S.E.X 2.1 no B+ on one channel [resolved]

[Bill S]

I built my 2.1 back in 2011. I've enjoyed it very much and it has quite a number of trouble free hours on it until now. I didn't use it for a few months and when I hooked it up again it had a intermittent loss of the LH channel. I could tap the chassis, there would be a slight crackle and the volume would return in the left channel but fade out within a few seconds. I figured it was a bad solder joint somewhere so I examined and redid a number of solder joints for that channel. When I hooked up the amp again I had lost the LH channel completely.

I put it back on the bench and checked voltages. I have lost voltage in the LH B+. I'm wondering if I have a UF4007 diode that became intermittent and has now failed. Can a diode become intermittent?

When I check voltages everything on the RH channel appears correct, albeit a bit higher than specified in the assembly manual. Line voltage measured 123 volts.
Here are my voltage readings  (by terminal number, and I am including voltages of the RH channel that is working) (all voltages are DC)
#6- 422    #26- 470
#9- -16      #29- 0
#11- 403    #31- 0
#12- 409    #32- 0
#16- 386    #36- 0
  A couple of times I did get a very brief reading of -17 on terminal 9 but out of maybe 25 times I checked I only saw the -7 two or three times and even then only for a moment as I touched my probe to the terminal. I did try moving things with my chopstick but that never seemed to indicate any bad connection.

Any suggestions of what to do?  Should I replace the UF4007 diode? Would  that cause the 470 v reading on terminal 26?

Thanks,  Bill

 

[Paul Birkeland]

The SEX 2.1 has a single power supply for both channels, so a bad rectifier diode couldn't only influence one channel.  I suspect you have a SEX 2.0?


In the SEX 2.0, 29 and 31 are connected with a 680 ohm resistor.  This is the issue I would focus on, as these terminals cannot be connected with that resistor in your amp based on your voltages.

 

[Bill S]

My  amp has the 680 ohm resistor between terminals 26 and 31 (and between 6 and 11 on the other channel)

Thanks,  Bill

 

[Paul Birkeland]

My  amp has the 680 ohm resistor between terminals 26 and 31 (and between 6 and 11 on the other channel)

Thanks,  Bill

But I'm saying electrically you don't. You have 470V on one side of that resistor and 0V on the other.  That would mean 700mA of current through that resistor and 300W of dissipation, so it would explode if it was well connected (and since you didn't mention that issue, it's not).

The resistor could be open, or one end of that resistor is loose.

 

[Bill S]

Paul,  First, I apologize. My amp is a 2.0.  I hadn't given it any thought in years. I see I bought it in 2011 just before the 2.1 was introduced and 2.1 was what stuck in my mind.

Secondly, thanks for your expertise, quick responses and willingness to help out an amateur like me. 
My problem was a bad solder joint at terminal #26. I redid that joint and my amp is back working and sounding great again.

Thanks again,  Bill

 

[Paul Birkeland]

Oops, I see I said 29 instead of 26.  I'm glad you were able to find the issue without too much trouble.

-PB