Vox NT15 various problems

[rockman627]

r13 to v3p7: bright= 57.4 k ohms; thick = 45 k ohms
r13 to v3p8: bright = ol; thick =ol.

[rockman627]

BTW the pcb now looks like this

[g1]

I'm not sure how hard it is to reconnect that board to test. It doesn't look that bad but I could be wrong. If it's not too bad, I would start reconnecting things one at a time from the R13 end and checking that the DC voltage at R13/C14 junction stays up. Leave R24 disconnected till last.

[rockman627]

I don't look forward to connecting a component, putting the board back, testing, removing the board, connect another component, putting the board back in, testing, 10 or 20 times. I will surely break at least one more pot like I did the gain control. But I probably will, but first I'm going to remove some more components with the board out.

But let's review:
I am reading about 22k ohms between r13/c14 junction and ground. Surely this should not be and it must be leaking current to ground which is throwing off my voltages past r13.
r13, r26, r9, and c9 are all lifted on one side, c14 and c17 are entirely removed.
From side 1 of r13 to side 1 of r26, r9, c9 I get 0 ohms. From side 1 of r13 to side 2 of r26, r9 and c9 I get OL. Doesn't that mean those circuit paths are ok?
So the c14 and c17 paths should be suspected?

I'm not rejecting the suggestion, I just want to try easier things before I get to that. Also I need my logical thinking confirmed or corrected.

PS those readings were in bright mode. In thick mode, r13 to ground changes to 10k ohms. All other readings are the same.

[rockman627]

On Tuesday I removed the bright thick switch and each pot. Nothing changed. Thursday about quitting time, I lifted r27, r28, and r29. I know I was supposed to do one at a time, but I got impatient and didn't think anything would change. Well I started reading OL between r13 and ground. When I get back next week, I will put back each resistor and see who is the offender. I hope the offender just needs replacement. Then I will resolder each component and check each time between r13 and ground.

[rockman627]

Okay it looks like r28 is the offender. So I started resoldering the lifted components. So I started to resolder the lifted components. When I got to r24, I started reading resistance again. This has certainly a brain twister. It doesn't help that the schematic differs somewhat from the board.

Some corrections:
Schematic On board
r25 r29
r36 r25
r18 (18k) r17
r32 r30

I'll try to post a corrected schematic when done.

Measurement with r24 and r28 in and out; bright first thick second.

r24 in, r28 in - 21.2k, 13k.
r24 out, r28 out - ol, 13k.
r24 in, r28 out - ol, 13k.
r24 out, r28 in - ol, 13k.

I'm trying to see some kind of pattern, but my brain is starting to fry.
c14 and c17 are still removed, vr2 is also out and needs replacement. r13 is still lifted.

That's it for now.

[g1]

Measurement with r24 and r28 in and out; bright first thick second.

r24 in, r28 in - 21.2k, 13k.
r24 out, r28 out - ol, 13k.
r24 in, r28 out - ol, 13k.
r24 out, r28 in - ol, 13k.

Can you repeat with R38 lifted?

[rockman627]

Will do on Friday. I took today off.

[rockman627]

I took all fresh readings today. Remember, r13 is lifted; c14, c17 and vr2 are removed totally from the board
r24 r28 b t
r38 in
in in 18.1k 13.2k
in out. 21.3k. 13.1k
out out. OL. 13.3k
out. in OL 13.3k

r38 lift
in in. OL. 13.4k
in out. OL 13.5k
out out OL. 13.3k
out. in. 21.3k. 13k

[rockman627]

I may have it, or maybe not. I again lifted r9, c9, and r26. This pretty much isolates r13s1. Then I started to measure resistance from r13 to any other component. I got to r24s2 and read 150kohms. Same with r26s2. After a lot of brain twisting, I looked and found a trace that went from c17s1 to r26s1. I thought that's my problem and scraped the trace. Well that cleared everything up: OL between r13 and r24 and r26 in bright and thick modes. But wait, you need a connection from r13 to r26. Instead of restoring the trace I hard wired from r13s1 to r26s1. Everything reads good now but when I read from r13 to ground, it starts off at 20k ohms and sloowly moves up to OL. Why, and is that a problem? I found the trace I scraped ran real close to c21 and I read 125k ohms from trace to c21.
Next Tuesday I will resolder everything and check all grounded components connect to ground, since I removed the copper tape. My head can't do anymore today.

[rockman627]

image widget

[g1]

when I read from r13 to ground, it starts off at 20k ohms and sloowly moves up to OL. Why, and is that a problem?

Not sure if R13 or C14 are back in circuit when this happened. If so, probably charging up C14 or C6 or even C18.

[rockman627]

Ok, I resoldered all components, put everything together, plugged into my light bulb limiter. No short. Plugged into my variac and set voltage at 120vac. Plugged in my test guitar got very low volume. I took new voltage readings. Here they are (r2.1 means r2 side 1 as per schematic; v3.4 means v3 pin 4):

r2.1 301vac
r2.2 290 vac
r1.2 329 vdc
r1.1 315 vdc
r17.1 277 vdc
r16.1 264 vdc
v3.1 143 vdc
v3.6 2 vdc
v4.1 229 vdc
v4.3 46 vdc
v4.6 142 vdc

So, things are a lot better except v3a plate has 2 vdc. Some other readings:

r13.1 241 vdc
r26.1 259 vdc
r9.1. 6 vdc
r9.2. 3 vdc
c9.1. 6 vdc
c9.2. 3 vdc

So, I'm losing voltage between r13 and r9/c9. Thursday I'll take the board out again and see what's up.

[rockman627]

Vox_nt15 blank 2.pdf
my updated schematic.

[g1]

So, I'm losing voltage between r13 and r9/c9. Thursday I'll take the board out again and see what's up.

If there's not another resistor there that is not shown on the official schem., I would guess there is a burnt or cracked trace there. Otherwise, seems like some good progress.