Voltages for T4B in LA3A?

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JW

Well-known member
Joined
Jun 8, 2005
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Portland USA
Here's the schemo: http://www.waltzingbear.com/Schematics/Urei/LA_3A.htm

I'm trying to figure out what the correct voltages are at the T4B pins because I got an LA3A in trade that apparently 'just needs a new T4B' so I ordered one from Drip, but before I install it, I wanted to try to make sure I have the right readings.

(edited:
I'm getting nothing on any T4B pins except pin 6, which is 28V)

Does this seem remotely right?

 
hi JW. Those numbers don't seem correct. You should have a much higher positive voltage at pin 3 being fed by the the sidechain autotransformer. It has been some time, but I remember that voltage in the range of 30V-100V, depending on the compression level.

What are your symptoms? Is it passing audio? Compressing?
 
Hey Greg,

Thanks. I havn't even wired it up to listen yet. It came without a t4B because it was apparently defective, so I'm hesitant to put the new t4b in and give it a go for the potential problem that made the old one defective.

What should I be using  for a ground reference?
 
Hey Greg,

Okay, just ignore the original voltages that I posted. I don't know why I was getting negative V, but now I'm using chassis ground for reference, and I'm getting

40V at C10 in the P.S.
28V at C12 in P.S.

At the T4B, I'm getting nothing on any pin except pin 6 which is also 28V

Does this seem correct if there is no T4B in there? Should I be getting voltage for pins 3, 5, and 7?




 
Anybody else?

For those familiar with the t4B, will I at least me okay plugging the new one in and giving a listen?
 
JW, your PSU voltages look ok. Have you determined if the unit passes audio? Assuming it does, there's probably a problem in the sidechain; most likely a bad capacitor. Often the LA-3As I receive that won't compress have bad caps... C6, C7, C8, C9, or C13. Also check transistors for junction shorts. You should have control voltage at pin 3 regardless of the T4B.

Grant, the PSU transformer has one 24VAC secondary. And based on the schematic fuse size of 1/8A @ 120V, that gives us a 15VA transformer. So a standard 25VA is overkill.
 
Hey Greg,

Thanks. I'll check to see if it passes audio when I get home.

(I'm an idiot) because I forgot to mention that the ground path that connects pin 8 of the T4B and C3, R44, R10, R12 etc had been fried. I repaired it a long time ago and then forgot about it. Could this be a symptom of Q6 shorting to ground or something?
 
Hi Greg,
Thanks for the PSU xfrmr info!  Very much appreciated.

Anybody know the voltages that the LA-3 runs at?  Does the LA-3A run on a single +24VDC regulated supply?
Just looking towards the future if we ever get one of the LA-3 projects happening in the drawing board happening (hint, hint). ;D

Thanks againg Greg!!  (thumbs up!)
-grant
 
Grant,

Yes, most parts the LA-3A run on 24V DC regulated, only the interstage driver transistors are feed unregulated DC (~35V). The PSU is on board - at least in the original and my clone.

One could have used a more "modern" approach with a 7824 or 317 but I followed the original design.

I am using a 24V 30VA torroid (so absolut overkill  ;)) but this was the cheapest and best available option for me.

I can imagine the original power transformer has a somewhat higher output, when I tried 1819 bulbs for the meter illumination powered by 24V AC they were way to dim, but I do not have an original to compare ...

  Andreas
 
Hi Andreas,
Thank you very much for the explanation and info on the PSU.  Greatly appreciated.
Hmmm...now when will an LA-3 project be coming our way?  (hint). ;)
Looking forward to your version two of your PCB in 2009!
It would be really cool to see both your version (original type) and Greg's version (hybrid type) appear next year.
There might be a bloom of LA-3 projects in 2009 and by the looks of it Luny Tune might have an LA-4 as well.
Exciting compression options coming in the new year.
Have a great Christmas!
Regards,
Grant
 
Hey Greg,

Okay, yes. The unit passes audio fine.

So, what can I do with my DMM to see which cap(s) is(are) bad? I guess, a better question is, when little film (or ceramic?) caps go bad, like C6, C7, and C13, do they ever get to a point where they won't pass a continuity test from one leg to another? How about lytics?
Just wondering. Seems like it's hard to determine a bad cap if it's not visually puffed out or something. I don't have anything that can test capacitance.

Should I just try a sweeping replacement of all those caps?

Thanks for your help
 
JW,

if your unit passes audio, I would assume Q6 is fine. Check the voltage at pin 3 of the T4B socket - as Greg has suggested (with or without the T4B plugged in). Put you Dmm to AC, you should get voltages up to 70V or 100V depending on the setting of your Gain pot, if so, your side chain should work correct.

Otherwise, if all voltages at the T4B socket are within spec, why don't you simply swap in your new cell, there should be no risk of ruining it ...

If everything else fails, you might have a look at the caps. I have seen all kind of types go both short or open, I would check the lytics first for a suspectiv low ohm reading.

   Andreas
 
Thanks Andreas,

I didn't realize I should be measuring in AC for the T4B sockets. The voltages I put before were in DC.
For AC,
I'm getting nothing at any of the pins except:
pin 1: 0.6V
pin 2: 0.6V
pin 3: 0.06V

I measured resistance across the caps that Greg mentioned and I'm getting:

C6: 847K ohms
C7: 1.5K ohms
C8: 7.3K ohms
C9: 68K ohms
C13: 219K ohms

Do C7 and C8 look suspect, or are those resistances reasonable?


 
JW,

I think I forgot to mention, you should check pin 3 with a signal present at the input of your unit (oscillator / music / ..) I don't now what DMM you have got, but you should get some AC voltage if you crank up the "peak reduction" control.

  Andreas
 
Hey Andreas

Okay, I'm getting AC voltage on pin 3 with signal running through it. It appears to be in the normal range. Over a hundred Volts with it maxed out.

So, I popped in the T4B, but it's not compressing.

Hmmm. I guess it's back to those original caps possibly being a problem? How do the resistances look for them?
 
Hmm, this looks like your sidechain is working, so I would leave the caps alone for now - none of them seems to be dead short.

So, to summarize:

- Your PSU is o.k. delivering 24V DC

- The gain make up stage is o.k. (unit passes audio)

- The side chain is o.k. (over 100V AC at pin 3 of the T4B when driven)

There is not much left in a LA-3A that may cause a defect  ;)

I suspect that either your T4B is dead (you have not mentioned if you popped in the old or the new cell) or connections between the T4B and other parts are open ... Check if pin 1,2,4,5 and 8 have conductivity to ground / check pin 7 to CCW lug of the gain pot.

What about your meter, is the needle moving with the switch set to "GR" and input signal applied ?

  Andreas
 
Interesting... you have CV at the T4B but no compression. As Terby said, check the T4B ground paths and make sure you're showing continuity between the "fried" points you mentioned.

Grant, if there's one DIY I really want to get to in 2009 is to finish my hybrid LA-3A. I really liked the way it sounded (it had more of a midrange bloom that I loved) and the interstage transformer Edcor wound for me worked great. It's just that I've had a steady flow of tech work coming in (still do) and I can't seem to find time to build things for myself. I've even got a PCB layout 90% complete.  8)
 
first i need to apologize to JW for hijacking his thread.  :-[  sorry JW
but i'm glad greg and andreas are here to help out with troubleshooting and my off-topic questions.

@ greg,
glad to hear that your LA-3 hyrbrid is still in the works for next year! (thumbs!)
anxiously awaiting both versions. ;D

happy holidays everyone!...and now back to your regularly scheduled troubleshooting.
warmest regards,
grant
 

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