Crack Upgrade Problem [solved]

[mSummers]

After installing the Speedball and having it working for 2 weeks, I decided to replace the last cap in the power supply with a Mundorf M Cap and the last resistor to a Triad C7-X choke as I have seen a couple of other members do.  While I had the amp apart, I also replaced the resistor between 15u and 21u with a Kiwame of equal value.

When I plugged it in to do the voltage check, the resistor between 12u and 13u immediately started smoking, so I shut the amp off immediately.  Any thoughts what the problem might be?

I know there isn't a resistance check after the speedball has been installed, but since I had done one after I added the speedball, I did another to compare in case that helps trace the problem.  The following are the readings that are different:

3/15 Speedball measurements:
T2 - 10.5m
T4- 6.8m
T7- 1.8m
T9 - 2.2m
T13 - 1.4m
B3 - 1.9m
B6 - 2.2m

3/31 upgrade measurements:

T2 - 268.7
T4- 268.7
T7- 0L ( no reading)
T9 - 0L ( no reading)
T13 - 269.1
B3 - 0L (no reading)
B6 - 0L ( no reading)

[Paul Birkeland]

Did you put a 2.7, 27, 270, 2,700, 27,000, or 270,000 Ohm resistor between 12 and 13?

[mSummers]

270 ohm between 12&13.  It reads 269.2 on my meter.

That's the problem...  I grabbed the wrong size.  I'll get a new one in the correct size and try again

[Paul Birkeland]

Yeah, 0.7mA flows through the 270K resistor. With a 270 Ohm resistor, 700mA flows through...

This takes the power dissipated from 0.15 Watts to 150 Watts, which would indeed make that little 1W resistor scream and let the magic smoke out!

[mSummers]

Thanks for the explanation, Paul.  I'm just glad I didn't hurt anything else with that mistake.

[mSummers]

Paul, I have a follow up question.  I installed a replacement resistor between 12U & 13U, and yes it is the correct value this time (270k ohm, 1w).  So, I did another resistance check to compare to the readings from my initial install and got the following differences:

3/15 Speedball Measurements:
T7 - 1.8M
T9 - 2.2M
B3 - 1.9M
B6 - 2.2M

4/4 Measurements
T7 - No reading
T9 - No reading
B3 - No reading
B6 - No reading

Does that indicate a problem that I need to solve before checking the voltages?  I hope I didn't mess anything up other than that resistor I started to cook.

[Paul Birkeland]

Nah, you've removed resistors and replaced them with C4S boards, so these impedances will naturally change quite dramatically. 

[mSummers]

Thanks Paul!  It passed the voltage check and sounds great