All things LA2a related

[Paul B]

I got 3.88K Gonna go hack that RCA cable now.

 

[CJ]

That's good. Are you going thru the VU switch?
If so, try disconnecting it and just use two wires from the meter to the resistor and transformer. This elimnates anything connected to the switch from loading the meter down.

 

[Paul B]

Yep Ive got the VU wired straight into the output. Gotta rehook the output jack. gonna run out of the cd into an adapter jack into a DI box and then into the Bloo.

 

[CJ]

No DI box necessary, but if you want...

 

[Paul B]

With the CD player I still need to blast it to distortionland to get it to register at 0db...

 

[CJ]

What are you feeding the output of the LA2 into?
Just for the heck of it, disconnect it and just drive the meter.

 

[Paul B]

There's no change when I try that..

 

[Paul B]

Does this tell you anything at all: V2 and V4 burn hotter than a pistol, but the AX7's are a faint glow and do not burn very hot at all. The voltage is running 6.5V across all the heaters...

 

[SSLtech]

[quote author="Paul B"]Does this tell you anything at all: V2 and V4 burn hotter than a pistol, but the AX7's are a faint glow and do not burn very hot at all. The voltage is running 6.5V across all the heaters...[/quote]
Both of mine are like that and they're both normal.

Keith

 

[CJ]

That's normal. They use a lot more juice than the AX's.

Measure the output of the transformer with your voltmeter on ac volts when you are getting 0 db on the meter.

 

[Paul B]

With the gain set on 70, I'm getting peaks that occasionally peg the meter. The voltage at the output was topping out at 1082 mV

 

[CJ]

That sounds about right.

OK, we have a healthy pwr supply, a good line amp, a meter that could be trimmed up a bit and a unit that is passing audio, albeit a bit distorted when driven hard. Which is normal for this circuit. 12AY7will help a bit.

Side chain time.

The side chain signal is taken off the input transformer via some resistors.

Check the two 68 k's and the 2.7k resistors against the schematic to make sure they are ok.

 

[Paul B]

the 68k's & 2.7 look good against the schematic..

 

[CJ]

OK, disconnect the output amp you are listening thru, put on a cd, crank up the gain till you get 0 db average on the meter, take out the T4 module, turn up the GR pot all the way,and see if you can measure any millivolts of cd signal pumping thru the input tranny. You can take the measurement off of the junction of the 68k and 2.7 k resistor, or off the top of the 100k GR pot.
The bottome of the pot will be grounded.

 

[Paul B]

do I put in V3 and V4 yet or am I still leaving those out?

 

[Paul B]

I did the above with V3 and 4 in. With the gain turned to 70, I got an average of 0db on the meter. Whether the PR was up or down the measurement at the 68k-2.7k juncture averaged about 85-95 mV and peaked (with the VU pegging) at 152mV.

I know nothing, but that seems awfully low....

 

[CJ]

That's a good enough signal to work with. Make sure the millivolts you have at the resistor junction makes it to the top of the GR pot.

 

[Paul B]

Yep its getting there. Measured about the same with a 155mV peak.

 

[CJ]

OK, lets fix this dang thing.

All we have left to do is follow the siganl thru two amplifier stages, into the EL panel, and then verify EL>LDR circuit in the T4 module. Once we are to that point, we will verify that the T4 is controlling the input signal and registering this reduction on the VU meter.

So plug in all the tubes and get on that first stage:

1) V3- 12AX7a - voltage amp.

Volt meter on DC Volts > Measure pins 3 and 8 to determine the health (bias) of this voltage amp.

Anywhere from 0.5 to 1 volt is OK.

 

[Paul B]

V3 pin 3 & 8 measure .84 V