Turning the chassis over for testing.

[Bingtou]

I did look for this information, but haven't found any references to my problem so far. I just finished building my Crack, everything went well, and I've done the resistance tests and all seems to be as it should be. This is a first DIY audio project for me, no experience with electronics or circuits other than building my own PCs for the last few years (which is hardly the same thing).

One thing during the resistance checks was that I got a reading of 93 on one of the RCA jack centre pins, and 101 on the other. I did the readings four or five times to be sure, and got the same every time. As they're supposed to be from 90 to 100 kohms, a) does the inequality matter, and b) does one of them being just outside the range matter?

Main question: I'm going to be doing the voltage check in the next week or so, and here's the only bit where the manual failed me - the build process was very clearly laid out. But for the voltage check instructions, I got lost here:

"Plug the IEC power cord into the power entry module. Do not plug the cord into the wall yet.
( )Attach the negative lead (typically black) of a volt-ohm meter to ground. A good spot to do so is at
terminal 12. Use of a clip lead to connect the black test lead to the ground will free one hand, making
testing much easier and safer.
( ) Turn the chassis over and reconnect the negative lead of the volt-ohm meter to the ground buss.
Switch the meter to read DC volts on the 400V or higher scale"

What's going on here: attach the neg. lead to ground....and then reconnect the neg lead..to the ground buss? I really don't follow this part....

More importantly, when I came to this point in the project, I realized that with the tubes installed and the power cord plugged in, flipping the chassis as I had been doing before, and inverting it onto the base to work on it is no longer going to be an option. How are people setting up the chassis to carry out the voltage check? There's live parts on one side and fragile tubes on the other, so this operation seems pretty crucial! 

I didn't see this mentioned in the manual or in the Crack-building video.

Thanks for any and all advice.

 

[ssssly]

Put the tubes in, plug the chord into the base (not the wall), flip it over and then attach the negative lead to the ground buss. Don't worry about the flip it over than flip it over thing.

I use some wood blocks to prop it up while it is flipped over. Have also used a pile of books or just kind of leaned it over on the transformer away from the tubes. The possibilities are endless.

The resistance variation is fine. Unless you hand pick all the resistors and pay close attention to keep the wire lengths the same you are going to get variations like that.

Enjoy

 

[adamct]

I have to agree. I found this part of the manual very confusing, and slightly nerve-wracking. The manual is clear all the way through, until you get to the very end where you will be dealing with live voltage, and then suddenly it gets confusing!

It also doesn't help that "ground buss" is a term that isn't used anywhere else in the manual. I'm sure this seems like an obvious term to those who have experience with this kind of thing, but I didn't know whether "ground buss" just meant Terminal 12 again, or something else. I searched the forums and found the answer (in means Terminal 12 again), but you shouldn't refer to the same thing two different ways, especially in quick succession.

The same thing applies with regard to placing the case on books to allow you to raise it far enough to flip the chassis for the voltage checks. Yes, there are lots of ways to do this, and yes, it should be obvious, but following the manual gets you into the mindset of "Do exactly what is written and follow the instructions to a "T"." When suddenly at the end, the manual expects you to improvise at the most dangerous stage in the process, it is confusing. While it may be a mental fault on the side of the novices (like me) who get confused by something so simple, I think that should be taken into account when revising this section of the manual.

FWIW, here is what I did: I put the case on books to raise it. I attached the power cord, flipped the chassis and lay it on the case. When the time came, I plugged in the power cable, then delicately reached underneath to flip the power switch. After doing the voltage checks, the amp (and tubes!) had gotten quite warm, and I didn't feel like fumbling around blindly, so I just unplugged the power cord from the wall.

Best,
Adam

 

[Bingtou]

Thanks very much to both of you, those replies have set my mind at rest over going to the next stage of testing.

Despite the slight information gap at the tail end of the manual, I did really appreciate the rest of it, and the photographs were incredibly helpful. I don't think this is going to be my last Bottlehead project.

There was one odd thing that came up during my build: the resistor between 22U and 15U has to bridge about 7 cm of space across (above) the power supply area. It was by far the widest connection made by any of the resistors. The actual resistors I got were correct spec. (270 ohm, 5 W), but clearly were not as illustrated in the manual (rectangular white ceramic). I got two that were the rounded type, and green. The problem there was that the leads on the resistor supplied to me were definitely not long enough to bridge this space.

The only way I could think of to handle that was by assuming that it is acceptable to solder a section of lead from another resistor onto the lead that was too short, to extend its length. I cut the extra from the brother of this resistor, which was easily long enough to go into its own designated terminals. This is just a quick fix I dreamed up to avoid the trouble and delay of getting a new resistor sent out from the US, but installing was easy. I don't see why it shouldn't work, and everything seems to have tested out fine.

I think the last point I've been left wondering about is why terminal 12 is recommended as the ground during voltage testing, as it's very congested. I can just about get an alligator clip onto it, but really only just. Why not terminal 11, which is empty? I suspect this is a noob question.