Fender EVH 5150 low volume channel 1 & 2

[mikeskory]

This is a 5150 III. I found several threads with a similar symptom but a different problem. I did find good schematics and will forward if somebody is interested.

Channel 1 and Channel 2 are very faint. Channel 3 appears to be normal. The original problem was "no heater voltage" which I verified. I temporarily fixed the heater problem by changing fuses, heating the holder and also soldering the spade connectors that attach to the pre-amp circuit board. I realize from another thread that I'll need to change the heater fuse holder.

But the problem now appears to be in V1. Only 1/2 is working.

[mikeskory]

Lol! Now we have something to go on. I left it on while working on something else. After about an hour it started making huge noises on channel 1 & 2. Channel 3 remained ok. I had just started scoping around for the noise...a nice firery in the pc board short developed between pin 5 (heater) and pin 8 of the v3 (ecc83 pre-amp). This tube appears to only serve channels 1 & 2.

[mikeskory]

Now we have something to go on. I left it on while working on something else. After about an hour it started making huge noises on channel 1 & 2. Channel 3 remained ok. I had just started scoping around for the noise...when I saw a big arc at V3. A pc board short had developed between pin 5 (heater) and pin 8 of the v3 (ecc83 pre-amp). This tube appears to only serve channels 1 & 2. I know what I have to do now. Pull the board, fix the arc and then check/replace all the components feeding the V3 tube. Has anyone seen this before on the EVH iii?

[mikeskory]

To wrap this up temporarily, I feel it is the tube sockets at V1, V2 and V3. I noticed it while injecting a signal at V3. The 128V that is supposed to be at Pin 8 was missing. I
re-tensioned the pins and it appeared. (remember V3 was the sight of that arc). That did not cure the problem however. I re-soldered the sockets for V1 and V2. It played for a bit then quite. At this point I could finesse the tubes and get it to play. I had to finesse V1 and V2 independently as both had problems. Now we'll see if the customers wants to go ahead with new sockets, new PC mount fuse holder and repairing bad connections...

[The Dude]

Glad you got it!

[mikeskory]

Glad you got it!

That wasn't it. In the pre-amp section there are 6 12ax7s. With all the tubes out, I get 440 to 480 volts at Pins 1 & 6. As I add tubes, those voltages got down. With all 6 in they vary between 160 and 230 volts with V1 the lowest. I now think I was merely disconnecting several of the tubes by wiggling them which cause a temporary rise in plate voltages. I could use a direction...

[nevetslab]

That wasn't it. In the pre-amp section there are 6 12ax7s. With all the tubes out, I get 440 to 480 volts at Pins 1 & 6. As I add tubes, those voltages got down. With all 6 in they vary between 160 and 230 volts with V1 the lowest. I now think I was merely disconnecting several of the tubes by wiggling them which cause a temporary rise in plate voltages. I could use a direction...

Looking at the schematic, the preamp tubes are either pulling current from the 'W' tap or the 'X' tap on the decoupled power supply filter section on the main board, being fed from the power supply board. From the cathode voltages on all the preamp tubes, you have 4.56mA total from the 'W' tap, and 7.45mA from the 'X' tap. The IR drop across R95 (10k) in the decoupling stage is 45.6V, and drops that 341V potential at 'X' down to 295V at 'W', which is mighty close to the stated 297V on the schematic.

While it does sound like you do have tube socket issues, and possibly plate or cathode resistor issues in the preamp stages, I'd also look to see what you have at the supply decoupling at P4 (blue wire from the power supply board), where it connects to C39/R96/R108 which is the 'Y' supply feed, then at 'X' tap between R95 & R96 at C40, then at the 'W' tap at C36.

And, as you're seeing more current being drawn then as shown in the schematic, you may have one or more tubes that are pulling too much current and need to be replaced/swapped out. I'd check the plate & cathode as well as the power supply decoupling resistors values (once you're powered down and have discharged all the supply caps) to make sure the resistor values are close to spec, and not significantly lower. If there are shorted cathode bypass caps (not common), that would show up during this check.

I've never been a big fan of full-size huge PCB's to put everything on one board, as chasing down issues like this is enough to make you not want to dive in and commit to solving the problem. It may be solder joint fractures at the tube sockets, plate & cathode resistors, decoupling resistors in the power supply, or even over on the power supply board that feeds C39 and all of the preamp supply circuits.

[mikeskory]

I'm back. I kept poking around and finally replaced the V1 socket. So now v1, v2 and v3 sockets are new. It plays all channels now and is fairly loud BUT the sound with a guitar is soft and distorted. I checked the factory bias point and set it for 80 MV The outputs are all Sovtek 5881. 475v on every pin 3. 469v on all pin 4s. V1 (12ax7) is 147v@pin 1 and 193v@p6. It sounds like it is biased wrong but the setting is right on at 80 mv. The pin 5s are all -53v at all the 5881. This just in: I pulled the 5881s and played the amp through the PREAMP OUT. Same yuchy sound

[nevetslab]

If you're going thru Ch 2 or Ch 3, there are 5 consecutive tube gain stages in series between the Gain control and the Volume control on those channels. I think on Ch 1, there's only 2 or may 3 stages between the Gain & Volume control (I don't have the schematic opened up in front of me). Ch 1 is obviously much cleaner than Ch 2 or Ch 3. I've had to juggle tubes around to obtain the quietest results with it. They are what they are.....I'm not a big fan of them, but obviously there's a big fan base for them. I just do my best to keep them running and optimized from a performance standpoint.

[mikeskory]

You've really helped me. It's good to get an overview from another mind. I'm going to work with the tubes. If I make headway I'll post it up for every one.
Thank you.

[Enzo]

Hi, Mike, I almost dropped by the other day, but was feeling dizzy, and cut my day short.

SO all three preamp channels play, but sound bad out the speaker. Preamp out signal is bad. By the way, did you plug a signal into the FX return to test the power amp alone?

My suspects? SInce all three pre channels seem to work, I'd look at the common circuit after them, the V7a circuit driving the FX send. Lower left on the main schematic page.

See Q11? That is a mute. If that fails or if the mute is stuck on, it can cause such symptoms. R156 is 330k in parallel the transistor. With the amp RUNNING, measure resistance of R156. I expect a couple hundred k or more. If you read like 200 ohms or some such, then the transistor is either bad, or it has lost the signal to its gate.

[mikeskory]

Hi, Mike, I almost dropped by the other day, but was feeling dizzy, and cut my day short.

SO all three preamp channels play, but sound bad out the speaker. Preamp out signal is bad. By the way, did you plug a signal into the FX return to test the power amp alone?

My suspects? SInce all three pre channels seem to work, I'd look at the common circuit after them, the V7a circuit driving the FX send. Lower left on the main schematic page.

See Q11? That is a mute. If that fails or if the mute is stuck on, it can cause such symptoms. R156 is 330k in parallel the transistor. With the amp RUNNING, measure resistance of R156. I expect a couple hundred k or more. If you read like 200 ohms or some such, then the transistor is either bad, or it has lost the signal to its gate.

The resistance of the R156 is 235K while amp running. I installed new ECC83 at V1, V7 and V8. It's about the same. I wish I could explain the symptom better. The amp is very loud. The distortion I don't like seems to be more noticeable as you hit the strings hard. THIS JUST IN: I found the problem. I reversed the 2 brown bias wires going to the amp board from the main board at P6 & P8. I labelled them wrong when I took the board out to replace the sockets. That hint about R156 got me to the trouble spot however.