Haunted by David Eden

[Sowhat]

Over two years ago I had a customer bring me a David Eden Traveler, the 300 watt one, I forget the model number with the complaint that the amp runs for about 10 minutes and then cuts off. I checked through the amp, hooked it up to 400 watts of 4 ohm dummy load, shot a 220hz sine wave at about 300mv into the input and cranked it monitoring the output waveform on the scope. I also stuck a temperature probe on the heatsink. After a bit I could plainly see that the heatsink temperature was not stabilizing and continued to get hotter and hotter until the thermal sensor tripped and the amp went silent. I finally found that I had a failing output transistor, interestingly enough the one that the thermal sensor was attached to. I replace all 4 output transistors, reset the quiescent, did the trial by fire test again which it passed this time and sent the customer on his merry way.

2 years later he call again and says "the thing is cutting out again". This amp led a hard life constantly on tour and worked perfectly for two years and then this... I said bring it in and I will figure out whats going on here. I figured the output transistors were going bad again.

I played it for a few seconds with my trusty 5 string bass and when I hit that low B the thing sputtered and went silent, I repeated this several times and got similar results. This time however it was a flakey gassy old 7025 preamp tube, so I replace that and did my trial by fire test as I did before and everything looked great, the temperature stabilized somewhere around 150F and the amp never cutout so I figured problem solved. I played it for about an hour just to make sure, called the customer up to pick it up and he was off again to beat the amp to death just as he likes to do.

A few days later I got a call from him... you guessed it, it's cutting out again. This time I had him bring in his whole rig from the pedal board all the way to the speaker cabinet plus all his cables and I went through it all in great detail. I could find nothing wrong, aside from that I could not replicate the failure in the shop no matter how hard or long I drove the amp. He took the amp back and called me a few days later and said it's working great, thanks!

Here comes the punch line... Two weeks later he calls me and says, "it's cutting out"!!!

Any ideas out there???

[Enzo]

Yes. Invite the whole rig back again. Let him try to replicate the problem. You need to isolate the problem to repair it. Do not assume this "cutout" is related to the previous. It may or may not be.

We don;t really know if the amp is at fault or not. When I hear "pedal board" that means to me a whole pile of connections begging to fail, not to mention the pedals those connections are on. Plug the guitar straight into the amp.

When it "cuts out", what exactly happens? Does the power light go out or not. Does the sound disappear COMPLETELY? Or does it just go really low? Does the sound go away for a moment, a second, an hour, what? Does the amp do ANYTHING else unusual before it does this? When it cuts out, is the amp overly warm to the touch?

You gave it a 4 ohm load, what load is his speaker system?

And of course, ball up your fist and whack the top of it hard. ANy reaction?

Got FX loop jacks or pre out and power in? They may be working, and a patch cord may make no difference, but MEASURE the resistance of the cutout contacts. ANy that are over half an ohm need attention. A contact resistance of only 2 or 3 ohms will not affect the signal, but the very fact that contact is not zero ohms means the contact is dirty. It may read 2 ohms now, but at some later time it will measure a lot higher and cause signal interruption.

[MarkusBass]

The last tim I've seen this problem it was as simple as a ballast resistor not contacting properly to the board. Tap on the ballast resistors and check FX Loop as Enzo says.

Mark

[Sowhat]

I did the jack cleaning, RCA test, pound the fist test and so on... sorry I forgot to mention those. The customer uses an 8 ohm cab so he's only using maybe somewhere around 1/2 the power. I loaded it a 4 ohms to really give it a workout. I thought it might have that old dirty FX jack problem too, but after a comprehensive cleaning no difference... as I said I have not been able to get it to cutout regardless in the shop. I had to clean out dirt and sand from the unit, especially seeing that his band just got back from burning man out in the desert. He has the thing in a road case and uses it in the case with the front and rear hatch removed, kind of standard bill of faire for touring groups. I asked him if he took out all the stored cords before firing up the amp, DE's breath through their sides and junk blocking the inlet or outlet could cause overheating. He said he did leave junk in the road case when he used it but I had him clear out the case before use and that did not seem to make any difference for him. Ballast resistors??? I'll check those out whatever you mean by that... maybe emitter resistors or any of the wirewound PS divider resistors I guess.

This problem is definitely thermally related because once it cuts out on him, letting it stand for a few minutes and then turning it on again fixes the problem for a few minutes. Asking him if it get hot or not is really a difficult thing to ask of him, he's playing a show, not troubleshooting his equipment. I like the possibly cracked or loose resistor thing, I'll have to check that out when he brings it back in.

[MarkusBass]

Yes, by "ballast resistors" I mean emitter resistors of the output transistors. Sorry, English is not my native language I it may be that I use the term incorrectly.

Mark

[Jazz P Bass]

Not incorrect useage, at all.

Technically, they Are ballast resistors.

[Chuck H]

Also, just because the problem seems thermal doesn't mean the amp is getting too hot. I've seen cold solder joints and dirty tube sockets that would repeatedly work on turn on only to cut out after some use. Have you tried whacking it?

[Sowhat]

Guess I'll never get to look at that amp again. The customer called me and said he wanted to bring it back in but he also told me he just got a new 800 watt DE mainly because his band has a new drummer and he's a hard hitter, that little 300 watt DE could not compete anymore. I guess it's in his pile of old busted junk now... we all have them, maybe he will bring it in but it's been several weeks now and no word.

[Jazz P Bass]

I would have resoldered the whole amp.

