EVH 5150 III 100W No Sound Blue Channel

[Redhoodboi]

I've got an EVH 5150 III 100w head, 6L6, that I've had this on and off problem with and I think(?) I have it down to somewhere in the switching circuit. For background, channel 1 works great, channel 3 works great. All of the pre-amp tubes are being utilized with just those 2 channels, so I have no suspicion the tubes are bad, and I've moved them all around and the problem stays the same. Channel 2 has VERY little sound. With gain and volume at 10 on channel 2/blue, its about as loud as channel 3/red on 0.5. I had someone "repair" it just for it to have the same problem a month later. The guy who repaired it didn't even know what he fixed, he just said he resoldered some joints and it came back to life, but since the same problem has come back again, it makes me think he didn't actually fix anything. This is the 3rd time the 2nd channel has gone out. The 2nd time it did, I poked around in the amp to no avail, took all the PC boards out to check solder joints to which they all looked ok and for whatever reason, it came back to life after reassembling it. That lasted about another month, then it just went out again tonight. I took the amp out, poked around again and it came back to life for a few seconds after measuring voltages on components between the 2nd and 3rd gain stage, it went out after about 20 seconds. There doesn't seem to be any AC voltage in the signal path on channel 2 between the 2nd and 3rd gain stage, but there is on channel 1, which leads me to believe it has something to do with the K2 relay that switches in the preamp for 1 and 2. I can attach the schematic if needed, but there's 0 volts at R35 and R80 on channel 2, but there is voltage there on channel 1. To me, it seems like there's an issue with the switching circuit because channel 1 and 2 share pretty much the same signal path and share the same tubes, but I'm no amp expert by any degree, so I figured I'd ask to see if anyone DOES happen to know what they're talking about when it comes to this stuff. I read somewhere that there's a common issue with the footswitch cable, but this issue is present with/without the footswitch plugged in. Any help is appreciated, thanks!

[Jon Snell]

From your description, I would replace K2 & possibly K3.

[Redhoodboi]

So there's 2 of each K relay. 2 K1s, 2 K2s and 2 K3s. There's only one of each per circuit board (K1, 2 and 3 on the main preamp board, K3 on the footswitch/fx loop board and K1/2 on the power supply board). Do all of these relays that share the same number work with each other? Like does the K2 on the preamp board work with the K2 on the power supply board and so on? Again, not an amp expert here. I can get around a preamp and power amp schematic but I'm clueless when it comes to relays/channel switching circuits

[Jon Snell]

So there's 2 of each K relay. 2 K1s, 2 K2s and 2 K3s. There's only one of each per circuit board (K1, 2 and 3 on the main preamp board, K3 on the footswitch/fx loop board and K1/2 on the power supply board). Do all of these relays that share the same number work with each other? Like does the K2 on the preamp board work with the K2 on the power supply board and so on? Again, not an amp expert here. I can get around a preamp and power amp schematic but I'm clueless when it comes to relays/channel switching circuits

K2-A is one half of the relay and K2-B is the other pole. K2-C is the coil for that relay.
There are a few logic ICs (J K Flip Flops) that control the switching transistors that change the state of each relay. Make sure the logic is working correctly. Follow the truth table for the flip flops and that will tell you if they are faulty or not.

[Mick Bailey]

A useful check to see if a relay is operating is to use a small compass stuck to the end of a plastic rod. I have some little plastic ones about 12mm diameter that were pressed into key fobs that are perfect for this; stick it on the edge so you can see the needle, set it close to the relay and the needle will deflect when the relay coil is energised. If there's no deflection, either the coil is open or it's not being energised from the switching circuit. Of course, it won't tell you whether the contacts are working, but you can use a DMM for that. You can get a commercial device that looks like a pen and turns on an LED, but I've always used a compass.

[Enzo]

The three relays on the main board act alone, the three on the power supply board also work on their own.

[Redhoodboi]

Haven't had the time to pull the board/test the relays, but channel 2 has also now just developed a nasty hum, along with its extremely low output. The hum goes away if I press down on the faceplate selector switch mechanism on the circuit board. The sound doesn't come back, but considering there's now probably an issue with the switch itself, you guys think there's any possibility its something as simple as replacing the selector? I also know nothing about JK flip flops so that's something I'll have to educate myself on

[Enzo]

But when you press on that, you are also applying pressure to the board it is mounted on. SO it could be a nearby solder joint rather than the switch itself. It pays to find that out before ordering a switch.

[Redhoodboi]

Alright I'm beyond confused now. Still haven't tested the relays or anything (however they do make audible clicks with all 3 channels, so thats something), but I was testing the preamp tube voltages and noticed that when touching pins 1 and 6 (plate pins), it makes pops VERY loud and clear when touching them with a multimeter probe, like the volume it should be at normally across all the channels, including channel 2. I also noticed while doing this is that the hum I was mentioning gets substantially quieter after touching those tube pins, but comes right back if I flip the standby switch on/off. There's still some hum, but its exclusive to channel 2. None of the tube voltages change across the 3 channels so that's ruled out. Sorry if I'm asking too many questions, I'd just really prefer to not spend another however many hundred dollars bringing it to someone for it to continue having this problem. It doesn't seem that anyone else on the internet has had this problem so I'm trying to get as many ideas as possible

[Redhoodboi]

Well uh, it works again. I gave up trouble shooting, put everything back together and it's alive again. Really confused at this point but I guess I'll roll with it til it stops again. I'll order the relays just in case as I don't really see what else it could possibly be

[g1]

I don't really see what else it could possibly be

A bad solder connection. If the problem returns, try chop-sticking around the area of the switch that made the hum issue come and go.

[Redhoodboi]

That's what I was doing for awhile, poking around to see if anything changed. The first and second gain stage had the correct measurements on a multimeter which is where the majority of the preamp differs between channel 1 and 2 (reminder, channel 1 and 3 work great, 2 was almost no sound) so thats why it's so confusing. After the second gain stage, the only difference between channel 1 and 2 til after the final gain stage is a resistor, 330k for channel 2 and 68k for channel 1, that switches with the K2-B relay and sends the signal to ground, so I thought either maybe the channel 2 resistor failed or the K2-A relay wasn't switching to channel 2 correctly. The resistor tested good, and I checked the solder joints in that area, and the channel 2 switch and they're all good, leading me to believe it's either the relay or as someone else mentioned here, maybe the ICs/transistors that operate the relays. I tapped around a fair bit and nothing changed, then it all of a sudden came back when I put it all back together, and that hum I was talking about completely disappeared. Made sure everything was properly tightened down this time, not sure if a loose screw would have anything to do with it but that's really the only thing that could've changed from not working to working since I didn't touch anything else