D-LA2A Support Thread

[snowow]

Does anybody have any tips on how to mount the Arco-trimmers?

 

[dmp]

Does anybody have any tips on how to mount the Arco-trimmers?

Search -> "this thread" -> arco
Gives a lot of results as many discussions have happened in this thread alone. Search the whole forum and you get a ton of results.
Most builds don't use it but use a fixed value cap instead of the adjustable cap OR a socket to swap fixed value caps.
First post also has links to pictures of builds so you could try to find a build with the arco caps installed.

 

[ron_swanson]

When I built my LA2A I just soldered the tabs to the turrets on the turret board. Arco caps are a pain and I found them to be unstable. Maybe just me, but I removed the arco cap in favor of a single value cap as suggested above.

 

[snowow]

Can somebody please check and confirm if these relays will work or not?


Miniature Signal Relays

• Dimensions: 20.2 x 9.8 x 12.5 mm
• Function: 2 x changeover contacts, DPDT
• Nominal voltage: 5 V/DC
• Nominal load: 1 A - 24 V/DC, 0.5 A - 120 V/AC
• Max. switching voltage: 60 V/DC / 120 V/AC

https://www.tube-town.net/ttstore/miniature-signal-relays-ry-05w-k-dpdt-5v.html

 

[snowow]

Pretty please? I really like to avoid having to desolder all the relays later if they won't work for some reason?

...I've never used realys before, that's why I'm a bit cautious.

 

[dmp]

Those relays look fine to me. they are the correct voltage (5v) and size

 

[rna]

I just rehoused my dla2a. I didn't have much money for the case originally so I decided she needed a make over after I turned my 2-1176 from 3U to 2U.

 

[delatsch]

So excited that I've finally finished my D-LA2A! I've wound the power and the two output transformers myself and you can check out how the unit turned out in the end:

 

[delatsch]

You can hear a short demo of it here:

YT Demo

 

[[silent:arts]]

Selfwound transformers? Cool

 

[delatsch]

Selfwound transformers? Cool

Haha, thanks Volker, the input ones are OEP, but I wound the power and the output ones. I must say that your PCB works perfectly and this has been such a great project to make! I will probably build another one and I'm planning on making a video where I show how I wound the OTs.

 

[musgrave]

I'm planning on making a video where I show how I wound the OTs.

I would enjoy that, I'm a transfo winding nube. I want to learn the craft well!

 

[innercityman]

So excited that I've finally finished my D-LA2A! I've wound the power and the two output transformers myself and you can check out how the unit turned out in the end:
View attachment 95023View attachment 95024View attachment 95025View attachment 95026View attachment 95027

Beautiful work and congratulations for the self wounded trafos !
Did you use the Don-Audio's WF1029X vu meters ? I've chosen them for my build and I'm actually finishing wiring it up but I wonder where to take power for the vu-meter leds... They require 12VDC. Can you tell me how you did it ?

 

[delatsch]

Beautiful work and congratulations for the self wounded trafos !
Did you use the Don-Audio's WF1029X vu meters ? I've chosen them for my build and I'm actually finishing wiring it up but I wonder where to take power for the vu-meter leds... They require 12VDC. Can you tell me how you did it ?

Thank you! Yes, I used those meters. The meters work on both AC and DC, and I just wired the meters of the AC heater taps like the original did. The light is dimmer but I find it even better like that because I run the same meters on 12V in my 1176 and the light is to bright and they generate a looot of heat. So, check if that's enough light, if not, you can buy a 6.3V light bulb that you can install in the meter. I am not sure if don audio has it at the moment, but they are like small modules held by two screws on the back of the meter.

 

[innercityman]

Hey Ya'll, it seems that I need some help... I've finished building a D-LA2A for a friend and I'm getting some issues...

