All things LA2a related

[college101]

I did notice that...but still, the reading of the AC voltage on the AC voltage red part( in the paper showing the voltages) it was still reading .003 for me...I have R4 and R37 counter clockwise as well....and across the neon is 55.0

 

[kazper]

55 DC across the Neon is what's shown.

I'd suggest printing off the chart in color and testing it all. Report back with whats out after you take a crack at why.

I'd look at your grounding too, make sure you didn't forget one to the common point. There is 11 ground points in the schematic, not sure about the layout that your following.

 

[Biasrocks]

I would verify that these DC voltages are present and then go on to the AC voltages.

Mark

 

[college101]

Ok let me a little more specific about what I have going on .... First off...I have the La-2a on a variac.. at 120...fuse in, and circuit on...zero adjust does not do anything to the vu meter needle(which is pegged at 20) Floating, but stuck at 20)  Neon lights up, and adjusts when I turn the knob(from non-lit to lit)  

no movement on the needle

Im concerned with transformers, the Sowter 1010...Im not happy yet...And I need some help...

For tubes, I am using 1960 NOS RCA's 12AX7A's(x2)/ (1) NOS 1958 RCA  6aq5a/ (1) 12bh7a tubes

Sowter input transformers 1009/1010 and Hammond 369JX power tranny


**So another thing I noticed  so far...C11 is .01 on the 68 schematic...but .1 on enverything else I have seen?

 

[[silent:arts]]

I would suggest forget the meter and compression at the moment:
first it must pass audio!

remove the T4B, remove the meter, remove the gain reduction amp tubes.

 

[college101]

I took the t4b out a long time ago...I am taking out all of the tubes...since nothing is working

 

[kazper]

http://www.twin-x.com/groupdiy/albums/userpics/LA2A-deconstructed.jpg

http://www.groupdiy.com/index.php?topic=16629.0

To help you look at what your building in smaller pieces Chrissugar did a excellent job in redoing the schematic posted above.

Since your tubes are out check your supply voltages and make sure there heater voltage and Anode voltage are reasonable and close unloaded.

Then follow what Silent Arts was telling you.

 

[simonsez]

I took the t4b out a long time ago...I am taking out all of the tubes...since nothing is working

You can't take out all of the tubes.. ??? Again, check for DC voltage first (in blue) with all tubes put on.

 

[college101]

So leave in V3 and V4?

Ok..so what do I need to look at start getting it to pass audio?  After going back threw cayacosta's set up...the main difference I am noticing, is really just the input and outout tranny set up

The in put tranny I have ( the Sowter 1009)-has 2 primary leads..which go to turret board#1 and then the secondary goes to junction point r5/r6 and the bottom lead goes to the ground..that is simple

The Output transformer...has 2 wires on the secondary...which top wire woll go to C5...bottom wire goes to ground with shield on chassis---  primary side has 3 wires...top wire goes to pin 10(which also connects to the positive  1+3  of meter--center tap to 9 and  bottom wire goes to pin 8 which also connects to 1+3 of the meter

Gain reduction is set by the zero adjust


What am I missing that I dont understand about the schematic?

 

[Biasrocks]

What am I missing...

Bueller?

Apparently an aversion to measuring voltages as requested.

Before you spout off about transformers and VU meters again, take all the DC measurements in the chart that I posted, with the tubes in and post them here. AC voltages are pretty much meaningless until you have verified that the proper operating voltages are present.

Mark

 

[college101]

I have to print the sheet off from work, but I will be measuring once i get it home and on my bench...

Im guessing as well...my ground for the power transformer is on the bottom plate of my chassis, while everything else is on the back plate near the tubes...I will install a new ground on the backplate and attach/link them together...

 

[college101]

Oh WOW ...Did I learn alot

First I re-solder'd my ground off of the main power tranny...then check'd the tube voltages!

c7B+=353 dc

12ax7a #1
pin 1=155 dc
pin 3=1.06 dc
pin 6=268.4 dc
pin 8=5.05 dc

12bh7a
pin 1=115.4 dc
pin 3=4.24 dc
pin 6=263.7 dc
pin 7=86.9 dc
pin 8=120.7 dc

12ax7a #2
pin 1=.116 dc
pin 3=.959 dc
pin 6=115.8 dc
pin 8=.956 dc

6aq5a
pin 2=85 dc
pin 5=111.5 dc
pin 6= could not get a valid reading-it just kept draining all the way down to zero

Tell me whats up guys...I still have to do some more checking...with the other voltage chart at turret point locations... more to come

 

[Biasrocks]

You have several spots that are significantly higher than what they should be.

