Crack build (basic) problems

[norneslo]

I spent last weekend doing the build, and after completing and doublechecking all the solder's, I checked the resistances, and it all checked out good.

I inserted the tubes to check for the glow, and powered up, and I got a glow on both tubes. After I powered it off and turned the plate over, I noticed some smoke, but I could not really determine the source of the smoke, at first I thought it may have come from under T14 or T15. I thought this might just be some solder flux.  I dared power on again, and did not see any smoke, so I started to measure voltages.  I didn't get too far, as I I don't see any voltage at all at the T1, T2, so I stopped.  I also noticed the LED's do not light.

After all this I noticed the two 270ohm 5w resistors have erased the writing on the outside. I set it all down for a couple days, and took a look last night, and touched one of those resistors, which split in half.

The other thing I have a concern with is that when I soldered the power supply terminal 7 that it moved. There was some strain on that tab from the wiring, and as soon as I heated it up it moved. I've taken some pictures and posted them to skydrive:

https://skydrive.live.com/redir.aspx?cid=99d34cacbb193f55&resid=99D34CACBB193F55!119&parid=99D34CACBB193F55!101&authkey=!ANDHeqmuiJXmdwM

Are there any measurements I could post at this point that can help determine the problem?

Thanks,

Ron

[Paul Joppa]

If you smoked the 270 ohm resistors (sure sounds like it!) then the high voltage power is shorted somewhere - that would cause too much current to flow through those resistors.

One way to do that is to install the caps backwards (the 220uF 250v ones in the power supply); they short out if they get reversed voltage. Another way is to make a wiring mistake (naturally) - those are hard to see once you've made them. But you can measure resistance from the power supply T13 to ground, it should be 270,000 ohms (270K) but if the cap is good it will take the meter forever to settle on that.

Do check that you have not inadvertently used the end lugs on the power supply terminal strips - those are bolted to the chassis and are not used.

[Doc B.]

Both (the other one looks split too) resistors being split open and the lettering erased from overheating indicates a hard short to ground somewhere after those resistors. They will both need to be replaced. Check that all of the excess leads sticking thru terminals are trimmed and not shorting against another terminal or the chassis. I would look most closely at the T11 thru T16 strip. Also, I can see only three of the four rectifiers in the photos on the T17-22 strip. Make sure the fourth one is oriented properly.

After you find the problem, make sure you replace the fuse, as it is probably blown.

I'm not spotting anything else in the pics so far.

[norneslo]

Gentlemen, thank you both for the help:

I rechecked T13, it took close to 10 minutes, but my meter climbed up to 270k.  I then snipped out the resistors and I can get a little better view, I see one mistake, on page 19, the correction in the manual for running the wire from terminal 4 to 14U was run to 14L.  Right terminal, just the lower opening.  Would that be the cause?

I double checked cap orientation, and I think they are all correct. I double checked the T17-22 rectifiers, and they look correct.  Oddly, the fuse is fine.   

Updated photo's with the resistors out of the way:

https://skydrive.live.com/redir.aspx?cid=99d34cacbb193f55&resid=99D34CACBB193F55!128&parid=99D34CACBB193F55!101&authkey=!ALW4eCOmF-feJgQ

[Laudanum]

14U or 14L ... makes no difference.  Bottlehead specifies whether to use the upper or lower point of a given terminal for reasons all mostly related to ease of connection.  But there is no electrical difference.

[Grainger49]

Looking at the resistor, there was a catastrophic problem.  Doc mentions a short from the power supply (the high voltage positive) to ground.  That is the kind of thing that would draw a lot of current.  And it should blow the fuse, which didn't happen.  That is odd! 

You have been looking at it for a while.  Call someone else in, you read what should be there and let them look.  Too often I look at a problem till I can't see it.  You might too.

I would have told you to check the capacitors, they can be damaged by this, but the second set of pictures makes them look good.  (when they are bad the end opposite the terminals isn't flat anymore)

[Paul Joppa]

OK, so T13 - the end of the power supply - is not shorted to ground. So somehow the high current to ground happens downstream of the power supply, and when the tubes are in operation.

Did you do that test with the tube in place, or not? Do it both ways to be sure it's not an internal short in the tube.

The 6080 is capable of making a near-short circuit, so if a cathode is grounded it could cause the observed problem. You already measured 3K from each cathode to ground, so that would only happen if there is an intermittent connection to ground - a stray bit of wire, for example. The cathodes only go to the large 3K resistors and to the output coupling cap so it should be easy to look that part over carefully. Look for a possible intermittent short between tube pins 6 and 7, as well. Check the tube pins as well as the socket, in case there's an internal short in the tube.

[norneslo]

Paul, That test was without the tube installed, so I'll repeat it and post the result.  Hey, some good news, I scored some 270ohm 5 watt resistors at Fry's!

Ron

[norneslo]

I probably spoke too soon.  With the tube out and the 270ohm resistors cut out at the moment I'm not getting a reading.  I can get the resistors back in there, but I've got to get the old wires out and clean up those solders before I redo them.   I'll post back but it will likely be tomorrow.

Ron

[norneslo]

FIXED!

After replacing the blown resistors, I gave the whole thing another once over and I found the short, it was the 22.1k resistor at T4 to T5.  When I put in the 3k resistor pair in I failed to trim that one resistor wire, and I bent the pin down and out of site, touching terminal 3. As soon as a I saw that I thought, man that's it! I trimmed that up, and rechecked all resistances and voltages, which checked out fine, LED's lit up, no smoke anywhere.

I'll start work on the speedball on Sunday. I need to re-stain the box, so I'll work on that tomorrow.  Thank you all, Paul, Doc, Grainger, Laudanum, for the help. 

[BNAL]

I know for me that when I run into a problem in a build it is so frustrating and I get very agrivated over it. When I finally figure out the problem it is such a great feeling.

Congratulations on figuring out the problem and enjoy the amp.

[Laudanum]

Good news my friend!  

Pay extra attention when assembling Speedball.  There are two different types of those 3 leg metal can transistors eventhough they both look the same.  Make sure you install them on the correct boards. Their "cans" are numbered so they are easily distinguished in that way.  Dont install them flush to the boards, leave them standing off the boards a little bit.  Careful of solder bridges between their pins as well.  Also, double check orientation of the LED's.  These are the most common mistakes that cause problems with Speedball.  Very simple to remedy by just paying extra attention when stuffing and soldering the boards.  It will work out fine.

[Grainger49]

A second on the congratulations and a second on everything Desmond mentions about the C⁴S boards.

[norneslo]

I thought I'd update - it took me longer than expected to get the speedball completed, mostly because I had an addiction to listening to the stock crack. On Saturday two old 6AS7G's arrived, and I got readdicted to listening and put off the build, and I finally got it together yesterday. In my case, all told it took 7 hours to put in the speedball, but I spent a good bit of time with the solder wick cleaning out all the old work and cut off leads.  Everything checked out first try on voltage readings, so I was able to listen some last night.  As described, the bass is cleaned up and very defined, which was a little muddy before with the 6080.  On Saturday, I found the 6AS7G's to sound much better than the 6080 and were more defined to my ear in both low-end and high frequncy, but with speedball, they both sound great, I'm not sure I could really tell a difference on my initial listens.

Once I get the finish to a point I'm satisfied with, I'll post up a photo or two in the gallery.   Once again I wanted to say thanks again for the help. 

Ron