Mod kit 102+ not giving any guitar output

[WillK]

Hi all,

I'm currently trying to repair my Mod Kit 102+. When I first assembled it, it worked great, but now something has gone wrong and I'm trying to figure out what. I had done an extremely sloppy job soldering on my first go-round, so I decided to replace a lot of the internal components so I could do a tidier job and hopefully avoid shorts in the future.

Sadly, I'm getting no sound when I plug my guitar in. I wanted to poke around with my oscilloscope so I plugged in a tone generator from my phone and was able to get some sound out, but way quieter than it should have been.

I know enough to do basic soldering, and how to not electrocute myself, but beyond that so far I've only just followed instructions in kits without really knowing the theory behind things. If anyone is feeling generous with their time, I'd love to get some suggestions of what to test out and ask some newbie questions along the way so I understand what's going on under the hood a little better.

Here is a link to the troubleshooting document that includes the schematic. https://www.amplifiedparts.com/sites...supplement.pdf
Both tubes are glowing merrily, and I've gotten as far as testing the DC voltages at the points specified in the PDF. They tend a little low, but not so much that something seems obviously wrong.

Any help is much appreciated. Thanks!

[Jon Snell]

A difficult thing to advise on except, work backwards from pin 2 of V2, assuming you get a healthy signal passing through V2, that rules out V2 issues, if not there is your issue. . That will at least point you in the right direction as the power loss will be obvious when you inject a small signal to the faulty stage.
Keep it simple and check the obvious first; voltages should be within 20% of stated.

[Helmholtz]

Please post all measured DC voltages at points specified. The differences may give a clue.

Also check if all controls are working with tone generator.

[WillK]

Thanks for the responses!

These are the DC voltages:

Test point	Reference	Actual
HV	251	249
B1	249	245
B2	198	191
V1 pin 1	126	132
V1 pin 2	0	0
V1 pin 3	1.1	0.9
V1 pin 6	132	127
V1 pin 7	0	0
V1 pin 8	1	0.9
V2 pin 7	242	237
V2 pin 9	197	190
V2 pin 2	0	0
V2 pin 3	5.3	5.7

All the controls seem to be working with the tone generator. Since it's quiet, it's hard to tell if the push-pull on the treble is working, but all 3 change the tone/volume when I turn them, and I can tell a difference on the push-pull for the volume and bass.

I put an unconnected cable into the input to open the switch, then connected the noise generator to the sleeve of the input and V2 pin 2. The output was louder, clearer, and sounded like the sine wave I was expecting.

[Jon Snell]

Go back to basics before you involve your thoughts in more ideas.
Is the output valve working?

[Helmholtz]

DC operating points look good, means that the tubes are working properly.

Now check cathode caps (22µ and 100µ) and their connection as well as the 0.022µ coupling cap.

Adjust your tone generator to an output of 0.16VAC and try to verify the AC signal levels given in the schematic/instructions.