D-LA2A Support Thread

[dandeurloo]

Here is a option.  If you plan on using one of my cases.  You can have the edcors shipped to me and I can ship the case and transformers to you to share shipping.  I have done that for a number of guys. 

 

[Sammas]

Hi Sammas,

I'm from Oz as well, I had a similar dilemma... The postage direct from Edcor in the USA is ridiculously expensive but the price of the transformers are ridiculously cheap...
The transformers for this project cost me $10usd each + $50usd shopping so $90 all up, I think that will still come in cheaper than the carnhills ...

Hey frazzman!

The transformers are just tacked onto an order of other parts, so I would be paying shipping with or without the transformers anyway.

I think I have found a nice chap locally that has some edcor 600:10k transformers for sale. I think I might just go with those.

Here is a option.  If you plan on using one of my cases.  You can have the edcors shipped to me and I can ship the case and transformers to you to share shipping.  I have done that for a number of guys.

I purchased the project half finished from a chap in Australia and it came with a case.
But thank you Dandeurloo! That is mighty nice of you to offer!

 

[salomonander]

hey
did anyone use the original utcs on this build? i need help with the pinouts. i have utc a10 and a24 and need to know how to hook them up. thanks!

ps: would it be smarter to mount them on the outside of the case (like back in the days) or inside? im wondering about shielding etc....

 

[saxmonster]

Where does one get a face plate like this at?

 

[melomane]

Hi there!

I have a problem with my D-LA2A.
I just done the assembly, but I have problem with my unit.

I made a video for convenience, check below the Youtube video please

Here is my configuration:
- T4B IGS
- Power Transformer from Silent Arts
- Audio transformer from Edcor

The problems:
- Left Channel: no sound, just a buzz...
- Right Channel: no compression G.R, problem with gain potentiometer when more than 50%...

I have double check the heater connection, and all connection (include for the 3 grounds connection + screen).
I don't know the problem 

Please help me  8) Thank you so much !

http://youtu.be/idRbu564NGs

 

[Dr Gris]

hey
did anyone use the original utcs on this build? i need help with the pinouts. i have utc a10 and a24 and need to know how to hook them up. thanks!

ps: would it be smarter to mount them on the outside of the case (like back in the days) or inside? im wondering about shielding etc....

I guess you've solved it by now, but I found this:

It appears that the 1968 schematic illustrates the A-10 (input tranny) primary labeling inverted - with tranny pin 6 hot. Older schematics as well as photos of reissue units illustrate tranny pin 1 hot.

If I'm correct, then this is the way to wire the input for XLR pin 2 hot:

XLR input pin 2 --> barrier strip terminal 1 (optional) --> HA100X/A10 tranny pin 1 (primary) HOT input to tranny
XLR input pin 3 --> barrier strip terminal 5 (optional) --> HA100X/A10 tranny pin 6 (primary)

(My layout reflects this, and this supports the reissue manual's instructions to use barrier strip terminal 1 for hot)

Now, the output side is always illustrated the same way in all the schematics (without the XLR) - barrier strip terminal 10 to tranny output pin 1 hot. However, photos of the reissues (with XLR's) indicate that XLR pin 2 is wired to barrier strip 8 (pin 6 of the output transformer), which would reverse the polarity from the output tranny.

This is how (in photos) the reissue unit's output is wired for XLR pin 2 hot:

XLR output pin 2 --> barrier strip terminal 8 (optional) --> A24 tranny pin 6 (secondary)
XLR output pin 3 --> barrier strip terminal 0 (optional) --> A24 tranny pin 1 (secondary) HOT output from tranny

(My layout reflects this too, and this also supports the reissue manual's instructions to use barrier strip terminal 8 for hot)

But, the trouble here is that pin 6 of the output tranny secondary is not the hot side.

Has the signal's polarity been flipped by the time it gets to the output transformer primary, so that reversing the XLR connections puts the signal back in phase on the way out?

If so, then it makes sense.

