[zzz]

(just a rant, I guess)

Received Nik's HP4 today. To cut a long story short, the left channel does not work the way it is supposed to. It is overloaded all the time (on idle with no input signal that's RUMBLE, not some wussy `hiss`) and when I tried to put some music on, well, my left ear is still hurting. The good thing is that W2002 seems to have escaped undamaged... Replacing tubes didn't do a single thing, just increased the warm-up (read `zero to noise`) period.

I called EAR USA and they are willing to give it a look, but they do not consider it to be under warranty, so I would have to pay for everything myself.

I have the schematic and I'm going to sit and compare the voltages on the left and right parts of the amp today, will see if I find anything (and if I can do anything about whatever that is I find).

 

[zzz]

Hm, when the amp is switched off, it lets the line out through. But only in the right channel, left is cut out. (/me looks at the schematic) Yes, the input is connected to the output through a couple of resistors, input/output transformers, volume control (which indeed is an Alps Blue), and one capacitor. Interesting... Less parts to check, I guess.

Is there any way to tests capacitors (except for when they are shorted, of course, then it's easy...)? Just curious.

 

[zzz]

Found a problem.

After looking at schematic and realizing that I was listening to the negative feedback circuit, I understood that the symptoms I have would occur if one half of the stereo variable resistor dies. And yeah, of course I have immediately found that one of its long legs (it's not attached to the board, rather to a separator between the power supply and an amp... I will post the photographs tomorrow) got disconnected from the board.

Phew

... Disassembling it to do the soldering job is not going to be fun, but at least the problem is simpler than I though it would be.

 

[eric343]

Good to hear you found the problem... that's what I call DIY

BTW, the way I test capacitors is to connect an ohmmeter across the leads (polarity correct of course), and watch the display. A capacitor should increase in resistance up to a certain point, depending on the capacitance.

 

[zzz]

Fixed it. That was the weirdest soldering job I've ever done (which probably reflects my lack of experience more than anything else), because it's impossible to take out the PCB without disassembling the whole amp, and it's literally impossible to disassemble it because some of the screws are hidden behind the PCB. So the only way for me was to heat the disconnected leg up and push it into its hole, hoping that it would melt the solder on the other side... Well, that worked, and as you can see, I am very pleased with myself. Maybe I'll have the guts to do the voltage conversion by the end of the week (it seems to require opening up one of the transformers).

Sound changes while I'm typing this, so I wouldn't post my impressions until at least a week later.

 

[Nezer]

I think that joint is a candidate for a cold solder joint in the future. At least if it starts to freak out again you'll have an idea of what it sounds like when it does.

 

[eric343]

That sounds odd, becuase they had to get the PCB in there SOMEHOW...

 

[zzz]

Quote:

Originally posted by eric343 That sounds odd, becuase they had to get the PCB in there SOMEHOW...

Well, for one there are tools that make it possible to work with screws in such positions. On the other hand, it seems possible that it was assembled first and then jacks were soldered to the board later (there are two boards connected by wires: main with tubes and another one with input/output jacks perpendicular to and standing on the first one). I'll post some photographs later.

Or maybe I missed some other non-destructive way to take the amp apart. But unless there is some magic button that removes all the caps (and then preferably puts them back again), it would have to involve a *lot* of work...

On a totally unrelated note: the faceplate is too reflective. I mean, it is effectively a mirror and it attracks fingerprints like there's no tomorrow. Wicked, just like W2002... I should get some wiping cloth...

 

[kelly]

zzz
Although you were quick to diagnose and repair your amp, I want you to know that I for one was fully intending to extend my sympathy. I realize you've already robbed me of this opportunity at comradery but felt I should share some sort of "man, nice expensive amp fried" bonding thing. Anyway, I'm glad you're up and running.

 

[eric343]

zzz: I've got some great tools, a fantastic digital camera, and very good soldering skills. Why don't you send it to me and I'll see what I can do?

 

[zzz]

kelly:

Appreciated, what else can I say.

eric343:

Hey, I'll be interning at MS this fall, so if you ever want to play around with the EAR (and help me mod it) in the September-December period, that could very well be arranged

.


Anyhow, here are some photos if anyone cares, inside and outside. I will get some more tomorrow.

 

[evilcthul]

Hey ZZZ,

Could you give us the schematics for the EAR HP4? Imagine DIY EAR!

 

[grinch]

Quote:

Originally posted by zzz Anyhow, here are some photos if anyone cares, inside and outside. I will get some more tomorrow.

holy cow does that thing look complicated. you've definitely got some guff if you can crack that puppy open and go at it with a soldering iron! just looking at the inside would scare me away from any idea like that.. that and the fact that it costs $2200+ to buy.

great job on the diy fix though, very impressive.

 

[eric343]

It's not *that* complex. I really like how Tim P. put the Alps Blue on a long extension to keep it away from the transformers- really good idea, that.

Just from looking at the first picture, I'd guess you have to remove the PCB with the jacks on it first, and then you'd be able to pull out the main PCB.

 

[zzz]

Quote:

Originally posted by eric343 Just from looking at the first picture, I'd guess you have to remove the PCB with the jacks on it first, and then you'd be able to pull out the main PCB.

XLR jacks are attached to the chassis with screws, and there is about 1cm of space between the little board and the enclosure. Makes removing it somewhat complicated.

Screw that connects the separator between PS and the amp board to the chassis is made in such a way that it scratches the paint on the outside when you try to remove it. Annoying.

At some point I might spend some time and figure out how to take it apart without damaging (or maybe I'll just email Tim, he seemed to be pretty helpful in answering my stupid questions before

), but for now I'm content with not knowing that.

This thing needs a stepped attenuator for volume control. The image is a bit shifted to the left right now and right channel cuts out before the left one. That's going to be my next upgrade, I guess. The little problem is that normal attenuators are wired in the opposite way (all the way ccw is infinite resistance, all the way cw is 0 resistance) of what HP4 wants. Moreover, as my experience shows, you NEVER want to have an infinite resistance in the volume control on HP4.