Resistance measurement anomalies

[JP]

I finished building my crack, and found a few places where the resistance was a bit off from what I expected.

* 13 - starts in Mohms and dropping (instructions state *will climb slowly toward 270k ohms)
* RCA jacks - white:99.7k ohms and red:105.6k ohms - what drives the difference/what can I do to get these two closer?  Does this matter?  The instructions state that these should be between 90-100k ohms.

Not sure it matters, but I'm using a Fluke handheld to take the measurements.  For earth, I tried a couple of different places - all show 0.1 ohm, and no differences in all of the measurements above.

I haven't moved to voltage testing yet - wanted to get this figured out first.

Thanks!

JP

 

[Doc B]

Those measurements are fine, you can move on to the voltage measurements.

 

[Laudanum]

The difference in the RCA jack measurements are due to differences between the two channels of the volume pot.  The tolerance isnt fantastic with the stock pot but you probably wont hear any differences in volume between left and right channels at normal listening levels.  Actually, what you are measuring isnt too bad at all with those pots.  I have several that I have had in the parts bins for a while and they all measure further off between channels than what you are seeing.

 

[JP]

Thanks all for the feedback.  I ran the voltage tests and all measured well - the only thing that was off a bit more than expected was the 90VDC lines coming from the 12AU7 - all of those are measuring ~75VDC.  I'm not sure if something that low is going to cause problems around the rest of the board with the 6080 - any feedback would be appreciated.  (If it's not obvious, my electronics skills/knowledge are a bit lacking.)

I've been listening to it and it sounds amazing - it wasn't until I was on Crack that I noticed that on Radiohead's Planet Telex from The Bends, the first note or two in the bass line has too much gain and is distorted, and the same goes for when Thom Yorke starts singing.

Can't wait to load up the Speedball this weekend.

Thanks again!

 

[Laudanum]

The 75 volts is a little less than the up to 15% allowed for variation in readings, either way.  Im not too circuit savvy either, I just happen to know about those pots.  Others may jump in to offer better help, but I think it is possible that tube variation could account for your being a little low at 75 volts, especially since everything else is measuring ok.  Could also be low line voltage which could fluctuate by the second ... it does here, badly this time of year.  You could try a quick check again earlier in the morning or late at night when demand is lower just to be sure it wasnt a sag in supply voltage.  Or possibly try another tube if you have one.  I'll let others chime in with smarter suggestions.  Atleast mine are harmless in the meantime.  Glad to hear you've got it up and making music. 

 

[Grainger49]

JP,

As Desmond says, the incoming voltage might be lower than 120V AC so the outgoing voltage from the transformer might be a tad low.  I would still give it a listen.  You are not that far off of the mark.

 

[JP]

I did a little more testing - I get the same readings when the AC Voltage is 117-118, so I'm going to assume it's just tube variation - I guess that means I can join the thread on tube rolling now!

Thanks again.

 

[JP]

Although this should probably go under a new thread, I'd thought I'd try here first.

I just installed the Speedball, and all of my voltage measurements are correct with the exception of the ones from the headphone jack.  When powering the unit up, the voltage slowly increase for a bit and appears to be normal.  Then my Fluke shows 0L or overload.  This happens for just barely a second, and then drops down quickly.  I'm able to glimpse ~40V, ~15V, 10V, and then values below 9V.  It does stabilize to 0VDC.  Interestingly, when I power the unit off, I see the same thing on the - side - it goes to -0L, and then -44V, -15V, and then stabilizes eventually getting back to 0VDC.

I went back to retest the resistance values, and everything is the same as it was before, with the following exceptions:
7: 0L (which I'm guessing is okay because this leads into the Speedball)
9: 0L (which I'm guessing is okay because this leads into the Speedball)
B3: 0L
B6: 0L
I can't figure out why B3 and B6 changed, but I have to guess that has something to do with the problem I'm seeing.  Any thoughts?

Appreciate any assistance with places to start troubleshooting - I'm not even sure where to start since the rest of the measurements are okay.

Thanks!

JP

 

[Laudanum]

Im sure that you know already that in the stock build, the voltage at the headphone jack when first powering up is supposed to rail up to about 9V and then quickly back down to 0 volts.  After installing Speedball and Film output caps, my voltage rails up to about 24 volts according to my meter, and then quickly back down to 0 volts.  It does it similiar to the manner you state, different from in the stock build it is a sudden, quick jump that seems to happen a few seconds later than it did in the stock build.  My quess is that it has something to do with how Speedball works (but thats a novice quess).  I asked about this in a thread and another Crack owner tested his and measured to 14 volts.  Im not sure that I trust my meter completely in making measurements that change that quickly, and the OL that your meter is showing may be a false reading.  Nevertheless, it's normal, atleast in my and another Bottleheads experience, that the start-up voltage at the headphone jack measures higher after Speedball install.  You could try to set your meter to manual range (if so equipped) instead of autoranging and see if you can read an actual voltage.  Another Bottlehead with more electronic knowledge would have to explain why the voltage climbs higher after the upgrades, I dont know why.  If all your other voltage measurments are OK, you are probably just fine.

Also, that startup voltage is much, much lower when a load (headphone) is connected to the jack.  You can see for yourself by making the measurment with headphones connected.