Best
//M

 

[salomonander]

thx! this helps a lot

 

[salomonander]

my pcb is already in the case so no way to check the backside. i have a question regarding the inputs and outputs.
+ and - are labeled left and right. so mass (pin1 on the xlr) goes in the middle of each molex ?

cheers

 

[salomonander]

ps: i did not insert any tubes or t4bs. but i measure around 370v on most tube sockets. seems wrong i guess.... any help..... thx!
i assume that something is wrong with r29? its certianly a 4.7k one in there... is the grounding the first starting point?

 

[salomonander]

ok so i sorted most things out. the unit works, compresses and sounds gorgeous. but my meter doesn't show the correct values in GR mode. i can clearly hear more than 15-20db of reduction while the meter never adventures further than 3-4db of reduction. no matter what i set the gain reduction to. anyone have an idea where i can start the search? i did replace rX25 with a trim pot.

cheers

 

[salomonander]

ps: if i may make a modest suggestion for the next revision of the board...
it would be amazing if the signal would travel to the t4b in bypass mode. switching out is great and clean. but whenever switching back into compression mode one gets a dynamic spike (a big one when doing lots of reduction) since the t4b has to catch up first...

 

[Bowie]

ok so i sorted most things out. the unit works, compresses and sounds gorgeous. but my meter doesn't show the correct values in GR mode. i can clearly hear more than 15-20db of reduction while the meter never adventures further than 3-4db of reduction. no matter what i set the gain reduction to. anyone have an idea where i can start the search? i did replace rX25 with a trim pot.

cheers

Sorry if I'm pointing out the obvious but triple-check all the resistor values that are in the path to the meter.  If it sounds like you're getting correct GR then something is going wrong between your sidechain and your meter.

 

[frazzman]

Hey Volker et. al,
Having some issues with one of my d-la2as... I had a bit of noise on one channel and had to pull the case apart to improve conductivity between all pieces. Anyhow in the process I've managed to mess up the output on my left side. I have a constant hum and very low distorted signal on that side. 

I thought perhaps bad molex crimp on output transformer leads but I've swapped it with working side and no difference. I have also tried known working pots / switches etc from working right channel but to no avail. GR metering is working properly on both channels and stereo link so I know input is ok but output is interrupted.

Any pointers on where to look for areas that may interrupt output ? I am thinking a dry joint somewhere but I've avoided taking the whole thing apart as I'm not too keen on removing the PCB... Best to discharge all caps before taking an iron to the pads around the molex connectors ? I've been lucky until now with any tube builds I've done not requiring any surgery. I'm paranoid about residual charge in the caps....I was thinking of just reheating all the molex connector pads? Sorry for the long post, I've spent way too long looking at this, appreciate any fresh insight

 

[band_master]

Hi all,
Working on a re-build of my D-LA2A. It was my first project and the orientation of the PCB was not the best way to go. Changed up the layout and re-wired everything including the heaters. Was up too late trying to troubleshoot some hum in channel 2. It does that thing where the signal cuts out and the meter spikes if you crank the gain. Had the limit/comp switch wired backwards, fixed that. Had the unit on for a while and noticed that R234 started to smoke. Check the heater wiring? other ideas?

cheers,
BD

 

[bernbrue]

Hi,
I would like to report my findings concerning oscillation. My input transformer is the original Sowter 1:4, Output is Edcor 10K/10K. I did the 25K pot 75K resistor mod for the input gain.

Background: My unit worked fine from the very beginning, except for the annoying 30kHz oscillation with higher gain settings causing meter pegging. I moved the output transformer farer away from the board and the xlr connectors which helped a bit. I consequently used moderate gain setting so that unity gain was almost achieved.

Today I tried various ideas to eleminate oscillation. I realized that the area around V1 and R11 (68K Feedback) is very touchy and that even moving my hand around V1 reduced oscillation a bit. CJ proposed to change the value of R11 in order to reduce negative feedback and gain. I finally put a 14K resistor in parallel to the existing 68K. That helped quite a lot but I still couldn´t crank up the gain to maximum without meter pegging.

