All things LA2a related

[CJ]

OK, Plan B.

Disconnect the wire that is on the side of the 4.7k power resistor, where it meets the diodes.

 

[wave]

The wire that goes to C7a +
Done

 

[CJ]

OK, pull all the tubes and the T4 module if installed.

 

[wave]

Done

 

[CJ]

Connect one of the red secondary wires to one of the diodes. It does not matter which red wire or which diode.

Hook your ohm meter neg lead to one of the C7 neg leads and the red lead to the junction of the 4.7k/diode connection. We are checking for a dead short before we power this bad boy up.

If no dead short, switch your meter to dc volts.

We are going to briefly plug in the unit for 5 seconds and then un plug it.
Use the power chord, not the switch. Turn on the power switch and then plug the chord in, just long enough to check for about 300 volts dc on the resistor.

Use alligator clips on the meter leads.


Tell me what voltage you measure.

 

[wave]

Measuring on the B21 turret I got -130 VDC (minus?)

 

[CJ]

Backwards diodes!
We found it. I think. Are your voltmeter leads in the right plug holes?

 

[CJ]

See if you can see the little arrow symbols on the diodes.

 

[wave]

My black lead to the meter is in common and the red lead is on the V,Ohm,diode capacitance input.
My diodes have a white line on one side is that the positive side?

 

[CJ]

The side with the lines should be connected together and to the 4.7k resistor.

 

[wave]

Diodes wired correctly now (I feel like an idiot)
I'm getting 340 VDC at the 4.7K junction

 

[CJ]

Good Good Good!

Be careful, without the tubes, those caps have nothing to discharge them, the will stay hot for days.

Put a 10k resistor on the end of your voltmeter plus lead and switch the meter to milliamps and carefully short them out.

Then hook up the other lead to the front of that 4.7k.

 

[CJ]

Don't feel bad. For the longest time I thought the stripe was the "plate" like in a vacuum tube schematic, and therefore should go to the transformer windings, which is , as we just found out, backwards.

The stripe is the cathode. Let the stripe remind you of a minus sign, which means the arrow is plus and should see the transformer secondaries in order to pass positive voltage.

Plug the tubes in one at a time and see if your fuse stays good.

 

[wave]

Will i be able to tell when theyre discharged

 

[CJ]

The dc current should slowly drop to zero.

You can also just ground one end of the resistor without the meter and carefully hook up the resistor to that 4.7k resistor at the diodes.

Then check the voltage on all the C7 caps with the voltmeter. 20 volts won't kill you.
Sometiomes they stay charged at a few volts by themselves.

Thats two Heinekens.

Now were getting to the really fun stuf, getting the VU meter to work.

If you want to continue tommorrow, I see your on Chi Town time.

I have a V72 choke shootout to do at home.

 

[wave]

First of all let me say
THANKS CJ!!!!
All tubes are in and lit, fuse is holding.
Should I start probing for voltages?
I can go some more if you can.
I have to go out of town to record some tracks tomorrow so I'll probably work on this thing as long as I can
:green:
By the way my neon is on!

 

[wave]

CJ YOU ROCK!!!! :sam: :sam: :sam: :guinness: :guinness: :guinness: :sam: :green: :thumb:

Voltages with tubes in
A24-25 = 3.5 VAC
B21=378
B20=316
A1= 53.7
A20=111
A14= 315.5
A17= 96.8
A19=142
Any thoughts?

 

[Scenaria]

when you look on the back of your box...

the two pots that are mounted back there should be turned fully counter clockwise...

make sure both of them are.

your numbers look reasonable...

no smoke is a good thing

 

[wave]

I believe I did have those pots fully counter-clockwise
Unfortunately I have to go out of town right now and can't resume the build until Sunday.
Thanks for all the help CJ,Scenaria, Analog Packrat
Have a great weekend :thumb:

 

[CJ]

Glad to help.

Here is a screen capture of the Jensen pdf that shows how to mod the LA2a for presumbly better sound.

The 1:1 transformer solves the many complaints of distortion with only minimal settings on the gain control. And there are some mods on the output stage.
NYD, alias Kojak, alias New York Dave, alias Coney Island, alias Groucho also has some cool mods for the output stage. It would be interesting to compare his with the Jensen, but I can't find the link.

This is a jpg if you want to save it so you don't have to search Jensen's schematics.
It is also at the LA2 icon at www.vacuumbrain.com with the cool graphics from Ron.

You don't have to use Jensen iron, but I hear it sounds good.
A 1:1 at the front with enough inductance not to load the source, or even a 1:2 or 1:4 would reduce signal below the stock 1:10 UTC A-10 HA-100X setup.and any good output iron will work.