NC3015 Troubleshooting Help

[donnieschex]

Greetings all...new member and first time poster. I recently finished building a Mojotone NC3015 kit, and I was hoping to get some troubleshooting help with a few issues. I've built a couple of other amps, but am by no means an expert (just decent at soldering lol). Schematic and wiring diagram are attached.

Sorry if this is too much info in a single post...I would really appreciate any help that can be provided. The problems I'm trying to fix are:


1) Plate voltages on the preamp tubes are low/off

My voltage readings all appear to be normal except for the plates on all 3 preamp tubes. I checked the wiring several times and it all looks correct. Not sure how to resolve this:




2) Only 1 out the 4 input jacks are working

I've checked the jack wiring several times without spotting any issues. I also verified the continuity of all the ground connections. The amp has both normal and brilliant channels...here's what I'm seeing with each:

Normal channel - Lo: Works as expected (and sounds great)
Normal channel - Hi: hum (no audio)
Brilliant channel - Lo: completely dead
Brilliant channel - Hi: Hum (no audio)

Photos of the jack wiring...hopefully they will help:




3) The Treble and Bass controls aren't working

While using the one working channel, I noticed there's no effect when I adjust the bass and treble controls. The top cut control works as expected. Here's a gut shot of the whole amplifier...again, all of the wiring and connections seem right, but I'm not sure how else to troubleshoot:



Thank you in advance for taking a look...I really appreciate it!

[Helmholtz]

Your "desired voltages" in the table are partly wrong/don't correspond to schematic values.
Please also post actual supply voltages at power supply nodes XA, XB, XC, XD for reference.

Your measured voltages don't look that bad, except for the cathode voltages on V1 and V2.
Check your meter/battery.

I think your main problem is with the input jacks. Check wiring and make sure the switching contacts work.

[donnieschex]

Thanks for the assistance. Here are the PT voltages:

XA - 361.7
XB - 332.9
XC - 283.9
XD - 203.3

I changed the batteries in my meter and got more accurate results. I also updated the values to match the schematic (click below...sorry, I reached my upload limit):

https://ibb.co/PW7v7zK

[Helmholtz]

Actually your measured voltages make much more sense than the schematic voltages, which look wrong.
With these voltages the amp should work ok.

As said, check the input jacks. It seems they don't switch as intended.

[donnieschex]

I did inadvertently overtighten the plastic nuts on the cliff jacks during installation...it made it really hard to insert the 1/4 cable, but loosening then hand-tightening them fixed that problem. I wonder if the jacks could be damaged from overtightening and then inserting the cable? I wouldn't have thought they would be that fragile. I ordered a few more, just in case

[Helmholtz]

Use your Ohmmeter to check if the jack switches work.

[donnieschex]

Continuity checks were successful on all 4 jacks...tip and sleeve have good connection from end to end when inserted.

[Helmholtz]

Continuity checks were successful on all 4 jacks...tip and sleeve have good connection from end to end when inserted.

That doesn't mean much.

Each jack has 2 built-in switches, look at the schematic.

[donnieschex]

OK...thanks for your patience. I re-flowed the solder joins on the normal channel and both of those jacks are working now. 1 down, 2 to go.

On the Brilliant channel, I verified the built-in switches are good. I even replaced one of the jacks that was suspect...still not working. I grabbed a quick video of what happens when I touch the connections on the tip side of the jack. Just loud hum and a popping sound when I touch:

https://youtu.be/0Z0cHocSfRE

Could it be a grounding problem? I verified all my ground connections are where they should be, and re-flowed all connections on this set of jacks, as well. The wiring is identical for both sets of jacks. There's really not much to it...a few resistors and a wire that runs straight to preamp tube #1. I must be missing something obvious here.

[g1]

Look at your jacks. All lugs should be pointed in the same direction. You have one pair flipped over, so they are no longer wired correctly.

[donnieschex]

Look at your jacks. All lugs should be pointed in the same direction. You have one pair flipped over, so they are no longer wired correctly.

Doh! Jack problem resolved lol...thank you. The forum has been so helpful; I'm grateful for all the assistance.

Now that the jacks work, there's another issue on the Brilliant channel. The volume and treble pots have weird "steps" when you turn them and the sound produced isn't as loud as expected...touching the first treble pot lug produces hum, and so does the first volume pot lug (but less so). Again, my normal channel is working perfectly. Here are a couple of videos demonstrating the problem:

https://youtu.be/QP18GiBX018

https://youtu.be/P2te0gDH9bA

Any suggestions? I'm getting real close here.