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Post subject: EVH 5150 iii Problem
Posted: Thu Jan 17, 2013 12:24 pm
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Brand new to the forum! Thought it would be a good place to rattle brain pans.

I have a EVH 5150 iii 100W head. Love it! Sounds amazing.....but.........it died.

Symptoms:
I was playing at bedroom level for a few mins and then the amp just faded away. I put amp to Standby, waited a minute and turned on again. Worked great for an hour or so.

A few hours later, turned amp on, let warm up, turned off Standby and nothing. Nothing at all. No crackle, no whoosh, no hum, no speaker pop, nada!

Checked input fuse = fine

The pilot light comes on, the footswitch is powered and the light for each channel works.

Looked at power tubes and had no glow at all. I have always seen a glow.

Hung head, sighed and possibly peed a little.

Pulled off back cover, pulled off preamp tube covers. Powered on amp, and found that two tubes were working. Thats it. Just V1 and V4. Said to myself that can't be possible, the should be powered in series. How does it jump from V1 to V4 withough powering V2 and V3.

Pulled up schematic and found that V1 and V4 are not in the power circuit with the other preamp tubes (P2, P3)

So, all I have so far is the heater circuit working on two tubes. Looks like they should be about 6V but i'm measuring about 13V AC from pin 5-9 on the tube pins.

I have to check one last thing tonight and that's fuse F1. It looks like its in line with P2 and P4. Thought I checked continuity last night on it, but it may have been a different fuse.

Question:

Has anyone dug into these amps yet? Dug into the schematic? Anyone had this problem?

Thanks for any and all input.


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Post subject: Re: EVH 5150 iii Problem
Posted: Thu Jan 17, 2013 6:44 pm
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smitkis, Pretty sure the 5150 has the heaters wired in parallel. And not series. That is why certain tube may light downstream of ones that do not light.

Sometimes, it's really hard to see the heater light up in 12AT7 and 12AX7 (depends on brand). Best to look at the tubes with no room lights.

The 12AX7's heater lines are in parallel between the two triode heaters. So, if you measure the AC voltage from pin 4 (or 5) to 9, you SHOULD get 12.3-13 VAC.

Sounds like you need to do some tube swapping. But, first... be sure that the output tubes are getting proper plate voltage (should be around 450-475VDC). And have correct idle bias (around 40mA per tube).

If 6L6GCs seem fine, start to swap each gain stage tube (12AX7 or 12AX7A) one-by-one with a known good 12AX7. See if you can get the amp to work. You may need to swap two 12AX7s (if there are two bad ones)--- etc.

If this doesn't work, you may have faulty wiring or soldering to sockets. Prolly best to have a tech look at the amp, at that point.

Good luck, keep us posted!


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Post subject: Re: EVH 5150 iii Problem
Posted: Thu Jan 17, 2013 10:34 pm
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In this amp, V1 and V4 have their own separate power supply for the heaters, and they are in series, approx 24 VDC, which should be approx 12 VDC from pins 4 to 5 on each tube. The "regular" heater supply, approx 6 VAC is for the other tubes; V2, V3, V5, V6, V7, V8, across pins 4 and 5 (connected together) to pin 9, and output tubes pin 2 to 7. Sounds like you lost the AC supply for the majority of the tubes' heaters, and still have the DC supply for tubes V1 and V4.

Check fuse F1, you likely have a bad output tube that caused this fuse to blow, possibly even one of the preamp tubes. Great time for a good tube tester. The alternative is to pull all of the tubes (except V1 and V4) and then replace the fuse, power up and check voltage (6 VAC), then replace tubes one at a time until fuse blows again, that tube is the bad one, save output tubes for last. Power the amp off each time before putting each tube back in. Odds are that it's an output tube, so to save some time, just pull the output tubes and replace the fuse and power up and see if the fuse holds. Hopefully, the circuit isn't damaged (R11 and R12), but possible.

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Post subject: Re: EVH 5150 iii Problem
Posted: Fri Jan 18, 2013 6:19 am
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Shimmy, interesting. Wonder why two signal tubes are wired in series. And the rest of the tubes in parallel? I've only seen such circuitry in some old-time hi-fi tuners

Do you have a link to this schematic?


Thanks!


