1

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

« on: August 07, 2025, 04:03:07 PM »

Do a sound test. If it works then it's your meter.

i did - it’s working perfectly - sounding really good! 🙏🏻

thanks for your help! On to the second, and then 2 Mp512 😁

2

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

« on: August 07, 2025, 12:58:36 PM »

From the new numbers, it looks like Q4 and Q5 are damaged.
You should try to replace them.

You are right! Q4 and Q5 seemed to have been damaged. I had D4 the wrong way around at the very start, could that have destroyed them?

Here are my latest measurements:

V+ to TP2
Without Transformer: 24.8V
With Transformer: 24.8V

Checkpoints:
A 2.5V
B 4V
C 22.2V
D 13.2V
E 1.9V
F 12.6V
G 0.99V
H 4.08V
I 0.33V
J 3.52V
K 22.2V
L 3.01V


So it's almost looking good! Just still not getting a loss when connecting the transformer. Could that be my multimeter being inaccurate?

3

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

« on: August 07, 2025, 10:18:56 AM »

Also i measured between TP1 and TP2 and it’s getting 7.3V
U1, U2 and Q6 is getting quite warm!
All this indicates that too much current is flowing in Q6. But this is contradicted by your previous measurement J: 0V (expected 3.6V). Can you check J again ?

So this morning I've taken all measurements again, trying to give you as much info as possible.

From Setup Guide Step 3: General Power Check
Transformer not connected: 24.9V
Transformer connected: 16.5V

Transformer Connected:
A 0.17V
B 2.9V
C 14.4V
D 8.2V
E 1.2V
F 7.61V
G 3.9V
H 9.5V
I 3.9V
J 7.9V
K 11.57V
L 7.4V


Transformer Disconnected
A 2.5V
B 4.08V
C 22.3V
D 13.25V
E 1.98V
F 12.62V
G 0.39V
H 1.11V
I 0.39V
J 0V
K 0V
L 0V

I noticed U1, U2 and Q6 only get hot quickly when the transformer is connected which makes sense?

It’s also strange that I used to be able to adjust the BIAS, then changed nothing and now it’s not adjustable and gets 7+ volts. 

Verified Transformer VTB1847 (disconnected):
Yellow: 11.6 ohms
Orange: 12.6 ohms
Red: 12.6 ohms
Brown: 13.6 ohms



VTB9045 (when powered on):

2-4: 13 Ohm
3-5: 13 Ohm
7-9: 141.6 Ohm
8-10: 142.4 Ohm

4

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

« on: August 06, 2025, 05:24:29 PM »

Check you didn't swap or reverse U1, U2 or Q6.
Triple checked, not reversed or swapped.

Look for a solder bridge around these components.
Could not see any bridges with magnifying glasses

Check you don't have a reversed electrolytic.
Double checked all components that have a direction - they look all good

Do you really mean TP1?
Apologies, I mean TP4 and TP2

5

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

« on: August 06, 2025, 04:10:03 PM »

Yes, R24 is 12.6Ohms.

i just remeasured from V+ to TP2 and now it is 16.6V with the transformer plugged in, and 24.9V with it not plugged in. So it is now dropping - even though I haven't changed anything. But obviously not it's dropping more than it should

Measuring from TP2 against both ends of R24, I get 24.1V

I also just noticed that U1, U2 and Q6 is getting quite warm!

Also i measured between TP1 and TP2 and it’s getting 7.3V - not adjustable as much as I turn

6

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

« on: August 06, 2025, 10:26:29 AM »

Thank you!

Just measured. 

Without the transformer connected measuring on the board I get 24V on brown but 0 on yellow, red and orange. 

With the transformer connected measuring on the pin on the small transformer connector board I get
Yellow: 11.6V
Orange: 12.6V
Red: 12.6v
Brown: 13.6V

Thank you for your help!

7

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

« on: August 05, 2025, 09:26:44 PM »

I have just run a few more tests and I _think_ i have an issue with R4 and R5. When I try to measure across them (i.e. check value of R4 and R5) I get a reading of 0... They should be 4k7.

For D1 I get no reading (it flashes at 1.2k), whereas D2 and D5 read at ~ 1.9k.
R6 and R28 get the right readings.

This is very annoying - would love to know if this could be a cause because I would have to desolder the entire transformer sitting on top of it 🫠

8

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