kdawg

Original LA3A issues
« on: May 07, 2007, 05:55:49 PM »
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


dripelectronics

Original LA3A issues
« Reply #1 on: May 07, 2007, 07:43:37 PM »
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.
g.

[email protected]
(not so good with PM's so dont pm...use email)
la2a ver 3 atoms manual : http://lomayesva.com/la2a_pcb_ver3.pdf
v72 manual : http://lomayesva.com/v72_pcb_manual.pdf

SSLtech

Original LA3A issues
« Reply #2 on: May 07, 2007, 07:47:49 PM »
Be careful... You don't want to burn your good T4...

Do you have a scope? -A meter?

Keith
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.

kdawg

Original LA3A issues
« Reply #3 on: May 07, 2007, 08:35:40 PM »
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

SSLtech

Original LA3A issues
« Reply #4 on: May 07, 2007, 10:02:13 PM »
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
"A waist is a terrible thing to mind"
Quote from: PRR
Ah, but that was 1999; we don't party like that any more.

Vetsen

Original LA3A issues
« Reply #5 on: May 08, 2007, 02:30:13 AM »
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
"The smoke from the flux is filling my lungs..."

David Kulka

Original LA3A issues
« Reply #6 on: May 08, 2007, 03:10:02 AM »
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.

gyraf

Original LA3A issues
« Reply #7 on: May 08, 2007, 04:35:18 AM »
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.
..note to self: don't let Harman run your company..

kdawg

Original LA3A issues
« Reply #8 on: May 09, 2007, 12:48:58 PM »
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

Barish

Original LA3A issues
« Reply #9 on: May 09, 2007, 06:16:24 PM »
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)


kdawg

Original LA3A issues
« Reply #10 on: May 12, 2007, 03:01:09 AM »
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

Barish

Original LA3A issues
« Reply #11 on: May 13, 2007, 07:15:43 PM »
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

kdawg

Original LA3A issues
« Reply #12 on: May 13, 2007, 07:41:48 PM »
$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

mich

Original LA3A issues
« Reply #13 on: May 13, 2007, 08:41:49 PM »
Quote
$170 for T4B? Boy I wish I had bought a few back at $90 from JBL.


We all do...  :?
"Tubes gone - darkness,darkness,darkness no colour ,no contrast." - Joni Mitchell

Barish

Original LA3A issues
« Reply #14 on: May 14, 2007, 04:40:34 AM »
Quote from: "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


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

pucho812

Original LA3A issues
« Reply #15 on: May 14, 2007, 10:37:44 AM »
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.
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.

Barish

Original LA3A issues
« Reply #16 on: May 14, 2007, 01:01:35 PM »
That's correct. I couldn't recall Ed Evans' name and quoted Clearmountain instead.

Thanks for the correction.

Mach

Dtube

Rebulilding T4B's
« Reply #17 on: May 14, 2007, 03:01:56 PM »
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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