My 2nd LA2--Problem (NOT) solved--Spoke too soon!

Ethan

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I'm almost finished building my 2nd LA2a but I'm having a little trouble finding where the root of this problem is... It works FINE as a line amp, but I there's no compression happening. No panel lighting up in the T4 either.
Strange behavior: when I crank the peak reduction pot fully clockwise I get a 2nd-order harmonic distortion--i.e., when giving it a 1K signal, after turning the peak reduction fully clockwise I get a 2K signal mixed in.
More strange behavior: the meter reads output just fine (as is expected), but when set to read gain reduction, the meter drifts up or down in no predictable fashion (not spontaneouslyt, but gradually)--even after the unit is warmed up and the zero adjusted.

Could anyone tell me where I should pay close attention to find why it's not compressing? I've tried just about everything. Voltages seem to check out give or take. Any help would be greatly appreciated!
 

Scenaria

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Check your grounding around the T4B.. pins 2, 4, and 8,,, also pin 3 coming of that .1uf cap

what do you measure in terms of voltage between pin 3 and 4?
 

Ethan

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Thanks Steve,

On the T4, pins 1, 2, 4, 6, and 8 are grounded. On both pins 3 and 4, I've got virtually no voltage. (BTW you meant 0.01 coming off pin 3 right?)
Still no joy.
 

Scenaria

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yea the one off pin 3..

I would hope there was no voltage on 1, 2, 4, 6 and 8 :green:

What do you measure on you B+ rail? goes through R34 and then up to pin 5 of V4... I would trace that back to the B+ to see if anything is funky in there..
 

Jerry

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I built an LA2a recently and had the exact same set of symptoms. I took my clone to a local tech who I know who had also built one to help figure it out. He suggested that I change the input transformer to one that has an input:eek:utput ratio that is closer to that of the original UTC units (I had used a Jensen with a 1:1 ratio). He explained that the LA2a is a feed-forward design that required a step up before the first gain stage (I have an e-mail at work that goes into more detail).

I got a new transformer from Sowter, put it in and put the project on the back burner. I'll hook it up today and see how much compression I'm getting - if any. The meter still doesn't work in GR mode.

Until I saw this post, I thought that I had two different problems, and assumed that the replacement transformer would fix the first one. Because the odds are against two people making the exact same two mistakes, it seems more likely that the two symptoms are caused by one mistake that we've both made. My guess is that since I know that the meter still doesn't work, the compression will also not work.
 

Ethan

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Upon measuring again, all the PS voltages are fine, but some of the others are off at the test points.

@ C4+R11 node, should be 27V, I have 17
@ R9+R13 node, should be 205V, I have 195
@ Pin 8 of V1 should be 61V, I have 1!!!!!!!!
@ Pin 6 of V1 should be 120V, I have 111
@ Pin 6 of V4 should be 100V, I have 56

My B+ is 252V

Where'd I go wrong :cry: ?
I wish I hadn't scrapped my first one for parts. I have one good thing going though, this new build is quiet as can be...now if I can only get it to compress... BTW I'm using the Sowter trannies.
 

Ethan

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[quote author="Jerry"]He explained that the LA2a is a feed-forward design[/quote]

From where I'm sittin' it's feedback as far as I can see. To be feed-forward wouldn't the amp have to be fed before the LDR?
 

Gus

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If you don't have a scope use the ac measurement on a DMM inject a signal at a fixed freq. measure at the plates of V9 and at the plate of V4 using the AC setting most newer DMMs can measure > 60hz

The compresion amp V4 V9 needs to be looked at check all r values and connections

That will check the signal path that lights the EL section of the panel. If that changes with the compress control and siganl level check if the LDR is changeing R with light changing.

The transformer should work you just need a bigger voltage drive. If it is working like a line amp the transformer should be fine.



If it is passing signal but no compression V4,V9 amp section

Pin 6 of V4 should be 100V, I have 56 this is what I would look at first check resistors, change tube if resistors are ok see if it changes

R14 being a 270 ohm might give you that reading or the plate R is wrong
 

Gus

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An Idea for builders of this circuit. Get a cheap EL night lite wire it to the output of V4 after C11 disconnect the T4 module adjust the compress control when injecting a signal or music the panel should change with the signal.

