Resistance check for newly constructed Crack

J

jon.morton

New member
I'm completing the resistance check on a newly built Crack headphone amp.  All reading are spot on except B3 which reads 2.46K ohms instead of 2.9K ohms.  The only connection at B 3 is a single red wire to T6.  The readings at T6 and T3 are 2.4K and 2.9K ohms.  All connections are solid.  Is the low reading at B3 acceptable?
Thanks, Jon from Alaska
 
The red wire from B3 is not supposed to go to T6.  Definitely don't turn the amp on until you move this wire over to T7, or you'll smoke one of the 2.49K resistors on the headphone jack.
 
I moved the connection from T6 to T7 and the resistance is spot on.  I moved on to checking electrical and I get nothing.  I tried 2 power cords, 2 fuses, 2 sets of tubes and get no signs of power.  I've tried several different electrical plugs at the house and get same results.  I also checked power reading across Crack plug receptacle connections and get nothing.  I confirmed with test light that there is power at the plug in on my work bench.  Any suggestions?  thanks, Jon
 
The IEC inlet has a live and neutral terminal on it (those are the two terminals that feed the power transformer).  If you don't have 120V between those two lugs, it's possible that the fuse isn't installed correctly.  It would also be a good idea to try measuring the voltage at the wall with your meter, just so you have a baseline for what the meter will show.

-PB
 
The fuse is in the little clip part of the fuse holder, not the square tube part, yes?
 
I've confirmed power at the wall outlet plus power getting through the power cord.  Fuse is installed in the holder and securely insereted into its receptacle.  the fuse appears to be good - I can still see the filament though I'll hit the local electronic store and get a couple more.

I checked for power across all combinations of terminals on the power cord receptacle on the Crack unit and get no reading.  I've got power to that receptacle but nothing coming through.

Thanks for your help.
 
Touch both ends of the fuse with your meter, 0 ohms (or so) the fuse is good, high ohms the fuse is bad.
 
Jon,

What about Dan's last question.  Several Bottleheads have gotten the fuse in the wrong part of the holder.

I second the use of a meter to determine the status of a fuse unless its rating is 0.01A.  In that rare case the meter will blow the fuse.
 
I stopped by the local electronics store and got a 5-pack of new fuses to ensure that wasn't the problem.  To remove any problem with the fuse holder, I snapped the fuse direct into the holder.  I confirmed there's power at the wall outlet and coming to the amp plug in. When I plugged in the power to the Crack and hit the on/off switch, the fuse immediately blew (quick white flash).

I spent time last night looking at connections and wiring and can't spot anything out of place or not in accordance with the directions.

Any suggestions?  Appreciate the help.
 
It's could be a miswire or a wire shorted to something that it shouldn't be. Or the fuses were under rated or fast blo. This is the point where I would use a magnifier and go over the whole circuit looking for a problem, with emphasis on the power connections and the power supply.
 
Try posting a few pictures.  Of major concern is the power wiring and the high voltage diodes & capacitors.  Shoot from a couple of angles so both sides of the diode string can be seen.
 
jon.morton said:
When I plugged in the power to the Crack and hit the on/off switch, the fuse immediately blew (quick white flash).

Any suggestions?  Appreciate the help.
Click to expand...

This is usually:

1.  A backwards cap
2.  A backwards diode
3.  A short (wires/leads touching which shouldn't touch)
 
Hello Jon,

I can't reply to your e-mail from home (new restrictions from Comcast).  Your octal socket is off by 90 degrees (see red arrow, top of page 13).

-PB
 
I corrected the orientation of the octal socket and resoldered connections.  I also found a mis-wire on the B+ power supply and corrected.  I've gone back through the entire instructions and feel confident of correct, solid connections.  When checking resistance, using terminal 12 as ground, the reading at terminal 13 seems to be hanging at ~500K ohms (my meter has scales at 200, 2000, 20k, 200k and 2000K so I'm using the highest and letting it run down).  All other resistance readings are spot on.

When I try the voltage check, the fuse still blows the minute the power cord is plugged into the wall, regardless of the power switch being on or off.  Any thoughts?  Thanks, Jon
 
I'm posting at the same time as PB, with a similar suggestion:

Measure the DC resistance between the two non-ground prongs of the power cord. It should be infinite with the power switch off, and about 13 ohms with the switch on. If you get less than that, then there's a short in the power cord or IEC wiring. You can pull the cord from the IEC connector and repeat the test to check the cord itself. What's left is very simple, the IEC socket, power switch, and the power transformer input terminals 1 and 2 - knowing where to look is more than half the battle!

Check the wiring of transformer terminals 6-7-9-10 as shown on page 26. If one of the pairs is reversed, the fuse will still blow immediately even after you fix the "power switch has no effect" problem.
 
Jon,

Look at the post below.  Make the measurements suggested.  It will help you find a missing ground.

http://www.bottlehead.com/smf/index.php/topic,4812.0.html
 
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