Resistance reading help

[slant6]

hello ... finished wiring up my Stereomour II and getting some consistently off resistance readings from a few terminals ... i've gone though and checked solders, rechecked solders and looked for solder bridges, went through the steps to make sure nothing was miss-wired and all "seems" well.

below are the terminals in question:

terminal 06  = 0.0
terminal 21 = .19
terminal 22 = 0.0
terminal 24 = 0.0
terminal 25 = 0.0

i'm about ready to start pulling it apart but wanted to check first if these particular terminals/readings might indicate where the problem may lie and I can focus on that area.

thanks!

 

[Paul Birkeland]

The center leg of the hum pot can sometimes be unintentionally be bent down and touch the chassis plate.  The resistance readings you have here are indicative of such a problem, and the fact that you don't have them on the other channel tends to rule out a measuring issue.

Whatever you do, don't "try out" the amp with this problem. 

-PB

 

[slant6]

thanks Paul - very much appreciated.

that was it ... the middle post was down enough to touch. bent it up and all resistance #s checked out.

went to test voltages and now blowing fuses as soon as I plug in. will look for more "bad touching" but after an initial check there is nothing obvious out of place.

if there are any tips to tracking down the fuse-blowing culprit please let me know.

thanks again!

 

[Paul Birkeland]

Fuse blowing with perfect resistance checks is almost always a backwards diode or capacitor.  Accidentally wiring one of the 130 Ohm resistors from the + node of a filter cap to the - node of the next would also create enough of a short to blow the fuse also.

-PB

 

[slant6]

yep - again you are correct - i had the banded side of the UF4007 rectifier
diode backwards - good catch.

i went to check resistances again before plugging in and sure enough I now have these funky resistance readings:

terminal 15 = 2.9 instead of 1.2
terminal 36 = 2.9 instead of 1.2
terminal 37 = 2.9 instead of 1.2
terminal 40 = 3.1 instead of 1.4

i'll keep looking to see what the deal is - i may just go through step by step from the beginning again. thanks for your help so far - any more thoughts on the new issue would be very much appreciated!

 

[Paul Birkeland]

There's a capacitor attached that can throw off the readings at those terminals, but there is symmetry in the amp, so for instance your measurement of 2.9K at terminal 15 should match what you measure at terminal 6.

There's a 2K and 3K resistor in parallel from terminal 16 to 36.  I would guess that the 2K resistor isn't installed or isn't well enough soldered to conduct during the resistance measurement.

-PB

 

[slant6]

ok - took a nice long break from this project ... revisited this morning, re-soldered a couple spots and all resistances check out - thanks for the tip, PB!

so ... confident that everything else was going to go well I installed tubes, plugged in and only the preamp tubes are firing. unplugged before checking anything else. no smoke, smells or pops when plugged in. i went back and rechecked connections — no obvious shorts/wires touching.

Would anyone suggest checking anything else to help narrow down issue?

Thanks!

 

[Doc B]

The most straightforward approach is to take DMM and meter the resistance at every part of the wiring from the filament winding terminals on the power transformer to the 2A3 filament pins (those are the bigger pins, 2 and 3), checking for continuity at every terminal.

 

[Paul Joppa]

Just a quick note - the 2A3 filaments (pins 1 and 4 are the fat ones) are nowhere near as bright as the driver tubes. Are you SURE they are not lit?

 

[Doc B]

Duoh! Yeah, 1 and 4.

 

[slant6]

thanks for the quick reply, guys ...

I was working by a window and perhaps I just wasn't seeing them light up. I can hear the hum of the transformer, I cupped my hands around the 2A3s attempting to create some shade but didn't see any orange at all so I switched it off worried I was  making something worse. I'll try again and recheck connectivity if needed ... ideally i was being overly cautious and i'll be responding with positive results.

thanks again!

 

[Paul Birkeland]

You should measure your voltages.  If the voltages are correct, the 2A3's are lighting.

 

[slant6]

ok ... so, I assumed the 2A3s weren't firing but sure enough they were - a sunny window makes it easy to see what you're doing but not so good to see if your tubes are firing. I just had a funky voltage reading on terminal 7 — resoldered a couple joints and all is well. I purchased the upgrades when i purchased the kit but will wait a bit to do those ... sounds pretty sweet as is. thanks for the help along the way - very much appreciated!!