First time Crack build: Voltage checks (help)

[tmiddle]

Hello,

My first attempt at building any sort of electronics and today I finished the wiring assembly. Soldering went well and I double checked everything the best that I could. I have just finished doing the resistance checks and they all matched up. On the voltage checks I ran into bad readings beginning with terminal 21

21 = 1.63 (need 206V)

A1 = 1.6V (need 90V)
A2 = 0 (good)
A3 = 0 (need 1.5V)
A4 = 0 (good)
A5 = 0 (good)
A6 = 1.2V (need 90V)
A7 = 0 (good)
A8 = 0 (need 1.5V)
A9 = 0 (good)

B1 = .96V (need 90V)
B2 = .92V (need 170V)
B3 = 0 (need 100V)
B4 = .82V (need 90V)
B5 = .76V (need 170V)
B6 = 0 (need 100V)
B7 = 0 (good)
B8 = 0 (good)

So I haven't plugged the phones in yet and have only powered it up to test. No smoke, no smells or signs of malfunction. Just not getting the desired voltages to the above terminals. The only troubleshooting that I know to do is visual and I just double checked the orientation of the 5 caps and they are all correct.

Please someone help me to find the problem here.........and thanks!
Thom

 

[jrebman]

Thom,

Just a couple of things to check first --  are the transformer and tube sockets oriented correctly and wired correctly?

-- Jim

 

[tmiddle]

Jim,

I just a found an unsoldered wire to B2 and fixed it, but when I went to recheck I was getting strange drifting readings (pretty low) and found that the fuse was blown. Have to track down some F1AL 250W fuses now.......better get several just in case.

thanks and I'll keep you updated.

Thom

 

[tmiddle]

Added the first voltage error as terminal 21 = 1.63V (need 206V)

OK, just thinking about the results that I got and the fact that later this evening I discovered an unsoldered wire on B2.
Then after repairing this, I discovered the blown fuse.

Is it likely that the fuse blew because of the loose joint at B2 and "before" the repair of the solder joint?
Did the fuse blow as I touched the probe to the loose joint on B2? (I'm hoping this was all my problems)
That might explain why all of my voltages from terminal 21 on until B8 were all very low and only the 0 voltages were correct.
I was unknowingly testing with a blown fuse!

I hope to get a new fuse in this tomorrow and see.

 

[Grainger49]

Did you answer this one?Many first time builds have gotten these components wrong

Thom,

Just a couple of things to check first --  are the transformer and tube sockets oriented correctly and wired correctly?

-- Jim

 

[jrebman]

Thom,

Another question: did you have the tubes in when you made the voltage measurements?  Did you check the posted corrections to make sure you have the latest information from where the manual had some errors?

Yes, it is completely possible to short something out with a test probe when measuring, but the thing that is really odd, if I'm reading things correctly, is where are the voltages coming from if the fuse was blown?  Things just don't seem to be adding up here.

-- Jim

 

[Paul Joppa]

One side of the power line is connected to the transformer primary even when the fuse is blown, and all windings are capacitively coupled to each other. And any stray magnetic fields can be picked up by the transformer. These are plausible explanations for measuring small voltages with a blown fuse.

I once had an amp that generated hum in the loudspeaker even when unplugged from the wall outlet. That's when I started thinking about these things....

 

[tmiddle]

Thanks to all for the suggestions and taking the time to post. I am listening now to my Crack with a pair of Sennheiser HD650's that I recently picked up. Very very nice amp and I can't help but smile.

The problem turned out to indeed be the fuse apparently blowing when I was performing the voltage tests. Today I replaced the fuse and all test points were within just a few volts of desired. I will enjoy the Crack for a while before I install the Speedball upgrade and try the power cord and tube swapping. I had purchased an Alps Blue Velvet pot for this project, but I backed out and went back to the stock pot after not understanding how I would make the connections to the small tabs on the Blue Velvet.

Thanks again,
Thom

 

[Laudanum]

Thanks to all for the suggestions and taking the time to post. I am listening now to my Crack with a pair of Sennheiser HD650's that I recently picked up. Very very nice amp and I can't help but smile.

The problem turned out to indeed be the fuse apparently blowing when I was performing the voltage tests. Today I replaced the fuse and all test points were within just a few volts of desired. I will enjoy the Crack for a while before I install the Speedball upgrade and try the power cord and tube swapping. I had purchased an Alps Blue Velvet pot for this project, but I backed out and went back to the stock pot after not understanding how I would make the connections to the small tabs on the Blue Velvet.

Thanks again,
Thom

Glad to hear you have it making music, congrats.  
Here is a possible option for the "Blue Velvet" ...  http://www.amb.org/audio/epsilon27/
I am not sure it will fit but you can look into it for later.   Aside from the mounting board, it is not difficult to wire in the Alps pot.  Personally, I used the same type wire included with the kit but in 22 AWG for the RCA input jacks to pot wiring.   But it can be done with the 20 AWG thats included.  Just a matter of wrapping the wire around the pin terminals of the pot with a little crimp with some small needle nose and then soldering.  Really pretty simple.  Give it a go if you feel so inclined down the road.

 

[jrebman]

Hi Thom,

Nice work! Now just put a hundred or so hours on it, let it and the headphones (if they are new) break-in a bit, and then install the speedball and then you'll be off on a whole new adventure.

Glad it worked out,

Jim