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Post subject: Re: EVH 5150 iii Problem
Posted: Fri Jan 18, 2013 8:33 am
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Well, isn't 5150 the police code for "crazy"? :lol:

http://support.evhgear.com/schematics/EVH_5150III_AmpHead_Schematics.pdf

The 50 watt version has a DC heater supply with three pairs of series connected heaters for the preamp tubes, while the PI and output tubes have the "regular" parallel AC heater supply. :o

http://support.evhgear.com/schematics/EVH_5150III_50W_Mexico_Schematics.pdf

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Post subject: Re: EVH 5150 iii Problem
Posted: Fri Jan 18, 2013 9:29 am
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More debugging has been done.

First I just swapped a different 12AX7 tubes into the V1 and V4 position. The new tubes lit up fine.

Went back to the schematic and noticed that step one to making this entire amp work is the F1 fuse (as mentioned a couple posts up). That power line supports every tube except the V1 and V4 oddly enough.

Checked the fuse; no problems.

Took fuse out, powered amp up and tested voltage onto the fuse holder. Nada!

Tested voltage to the socket where the fuse holder leg fits through. Tada!, VOLTAGE! (~6V)

What we have here is a crap solder joint. It does not seem like I'll be able to touch it up from the top side though. So the board needs to come out.

And that is where I am stuck now. Has anyone removed the power supply board on these yet? There are a bunch of screws retaining it to the chassis, but after that, there seems to be something else holding it down. I can't seem to get it free.


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Post subject: Re: EVH 5150 iii Problem
Posted: Fri Jan 18, 2013 9:57 am
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:?

V1 and V4 were already lit-up, so a waste of time replacing those tubes. Good job finding the poor connection in the AC heater circuit. The knobs and nuts for the three presence pots have to come off also, in order to remove the board. Make sure that the caps are discharged (check with a meter) before removing the board. :!:

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Post subject: Re: EVH 5150 iii Problem
Posted: Fri Jan 18, 2013 10:08 am
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Knobs and nuts are removed already. I was going to remove power and standby switch and power plug too just to bring all wires inboard to help fit the board through the chassis.

Could the power transformer have some sort of attachment to the board through the chassis?


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Post subject: Re: EVH 5150 iii Problem
Posted: Fri Jan 18, 2013 10:26 am
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Not sure, could be. What about the screws/nuts holding the transistors to their heat-sinks?

Looking at the layout, there might be at least seven screws holding the board down. :idea:

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Post subject: Re: EVH 5150 iii Problem
Posted: Fri Jan 18, 2013 11:16 am
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I for sure got all the screws but the transistor nuts did cross my mind. There is a small aluminum heat sink, but it may go through to the chassis. I'll take them out tonight and see if it makes a difference. I also may try to bring home a small solder iron with a super skinny tip and resolder from the top side. I don't want to do a sloppy job though.


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Post subject: Re: EVH 5150 iii Problem
Posted: Fri Jan 18, 2013 11:19 am
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Looks like the holes are through-plated, so you might be able to do a good job feeding in more solder from the top.

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Post subject: Re: EVH 5150 iii Problem
Posted: Fri Jan 18, 2013 11:25 am
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Yep, they are. Here is a pic.

Image


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Post subject: Re: EVH 5150 iii Problem
Posted: Fri Jan 18, 2013 8:33 pm
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Did you try soldering from the top, or was the melted insulation already there? If so, any success? Been there, tight spot. :o

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Post subject: Re: EVH 5150 iii Problem
Posted: Fri Jan 18, 2013 9:50 pm
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I call those "solder hickeys." :D

Been there many-a-time. Usually leaves a telltale rubber residue on the upper part of the soldering iron.

I use heat shrink insulation over the scar.

"Aw... the sweet smell of rubber, in the morning..."

:mrgreen:


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Post subject: Re: EVH 5150 iii Problem
Posted: Mon Jan 21, 2013 9:05 am
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Dammit!!!! I was hoping nobody would see that! Haha.

Yeah, that was me on my first attempt when I was just trying to re-float the solder. On the second attempt I brought home a nicer, much smaller solder iron from work an even pulled all the wires away from the fuse block.

It worked though. I touched up all four points and I'm back up and running. I was pretty disappointed in the workmanship on Fender's part though. When I heated up the first leg, the block actually came free. It felt like the other leg didn't have any solder at all.

Either way, it worked and I hope it stays that way. It's a great amp with great sound and I'm pretty happy I didn't have to run it into the shop.

Thanks for all the help!


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