1

MP573 Microphone preamplifier / MP573 LED not behaving correctly

« on: August 10, 2025, 09:15:44 PM »

Hi Jean-Pierre,

I've just finished my second MP573 and it went together a lot smoother than the first!

The only issue I can't seem to work out is my LED. At first I had it the wrong way around (or so I think), so it only ever started glowing red but only at really high gains.

Now, it seems to at least go green first and then red, but I need quite a bit of input gain + trim to get it to come on.

I've checked all resistors that I thought are relevant (R48 - R48) and they have the right values.
I've checked the Diodes D6, D7, D9 for orientation.
I also checked the solder points for the U3 (I believe this is controlling the LEDs?) and they all look good too.

Would love some pointers on where to look.

Thanks
David

2

MP512 Microphone preamplifier / Mp512 kit components question

« on: August 08, 2025, 10:14:57 AM »

Hi,

Just started itemising the resistors for the MP512 kit.

1R0 - measures 1.7 and 1.8

5R6 - is the color code correct in the document? It says grn,blu,blk,slv but I can only find grn,blu,blk,slv,brn which measures 6.4

2K2 - got 1 extra

10K - got 2 extra



The extra ones are obviously not a problem but wanted to double check on the first measurements and the missing one.

Thanks a lot ??

3

MP573 Microphone preamplifier / MP573 Voltage not dropping when Transformer is Connected

« on: August 05, 2025, 06:37:22 PM »

Hi all!

I just found out that this is the better way to get help, rather than poor JPK getting bombarded with emails 🫠

I have finished my first MP573 build and am in the test phase.

After an initial hick-up with my TP2 -> V+ measuring being lower than ~24V, I have now fixed that help with Jean-Pierres help.

Now, the next issue seems to be the VTB1847 transformer not actually getting any power, because once I connect it I still get the exact same voltage from TP2 -> V+.

Things I've done:
- Double checked soldering connections
- Done a continuity test from the transformer wires to the circuit board
- Checked Q4...Q6, R40...R47, C24...C29 (as found in another thread) - couldn't find any issues visually.
- Checked the transformer resistances when unplugged. Readings below:

CARNHILL VTB1847 (unplugged) :

• Yellow-Orange : 8 ohms
• Red-Brown : 7 ohms
• Blue-Green : 21.6 ohms
• Violet-Black : 21.5 ohms

I also measured the test points according to this thread, here are the results:
A: 2.52V (expected 2.5)
B: 4.07V (expected 4V)
C: 22.3V (expected 22V)
D: 13.2V (expected 13.3V)
E: 2.0V (expected 2V)
F: 12.6V (expected 12.7V)
G: 0.4V (expected 0.9V)
H: 1.10V (expected 4.2V)
I: 0.35V (expected 0.35V)
J: 0V (expected 3.6V)
K: 0V (expected 22.3V)
L: 0V (expected 3V)

So it seems G, H, J, K, L are causing issues potentially

What else should I try next?

Best

David