[Cecil]

Hello,

I've just finished building and testing my mp573. Everything works fine except the trim pot.
At the extremes (-10/10+) the signal passes for all inputs at all switch positions, 
but when the trim pot is set at '0' the signal is clipped and extremely distorted.
The gain difference at -10 vs 10+ is not any different either. Same metering on my interface at -10 as 10+.
I'm assuming this is a faulty pot.

[JPK]

Look for a bad solder joint or a solder bridge on the pot pins.
With the trimpot set to 0, jumper CN1 removed can you measure the resistance between the center pin and both side pins on the 2 sections of the trimpot?

[Cecil]

All the solder joints are sweet. No dry joints or bridges.
'0' position resistance measurements

Transformer side closest to LED - 2.74k 
Transformer side farthest fro led - 2.46k

Faceplate side closest to led - 565
Faceplate side farthest from led 001

[JPK]

The pot seems OK.
Can you check the voltages ? 
https://www.soundskulptor.com/forum/index.php?topic=5.msg5#msg5

[Cecil]

With the gain at 10, the trim at 0 and 1200ohm

a-2.3
b-3.8
c-22.1
d-12.5
e-1.8
f-11.9
g-0.89
h-4.33
i-0.32
j-3.79
k-22.1
l-3.29

[JPK]

Your voltages are OK.
Check that the caps C15, C16, C17, C21, C25, C26, C28 have good values and solder joints.
Is the issue occurring in all the trim pot positions except the extremes?
Is it the same for all the gain switch positions?
Is there a 20 dB gain increase between the 2 extreme positions?
Is the gain switch working in all positions.

[Cecil]

The trim pot distorts at all points except the extremes with the signal degrading completely at the 'O' point. 
It's the same a t all gain switch positions
There is no gain increase between -10 and +10 on the trim knob.
The gain switch works in all positions

[Cecil]

I'll check the Caps tomorrow

[JPK]

Also redo a check of the trim pot.
Check that the ohm value increases progressively from 0 to the max, between the center pin and the side pins when you rotate it.
On the faceplate side pot, the farthest pin from led is connected to the center pin. This is normal.

[Cecil]

between the outer pins of the faceplate side potentiometer 
0 ohm at +10 
smoothly to 600 at 0
990 at-10
between the middle pin and 'centre' pin (farthest from led) 0 ohm at all positions

closest to led pin and middle pin on other pot 0 ohm to 600 to 989

[JPK]

closest to led pin and middle pin on other pot 0 ohm to 600 to 989

This doesn't match with your other post where you said:

Transformer side closest to LED - 2.74k
Transformer side farthest fro led - 2.46k

If the last measurement is true then the value of the transformer side pot is wrong.

[Cecil]

Ok, the second measurement was with the jumper still on CN1. Is that why?
Would an incorrect Pot value would cause the issue I have?

[JPK]

Yes, redo the measurement without CN1 in.

[BC184]

This ended up on my bench... It had 3 bad tantalums; I replaced the fourth tantalum just because. All were replaced with Panasonic FC series aluminum Electrolytics.
C24 was bad, C20 and C27 were installed backwards (plus to minus). 

Also the first relay that switches between the xlr input and the front panel HiZ input was stuck. I replaced it with a Magnecraft TQ relay.

Seems reasonably happy now.
cheers

[JPK]

Good work!