La2a problem - Touch down ! Problem resolved

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Jazz

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USA - Portland Oregon
Hello folks, I have a problem on an La2a that I have not been able to track down yet and, was wondering if someone might have a clue or tip that could help me resolve this. Here is some info.

All the voltages at the standard test points on the schematic / tube sockets appear normal. What's happening is the VU meter starts to flutter back on forth when the gain pot is turned. I think I'll change the pot because I just noticed that you dont even have to turn the pot to trigger this fluttering. Just snapping it with the finger can also cause it to flutter.

As you may know I've built about six of these so far and have had only one that had a ground problem. So I'm scratching my head. Off to change that pot now.

Nope, not the pot. Thought I found it. Still flutters.



Thanks, ~R

:shock: :thumb: :green:
 

dripelectronics

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Dec 12, 2005
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five o fresh
that would be a problem located in one of the components that go to the T4 ground , meaning somethings not grounded right.
.

resolder all of the components that connect to that ground.


g.
 

Jazz

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USA - Portland Oregon
Meter is in +4 mode

When the gain pot is at 0 zero the meter is fine. When the gain pot is turned slightly the meter flickers back and forth.
 

JdJ

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Jan 13, 2006
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Mine kind of does the same thing, although I haven't heard it affecting the sound. Touching the front panel doesn't seem to do anything to affect it as it does with yours.

I have always suspected that it was caused by some oscillation- I just haven't had time to troubleshoot. Maybe try:

http://www.groupdiy.com/index.php?topic=43559&highlight=la2+oscillation#43559

On mine I didn't use the grid stoppers and I am using the Sowter iron, so I was thinking it might have something to do with that...

Grid Stopper Info

Let us know what happens!


Thanks,

Josh
 

Jazz

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USA - Portland Oregon
It actually does not flutter when the pot is flicked with the finger, it does have to be turned...
I was wrong about that. Will look into this,thanks Josh
 

Jazz

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Oct 29, 2006
Messages
95
Location
USA - Portland Oregon
V1
1 - 118v
3 - 1v
6 - 125v
8 - 1.1v

V2
1 - 97v
3 - 4v
6 - 220v
7 - 73v
8 - 96v

V3
1 - 99v
3 - 0.8v
6 - 98v
8 - 0.8v

V4
2 - 6v
5 - 155v
6 - 115v
 

David Kulka

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Dec 10, 2006
Messages
229
Location
Burbank, California
I would first look for DC on the high side or the wiper of the pot. There should be no DC on this pot but if any is seen, determine the source will lead you to the problem.

if it's not DC it's probably oscillation. Check lead dress, grounding, etc.
 

Jazz

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Joined
Oct 29, 2006
Messages
95
Location
USA - Portland Oregon
Thanks for your reply David. Well..... I feel a bit stupid but then I never claimed to be skilled in this area...after all I'm a musician. This is my point.

How the heck do you do that? I do have a scope. If you have time to explain it that would be great, If not, no worries. I've tried all the advise listed so far ...but only a fast version of moving wires a bit with chop sticks. I would like to try
that some more. But then all my other units did not have this problem and I
have not changed much really. If anything they are wired better I would think.

I don't really know what oscillation would sound or look like but I do feel it is oscillation.

Description of what's happening on more detail.

A) Gain knob is on zero, when you increment it a bit the meter flutters back and forth like a ball bouncing back and forth between two walls that are close to each other.

Sonically, there is distortion. If I apply compression it sounds almost normal. I've checked wiring very carefully and inspected the unit under a magnifier several times.



~R
 

Jazz

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Oct 29, 2006
Messages
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Location
USA - Portland Oregon
Bad capacitors C2 and C3 0.1uf/400v

Ok this is the story. I had a box of these old caps that I got from a surplus store when I purchased a box of stuff....well I wanted to try these caps to see how they are. I suspected these damn caps three weeks ago.....so I pulled out the capacitor tester and they both tested out at ~ 0.15uf

Then I pulled out a couple of my orange drops caps and they tested out at 0.1uf right on the nose..... so I can assume my cheap old cap tester is not so bad. Changed the caps and the problem vanished.

What a pain in the butt. Thank you all for your advise, I really appreciate it.

~R

Now I can sell this clone on ebay. JUST JOKING :green: :green: :green: :green: :green: :green: :green: :green: :green: :green: :green:


So anyone who plans to build an La2a.

Don't cut corners on the caps. It only took 0.005uf of an offset to cause the La2a to get upset, Is that math right? I dont know I cant see straight almost time for bed
 

Whizard

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Oct 28, 2006
Messages
62
[quote author="Jazz"]Bad capacitors C2 and C3 0.1uf/400v




So anyone who plans to build an La2a.

Don't cut corners on the caps. It only took 0.005uf of an offset to cause the La2a to get upset, Is that math right? I dont know I cant see straight almost time for bed[/quote]

That is very good advice!

I would go a step further and say "never cut corners on any components"!

Whats the use of spending lots of hours making supposed high end clones or other constructions if you waste the result with lack of quality components. Because thats what the real high end equipment are made off.
Its not just only schematics...
 

Purusha

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Jan 14, 2006
Messages
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Location
Somewhere on the Planet
V1
1 - 118v
3 - 1v
6 - 125v
8 - 1.1v

V2
1 - 97v
3 - 4v
6 - 220v
7 - 73v
8 - 96v

V3
1 - 99v
3 - 0.8v
6 - 98v
8 - 0.8v

V4
2 - 6v
5 - 155v
6 - 115v


That's funny. I have almost the same voltages across the tubes in my LA-2A. I gues I'll have to check the cap values as well.
 

emrr

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Apr 12, 2006
Messages
7,787
Location
NC, USA
Proabably has nothing to do with the offset in cap value, and everything to do with DC the caps were leaking.

I see this all the time with old caps (even NOS), and not found a clear way to detect it other than a cap change revealing a bad part, which still test good with a meter.
 

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