All things LA2a related

[Greg]

With tubes:

Heaters: 6.73VAC

Measured (Ideal reference):
B21: 362.9V (360V)
B20: 294V (275V)
A1: 52.6V (60V)
A20: 93.5V (131V)
A14: 275.8V (216V)
A19: 128.5V (105V)

What do you think? Are A20 and A14 too far off? Thanks.

 

[CJ]

A lot of us don't have the Bloo schematic, so tube socket numbers are cool.

 

[Greg]

Yea good point... I passed some audio through her with no T4B and the sine and square wave look good. This thing has a ton of gain !!!

 

[CJ]

That's what they say. :grin:

Distortion is not pretty though.

 

[Greg]

There she blows (with tubes and no T4B):

If it's not listed, then 0V...

V1:
1: 138
3: 1.51
6: 117.3
8: 1.37

V2:
1: 227
3: 22.3
6: 275.5
7: 143.2
8: 228

V3:
1: 101.7
3: 0.875
6: 101.7
8: 0.875

V4:
2: 8.94
5: 90.2
6: 127.2

 

[CJ]

Looks like you nailed it on the first try.

Plufg in that square can thingy with the brown amphenol base.

 

[Greg]

V2 is screwy, but I took a quick look and I think I found my mistake already.

 

[CJ]

V2 is busy, thats for sure. Especially with point to point.

 

[Greg]

I think V2 looks better now:
1: 104
3: 4.13
6: 227
7: 80
8: 110

T4B time !!! :thumb:

 

[Greg]

I don't think I'll work on this anymore tonight, but it's passing signal but no compression yet (with T4B). When I increased peak reduction, I noticed the neon starting to flutter. I'll double check some wiring then post back.

 

[Greg]

So decided to run some audio through the unit just to see how it sounds. I did this with and without the T4B. First off, the LA-2A is very quite and sounds great as a line amp. I hear no buzz or hum or anything like that, so that's good :thumb: .

Unfortunately, I have no compression. My voltages still seem to be ok, so I'll concentrate on the sidechain and perhaps the gain reduction wiring. One thing I did notice, and this happens with or without the T4B. When I switch to the GR metering mode, the neon becomes less bright.

I don't know if I have a bad T4B... when I put it in, the unit behaves just as if it wasn't there. Any advise is appreciated. I'm gonna sleep on it.

 

[Greg]

Sorry to keep posting, but I did a little more work on the LA-2A. I put the T4B back in without the lid in a dark room. Then I shined a little light on the T4B, and sure enough, the meter deflected and the signal was reduced... ie compression. So I guess that either means the EL panel is bad or the control voltage isn't getting to the EL panel.

From the schematic, it appears that the control voltage is at pin 3 of the T4B. Is this correect and if so, is it AC or DC? And what's the typical voltage range.

I'd like to test the EL panel, make sure it works, then I'll know the problem lies in my wiring.

 

[Rafita]

Ok so after reading the whole thread , i got mine working , the only differences were on V4 where i got

pin 5 ( r34 ) 92 volts
pin 6 ( r35 ) 122 volts

Still the same readings but its working

Hey Greg read this !

on page 53 AnalogPackrat quote:

"""" This is the older config with R34 = 22k and R36 = 1k, right? You're not using C13 and R38 which were added in a later rev, right? Recheck R34 and R36 as well as R35. Something isn't right with the wiring around V4.

BTW, I built the newer config with the extra stuff on the screen grid of V4 and 10k R34, 470R R36 """"

On page 54

"""" He also accidentally had a combo of old and new circtuitry around the 6AQ5. Now he's back to the old version with no R38 or C13 (his choice).

Paul has also taken the cover off of his T4B with the unit in place and the power on. Shining a light into the thing made his meter (in GR mode) move much more than his input signal would even with max peak reduction setting. Maybe you're right and he's just got too weak of a signal. """"


if you using the bloo manual as I we are using the old circuitry where
C13 and R38 is'nt used .

.

So be sure also that youre feeding a hotter signal , and it will move.
Also check your wiring on the rotary ( i've redone it like 4 times ) , and then discovered that my signal was weak so my needle wasn't moving

thank's to AP , Dissonant String , Paul B , CJ :thumb:

 

[AnalogPackrat]

Greg,

I don't have my Bloo manual with me (I wrote my voltages on the schemo in the back). I'll try to bring it in tomorrow, but hopefully you'll have it solved by then. Rafita is right--let us know which version of the V4 wiring you used. There are two different ones with changes affecting R34, R36, R38?, and C13.

You can tell if your EL panel works by popping the cover off the T4 and then running some dynamic music throught the unit. Turn off your room lights and crank up the Peak Reduction. You should see the EL panel flickering with the music. If it isn't, then either the T4 is screwy or the side chain is messed up somehow. Since you saw meter deflection with the flashlight trick, your CdS photocell for metering is probably OK. You can check the other one by pulling the T4, hooking up your meter between pin 8 and pin 7 and shining a light on the cells.

Good luck!
A P

 

[rove]

I got my lamp in, supplied by Radio Shack (thanks CJ!), and it works fine. A little reticent to drill a new hole in the case, so for now, just taped in with electrical tape, but I can see that meter! I also went ahead and replaced R9 and R13 with 100k resistors (had a 12ay7 in for V1 for a while, just hadn't changed the resistors).
Question, though: a problem I had before I put in the bulb, it seems the meter has a screwy connection as sometimes it loses all power and requires a few gentle taps to get back to work--the sound isn't affected and it almost seems to have something to do with how well the case is closed... I have checked and rechecked the wiring and re-did the wires to the zero-adjust pot as tapping at that seemed to make a difference. Any tips on how else to troubleshoot this? thanks.

 

[AnalogPackrat]

Rove,

Was your meter problem just in GR mode or all modes? Check for a cold solder joint on R4 or R24.

A P

 

[Greg]

Thanks guys... I did some reading this morning and I will do what you have suggested... Checking to make sure my signal is hot enough and checking the components around V4. I'll post back.

[quote author="AP"]Since you saw meter deflection with the flashlight trick, your CdS photocell for metering is probably OK. You can check the other one by pulling the T4, hooking up your meter between pin 8 and pin 7 and shining a light on the cells.[/quote]
I think both CdS cells are working. Because as the GR meter showed gain reduction, the signal on the output reduced proportional to the meter. The unit was compressing with the flashilight trick. :thumb:

 

[rove]

The meter problem is in all modes. I just went over those points and guess what? the meter isn't working at all now. Not really sure where to begin. One thing that may be an issue is that I am finding it difficult to get the wire from the mini turret board to solder well to the tabs of the board (though it appears connected just seems it could be better), any tips there?
thanks.

edit:
it appears that was the problem all along, the wire from the last turret on the mini-turret board to the tabs that attach to the meter was not well connected. I solved this by attaching a short wire with a lug on it that then was screwed onto the screw terminal, definitely attached now! and the meter is working fine in all modes.
thanks again.

 

[AnalogPackrat]

Rove,

I vaguely recall having trouble with that same solder joint. I just socked the heat to it and made sure the flux got all over it and it finally relented. Your solution is probably better. I'll keep an eye on mine.

A P

 

[Greg]

Just an update...

Just got home from work, popped in the T4B, put in a 1.3V sine wave, turned both R3 and R37 fully clockwise, and guess what...

MY LA-2A WORKS !!!

I guess it was a combination of too low of input, and the way I had the pots on the back set.

Thanks guys for making me realize that my LA-2A worked in the first place. Still need to do some calibrations and get this thing tweaked up. But first I'm going to get some audio through it !!!

:thumb: :guinness: :sam: :green: