WO 400 S2 R-Ch Distortion at Half Volume

Zstangl

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#1
I just finished a WO build on a 400. The left channel sounds great. The right channel is quieter by 25% and also clips or distorts in some way at approximately 50% volume.

I don't where to look. Any help, advice, or suggestions would be very welcome.
 

Gepetto

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#2
Some great, high resolution photos of everything you have done would be an excellent starting point. Have you carefully measured the +/-15V on the test points provided on the control board to see if you have that on both channels? Is your control board a RevE (single set of +/-15V test points) or a RevG1 (2 sets of +/- 15V test points)

I said carefully, don't blow anything up in the process.
 

Zstangl

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#3
Some great, high resolution photos of everything you have done would be an excellent starting point. Have you carefully measured the +/-15V on the test points provided on the control board to see if you have that on both channels? Is your control board a RevE (single set of +/-15V test points) or a RevG1 (2 sets of +/- 15V test points)

I said carefully, don't blow anything up in the process.
Thank you for taking the time to respond.

Electronics have been an On/Off hobby since I was very young. It was my father's favorite past time so I had access to a modest electronics workshop throughout my childhood. Fwiw, soldering was always my favorite thing to do. I don't even remember learning how. My earliest memory of soldering was in the 3rd grade when my dad and I built a Tesla coil for the science fair.

I have not had an opportunity to photograph my project, and, with all due respect, I'd like to move past a visual inspection for just a moment. I'd be happy to circle back after some further discussion.

I took on this project as a way to further my understanding. I learn best when faced with a problem to solve. I have one in front of me now and I would LOVE some direction on what steps to take and in what order. Again, my goal is to learn and self perform the process that an experienced hobbyist follow.

Step 1: Visual Inspection. Look for obvious issues with build quality.

I performed a heavily detailed visual while sorting out a separate issue last month. I will never claim to be 100% sure of anything, however, for now I am confident in my component placement and solder integrity.

Step 2: Double check all measurements and data from Joe's instructions. Here is what I've checked and measured.

B+ to G is ~85v
B- to G is ~85v
15+/- on pre board is spot on
Bias+/- is 370/360 on both channels
No DC voltage on the output terminals
I checked the bias transistors that are clamped to the chassis. I followed a procedure I found online. The results confirmed proper operation but I'm not confident as this was my first time checking a transistor.

Question: Are the Bias test points on the pre-board the same as on the back plains? I assume so because I measure the same values.

This is all I know how to do. Please share with me what YOU would look at next. I have a DMM and a Tektronix scope that I'm dieing to use. I also have a signal generator if that helps.

Here is a better description of the symptoms.

Both Channels sound amazing at low volume, but the right channel is quieter than the left by maybe 25%. Even still, this WO amp is a noticable improvement over the stock 400 I'm currently using.

As I increase the output to 50%, the right channel begins to distort. It sounds like clipping but I'm not certain. Shortly after, around 60% volume the left channel distorts in exactly the same way. The distortion occurs very low in the frequency range meaning the lowest, most demanding notes are affected first. I hate to describe this issue in such simple terms but it feels like the amp runs out of power.

In my very amateur assessment, I've been leaning towards the power supply. The caps are new from WO. The rectifier is original. A 50a rectifier is on order based on advice from another thread.

OMG, as I was writing this, I remembered dropping the transformer HARD last summer. I'd completely forgotten about that.

Tomorrow I'll swap it out for one that's proven to work. In the meantime, I'd love to hear some input.

Thanks for listening!!
 
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#4
Monitor your rail voltages while increasing the volume and see if they droop before swapping transformers.
 

laatsch55

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#5
The power supply is common to both channels. Doubt it's the transformer.
PLEASE provide pics..
 

George S.

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#6
This is a Series 2. Therefore it has attenuators. Include a good photo of your attenuator wiring from the RCAs to the pots to the control board. We assume you cleaned them with Deoxit or replaced with new? These were checked for equal resistance and wiring is correct?
 

