All things LA2a related

[college101]

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.

I checked r28,r35, r13 and r16...All checked out as correct

Added the jumper from iec plug ground to power tranny ground...

 

[Biasrocks]

Okay, good.

The 100R resistors are there to create a center tap for the PT transformer heaters, has nothing to do with the B+ supply.

What's the DC voltage coming out right after the rectifiers, just before the 4K7 2W resistor?

Mark

 

[college101]

OK..I dont have any 100 ohm resistors...so I will have to find themn/order them

BUT I have 354 V DC after the two diodes before R29( this is a reading I just did a minute ago_not the previous one)

 

[Biasrocks]

That's much better, what is the reading right after the 4K7 2W resistor.

If you have some other low value resistor, use those instead on the heater center tap for now.

Anything between 10R & 100R  is cool or even a pot will work. Wiper to ground, sides to the heaters. This is simply to reduce hum on the AC heaters.

Mark

 

[college101]

There is no hum right now at all...Its completely silent

Before I added the 100K resistors, it was making any hum then either...
Obviously, I need to add the 100 ohm which I did not realize at the time 

Is that my problem? I need 100 ohm instead of 100k?

 

[Biasrocks]

What is the DC reading right after the 4K7 2W resistor

 

[college101]

Tons of readings for you

FS=Front  side   BS=Backside

317.7 vdc at the r29 bs

R33 FS=317.6 vdc
R33bs=116
R36bs92.5
R17fs=317
r17bs=264

R15fs=87.1  bs=116
R18fs=121  bs=116

R11fs=21.90
R13/R9/R16 fs=278
R16bs=318
R28fs=318.8  bs=54.4


R25=5.05
R26=44.5
Volts Dc up to this point

On the hot wire of the IEC plug..I read 58.5 AC

I started to take pics, but battery died...GREAT..so batts are charging...try again tomorrow after work

Whats next?

 

[Biasrocks]

B+ is still ~50VDC too high.

We've verified that you're getting the right DC voltage before R29 so everything up to that point is good.

Follow the B+ path in the schematic, starting at R29 (triple check that R29 is in fact 4K7 and not 47R,470R,etc), check all of your solder connections and resistor values along that path. There are reference voltages on the schematic as well.

R29,R28,R16,R13,R9,R17,R33,R34,R35,R38 covers the B+.

Schematic is on the last page.

http://www.jblproservice.com/pdf/Vintage%20JBL-UREI%20Electronics/UREI-LA-2A%20manual.pdf

Mark

 

[college101]

I live on this schematic!

The only thing I question, and its because I order'd the parts 2 months ago(and Im at work, not infront of the la-2a)..is...I question if R34 is a 2 watt 10k resistor...Other then that...everything else will match... Tonight, I will re check each one of these by cutting out of the circuit, and measuring...

 

[Biasrocks]

Tonight, I will re check each one of these by cutting out of the circuit, and measuring...

No need, just verify the color codes against what the schematic says and make sure your solder joints have continuity.

Mark

 

[college101]

OK! all the contnty tests passed and I rechecked joints...got some pics to post up  BUT I did discover something...the Powere Transformer "Grey" core ground is not included in the version of the tranny I have...IS this the problem?  I replaced two of my MFR 470k resis with carbon comps...Pics in a sec

 

[Biasrocks]

If the Grey wire is the 6.3v (heater) center tap, then no it's fine.

Mark

 

[college101]

ummhh cant get my pics to load...searching for answers

The grey wire off the PT was just to ground the core

 

[college101]

 

[college101]

 

[college101]

 

[college101]

 

[college101]

Lit up...pegged at 20

 

[Biasrocks]

Does the neon light up when you power on? Have you tried the zero adj on the front panel?

Mark

 

[college101]

YES...Picture shows front panel light on....inside Neon is on as well...It does turn off when I turn if the opposite way...but the needle does not move

I realize the one pic of the neon shows it off, but I dont have the 2a on the variac...with the power being "ON"

OK..I had a 470 K  resis in there at first, then when I realized my mistake..I got a 470 ohm resis in there now..I will re-check voltages tonight as well cuz Im not sure if I measured before or after I changed that value...Im hoping it was before, and I get a better reading tonight