First, you must get the B+ down to around 245VDC.

I would suspect 2 things, A) your power transformer is putting out too much voltage or B) the power resistor is the wrong value (s/b 4.7K). Check your voltage just after the rectifier to see how much your getting there (according to the schematic ~350VDC) and then check the resistor value.

In fact, it might not be a bad idea at this point to re-check all the resistor values to make sure you haven't inadvertently installed a wrong value.

Mark

 

[college101]

Biasrocks- If you are talking about R29...It is a 4.7k 2 watt mfr...

on a side note...I have no ground off of the input tranny- The Sower 1009 has a center tap on the Primary side, but no ground that connects the core(Will this be causing the audio signal problem?)

C7B= shows 353 VDC... I will re-measure again turret points later tonight

I also noticed in cayacosta's layout...he has the power transformer set up, as the the two black wires tied together(seperated to two different turret points, and then linked together, with one side going to the switch...(Wouldn't that cancel out that side of the Tranny?)--* I followed the 68 schematic and set up the top side to have the top of each winding to go in on the Blue and Black wires of the tranny..((How the 369jx is set up)) as 4 and 3...and then the wh.blu/wh blk as 2 and 1 on the negative side connected to the fuse

The only thing I question, is maybe the transformer primary is upside down then? I need to move the black to 4 where I have it at 3 right now...but the 68 schematic does not reference which tranny it is using...  I would think this would not make a difference, but maybe it does?

 

[kazper]

Biasrocks- If you are talking about R29...It is a 4.7k 2 watt mfr...

on a side note...I have no ground off of the input tranny- The Sower 1009 has a center tap on the Primary side, but no ground that connects the core(Will this be causing the audio signal problem?)

C7B= shows 353 VDC... I will re-measure again turret points later tonight

I also noticed in cayacosta's layout...he has the power transformer set up, as the the two black wires tied together(seperated to two different turret points, and then linked together, with one side going to the switch...(Wouldn't that cancel out that side of the Tranny?)--* I followed the 68 schematic and set up the top side to have the top of each winding to go in on the Blue and Black wires of the tranny..((How the 369jx is set up)) as 4 and 3...and then the wh.blu/wh blk as 2 and 1 on the negative side connected to the fuse

The only thing I question, is maybe the transformer primary is upside down then? I need to move the black to 4 where I have it at 3 right now...but the 68 schematic does not reference which tranny it is using...  I would think this would not make a difference, but maybe it does?

Take some pictures too... It may help us help you.

 

[Biasrocks]

Your input transformer has nothing to do with the B+.

Also check, R28 (470K), R35 (220K), R13 (220K), R16 (68K).

And check to make sure that your star ground is actually grounded to the case and
the electrical outlet.

Mark

 

[college101]

Will check for the star ground(I can install a jumper wire interconnecting the plug ground to the power tranny ground on the back part of chassis).... Biasrocks

Kazper- I have my camera now, fiance took it the other day...Pics will be up tonight after school

I have to go out on a limb and ask   Are my tubes bad? The readings I took are way off from the spec sheet..?

 

[Biasrocks]

Yes, on the jumper.

My eyes are glazing over as we speak.

As I said before, you're B+ is ~100V's to high, solve that problem and then move on.

You have to approach this in a logical manner and eliminate one thing at a time; not willy nilly try this, maybe it's that.

You've found a problem with the B+, solve it and move on to the next thing.

Mark

 

[college101]

I agree Mark! I want to go threw each item one at a time...Its much easier to keep track of...

My B+ is super high! I have 2'-47/450 caps in...maybe they need to be lower'd to reduce the B+? I have some 40/450 caps, BUT they are Axial instead of radial, and take up a huge amount of space... due to the space inside the chassis, I moved the caps to be side mounted inbetween chassis face and back plate near the IEC plug... Pics tonight will help explain this...

To make a statement...I know for a fact that the plug is grounded to the bottom ground, but not linked to the new back plate ground that I put in yesterday...which i will link tonight...

Do you think I need to remove the grid stoppers and re-measure the voltage at c7b again?

Also, what values are you using on C4? (with the part I was able to order)I can set a value between 40~280...not the 50~330 from the spec...Maybe this is part of it?

Thanks again for helping!

 

[college101]

OH...I have two 100K res right now on the green heater supply...Have to switch that out to be 100 ohm res!

WIERD-just seen a post from NewYorkDave, and then it disaprd?