Vox NT15 various problems

[rockman627]

I believe I got it! Here's what I did:
I originally hardwired from r13.1 to r26.1, but I was losing voltage. Then I went from c17.1 to r26.1; still no good. I added a hardwire from r13.1 to c14.1; that did it. I'm getting sound from all settings (bright/thick, pentode/triode), and the tone knobs work, as well as the gain and volume knobs. I'm going to keep this here for a week to test it before I take new measurements and close it up.

[rockman627]

On the schematic, what does +1v97 mean? Is it like 1k5 ohms which mean 1.5 kohms? Why can't we stick to one standard?

[g1]

On the schematic, what does +1v97 mean? Is it like 1k5 ohms which mean 1.5 kohms?

Yes, that is correct, 1V97 is 1.97V. For myself, I prefer the method without the decimal point. You don't run into any issues where the decimal or comma become confusing in the European usage. I think of the 1K5 style as 'metric' but that is probably not correct.
Also, the decimal point can be hard to see on schematics, or is sometimes missed if there is not a zero in front for values less than 1.

[rockman627]

Sounds logical. Alrighty then.

[rockman627]

Ok the site is back and I'm at work with my notes. First up is my latest voltage measurements. R2.1 is r2 side 1 as per schematic. V3.7 is v3 pin 7. Here goes:
r2.1 301vac
r2.2 292vac
r1.2 341vdc
r1.1 329vdc (all the rest are in vdc)
r17.2 280
r16.1 261
r13.1. 251
r31.1 43
v3.7 0v89
v3.1 128
v4.1 235
v4.3 44
v3.3 0v8
v2.7 334
v1.7. 342
v2.9. 329
v1.9 329
r9.2. B 132. T 118
r24.1. B 0v89 T 0.71
v4.7. B 2 T 1v7
v4.8. B 3v3 T 3
v4.6. B 175 T 174

So it looks like everything is within spec. Took it home, played with it a few hours, and it sounds pretty good. Crunch sounds are very good. Drive sounds are sweet. Clean sounds are nice, but maybe not loud enough for full band gigs.

[rockman627]

Ok, to summarize:

• original problem was loud hum or no sound
• voltage readings indicated c14 was a problem; c14 and c17 replaced, no improvement
• ohm readings between c14 and ground; these readings increased as I tried different things
• removed glue; no improvement
• lifted r13, r9, c9, and r26; unknown connection to ground still there
• started ohm reading between r13 and all components; found it at r24
• pcb connection from c17 to r26 ran close to r24; cut trace and hardwired. Unknown connection to ground gone but still problems.
• No connection from r13 to c14 ( it was there before, I probably wrecked it removing the glue); hardwired it. Amp works good now in all modes.

This was one tough cookie for me. I suspect conductive glue was the culprit and caused the spark by r24, which caused a permanent connection to ground. This problem took a long time to solve and I'm glad I stuck with it, and I am so grateful for the advice given which pointed me in the right direction.

[Delta362]

Glad to see you solve it and what you found along the way!