Finished building the crack, but...

[mqbush]

My resistance measurements are good, I've looked over all of the connections (and re-soldered a couple), the tubes glow when I plug it in and turn it on, but my voltage measurements are all wrong.  Here is the checklist:

Terminal  Expected Voltage  Measured Voltage
1                    75-90            -1.47
2                        170            -1.47
3                          0                  0
4                      170              -1.46
5                    75-90              -1.45
6                          0                  0
7                      100                  0
8                          0                  0
9                      100                  30mV
10                        0                -0.2mV
11                        0                  0
12                        0                  0
13                      170              -0.66
14                        0                  0
15                      185              -0.66
20                        0                  0
21                      206              -0.66

A1                        90              -0.95
A2                          0                  0
A4                          0                  0
A5                          0                  0
A6                        90              100mV
A7                          0                  0
A9                          0                  0

B1                        90              -0.96
B2                      170              -0.77
B3                      100                55mV
B4                        90              -0.96
B5                      170              -0.76
B6                      100              56.6mV
B7                        0                  0
B8                        0                  0

Also, I do not see any light from the LEDs.

Help!

 

[Paul Birkeland]

Glowing tubes is a good sign, that means the transformer is working properly.

Set your meter to AC volts at the highest scale, then measure the AC voltage between 6 and 7 on the power transformer?  How about 9 and 10?

Can you confirm that the wire colors are correct for the twisted pair that connects these two windings on the power transformer? (6 goes to 9, 7 to 10)

A very common cause of something like this would be a loose or missing wire in the power supply, or going from the power supply up to the amplifier.  One way to triple check is to pull on the wires and components to look for loose connections.  (Do this while the amp is off of course)

-PB

 

[mqbush]

I appreciate the help.

6 & 7 shows 175 VAC.
9 & 10 shows 175 VAC.

I verified that 6 goes to 9, and 7 goes to 10.

I went over the wiring & connections again, but I am not finding any loose or missing wires.  I also followed Grainger49's ground tracing process, and everything checked out.

 

[Paul Birkeland]

OK, next measure the ac voltage between terminals 19 and 18.

If you see 175V there, then measure the DC voltage between 20 and 21 (carefully).

You can also post a photo of the power supply area up here, there may be something obvious.

-PB

 

[mqbush]

The voltage at 19 & 18 is 169VAC.
The voltage at 20 & 21 is 0.75 VDC.

I'll see if I can get a good picture of it to post.

 

[Paul Birkeland]

Yeah, a photo at this point would help.  It's possible that the diodes aren't properly oriented, but that region would be where your issue is.

-PB

 

[mqbush]

Ok, I took another look at it this morning, and found a bad solder joint on terminal 18.  I resoldered it, along with a couple of others.  Now I get these voltage readings:

Terminal  Expected Voltage  Measured Voltage
1                    75-90              149.8
2                        170              166.5
3                          0                  0
4                      170                166.5
5                    75-90                79.4
6                          0                  0
7                      100                  150
8                          0                  0
9                      100                  102.9
10                        0                  0
11                        0                  0
12                        0                  0
13                      170                166.3
14                        0                  0
15                      185                189.7
20                        0                  0
21                      206                213.3

A1                        90                79.3
A2                          0                  0
A4                          0                  0
A5                          0                  0
A6                        90              149.7
A7                          0                  0
A9                          0                  0

B1                        90              150
B2                      170              166.2
B3                      100              149.7
B4                        90                79.3
B5                      170              166.2
B6                      100              102.9
B7                        0                  0
B8                        0                  0

It looks pretty good except for terminals 1,7,A6,B1 & B3.  I also noticed the LED connected to B3 is lit now, but the other one is not.  I tried resoldering the two connections, but no change.  Is it a problem with the LED, or someplace upstream in the circuit?

 

[Paul Birkeland]

Alright, you're getting closer.

Are both halves of the 12AU7 glowing? 

The LED not lighting on A8 isn't usually an indication of a bad LED, but rather an issue with the wiring to A6 or A7 (like not having the jumper between ground lugs on the volume pot, a bad solder joint on the 22K resistor that feeds A6, or a bad heater connection).

Unfortunately, builders always see an LED not glowing and assume they need to heat it with their soldering iron until it lights, which will break the LED.

-PB

 

[mqbush]

Both halves are glowing, or at least I see two glowing points at the top of the tube.  I checked/resoldered the connections on the A socket, and rechecked the wiring, but did not find anything amiss.

I also measured the voltages across the LEDs, the lit one shows 1.5 VDC, the dark one shows 14VDC.

 

[Paul Birkeland]

Do you see 14V while the amp is running?

If so, that's a wiring issue with the wiring to A7 or the ground connections at the volume pot.

-PB

 

[mqbush]

Yes, I measure 14V between A8 and the center terminal while the amp is powered.

I've checked/resoldered A7, the center terminal, all connections on the pot, all ground connections, T3...

I've traced the ground continuity from the A pins, to the pot, to the headphone jack, all the way back to T12.

Still only get one LED to light, the other still has 14VDC across it.

 

[Paul Birkeland]

You can try replacing the LED, it's possible that it got overheated while being installed.

-PB

 

[mqbush]

Is there a specific version of the HLMP-6000 I should use?  I see several through-hole variants on Mouser.  Also, if I replace it, won't putting 14 volts through it be an issue?

 

[Paul Birkeland]

It's showing 14V across it either because it's melted, or because the wire going to A7 isn't providing a good path to ground (this is why I keep coming back to the jumper that connects the two ground lugs of the volume pot).

You want the HLMP-6000 with no additional suffixes.

 

[mqbush]

Ok, I'm going to bet on a bad LED, since I've checked/measured/resoldered the ground side multiple times at this point.  I've ordered the replacement, once it comes in I will let you know how it goes.

Thanks for all the troubleshooting help!

 

[mqbush]

It was the LED.  I got a replacement, popped it in, and reran all of the resistance checks just to be sure.  After turning it on, I saw both LEDs light for the first time.  All of the voltage checks were within tolerance, so I tried it out with my Crack Headphone kit, and it sounds fantastic!

Thanks again PB, your help was invaluable.