All things LA2a related

[orangechili]

Ok,
I get 57ohm on the primary and 300 ohm on the secondary of the input transformer with no signal going in and the unit powered on.

 

[orangechili]

I just won an auction for vintage UTC A-10 input transformer for a reasonable price so I'll see when it comes later this week if it buys me any more signal going in. I have a feeling it might sound a bit better than the Edcor as well

 

[CJ]

great!  you should have less hum with the UTC, and more of a vintage sound,

looks like the edcor is wired correctly.

you can always build another LA2a and use it for the OPT,

try removing tube V1 and checking voltage on top of Gain pot,

 

[orangechili]

Cool! I also have a few other projects I can put the Edcor to good use with. I get no voltage at all on top of the gain pot with V1 removed.

 

[CJ]

ok almost done finding problem,

test 1>

 

[CJ]

if plenty of voltage at R6 in test 1, do test 2,

if plenty of voltage at loose end of R7, plug in T4b and re check volts-ac,


test 2>

 

[orangechili]

Ok,

Test 1:
I get 5.4 volts AC at the loose end of R6 when desoldered and sending a 500hz ac signal in.

Test 2:
I get .300 volts ac at the loose end of R7 when injecting a 1 volt AC 500hz signal. I didn't bother plugging in the T4B as this seems real low…

 

[CJ]

good deal, the problem is in the switch area,

try un-soldering the comp/lom switch from the circuit and see what happens,

maybe a solder short, check under the turret board for a golb of solder that might have leaked thru the turret,

 

[orangechili]

Ok, I disconnected the comp/limit switch on the T4b and insulated solder terminal side, I now get 3.3v ac coming off the disconnected R7(2.7K) from the last test. What perplexes me is this is a brand new spst carling switch and I verified it was switching R7 in and out of the circuit with a continuity meter flipping it between the 2 modes and testing it on my meter. Solder points between switch weren't weird and verified good many times… I don't what to do from here? i guess try redoing the switch wiring until i don't get a drop coming off R7?

I verified no shorts under turret board.

 

[CJ]

leave that switch out of the circuit for now and do that side chain check that was done in red ink a few posts up,

re-solder R7 of course, check your VU meter in Gain Reduction position and see if that needle moves now,

leave the switch bolted to the chassis and ohm it out from ground to both terminals,

 

[orangechili]

Ok,
I taped up both free ends of switch left switch bolted to faceplate. First off, wow, there's the signal I've been looking for! My GR and +4 meter's don't agree yet as i didn't calibrate R25 and just have a 33k in there so it's hard to get exact measurements but side chain amp does appear to work very healthy when getting a 1.23 volts signal on OPT now. Voltage at peak reduction is around 100mv, 3-4 volts on C9. 55-60 volt range on C11(I have a .01uf there instead of .1uf per Cayacosta's layout). In +4 mode turning up the peak reduction control I can see the needle move down quite a bit. There seems to be a 5db difference between +4 and GR modes(trimming with zero adjust doesn't help)

 

[CJ]

Great news!

you seem like a smart guy so i bet you can handle it from here,

i never use that comp/lim sw, does not seem to do much,

so it sits in the best sounding position on the back,

can you help me with this weed eater that won't start?

ok, cue McGarrett>  ;D

 

[orangechili]

Hahahaha, Thank you you so much CJ! I'm gonna go get working on it ;D

 

[sr1200]

http://youtu.be/OOm2EeggHug

I made a short video to show my issue with my 2u la2 build. The meter is going crazy from oscillation. While testing voltages (all are within spec) i noticed when I touch a certain wire/turret on the board the oscillation stops and the needle calms down. Any thoughts as to where i can look to remedy this. Could it be a bad orange drop cap?  Or just proximity issues? Ive tried cleaning up the wires and spacing them out, but to no avail.

popped in some grid stoppers and oscillation went away... now to find out what else is wrong with this pig.

 

[beatnik]

Hi guys, today I finished my first LA-2A.

Before inserting the T4B I checked all the voltages as I found on the forum. All the readings were in the right spot.

The compressor passed signal and gain pot worked. I zeroed the meter and verified that meter switch was working switching between out and gr.

But I also noticed there was quite some rolloff in the lows and low mids.

I then installed the T4B and hoped for compression happening, but it doesn't.

Following some instructions i found on the forum, I took off the T4B cover and sending a hot signal i checked for the el panel illuminating. Again, didn't get the expected result.

So basically the compressor is not compressing. I already checked the wiring various times and everything is ok.

Without T4B installed I read 0V DC on all pins, apart from pin 5 which goes from 7.5V with meter in gr mode to 23.9V in vu mode.

Please I hope anybody more expert than me with this circuit to give some indications on how to proceed in the debugging.

If you need more voltages just tell me what I have to measure and I will report.

Thanks in advance

 

[beatnik]

I also tried two different 6Aq5 tubes cause i read if faulty results in no compression, but nothing changed

 

[sr1200]

Im in the same boat beatnik, I just got a new case, im going to RE-wire the whole thing and see if I made a mistake somewhere.

 

[beatnik]

doh!

you know what it was?

had pin 3 and 4 on the in transf not connected together, so the signal was to low to reach the threshold and also i had that hi-pass filter effect.

now it's compressing and sounds amazing! i used UTC transformers and original UA T4B.

good luck with yours!

 

[beatnik]

the last thing left is the meter illumination.

i have a meter with 12V lamp.

the project shows the meter light takes voltages from the heaters 6.3V.

Can I just supply the 12V lamp with 6.3V and it will work or I need to make some changes?

 

[rainton]

the last thing left is the meter illumination.

i have a meter with 12V lamp.

the project shows the meter light takes voltages from the heaters 6.3V.

Can I just supply the 12V lamp with 6.3V and it will work or I need to make some changes?

It will work, but the lamp will be dimmer...