[KungFuJosh]

Hi,

I completed my MP573 today. I tested all components before install, except for the thermistor, relays, and IC chip. Everything I tested was fine.

All my DC voltages are perfect per this thread: https://www.soundskulptor.com/forum/index.php?topic=5.0

My transformer impedances are also correct.

When connected to an interface, the only meter response is when clicking through the preamp switch's 6 positions, which makes me think there's output.

But, I get no input metering response from the MP573's LED, except when first powering it on.

I've verified the orientation before, during, and after building this of all the polarized and direction-oriented components.

I have an AWG and an oscilloscope. I fed a 1Vpp sine wave through the input of the 501 (confirmed working with a different preamp I built), and I get nothing at TP1. 

Please let me know what I should do next to find why I have no signal. My guess right now is an issue with the relays, but I'd prefer some ideas before I go removing the input preamp. One reason I think the relays might be an issue, is because I did not hear any switching when inserting a guitar cable into the DI jack. But this is obviously not the best measure since it's not always audible.

Thanks,
Josh

[JPK]

Check if the DI input is working.

and I get nothing at TP1.
The local ground is on TP2, different from the lunchbox ground.
Also check on the pin 7 of the input transformer.
If nothing arrives here, you may check if you can measure the primary resistance of the input transformer (48 ohms) from the input XLR pins 2&3. To do this the pre must be powered in order to get the relays properly switched. Jumper on JMP3, impedance switch on 1200

[KungFuJosh]

Check if the DI input is working.

and I get nothing at TP1.
The local ground is on TP2, different from the lunchbox ground.
Also check on the pin 7 of the input transformer.
If nothing arrives here, you may check if you can measure the primary resistance of the input transformer (48 ohms) from the input XLR pins 2&3. To do this the pre must be powered in order to get the relays properly switched. Jumper on JMP3, impedance switch on 1200

The DI doesn't work either. I was trying to trace the audio signal sine wave at TP1, got nothing there.

I get nothing on pin 7 of the input transformer.

Measuring resistance between pin 2 of input jack, and pin 2 of input transformer, I get nothing.

Measuring resistance between pin 3 of input jack, and pin 3 of input transformer, I get nothing.

I also get nothing if going from pin 2 input jack to pin 4 input transformer, and nothing from pin 3 input jack to pin 5 input transformer.

It doesn't appear to me that the relays are doing anything? They are oriented correctly. According to the silkscreen footprints and your instruction photos.

Yes, the jumper is in place on JMP3, as pictured in your instructions. Powered on, with imp switch at 1200.

Thanks,
Josh

[KungFuJosh]

I can trace the sine wave 1Vpp at R1 and C2, and R2 and C3, with the input AWG and scope probes referenced to ground. It appears to have signal up to the relays, but not after.

Thanks,
Josh

[KungFuJosh]

I removed the input transformer, and reflowed/added solder to the relays. The preamp is working great now, all voltages as expected, and everything works including the DI.

I wish I hadn't cut the relay pins short. I think that advice should be removed from the build instructions. There's plenty of clearance under the input transformer for the pins to be full length. This issue could have been avoided. If anything, I would suggest bending the pins down over the pads for added clearance, and mechanical support.

Thanks,
Josh

[JPK]

I wish I hadn't cut the relay pins short.
Agreed.

If anything, I would suggest bending the pins down over the pads for added clearance, and mechanical support.
I wouldn't do that because relay are mechanical parts that may fail someday and need replacing.
Bent pins would make the extraction more difficult.

[KungFuJosh]

I wish I hadn't cut the relay pins short.
Agreed.

If anything, I would suggest bending the pins down over the pads for added clearance, and mechanical support.
I wouldn't do that because relay are mechanical parts that may fail someday and need replacing.
Bent pins would make the extraction more difficult.

Good point. I won't cut the pins on the next one. But you should update the assembly guide, it says:

5. Relays
Add RLY1 to RLY3. Cut the pins ultra-short.

I'll ignore that next time. 

Thanks,
Josh