This might be an easy way to test the EL gain stage without a scope.

Bad Idea Good Idea?
 

Ethan

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Thanks Gus for taking the time.

-R14 is 2.7k as it should be.
-I changed the V4 tube and now I have 65V @ pin6, but still not near 100V
-@ C11+R34 node I get 154V
-@ pin 2 of V4 I should have 7V, I have 6.8 which seems within OK range.

Any more suggestions?
Thanks so much.
 

Ethan

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FOUND IT!

There was a "nonexistant" connectnio from Pin 7 of V4 to the limiter response pot. I purposely omited the stereo adj. pot and conveniently forgot to make the connection. Sometimes ya feel like an arse!

Thanks everyone for being so generous with your time helping me!
 

Gus

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What happened with V1?

that fix should not have anything to do with the voltage reading at V1.
 

Ethan

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ahhh!~~

I completely forgot about that. I just remeasured and I still have 1V at pin 8 of V1. Gus, what should I check? I'm goign to try swapping tubes and see if that does anything...
Well, it's not the tube. I just tried it with 3 other 12AX7s

Updated Measurements:
@ C4+R11 node, should be 27V, I have 17
@ R9+R13 node, should be 205V, I have 195
@ Pin 8 of V1 should be 61V, I still have 1!!!!!!!!
@ Pin 6 of V1 should be 120V, I have 111
@ Pin 6 of V4 should be 100V, I have 65
 

Ethan

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An Update:

I still haven't tracked down the root of the voltage problems. But it still seems to function relatively normally with a few exceptions.
-The meter in GR mode is retarded. It'll go full -20 semi-regardless of input level or compression amount. If the audio is more than 2 seconds long it'll peg -20. The recovery back to zero is slo-mo. The output metering is normal.
-Even if no audio is fed, if I turn the Peak Reduction pot past 12 o'clock, there's a faint squeal and it "compresses," the meter goes down to about -10 as if there's signal and it's being compressed.
-The unit is DEAD SILENT in terms of noise, but every now and then (not because of any pot or switch movement) there will be this low level crackle, then it'll dissappear completely and come back later.
Yes, an LA-2a is quirky, but still not this quirky.

Any help is always much appreciated....especially if it saves me from pulling out the rest of my hair :? --baldy
 

Scenaria

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hmm

65volts would be what your neon regulates at... no? but your seeing that value on V4..

are you confident about your rotary switch wiring?
 

mik

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I've got same problem with my clone: cold solders all around!!!! :oops:
 

cayocosta

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It looks like pin 8 of V1 is supposed to be around 1 volt.

Also, there's quite a range of voltages depending on the build:

JBL '68 Schematic Voltages:

Pin 8 of V1 = 1.1V
Pin 6 of V1 = 120V
Pin 6 of V4 = 100V

Jahnsen Voltages:

Pin 8 of V1 = 1.08V
Pin 6 of V1 = 101V
Pin 6 of V4 = 70V

I didn't really pay attention to my voltages other than the B+ rail, which was in line within 10% or so.

Your voltages might be okay actually Ethan.
 

cayocosta

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With the T4 out of the circuit:

Run a signal through the unit and set the gain to zero and the gain reduction to somewhere around 12:00.

Read *AC* volts from pin 3 of the T4 socket to chassis ground - it should show the signal up to 70 volts or so.

Your trouble might be with the removal of the stereo pot - it also feeds the signal to the grid of the 6AQ5. You could also look for the AC signal volts there - Pin 7 grid of the 6AQ5. (With an input signal and gain reduction up).

If there's no AC signal at pin7 of the 6AQ5, then there's none from the plate (pin 1) of the 12AX7 that's feeding it. In that case look for the AC signal at the grid (pin 7) of the 12AX7. If you still can't find it, keep going backwards through the circuit looking for that signal.

You might want to stick that stereo pot in to make sure that's not the problem.
 
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