Cursed kit part 2 - no sound in right channel

[PassionForSound]

After finally fixing this kit and passing it back to my friend who originally built it, the balanced/unbalanced switch on the right channel impedance switch fell off (no idea how). My friend reattached it, but there is still no output from the right channel.

All voltages test fine.

I've reflowed all of the switch joints onto the impedance PCB.

I've also checked continuity of the incoming signal path and there seems to be continuity from RCAs to volume pot and then to terminal 14.

Not sure where to look next...

 

[Grainger49]

I am beginning to believe it is your fried who is cursed, not the kit. 

Does he have 5 thumbs on each hand?  Does he have a dark cloud that follows over his head at all times?

The PCB is the heart of that switch.  If the board is broken it will be best just to get another.  If it is the connecting wire you will probably "Get 'er done" by getting them back where they belong.

Good luck!

 

[Paul Birkeland]

Can you download a 60 Hz test tone and take some AC measurements inside the amplifier?

The first would be between ground and the center pin of each RCA jack.  I recommend turning your source all the way up to make the voltage easy to measure.

Next, turn the level control on the SEX all the way up and measure all the lugs on the pot.  Two of them should be 0V (ground) and the other four should be about the same.

If those all test well, we can move along to the next check.

Frankly, it is a very, very bad idea to put an impedance matching kit on a SEX kit that isn't operational.  This is the exact reason that we don't have instructions to do this right away, as now we will have to perform some extra checks to see if this is just a switching issue or is some kind of operational issue in the amplifier. 

 

[PassionForSound]

Can you download a 60 Hz test tone and take some AC measurements inside the amplifier?

The first would be between ground and the center pin of each RCA jack.  I recommend turning your source all the way up to make the voltage easy to measure.

Next, turn the level control on the SEX all the way up and measure all the lugs on the pot.  Two of them should be 0V (ground) and the other four should be about the same.

If those all test well, we can move along to the next check.

Frankly, it is a very, very bad idea to put an impedance matching kit on a SEX kit that isn't operational.  This is the exact reason that we don't have instructions to do this right away, as now we will have to perform some extra checks to see if this is just a switching issue or is some kind of operational issue in the amplifier.

Thanks Paul. I'll try these tests.

For the record, the kit was 100% operational when I handed over the kit to him so TH switches were installed to a working kit and tested perfectly prior to me returning the amp to my cursed friend.

Grainger, I have no explanation!? Even if I had botched 3 of the 6 solder joints on the switch, it should never have fallen off (as in completely detached from the PCB). I'm wondering now if the chassis was dropped. I'll look for signs of a broken PCB before trying Paul's recommended tests.

 

[PassionForSound]

Before going into significant testing of the circuit (which was previously fine) I decided to use a separate wire bridge a few things to see if I could isolate the part of the circuit that was at fault.

I pretty quickly discovered that the impedance PCB (or one of the components is indeed the issue). If I bridge a wire from one of the positive input terminals of the right channel impedance PCB to the positive output of the same PCB (i.e. bypassing the PCB and mounted components), the sound comes good. Throwing both / either of the impedance switches, however, has no impact at all so it's not just a faulty connection on one of the inputs as I briefly suspected.

In his own words, the owner of the amp forced the switch back into place when it "fell off". I can't see any signs of damage to the PCB and all 6 pins of the switch are protruding through the PCB and are securely soldered.

There is no visible sign of physical damage to the PCB, but I'm wondering if the forceful reinsertion of the switch could have broken one of the traces in the PCB. I've attached images of the top and bottom of the culprit PCB.

 

[Paul Birkeland]

Hmm, sounds like replacing the switch in question would be the prudent thing to do.