Universal Audio 4 - 710d Troubleshooting

[nedonnelly]

Hello electron wizards!

Need a little bit of help troubleshooting a UA 4-710d pre. There is a single channel (Channel 1) that is acting up, and I'm looking for thoughts on places to look. I have isolated the problem to the MIC-IN section of channel 1, but that is the extent of where I'm at currently. The front panel HI-Z and the rear panel Line In are all fine, but the MIC-IN is where things go awry. The problem is this: there is a noticeable reduction in volume (dB) when sending signal through the Channel 1 Mic Input. It doesn't matter if the signal is using phantom power or not. The mic input always gets a reduced signal. I have tried every one of the trimpots (gain, level, THD) to see if there is a noticeable difference, but none of them have yielded much of a result.

You can see the volume difference here:


Unfortunately, I can only find the block diagram online for the 710d, but maybe someone more adept at schematic scavenging can point me towards one.
Here is the block diagram if it helps:


Here is a gut shot if that helps too:
View attachment IMG_3542.jpg

Thanks in advance for the information!

 

[mjrippe]

Try re-seating all of the connectors, I have seen that solve problems in UA gear before. Next look at the solder joints, then check continuity from the mic XLR through the pad switch and on to the mic/line switch.

Funny how they call it both a "HP Filter" and a "Lo-Cut Filter". Both are correct, just pick one!