the FET ZCON1 Hi-Z plugin next.  It's getting close!

jsteiger

I ended up replacing both relay PCB's  and one  of the toggle switches I damaged while troubleshooting  so now I have a functioning  312DI......well almost anyway.  The DI doesn't work while in post 2622 mode but it does while not engaged and the mic input works and sounds great.  I hear the click on the relay when I insert a 1/4 inch plug and when I toggle the 2622 switch so I'll look at the FET ZCON1 Hi-Z plugin next.  It's getting close!
Also make sure the 1/4" plug is mono. A stereo plug WILL NOT WORK with the relay switching of the VP312DI.
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