All things LA2a related

[Zee1usa]

[quote author="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[/quote]


Bingo !

I am getting 115vdc...close to 90vdc

B..I...N..G...O.... bingo was his name O.... sing along.... :sam: :green:


Ok, let me scratch my head now.

Why is R2 causing this?? :?
OK, Maybe I just needed to rotate R2 a bit? duh?
I'll hook it back up and rotate.. see if changes.
Uh... maybe thats what that pot controls (that there voltage)?

Hooked it back up . Turned R2 FCC (full counter clock wise) and Im getting 85vdc. That is close enough to 90vdc for me.

I am a happy camper now, thanks to Cayocosta and Scenaria!!


Thanks Guys for talking me through this !!

ISOLATE ..the key word.

Now, time to plug in the Coveted T4B module and pray to god no smoke pours out.

Z :guinness: :green: :guinness:

 

[Scenaria]

thats still not right..

you need to look over your wiring from R2 over to the T4b area

that voltage shouldnt be changing with the pot

do you have the X on the T4b socket grounded to chassis?

 

[Zee1usa]

[quote author="Scenaria"]thats still not right..

you need to look over your wiring from R2 over to the T4b area

that voltage shouldnt be changing with the pot

do you have the X on the T4b socket grounded to chassis?[/quote]


I was wondering about that. I should have asked. I was a little unsure
about the "X" or "Star" being grounded to the chassis.
instead i grounded the little tab from the T4B socket to ground for some reason thinking it was the "star" point you were talking about , but wondering if I was the "X" that should have been tied to ground.
Thats what I get for not aksing.

I corrected the grounding at the T4B. I still have the same 5vdc at R33, but I am headed in the right direction (from R2 over to the T4b area) . :thumb:

Thanks Guys,

Z

 

[wilebee]

OK I just got mine fired up and I'm having voltage problem at v4 at pin 5 (A20) I'm getting 160v and at pin 6 (A19) I'm getting 53v most everythng sees right I don't thave the t4b in yet whats up I'm using a 6005 for the 6aq5 Wil

Wilebee

 

[CJ]

Z, make sure one end or R2 is grounded.

 

[cayocosta]

What's the voltage on the other side of those resistors - and what value resistors do you have there.

 

[CJ]

Willibee, those voltages don't look too bad. Which one bothers you?
They changed the screen schematic on that tube over the years, which schematic are you using?
cj

 

[Zee1usa]

[quote author="CJ"]Z, make sure one end or R2 is grounded.[/quote]

My wiring is correct on R2 pin 3 goes to turret stud by T4B. R2 pin 1 is the shield which goes to the ground tag which is at the bottom isolated from the turret stud itself. Also R2 is grounded to the chassis.

T4B socket appears to be wired correctly.

:?

Thanks CJ !!

Z

 

[wilebee]

cayocosta: the voltage is 276
cj the schematic is s a good question I've got 2 of them here one sezs r29 is 47k the other sez 4.7k my voltage divider is r29 4.7k r34 10k and I am using r38 22k off the grid of v4 to the positive terminal of c7 d . I don't really understand why the 22k is there other than a voltage divider except its not referencing ground. Wil

sorry zee1usa I'm not trying to interupt you help Wilebee

 

[CJ]

Ohm out your R2 ground to be sure. Also, grounding the pot does not ground Pin 1.

Also, check for a short between pin 7 and 6 with ohm meter.

 

[Scenaria]

R29 should be 4.7k

Use a 22k for R34

and a 1k for R36

if you insist on using a 10k for R34 then change R36 to a 470 ohm

 

[Zee1usa]

[quote author="CJ"]Ohm out your R2 ground to be sure. Also, grounding the pot does not ground Pin 1.

Also, check for a short between pin 7 and 6 with ohm meter.[/quote]


Thanks CJ !!

I should have known fer christ sakes, *none* of my pots were making a good ground. I need to use a star washer so it will dig through the paint and make sure each pot gets a solid grounding to the chassis. I got a VERY BAD habit of using the "beeper" for continuity test.
:roll:


Thanks again,

Z

 

[wilebee]

Steve Thats what I had to begin with minus the 1k for R36 so 4.7 then 22k then 1k for r36 will do Wil

 

[wilebee]

ok r29 is 4.7k r34 is 22k and r36 is 1k ive got 87v at A20 and 57v at A19 the manual says 130v 90v that doesn't seem close ? Wil

 

[Scenaria]

turn the pots on the rear of the chassis at the halfway point... then measure them again


im thinking that your diff in voltage might be because your not using a 6aq5?

oh and check to make sure you have the 2.7k on pin 8 of valve 1 and the 470k on pin 7 on valve 1

 

[CJ]

Willibee, measure your bias voltage. This will tell you whats going on. This is the voltage from cathod to ground. Turn your two front pots down all the way. If you have oscillation, this might stop it. You guys passing signal without gain reduction?

Measure your B+ voltage going to the 6AQ5 and 12 AX7 you guys.

 

[Zee1usa]

OK, Thanks to my trusty Harbor Frieght Rotory Tool ..AKA..Dremmel. $14.99 (gotta get the diamond 1 inch bits...they work best)

I got a good Chassis contact to all my pots. Your gonna Laugh, I dremmelled out some paint around the trimmer pot on front too and forgot there is no knob on there... :shock: hahah....oh well , I got a cool Radio Shack knob that looks perfect there. :wink:

Usually with a pot you get the star washer, but the cheap Alpha pot you just get 2 flat washers. I need to go to the hardware store to pick up some star washers to have on hand. (shopping list)

Now I have 114vdc (24/7) regardless where i rotate the "Peak Reduction" knob. So I think I am in buisiness. Lesson Learned big time. I already knew about proper grounding of the pots to the chassis, so I kick myself in the ass.


So far so good....Moving Forward.

:guinness: :thumb:

Thank you Guys !

Z

 

[Zee1usa]

Oh ya WILEBEE ...I am using a RCA NOS 6005 also which I ordered from Triode Electronic on the net. I ordered the 6AQ5 and thats what they sent me.

""sorry zee1usa I'm not trying to interupt you help Wilebee""

No problem at all WIL ! I think it is funny that you, fum and I are all workin on our La2a's this weekend.

Z :grin:

 

[Scenaria]

so is it working correctly now?

oh and for future knowledge... a little scrape of the paint is all thats needed... the dremel was way overkill.. and only needed on the backside. :\

whats odd is ive built a half dozen of em without ever having to scrape off paint..

 

[Zee1usa]

[quote author="Scenaria"]so is it working correctly now?

oh and for future knowledge... a little scrape of the paint is all thats needed... the dremel was way overkill.. and only needed on the backside. :\

whats odd is ive built a half dozen of em without ever having to scrape off paint..[/quote]

Ya, its because my alpha pots did not come with star washers.
only 2 flat washers. There was no way in hell the 2 flat washers were going to bite through that thick bloo paint. I am sure you were using star washers on yours and thats why you did'nt have an issue. If I would of just used star washers I probably would of been fine. The problem is I was wanting to fire this thing up...impatient....I had not had a chance to get to the hardware store (35 min drive and snowing) last night to get some starwahsers. I figured if i just cleaned off some paint around the potentiometer holes I would be good to go.


The unit is making alot of hiss, static etc. which i can move my wiring around a bit and take care of that, but At the moment it appears to not be passing audio. I need to hook it up to my scope and signal generator
and try to see whats up.

Z