LA2A Issues

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Alrod

Active member
Joined
Oct 1, 2014
Messages
29
I recently purchase a Teletronix LA2A that according to UA, was manufactured around 2002 -2003.  I knew there were some issues with it when I purchased it, but i got it a really good price.  The first thing i noticed when i plugged it in and ran a signal through it, was that it distorted regardless of the peak and gain knob positions.  I tapped the tubes with an erasure end of a pencil and found that the 12BH7 tube was bad.  I replaced it and was treated to some pretty amazing sonic ear candy ;D

I ran it for a few hours that evening with no issues.  The next day, i started recording with it.  My signal chain is BAE 1073MP > LA2A > Lynx Hilo.  After a while, the sound would start cutting out to about 1/4 to 1/2 of the set volume.  It would cut in and out sporadically.  I went ahead and replaced all the tubes except for the 6AQ5 (on order).  I also replaced the T4 module with a kenetek T4B.  The reason I replace it was because the unit does not experience the problem when the module is unplugged.  I also made sure that the R37 pot was set to flat.  I am still having the same volume  issue.  Could the 6AQ5 tube be the culprit?

Is there anything else I can test?
 
Yes it could, but it's quiet strange that another tube gone bad. Can you measure the heater voltages, and take also your B+ voltage (after the diodes). Maybe a ground trouble!? Ah and take your voltages on the 6aq5. Refer to the schematic. I could give you some point of reference, too.
 
Thanks for that info.  unfortunately, this is my first LA2A.  I am not familiar with it at all.  What you are suggesting is outside of my skill set. If I had detailed instructions, i could probably do it.
 
open your la2a. Place one probe of you DMM (voltmeter) on the case (somewhere where there is no paint) and place the other probe where you want to take a measure. Take a look at the picture linked. It's a layout of a LA2A. Yours should looks similar to it. So, first thing first, place your second probe just after the diode CR1. You should get somewhere in the 345-360V
 

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Thank you so much for this!  I am at work right now, but i will measure the CR1 voltage when I get home this evening.

My new 6AQ5 tube should arrive either today or tomorrow.  I understand that I will need to have the unit rebiased when I change the tube.  Is it absolutely neccesary to use an ocilliscope for the rebias?  If not what would be the process using a DMM?
 
Alrod said:
Thank you so much for this!  I am at work right now, but i will measure the CR1 voltage when I get home this evening.

My new 6AQ5 tube should arrive either today or tomorrow.  I understand that I will need to have the unit rebiased when I change the tube.  Is it absolutely neccesary to use an ocilliscope for the rebias?  If not what would be the process using a DMM?

at first i'd suggest to measure voltages (of heater etc... )
 
I took some voltage readings, and here is what I came up with:

CR2 (1n4007) = 337.4V
CR1 (1n4007) = 337.6V

R29 (4.7K/2W) = 322.4V
- There is a 9 kOhms, 5% resister connected between the R29 and CR1 turrets.

6AQ5 (v4)
pin 1 = .1V
pin 2 = 8.3V
pin 3 = 0V
pin 4 = 0V
pin 5 = 67.7V
pin 6 = 115.4
pin 7 = 0V

12BH7 (v2)
pin 1 = 107.4V
pin 2 = 0V
Pin 6 = 239.8V
Pin 8 = 111.5V

12AX7 (v1)
Pin 1 = 116.9V
Pin 3 = 1.2V
Pin 6 = 126.5V

Please note that the 6AQ5 voltage readings are with the original tube.  The new on hasn't arrived yet.  Any ideas?
 
Please forgive my ignorance.  I am looking at the circuit in front of me, the front turret  (closest to the front panel), reads 322.4V and the rear torret reads 287.9V
 
287v should be in the ballpark.

Concerning the 6AQ5, some of your values seems a little off. Reference values are in bold

pin 2 = 8.3V (7 - 7.5V) here it's ok
pin 5 = 67.7V (135v) this value if off.
pin 6 = 115.4 (100v) here it's ok too.

Can you take voltages on Tube 3 please? Also, set your dmm in AC and measure the heaters.


 
I appreciate all of your help.  Thank you so much!

I will do more testing when I get home this evening. 

Concerning the 6AQ5 pin 5, could that be causing the problem I am having with the volume attenuating?  Also, when you say "measure the heaters", are you refering to the tubes? Again, please forgive my ignorance.  Up until now my electronics experience has only been with building guitar effects pedals.  I find the LA2A circuitry simple, yet facinating.  I know enough to realize the lethal voltage I am dealing with and take the proper precautions.
 
yes voltages get pretty high in this little comp :) concerning the heaters, you could put your probes on each side of the turret where the filaments wires from the power transformers are connecting. Probably the 2 first turret pin at the far top right. Probably they will be green wire. You shoudl get something around 6.3VAC
 
Here is what I got:

12AX7 (V3)
pin 1 = 102.8V
pin 3 = .8V
pin 6 = 102.8V
pin 8 = .8V

Heater Voltage
6.95VAC
 
It could be the 6aq5. Check all connections to be sure there is no short. If possible, take some pictures please. Take the schematic, along with the layout i linked before and trace all componants and connections. There could be have a defective cap somewhere.
 
I finally got my new 6A5Q tube in. The new voltage readings are as follows (old readings are in parenthesis):

pin 2 = 7.87V    (8.3V)
pin 5 = 85V    (67.7V) 
pin 6 = 112V    (115.4)

Is the pin 5 voltage still too low?

At any rate, I still have the issue with the signal volume dropping randomly.  It's strange because it can happen within a few minutes of the unit being turned on, or it can happen after an hour.  I did notice that if I DI into my BAE 1073MP and run that signal into the LA2A, the volume drop happens almost immediately, and the volume drops to zero (no signal).  When I play a string or chord harder, the signal comes through. 

When I run a mic through the XLR input, and after the volume drops, if I snap my finger up close to the mic, the signal goes through. 
 
I'm thinking about a defective capacitor, somewhere in your circuit. trouble is, you probably do not have a scope so we can not really know where in the circuit it happens. maybe around C3/4/5, or into the compression part of the circuit..!!??
 
My unit does not have a C5 capacitor in the circuit.  What is the proper step-by-step way to testing capacitors in the LA2A so that I can start testing the others?
 
check that out:

http://www.youtube.com/watch?v=R0Cy9wfrMFQ

check that post too, talking about how bad capacitor may sound. http://groupdiy.com/index.php?topic=47018.msg590696#msg590696

You could check C11, or completly replace it. Since it's a cheap one, and connecting between your T4B pin 3 and your 6AQ5 pin 5, it's possible this cap is bad
 

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