All things LA2a related

[Zee1usa]

Thanks Cayocosta,

On V3. pin 8 is wired to pin 3. Pin 3 goes to R31 (1K) . R31 is in paralell
with the .o1 and .02 orange drops which are tied to ground (R36) . Ground is good.

:thumb:

One thing I wonder about is that the "center" pin of V3 is not tied to anything.

FYI:
On V3 pin 4 and pin 5 are tied where on of the heater wires are iconnected, and the other heater wire goes to pin 9.

On V3 pin 2 and pin 7 are tied and go to R2 (peak reduction)
Potentiometer pin 2. Shield is connected on R2 but not at the
other end.


Your responce was much appreciated.

Z

 

[Scenaria]

two things to check...

R12 and R14

be sure that R14 is going to pin 7 (470k)
and R12 is going to pin 8 (2.7k)

also check R36...

If your using a 10k/2w for R34 then be sure that R36 is 470 ohms

if your using a 22k/2w for R34 then be sure that R36 is a 1k resistor..


also... make sure that R3 and R37 are turned at midpoint...

and as ron said.. becareful in there with the high voltages... allow time for everything to discharge and measure with your DMM BEFORE putting your hands in the box

 

[cayocosta]

The 260VDC sounds about right on the B+ side of R33, so your B+ is there.

Before you go any further, do all your heaters light up?

If not, stop.

If the heaters do light up, swap out the tube for another 12ax7 and see if that gives you the 90VDC reading to chassis ground at pin 6 of the tube socket.

Also, measure the voltage drop across R33 itself, you should normally see around 170VDC.

Check to make sure none of the pins on the tube socket are touching anything else.

 

[Zee1usa]

Thank you Scenaria ! All those look good.

Thank you Cayocosta ! Yes, all of my heaters light up. I swapped the 2 12AX7 for kicks and it did not make any difference.

I will check all the pins on the tube sockets for continuity to make sure none of them are broken for some reason.

I realize it is quite common to get bum tubes, so i'll keep that in mind.

Thanks Gentlemen! Going forward I appreciate your helpful comments,

Z

 

[Zee1usa]

and as ron said.. becareful in there with the high voltages... allow time for everything to discharge and measure with your DMM BEFORE putting your hands in the box

Oh Yeah Baby!!




[quote author="cayocosta"]
Also, measure the voltage drop across R33 itself, you should normally see around 170VDC.

Check to make sure none of the pins on the tube socket are touching anything else.[/quote]


R33 B+ is 250 vdc and the other side of R33 that should be reading about 90vdc is reading 5vdc.

Voltage drop across R33 (220K) is 245vdc

None of the pins are shorted, I checked that very carefully.

Thanks Guys,

Z

 

[cayocosta]

This socket is pretty basic - not much going on to cause trouble.

Check continuity from the ground side of R31 to a spot on the chassis that's not painted - this will ensure it is indeed grounded. Make sure that pins 1, 2, 6 and 7 on the socket do not show continuity to chassis ground.

Also, you should read 1k ohms from either pin 3 or 8 to the chassis.

I keep editing this post!

 

[Zee1usa]

[quote author="cayocosta"]This socket is pretty basic - not much going on to cause trouble.

Check continuity from the ground side of R31 to a spot on the chassis that's not painted - this will ensure it is indeed grounded. Make sure that pins 1, 2, 6 and 7 on the socket do not show continuity to chassis ground.

Also, you should read 1k ohms from either pin 3 or 8 to the chassis.

I would replace R33 at this point.[/quote]

R33 (220K) is good. pulled it and re-measured it.

V3 pins 3 and 8 are tied then go to the 1k which is then grounded, but pin 3 and 8 are not directly going to ground. They goto the 1K then on the other side of that 1K goes to ground.

Is this the issue?

Thank you,

Z

 

[cayocosta]

Yes Z, I was wrong about pins 3 and 8 to ground, I edited my post to check for ground to chassis from the ground side of R31.

I also edited out the suggestion to replace R33 after your voltage drop was posted.

 

[Zee1usa]

[quote author="cayocosta"]This socket is pretty basic - not much going on to cause trouble.

Check continuity from the ground side of R31 to a spot on the chassis that's not painted - this will ensure it is indeed grounded. Make sure that pins 1, 2, 6 and 7 on the socket do not show continuity to chassis ground.

