Great River MP-2 , info, schematics & discussions

[G-Sun]

What fet is this? Any fet? Is it on the boom. Is it on the pcb?
.. I guess it's two gain-fets on the pcb/boom,
and that I'll just now balance + and - side, right?
Ok, so need to order a 2k pot then

Take a look at the schematic. There's only two FETs in the whole circuit and they are right above R46. Q1: 2sk-389
The small trimming using R46 balances the FETs to calibrate the amp.

Thanks!

 

[G-Sun]

What's min/ideal/max dc voltages from psu?
(Before the onboard regulators)
I'm unsure of the voltage of the boards them-selves, what is it? +-18V?

 

[okgb]

I believe  regulators only regulate down, so the raw dc should be at least 24v

 

[dmp]

The boards use +/-24v
The voltage into the regulators needs to be at least 24v plus a little bit for the regulators to drop. Then the higher the input voltage it is, the more heat they have to dissipate. If you are regulating 24v from a 34v source, drawing 0.1mA will require the regulator to dissipate 0.1*(34-24) = 1 Watt

 

[G-Sun]

Hello!

I've put together my MP-2
It's passing audio, but only barely.

Noise is
ch 1:
-88 dbfs after AD, min gain
-46, max gain
ch2:
-73 dbfs, min gain
-42, max gain

I can see the meter moving when speaking into my sm58 on full gain on both channels.
But almost it's hard to hear anything.

Voltages:

AC in: 29,5/29,4 V

DC out: 
+24,9V
- 24,8V
+51,7 V

Images: https://www.dropbox.com/sh/qikvee1ibn9hcad/AABo8iEqQyuOp4inmSwUND88a?dl=0

 

[G-Sun]

Regulators
Ch1
lm337: in -24,8, out -23,2V
lm 317: in24,8, out 23,2V

Ch2
lm337: in -24,8, out -23,3V
lm 317: in24,7, out 23,2V

Now, where to go from here?

 

[G-Sun]

Inputs checked with 13kohm on input transformers secondary

 

[G-Sun]

BTW: If anyones would like to take over this unit, I'm open for trade/sale.
Because I'm really covered regarding preamps.

Interested in LC53A, Pulteq MEQ-5, a utility comp (not 1176)

 

[josh]

Howdy all,
Is there anyone still selling pcb's for this project?
I checked ptownkid's site and pcbgrinder, but nada.

 

[G-Sun]

Howdy all,
Is there anyone still selling pcb's for this project?
I checked ptownkid's site and pcbgrinder, but nada.

Your best bet is black market I guess.
Matador handled the pcbs last time I belive.

 

[G-Sun]

So,

I have two channels passing audio (barley), and seems to be similar.
So no single component failure or bad wiring/soldering

I'll check all components, value and placement.
Any more ideas? (Not done yet)

 

[G-Sun]

Tested unbalanced out. Same as balanced.
Seems like I have 20db signal to noise ratio (reading from meters).

Checked diode and cap direction. Fine

 

[dmp]

Seems like you must have a mistake on both channels. The first thing I would eliminate is the input section. Doe the signal from the XLR make it to the secondary of the input transformer?
If you don't have a scope, you could use a probe into headphones with some signal applied to the XLR.
After you've confirmed the signal makes it through the input section, see if the gain switch affects the output signal as it should. Does the level change as you turn the gain switch?

 

[G-Sun]

Thank you very much!

Seems like you must have a mistake on both channels.

Yes, so it's probably not a bad solder-joint or loose wire or something like that.

The first thing I would eliminate is the input section. Doe the signal from the XLR make it to the secondary of the input transformer? If you don't have a scope, you could use a probe into headphones with some signal applied to the XLR.

I see the meter move when connecting a sm58 to the input and from my output to AD.
Should I use a probe to verify the quality of signal on IN-transformer secondaries?

After you've confirmed the signal makes it through the input section, see if the gain switch affects the output signal as it should. Does the level change as you turn the gain switch?

Yes,  turning gain-knob changes output noise and signal level.

 

[dmp]

Should I use a probe to verify the quality of signal on IN-transformer secondaries?

Yes, I think you need to do more than see if the meter moves. You want to know where you aren't getting the gain you are supposed to be getting. Can you make a probe to narrow down where the signal isn't as strong as it should be?

 

[G-Sun]

Thank you!

Yes, I think you need to do more than see if the meter moves. You want to know where you aren't getting the gain you are supposed to be getting. Can you make a probe to narrow down where the signal isn't as strong as it should be?

Ok, I made mono probe.

From my daw, a sine-wave, -6dbfs. ADA8000 out, reading 1,23V ac
Ah.. probe not working.
Making some wires to my headphone.
See it's step up 5:1, so takes signal to -12dbfs

Ch1 and 2:, J3: Clear signal
(Couldn't get a proper ac-reading)

 

[dmp]

If the signal is good at the transformer secondary (I'm not sure based on the wording of your post), move on in the circuit.
Looking at the schematic, I'd check the junction of R8 & R9 next - which is the input to the first transistor stage.
Then the base of Q7.
If you keep moving through the circuit you should find the place where the signal drops off.
And then, I bet you'll find a wrong component value, like a 49K9 in place of a 49R9, or something like that.

 

[G-Sun]

If the signal is good at the transformer secondary (I'm not sure based on the wording of your post), move on in the circuit.
Looking at the schematic, I'd check the junction of R8 & R9 next - which is the input to the first transistor stage.
Then the base of Q7.
If you keep moving through the circuit you should find the place where the signal drops off.
And then, I bet you'll find a wrong component value, like a 49K9 in place of a 49R9, or something like that.

Sorry for unclear wording.
Yes, it's good on transformer sec.

I'll move on as you say.
Should I use ground for negative?

I'll try to make a new probe, as else I need an extra set of hands

Thank you very much!

 

[dmp]

Yes, just put the negative on a ground point.

 

[G-Sun]

Ok,
I've made a ne probe. Working

Now, there's one problem. I have bom, schematic and placement/value.
But I don't know what on schematic is where..

I'm trying tracing from pcb, but it's dualsided.

Ok, R8 is 562k, only one place.
But.. I'm confused on what goes where..

Q7 I can find. Base,  is that middle pin? Hard to get to.