Original LA3A issues

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kdawg

Well-known member
Joined
Jun 9, 2004
Messages
129
Location
California
I have a pair of LA3A's that are in good shape, one unit is working and the other is not compressing. I swapped the T4B's and that makes the working unit not work, but it doesn't fix the non-working one either.

I took the can off of the suspect T4B and it seems to be changing resistance to ambient light, so I believe the LDR's are OK - but I haven't checked if the light source is working. How do I test and what can the light source be replaced by to maintain the original-ness of this?

Beyond the T4B, I'm a little suspicious of the T4 transformer, I found what looks like part of a winding lying around the transformer. Whats my best plan of attack here? At least it is working as a preamp.

thanks
-kdawg
 
not so familar with the la3 but to test the
open the t4 , turn down the lights and pump some good loud
music through it (make sure the volume coming in is loud enough ,
people often make that mistake) .

and turn up the compression .

see if the light panel is working ,
if not ,

swap in the other t4b (lid off also) and
see if the light panel is lighting up .

if its not , then it's something else.

if it's your light panel , just order a new one from LSI and solder her in ,
(be carful not to melt the lead out of the plastic of the panel,
made that mistake , and killed a good panel :( )

g.
 
Yes - I have test equipment - scope, signal gen, voltmeters, mountain dew. I do repair work almost everyday, I'm just not too familiar with LA2A/LA3A's... although I have Drip's LA2A that I've been meaning to get together once the cheaper T4B's come around.

I'll poke around once it gets dark :wink:

-kdawg
 
Okay EEs eezee if you got the gear.

The LA3a is like the LA2a but with transistors and a step-up tranny to drive the T4 sidechain. Look at the schematic and it'll all make sense with the possible exception of the DC-biased transformer winding... but if you're passing signal, that bit is fine!

Keith
 
All of the El panels I have played with illumiate with 20-30V AC. I have tested EL panels in T4Bs this way. Proceed at your own risk though.

Vetsen
 
I've serviced hundreds of LA3A's and have never seen a faulty transformer in one. Start by recapping the unit, if that has never been done. If the T4B is good, chances are good that fresh caps will get the unit working again and in any event it needs to be done anyway. If you still have problems look for a power supply issue, bad transistor or a faulty solder connection.
 
if it's a T4 EL-panel -problem (that is, if one good unit will light green when compressing, and the other, broken one not), then check the el-panel with an ohm-meter. If it's good, it should read open-circuit like a capacitor - but tend to fail by making a short.

If T4 is failed by shorting the EL-panel, there's still a small chance to revive it by "burning" out the short - by applying a spike of high current. I have used a 40V bench supply fitted with a 4700uF electrolytic for this, and revived two panels out of three with this symptom..

use this method at your own risk!

Jakob E.
 
I'm still in the progress of testing but I put signal through and did some swapping of T4B's - no change. Also, either one doesn't light up in this LA3a, I haven't tried the working LA3A - that is next to test the T4Bs.

I checked pin 3 signal to the T4B - with it unplugged I get a good amount of signal, but once a T4B is plugged in, it goes down to barely anything - in the dark or light, open or closed. I did crank up the gain too, you can hear it in the transformers.

I will try some recapping, I'll do some of the electrolytics cans first.

-kdawg
 
Replace all the caps dude. You can't go wrong. I mean I have a pair of them too, originals, and they have a loooong way before breaking the T4B or tranny down. Virtually undestructible units. They never broke down, so never needed to really fix them. But started to lose top and bottom ends so just recapped them recently. The originals have those yellow Frako caps in them, which rot over time and become the real source of problems, which was almost the case in mines as well. I happened to intervene before they gave up.

I still keep the 6 of T4Bs I'd bought from David Kulka a while back as a safety, though. A pair will go into LA-2A clones and the other 4 will remain as spares.

Thanks for the deal by the way, David, a hello to beautiful Burbank, good to see you here :thumb: :guinness:

Mach (aka Barish)
 
Argh - I was testing putting another filter cap across one the power supply caps and forgot to discharge it first, so I zapped the transistor in the power section. New caps and transistor on order :)

So I verified one of the T4B EL panels is dead. Definitely shorted and won't light, and I tried Gyraf's little trick - now it's very not shorted but not going to ever work again either :?

What's my best choice here? I don't want to spend $100 from LSI for 1 panel... will UA sell just the panel at a cheaper price? They are only a few hours from where I live and I've ordered parts from them before, but they were expensive.

-kdawg
 
I don't think that it's worth fecking around trying to fix the bloody thing while trying to save a few bucks on it. It's a false economy in my opinion. Just replace the whole thing and move on. I suppose you could buy an original unit from David Kulka's http://www.studioelectronics.biz (which is what I did a while back.)

I also highly recommend the famous "Clearmountain" mod by replacing the R14 with 14K or 15K (mine have 2x 27K in parallel) while you are at it, if you haven't done it already. It runs it really quieter. Also remember to use a 620R 2W resistor as a terminator between +/- and COM pins, again, if you haven't done it already.

Mach
 
$170 for T4B? Boy I wish I had bought a few back at $90 from JBL.

I did do the R14 mod to 15k from 220k and 220 pf feedback cap to 12 pf. What does the 620R 2W do? Is that across the input or output?

-kdawg
 
[quote author="kdawg"]$170 for T4B? Boy I wish I had bought a few back at $90 from JBL.

I did do the R14 mod to 15k from 220k and 220 pf feedback cap to 12 pf. What does the 620R 2W do? Is that across the input or output?

-kdawg[/quote]

Across the output.

The output of the unit is not terminated so the transformer wants to see 600 ohms, otherwise it causes transient response anomalies in the preceding circuit (because whatever the winding impedances are, the circuit behind the primary will see whatever the load is across the secondary, times the ratio, so if the circuit is designed to see 600 ohms in there and you connect it to a load that is 10K+, which is the current standard input impedance, you mess up the whole balance of the circuit).

Normally the specified terminator is 604R, but an easier-to-find 620R or 680R will also do.

Mach
 
It's not the clearmountain mod. It's actually the ed Evens mod which he did for clearmountain and the studio they was working for in NYC. I have that info on it around here somewhere if anyone is interested.
 
That's correct. I couldn't recall Ed Evans' name and quoted Clearmountain instead.

Thanks for the correction.

Mach
 
Anthony at ADL rebuilds T4B's for a flat rate of $65. I talked with UA last Fall about buying their T4B equivalent - they rec'd Anthony instead. I just got one back from him - the work looks very clean; but I haven't put it in a LA3 yet to try it.

-Darren
 

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