Just finished my S.E.X.

S

SilverS

New member
I'm very happy to announce my completion of the S.E.X. amp! It took about a week of work and I'm (at the time of this posting) currently waiting for the caps to discharge so I can flip it over and bring it up to my room for a listen. But before I go about celebrating, I just wanted to confirm a few things:

First, most of my voltage readings showed the "A" side being +10% and the "B" side being -10%, though this probably has something to do with where I put the negative lead, or is a result of the following.
Second, I was given a couple parts that vary from the guide, including a 1.2uF capacitor where I was supposed to install a 1uF capacitor.
Third, it seems one of my tubes is shorter than the other, is that a common occurrence or something I should be worried about?

Also, many thanks to Eileen for shipping a replacement power switch so quickly! It actually showed up right as I was finishing the final solder joint.
 
There was a shortage of 1.0uF caps for a while. I don't think anyone will notice any difference, the circuit is fairly insensitive to that value - as long as the two sides are the same.

I expect that if you swap tubes from side to side, the voltage deviations will follow. That's more likely to be tube to tube variation than brand or shape related - we've found very little difference between different 6DN7 brands. Some people hear enough difference to prefer the coin base variety, but even then the difference is pretty subtle.
 
Alright, the only two issues seem to be the 120Hz hum in the left channel and buzzing bee sound on both channels. Work for tomorrow.
 
To start looking for hum and buzz, disconnect everything except the speakers or headphones and turn the volume all the way down. That isolates the electronics from possible external noise sources.
 
I ran the voltage tests again to double check myself and got some interesting readings back on some of the joints:

A7 and B7 both read 10V~
C4 read 4V~
Terminal 2 read .2V~
Terminal 9 read 145V~
Terminal 10 read .5V~
 
Alright, minor update on my situation:

There is no longer a buzzing sound
The 120MHz hum is still present, and I don't quite understand how I can go about fixing it
The right channel is no longer transmitting, but I have a feeling I know how to fix that.
 
A7, B7, C1 and C4 should all be connected to chassis ground by the wire from C1 to safety ground, and you should read nearly zero voltage, AC or DC, at these points. Check wiring and soldering.

Your readings at 2, 9, and 10 appear to be AC; the manual gives appropriate DC measurements but not AC. But T9 (connected also to T12) should be at AC ground through the 1000uF capacitorT12 to T15. Check those connections and solder joints, also the orientation of the capacitor. T10 is on the ground buss, ultimately connected to T18 which is screwed to the chassis plate so there should be no voltage, AC or DC at these points. (Are you sure the measurements are volts and not millivolts?)

By the way, the hum is at 120Hz, not 120 megaHertz!  :^)  Errors in the multiplier are easy to make, we all do it from time to time, but they can make diagnostics difficult...
 
Alright, the hum is now gone and the amp is sort of transmitting on both channels, but there seems to be an even worse problem to replace it. Its unbelievably quite even at max volume.

 
Well, we're making progress ...  sometimes these things get complicated but we almost always get it sorted.

So, what inputs have you tried, and what are the outputs (speakers? headphones?)?
 
My input's I've tried are my iPhone and my computer's soundcard, both being run through my quickie before hitting the amp.

I'm using a pair of Grado RS1i's for listening
 
OK, you've passed the resistance and voltage tests so most of the circuit is correct. Resistance doesn't check capacitors, so take a look at the solder joints at both ends of the two 0.1uF coupling caps, and the two 1.2uF parafeed caps - those are the only ones that are in the signal path.

If it's not there then it is likely to be at the input (level control wiring) or at the output (phone jack and speaker terminal wiring. Look them over carefully, poking with a chopstick perhaps, to see if anything is miswired or a bit loose.

If that doesn't turn anything up, we may have to trace the circuit. That means a source of fairly steady sound. I'd look for a computer sound generator to make a sine wave signal which we can trace, but a highly compressed pop song will also work. For starters, measure the output from the computer, with meter on AC volts - you should have something like 0.10v to 1.0v. Then we'll go directly to the SEX amp, to keep the chain simple. Be sure to start with the volume control turned down - once we fix it, the Grados will blast you!

Do the checks above, and if necessary I'll post some places to check and voltages to expect.
 
Back to square one it seems, the right channel is not working and the left channel is giving me the 120Hz hum (but at least it's transmitting sound).

:(
 
If the behavior changes intermittently without any changes to the circuit, than it is highly likely you have a bad or missing solder joint. A bright light, magnifying glass, and plenty of time are your friends in that case!
 
Sometimes a friend is the right friend for this case.  You may be looking at the problem and seeing something else.  After you do an inspection, if you find nothing let a friend do it too.
 
Could not agree more, Grainger.  Even a non-techinical friend brings fresh eyes to the table.

I can't count how many times I've over-looked the same obvious problem, then had someone else spot it for me.  After you've worked on something closely for a while, I suspect everything starts to look just as it should, even if it isn't correct.
 
Thanks for the suggestions, I'll have my dad give it a look over and see if he can find anything. I'm assuming he should just trace the circuit as best as possible?
 
I meant that your dad should look for odd looking solder joints and connections that have no solder.  If he can trace circuits that is good too. 
 
If he can trace the circuit, that would be great.  Alternatively, you may just want to read each of the wiring steps to him in order, and have him follow along in the actual amp.  That will end up giving you a terminal-by-terminal look at each connection, I think.  Take the steps in order, and don't move on to the next step until you are both satisfied that the one you are working on is correctly executed.

A little time consuming I realize, but you should find anything that looks suspicious to you.
 
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