J!m

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#7
Am I reading that correctly? The attenuator pots are directly between output and RCA connections?!? If so, I’d want a hard bypass switch on that. If it was my amp anyway.
 

George S.

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#8
Naw, the attenuator pots are between the RCA input jacks and the control board. Remember Lee's First Rule, "The wiper goes to the board". My first time rewiring these confused the fock out of me. Mark Comer had a great write up on these a good while back.
 

WOPL Sniffer

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#9
Electronics has been a hobby since you started soldering in the 3rd grade but you have never tested a transistor before? This latest batch of new guys come up with some doozies for sure. And "I'd like to move past the visuals for now"??? We catch a lot of mistakes just by having a few more sets of eyes checking. As for the B+ and B- measurements, what is "~85 and ~85"????? Never heard of that before.
 

laatsch55

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#11
And irlf you are dieing to use that scope hook it to the outputs and see what happens to the sinewave... got a signal generator?
 

WOPL Sniffer

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#12
By NOT adding pictures, you are basically saying to the guys here to help, "Help me, but I'm not helping you help me".....
 

Gepetto

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#13
Just for reference guys, Zach has RevD PL400 backplanes installed and a RevG1 control board installed based on his order history. This may head off some additional questions you may be wondering about. I am assuming that Zach installed what he ordered.
 

Gepetto

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#14
Zach
The test points for bias are the same whether you use the backplane or control board test points.

What speakers are you driving when experiencing the distortion you are noticing?
 

Zstangl

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#15
Monitor your rail voltages while increasing the volume and see if they droop before swapping transformers.
This is the kind of direction I'm after. I'm working another 12+ hour day. I'll put the peddle to the metal and see what happens to the rail voltage. Is there an acceptable amount of voltage drop?

Thank you for your input!
 

Zstangl

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#16
The power supply is common to both channels. Doubt it's the transformer.
PLEASE provide pics..
Both channels are affected as mentioned in my post. The distortion appears in the right channel before it appears in the left. Weird.
 

Zstangl

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#17
This is a Series 2. Therefore it has attenuators. Include a good photo of your attenuator wiring from the RCAs to the pots to the control board. We assume you cleaned them with Deoxit or replaced with new? These were checked for equal resistance and wiring is correct?
I didn't have the attenuator wired correct initially but i discovered and corrected that issue a few weeks ago.

I wanted to replace them but couldn't find something I liked with the same resistance. I cleaned them thoroughly with Deoxit.

For trouble shooting purposes, I've removed them from the circuit.
 

Zstangl

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#18
Damn Sniff, did someone $hit in your raisin bran this morning.

but you have never tested a transistor before? This latest batch of new guys come up with some doozies for sure.
The ability to solder and the need to test a transistor are not mutually exclusive. Hell, they're barely related. As I mentioned in my post, electronics as a hobby have always take a back seat to other interests. See below for an example of a hobby that I've focused on instead. I designed and built the entire thing by hand IMG_20180921_190146.jpg

I'd like to move past the visuals for now
I want this project finished more than anyone here. The easy path would be to have someone else just tell me what's wrong. In case you missed it in the original post, I put this kit together to further my knowledge. Confronting and working through this problem will do more for my advancement than anything else.

As for the B+ and B- measurements, what is "~85 and ~85"????? Never heard of that before.
You're a native English speaker and have never heard of a Tilde?????? It looks like this ~ and it means "approximately". Glad you had the opportunity to learn something from me, I just wish I could say the same.
 

Zstangl

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#19
And irlf you are dieing to use that scope hook it to the outputs and see what happens to the sinewave... got a signal generator?
I'll absolutely do this as a next step. I just picked up a signal generator a few days ago. I'm curious, is there a frequency that's preferred over another? I can't imagine there would be unless an issue were occuring at a specific frequency. But I don't know what I don't know so I have to ask.
 

Zstangl

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#20
By NOT adding pictures, you are basically saying to the guys here to help, "Help me, but I'm not helping you help me".....
I asked for help in learning how to troubleshoot a problem. I'm not interested in being spoonfed the answers. You and I are not the same.
 
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