Then I had a real "heureka" experience. The shielded wires for the gain reduction pot and gain pot were connected wrong, well not wrong but the shield was conducting signal and a red wire was meant to work as shield. What a silly mistake!!!! I rewired it and now I can set the gain pot to maximum without any oscillation at all. I measured the compressor with RMAA and the specs are more than convincing. With the paralleled negative Feedback resistor THD is 0,382 %. Not that bad. All other values including absolutely flat frequency response are excellent. I´m more than happy now with my Dual LA2a!!

People having these problems as well: CHECK YOUR WIRING (verify that shield is connected to ground!!!) and REDUCE FEEDBACK RESISTOR.


regards
Bernd

R234 has to be 3W. Check value of R235 and voltage rating of C213 as well. Try changing the 6AQ5 with the working channel
regards
Bernd

 

[band_master]

I checked wiring and made sure polarity on the pots and molex connectors were good. tried moving around the audio transformers. checked voltage drop across r229 getting 350v before but only 216v after. checked heater voltages on 6AQ5A and am getting 6.7vac across 3 and 4 but also getting 6.7vac across 4 and 5. Double-checked the heaters and a solder joint was poking through the PVC coating on the heater wires causing a short. Fixed that and now it's not smoking anymore. Still getting more noise floor in channel 2 and audio cuts out and meter pegs if you turn gain past 8 or 9...at least voltages across r229 are good now. 350v before and 250v after.

 

[bernbrue]

getting 6.7vac across 3 and 4 but also getting 6.7vac across 4 and 5. Must be a short? will have to double-check heater wiring again.

ATTENTION!!!! pin 5 is the anode that means there is high voltage. When there is a short between 4 and 5 you found the reason why R234 has burnt.
regards
Bernd

 

[band_master]

thanks, bernbrue. yep i edited my last post before i saw your reply. I fixed the heater connections to 6AQ5A. R234 and R229 still read good on my meter, but there is evidence of overheating on either side in the solder joints. do you think component failure caused by the heater short could be the cause of the signal drop out problem with the gain maxed out?

i tried swapping tubes and gain/reduction pots across channels. maybe i should try swapping transformers...attached is a pic of my current orientation. i have seen this work for other builds, but am considering moving the audio transformer to the left of the PCB on the opposite side of the toroid and out from under the XLRs as the left channel's noise floor is much lower than the right.

 

[frazzman]

Hey guys,

Still battling with my issue on the left channel of my D-LA2A. Everything initially worked fine but I developed a constant hum and low distorted audio on this channel. This is really weird and something I have never experienced on any other builds ... Tonight I found that if I press my thumb semi firmly against the PCB just under the "LIMIT COMP" Molex connector -- the problem goes away...!!! (see attached pic)

Audio then passes cleanly and behaves just as the right channel does. Once I release my thumb, the hum returns instantly and the problem returns...

I spent the last few days thoroughly checking everything on this channel
- I moved 'known working' pots/switches from the working side
- Tested in and out transformers across to the other channel - no issue
- Removed PCB and reheated all pads
- Removed powder coat on the casing around all switches / panel edges to ensure proper conductivity
- Checked for grounding with DMM, looks fine.
- Checked my working channel and found that audio passes cleanly with all pots and switches disconnected (and not just in bypass mode), so I can even eliminate all of those on the non working channel... problem still persists

I was initially convinced that this had to be a bad molex or transformer wiring issue but tonights finding disproves that...

The only thing I can think of is a bad connection on the heater wiring, the area I'm pressing is very close to the tube socket the heater wiring runs to. The confusing thing is, this wiring seems solid - can measure 0hm from the end of 100r resistors through to the end of the heater wiring.

I created an audio export as well so you can hear the problem too:
https://dl.dropboxusercontent.com/u/108741277/la2a-issue.mp3

I can create a video as well if it helps to explain.

You can hear in the clip, the hum is present, soon as I press down on the board, you hear a pop and the audio passes correctly, then 10 seconds or so later I let go of the PCB and the problem returns...

Really appreciate any ideas anyone has... does this sound feasible?

 

[[silent:arts]]

Is the problem still there with only the LIMIT / COMP unconnected?
(anybody uses this?)

the LA2A is very sensitive to unshielded / untwisted cables!