First of all I've been able to make voltage measurements and all seems to be good referring to component breakout V2 readings document... Neon lamps glowing up but I measured 54VDC and 57VDC only, is that ok ??? I've read that they require at least 65VDC right ? Tubes shine. I have signal coming in and out when powered off, on bypass mode and switched on, and signal coming and growing up as turning the gain pots, so I can conclude that my relays work. New and matched pair of T4Bx from IGS audio. Brand new matched JJ and Electro Harmonix tubes. Sowter transformers.
I've been able to zero adjust gain reduction meters easily.

My issues are that I can't see the meter needle moving up when applying gain on both channels, no compression either on both channels and a hum on both channels too.

I've set the stereo link trimers to counterclockwise position as suggested and R37 fully clockwise. I've put trimers at R25 set at about 33K. I used shielded mic cable for pots and xlr connections. Connected pots shield to molex pin 1 as suggested.

I guess I have some wiring issues. Any help and advice would be appreciated.

 

[beatnik]

might be the problem is in the sidechain amp. what voltages you have on the 12ax7 and 6aq5 ?

check the wiring to the peak reduction control is correct

 

[innercityman]

might be the problem is in the sidechain amp. what voltages you have on the 12ax7 and 6aq5 ?

check the wiring to the peak reduction control is correct

R31 0.77/0.78
R33 80.3/84.8
R34 136.2/160
R35 5.18/5.94
R36 111.2/91.2

peak reduction wiring is correct. I followed the wiring diagram

 

[beatnik]

Voltages seem ok. Can you post a copy of the wiring diagrams ? That might help

My issues are that I can't see the meter needle moving up when applying gain on both channels

Do you mean the meter is not working in output mode ? That should be fairly easy to troubleshoot, on the La-2a the meter just feeds directly off the output signal. Maybe there is an error on the wiring of the meter selector switch ?

Regarding no compression, if the stereo adj is set fully off the side chain signal is cut out. I know you are mentioning you set the trimpot as per the caibration instructions, I would try turning it the other way around just in case.

Best way to troubleshoot this would be with an oscilloscope and tracing the signal throughout the entire side chain amplifier to find where it gets lost.

 

[innercityman]

Voltages seem ok. Can you post a copy of the wiring diagrams ? That might help



Do you mean the meter is not working in output mode ? That should be fairly easy to troubleshoot, on the La-2a the meter just feeds directly off the output signal. Maybe there is an error on the wiring of the meter selector switch ?

Regarding no compression, if the stereo adj is set fully off the side chain signal is cut out. I know you are mentioning you set the trimpot as per the caibration instructions, I would try turning it the other way around just in case.

Best way to troubleshoot this would be with an oscilloscope and tracing the signal throughout the entire side chain amplifier to find where it gets lost.

thanks for your help. Long time I think about buying an oscilloscope but don't have one yet.

Voltages seem ok. Can you post a copy of the wiring diagrams ? That might help

I've used the one in the compiled build document on 1rst page of this thread. My wiring follows exactly this diagram. Here its is...



Do you mean the meter is not working in output mode ?

Yes, vu meter not working in output mode. In gain reduction mode, needle goes to 0db but no compression. I've tried with the stereo adjust trimmer set other way but no change. I've took measurements on T4Bs socket and I get 24vdc on one pin (don' know which number it is) when on output mode. When switching to gain reduction, voltage drops to 7vdc.

Few pictures of my build...

 

[beatnik]

maybe wrong value resistor in series with the meter ?
on the la-2a the meter is just taking the unbuffered signal from the output.
If there is healthy audio signal on the output of the compressor there is nothing else that could be wrong except the wiring to the meter, the switch, the series resistor, or the meter itself.

regarding the hum you mentioned, the AC wiring from the power transformer should be kept as far away as possible from high impedance circuitry.
It seems from the photos both the mains wiring and the lamp supply are very close to the gain pot. Any noise picked up there will be amplified in the following stages.
The wiring to the limit/comp switch is also in the input grid circuit of the first amplifier stage, I believe on the original La-2a this wire is shielded.

It would help to have the mains switch on the side of the front panel. I have seen some commercial units using a shielded microphone cable for mains wiring going to the on/off switch in the center of the front panel.