But that's just me.

[jamesmafyew]

If he got the WT800, then I hope he got one of the C-revisions. I have a similar story to yours with one of the A-revision WT800s, where it just kept coming back. And actually a WT400 as well, as I recall. In all cases the thermal protection circuit seemed to be 'overactive'. Eden was obviously aware that there was a design problem, since they made significant changes to most of the big high-powered models in the last ten years... but it's not clear from the changes they made what the specific root of the problem was. Could have been the protection circuit itself, or the cooling circuit, or it could have been just that the amps ran unreasonably hot -- they attacked on all of those fronts.

In the later revisions of the WT800 specifically, they added more parallel output transistors and reversed the fan direction, and rated it for minimum 2 ohms/side instead of 4. This seems to have cured the overheating problems in that model, but reversing the fan in earlier models seems to make the problem worse. The 800 I worked on I modified with a more powerful fan, and altered the fan supply so that it gave the fan maximum voltage as soon as it clicked on. That amp never overheated after that, although it did come back many more times with an intermittently shorting zener diode, cold/cracked solder, failed-open driver transistor, high-ESR electrolytics in one of the power amps, and a fried opamp in the preamp. None of these problems ever blew the fuse, just caused gross noise and distortion on the output, which of course rarely was reproducible on the bench. I ended up just disgusted with the build quality, especially with all the haphazard and inconsistent 'factory mods' snaking around on the underside of the PCBs.

[Sowhat]

Not to beat a dead horse, but here we go again...

The WT-300 is sitting in the junk pile and he picked up a WT-800. He played it for about 2 hours and then BAM!!! it went quiet and distorted and that was that. When it arrived in the shop I could see that he had bridge mode engaged and biamp disengaged. He stuck his 4x10 and his subwoofer into the high amp channel, flipped the switch and started to wail away. I asked him if he read the manual for the amp and of course, he never saw one. I calmly informed him of the special handing necessary for a Bridge/Bi-amp/Stereo amplifier and that bad things can happen if he don't configure and connect it properly, which he unfortunately didn't. The long and short of it the high amp blew an output transistor. I'm just going to replace all of them, drivers too and start with a clean slate. This amp has blown before as indicated by the unclipped replacement transistors, solder flux marks, stripped screws and like that... thankfully no lifted traces. Maybe now that he knows how to operate a bi-amp he will have a good long trouble free experience with it.

[Chuck H]

That's classic. I'm feeling guilty now for being a guitar amp designer. IMHE and WRT guitar amps pretty much nothing on the back panel is going to be understood other than the speaker jacks. Which should be labeled "SPEAKER" if you ever expect the amp to be plugged into one. That's probably why effects loops are "send"/"return" too. If they were marked "input"/"output" there would be all manor of problems. And anything more than a power switch and a volume knob on the front panel becomes a mental exercise for the average musician. These guys spend their whole life around this equipment and never even try to learn a thing about any of it. Since guitar amps don't stray too far from the power switch, volume knob and speaker jack formula my task is a lot easier than a bass amp designers. If I had to design guitar amps that actual guitarists were going to use that had to include crossover and bi amp circuits I swear that I wouldn't even bother. Perhaps five percent of musicians are smart enough and take the trouble to understand the proper operation of their gear. The rest are as dumb as a sack of rocks and even less interested in learning anything about something like "speaker impedance".

[Leo_Gnardo]

I could see that he had bridge mode engaged and biamp disengaged. He stuck his 4x10 and his subwoofer into the high amp channel, flipped the switch and started to wail away.

I've disabled LOTS of bridge switches. All Mikey Muso thinks is "I heard it gives you more power. 2, 3, 4 times as much. Must be good. Push the bridge button." Not only that, but when one amp channel fails, it normally kills the other one within a millisecond or 2. Then there's the case of flinging speaker cables around while signal is applied to the amp. Hard way to find the handle of the speaker plugs isn't ground - but carrying signal - lots of it. Enough to get a shock, or lunch the amp if it comes into contact with ground.

Still doesn't help when the muso runs all the speakers off one channel. User needs to be sent to re-education camp. Good luck!

[Sowhat]

doh!!!!

Just to complete this saga, I got the replacement transistors in today and I masterfully installed them clean as a whistle. You would never know they were replaced, then... I looked at the amplifier module and then looked at the other amplifier module and then back and again... S%$t! I rebuilt the wrong amplifier! I did an excellent job but I fixed the amp that was not blown. I quickly ordered up another set of drivers and outputs and called the customer to inform him of my blunder. We both had a laugh on it and I said " well, now you get two rebuilt amplifiers for the price of one." The slight delay in delivery is not a problem for him so I'm thankful of that but it goes to show that even a tech with nearly 50 years experience can still screw up.

[Sowhat]

One more little thing...

While waiting for another set of outputs and drivers to arrive I stupidly decided to take a good hard look at the amplifier modules. David Eden has made a lot of substitutions and changes to these amps since the last time I worked on some and one thing that pops out is a mystery trimmer pot. It's a 1 meg 20 turn tocos and is situated as shown in this picture.



I'm guessing that it's some sort of symmetry adjustment but the schematic, or should I say cocktail napkin sketch that DE supplies don't show it along with a bunch of other things. Has anyone encountered this and knows what it is for and how it is adjusted? In the absence of some concrete information about it I intend to just ignore it but being able to check if it's doing it's job correctly would be the responsible thing to do before returning the amp to my customer.