Also, you should read 1k ohms from either pin 3 or 8 to the chassis.

I keep editing this post![/quote]


All this checks out good. I am getting 1.081 K ohms across R31 and from pin 3/8 V3 to chassis. Damn those Carbon comps are sloppy!

thats like 8% tolerance out. Probably does not matter.
oh ya and re soldered it about 5 min ago but it feels cool.
I know those are heat sensitive.

Thanks :guinness: :wink:

Z

 

[cayocosta]

Try disconnecting the .02 cap from pin 1 and see if that makes a difference with the voltage.

 

[Zee1usa]

[quote author="cayocosta"]Try disconnecting the .02 cap from pin 1 and see if that makes a difference with the voltage.[/quote]


Hmmm... Interesting.. it is still 5vdc when i disconnect the .02 (C9)
from pin 1 of V3. Voltage did not change at all.

So this should shed some light on the matter.! ?


Thanks :thumb:

 

[cayocosta]

Sorry Z, I wish I could pinpoint your problem a little quicker for you.

By the way, are your other voltages in line?

Back to the heaters - from pin 4 or 5 to chassis ground should read 3.15 VAC - that's AC. Same with pin 9.

Running out of options, I would replace R33 anyway now.

 

[Zee1usa]

[quote author="cayocosta"]Sorry Z, I wish I could pinpoint your problem a little quicker for you.

By the way, are your other voltages in line?

Back to the heaters - from pin 4 or 5 to chassis ground should read 3.15 VAC - that's AC. Same with pin 9.

Running out of options, I would replace R33 anyway now.[/quote]


Thank you!!

3.29 VAC V3 pins 4/5 to chassis. 3.29 VAC pin 9 to chassis.

Turret Voltage readings:

A25 to A25 (heaters) 6.6VAC

B21 (R29, 4.7K 2w) = 350vdc

B20 (R34,10K 2w) = B+ 250vdc

A1 NE2 (glowing neon bulb) = 53.7

A20 = 110vdc

A14 = 207vdc

A17 = 5vdc !#$%!%&!!!! should be 90vdc

A19 = 97.5vdc


Thats what I have. They all look good except the R33 to V3 pin 6
reading that is 5vdc . From the info I have should be 90vdc there or
somthing closer to that.
:?


Back to the grindstone...

Thanks!!

Z

 

[Scenaria]

you do have the 1k between pins 2 and 7 on V3 right?

lift one leg of that 220k (r33) and measure its resistance.. make sure its a 220k (dont assume the color bands are correct)

oh and make sure you have the jumper strapped across A11 and A14...it wont cause the problem your having but it is something I missed in the docs

 

[Zee1usa]

[quote author="Scenaria"]you do have the 1k between pins 2 and 7 on V3 right?

lift one leg of that 220k (r33) and measure its resistance.. make sure its a 220k (dont assume the color bands are correct)[/quote]


Thanks Scenaria !

These 2 things have ben verified.

I do have a 1k between V3 pin2/7.. R33 was measured at 220K with my fluke meter. I took it out of the circuit and remeasured it.

I will make sure I have A11 tied to A14. Thanks !

Z

 

[cayocosta]

On V3 pin 2 and pin 7 are tied and go to R2 (peak reduction)

Yes, tied with the 1K right?

 

[cayocosta]

Did you replace that 220k?

 

[Scenaria]

one other thing to try....

the shielded wire from #2 (the reduction pot) to A15....

disconnect from A15 to see if theres any difference in your V reading

 

[Zee1usa]

[quote author="cayocosta"]

On V3 pin 2 and pin 7 are tied and go to R2 (peak reduction)

Yes, tied with the 1K right?[/quote]


Yes, its a 1K..

I changed R33 (220K) no difference.

I checked continuity of the V3 socket for kicks and it is ok.

One thing that i noticed is when I swapped the tubes..
I get 7vdc with one of the 12ax7 tubes.
When I install the other 12ax7 (Ecc83s/jj-tesla)
I get 4.3vdc which is exact. (I was quoting 5vdc to keep it simple).


Thanks,

Z

 

[cayocosta]

Yes, try Scenaria's suggestion to remove the input to the grid.

Also, what voltage do you read from pin 3 or 8 to chassis ground? Should be less than 1 volt DC.