Sound Skulptor forums

Build help and troubleshooting => MP573 Microphone preamplifier => Topic started by: FF COLT on March 16, 2021, 03:49:00 PM

Title: No input when testing
Post by: FF COLT on March 16, 2021, 03:49:00 PM

Hello, I did the LA502 on one shot but the MP573 was my first DYI, so I did some mistakes I guess because I got no input (either on xlr or jack). I can hear some click when switching to the 1200 ? and the gain / trim reacts. 
The 1st generics tests was ok but here's the results of the error procedure:

A: 2v70
B: 4v16
C: 23v
D: 13v92
E: 2v08
F: 13v33
G: 0v50
H: 1v07
I: 0
J: 4v27
K: 0v41
L: 0v15

thanks for your help.

Title: Re: No input when testing
Post by: JPK on March 17, 2021, 09:27:39 AM

Check the solder joints on R24.
Check the connections of the output transformer: On connector CN2 socket and wires.
Check the output transformer DC resistances: https://www.soundskulptor.com/forum/index.php?topic=5.msg5#msg5

Title: Re: No input when testing
Post by: FF COLT on March 18, 2021, 10:47:56 PM

Hi, I've checked the solder joint everything looks ok

I've done more tests and I can have an output line (through the daw) but the signal is very low, I had to push the preamp and gain at max.

here's the output transformer DC resistances:

CARNHILL VTB1847 (unplugged) :

• Yellow-Orange : 7,9 ohms
• Red-Brown : 6,7 ohms
• Blue-Green : 21,4 ohms
• Violet-Black : 21,4 ohms

CARNHILL VTB9045 :

• 2-4 : 24,7 ohms
• 3-5 : 24,7 ohms
• 7-9 : 132,2 ohms
• 8-10 : 133 ohms

Thanks[/list]

Title: Re: No input when testing
Post by: JPK on March 19, 2021, 10:20:01 AM

What DC voltage do you get on V+, TP2 and both sides of R24, relatively to the lunchbox ground (pin 13 of the back connector)?

Title: Re: No input when testing
Post by: FF COLT on March 20, 2021, 06:18:57 PM

Sorry, but I don't get it?

I should put the negative to V+ and then with the postive on TP2, R24 and pin 13? 

Title: Re: No input when testing
Post by: JPK on March 22, 2021, 10:06:18 AM

The negative should go on the 0V pin.

Title: Re: No input when testing
Post by: FF COLT on March 23, 2021, 09:07:07 AM

so here's the results:

V+: 15.8
TP2: - 9.13
R24: 15.1 / 15.9

Title: Re: No input when testing
Post by: JPK on March 23, 2021, 06:09:57 PM

Check the DC voltages (relatively to the 0V pin) on the output transformer brown wire and yellow wire.

Title: Re: No input when testing
Post by: FF COLT on March 24, 2021, 09:15:45 AM

brown: 0
yellow: 0

don't know if it's ok or not?
thanks.

Title: Re: No input when testing
Post by: JPK on March 25, 2021, 08:28:52 AM

Not OK!
The brown wire is connected to the +16V via R24.
Did you need to de-solder some components during ther build?
In such a case check that the vias that connect the top and bottom pads together are still there.
With the connector unplugged, measure the resistance between CN2 lowest pins pair and V+ : 12ohms.