D-LA2A Support Thread

[bernbrue]

Hi Ben,
there must be a misunderstanding. I attached a picture for checking the heater voltages.
regards
Bernd

 

[benlindell]

So I should be reading 6.3 between pin 4&5? I'm getting 6.3 on the terminals in the pairs like you have drawn.

 

[bernbrue]

Yes, except for the 6AQ5, which has its heater on pin 3&4. Check the heater wiring twice and compare it to the picture. Then check the pot wiring.
regards
Bernd

 

[benlindell]

Here's what I'm getting, referenced to star ground. Thanks for your help and patience with this, I feel there must be something silly that's going on, especially since this worked before I put it in the case.

 

[[silent:arts]]

Ben,

I don't know anything about your "black marking" scheme ...
but there is some inconsistence there.

I'm sure you disconnected it from your PSU transformer while putting it in the case

 

[bernbrue]

Ben,
Volker is right, double check the power transformer wiring, especially the  flying blue lead. I saw a bad connection at one of the XLRs in the picture, too.
regards
Bernd

 

[benlindell]

Ben,

I don't know anything about your "black marking" scheme ...
but there is some inconsistence there.

I'm sure you disconnected it from your PSU transformer while putting it in the case

Disregard the black marks, I made them so I knew how I hooked it up when I initially put it in the case.

I did disconnect it after it working while putting in the case . I'm really confused as to how I could get 3.3V though.

I just unplugged the wires from the terminals and measured 6.7V between blue and green for both sets. Then when I put them back in the terminals I get the same results as before when referenced to star ground (3.4, 3.4, 6.4, .3), but still 6.7V when measured between the pair in the terminals (I hope that makes sense).

Ben,
Volker is right, double check the power transformer wiring, especially the  flying blue lead. I saw a bad connection at one of the XLRs in the picture, too.
regards
Bernd

The flying blue wire is the pilot light which I'm leaving disconnected for the time being. What issue do you see with the XLR?

 

[[silent:arts]]

Ben, referenced to star ground arround 3,3V each heater is correct.

 

[benlindell]

So then is it wierd that the other one reads 6.3 and .4?

 

[[silent:arts]]

So then is it wierd that the other one reads 6.3 and .4?

yes

and please measure the high voltage.
while I don't know your PSU transformer it looks uncommon, like out of phase.

PS: watch your grounds. one channel isn't grounded correctly

 

[benlindell]

So then is it wierd that the other one reads 6.3 and .4?

yes

and please measure the high voltage.
while I don't know your PSU transformer it looks uncommon, like out of phase.

PS: watch your grounds. one channel isn't grounded correctly

I'm using this transformer, should be fine right? http://www.antekinc.com/pdf/AS-1T250.pdf

I'm getting 350VDC on the right side of R129 and R229. I went through http://www.twin-x.com/groupdiy/albums/userpics/LA2A-deconstructed.jpg and all my high voltages were really close (with tubes installed on one channel) just a few volts low across the board.

both channels are tied to star ground, I just didn't use the quick connect that's recommended, I soldered a wire in there, just to be safe I just double checked the continuity on that too.

 

[benlindell]

In the interest of thoroughness I swapped the pairs of blue and green a bunch and no matter what combo, I can only ever get 3.4, 3.4, 5.9, .4)

 

[Cardinen]

Finally finished my unit 






I've replaced R25 with a trimmer, and now the VU tracking is quite good.
The right meter moves a little in Gr. mode but is not a problem at the end; tried to change the lamp with a zener did not solved the problem.
Anyway i'm really happy with this build.

 

[bernbrue]

In the interest of thoroughness I swapped the pairs of blue and green a bunch and no matter what combo, I can only ever get 3.4, 3.4, 5.9, .4)

Ben,
measure AC coming from the power transformer. Therefore disconnect green and blue wires form pcb and measure the AC voltage between green/blue. Check the heater wiring underneath the pcb (bad solder connection, broken wires, scratched isolation, wrong pins, etc.). When everything is correct, reconnect the blue/green wires for one channel only, measure voltages once again without the tubes plugged in. If voltages aren´t correct there must be a mistake with R1&2 or R3&4 (resistor values, badly soldered, etc.). If voltages are correct, measure with one tube plugged in and so on. It shouldn´t be too difficult to find the mistake.
regards
Bernd

 

[benlindell]

Ok will do. I do get 6.7V between both pairs of the blue and green wire when not connected to the board. Just a note, when I measure either one referenced to star ground I get 70V, is that not a good thing?

 

[[silent:arts]]

Ben, while there is still something wrong with your heaters (where the colors doesn't matter, it is AC),
change your high voltage PSU connection zo white/yellow white/yellow.

look at the schematic and your transformer PDF!

 

[bernbrue]

Just a note, when I measure either one referenced to star ground I get 70V, is that not a good thing?

Just ignore the 70V for the moment and measure between the leads/pins. Now connect the transformer leads to the board and measure without tubes plugged in.
regards
Bernd

 

[bernbrue]

Hi Volker,
I had a look at the tx pdf. You´re right.  Besides that there is more going on. Greetings to Berlin.
regards
Bernd

 

[benlindell]

Ok so I get the right value between the leads, then when connected to the board without tubes I get the same problem. I think it may be time to take it all out of the case again so I can get to the bottom (pun intended) of this.

For the high voltage, the board is labeled 250 GND GND 250 (going top to bottom). White is 250 and Yellow is 0 on the sheet so thats why I did White, yellow, yellow, white. Why does it need to be white yellow white yellow?

 

[[silent:arts]]

For the high voltage, the board is labeled 250 GND GND 250 (going top to bottom). White is 250 and Yellow is 0 on the sheet so thats why I did White, yellow, yellow, white. Why does it need to be white yellow white yellow?

look at the original schematic, which uses a 500V center taped transformer.
the labels on the PCB are fine, but for sure don't match the PSU labels.
for your specific transformer:
the first 250V label is white from the first secondary.
the second 250V label is yellow from the second secondary.
first secondary yellow + second secondary white = center tap.
wired like you have it at the moment it is out of phase.