All things LA2a related

[Kingston]

It's called a white cathode follower: http://www.tubecad.com/october99/index.html (more details starting page 4)

additional notes, http://www.tubecad.com/may2000/page16.html

 

[college101]

OK guys..I am having a lot of problems with my DIY la-2a...no signal threw the input and the meter needle is stuck on 20! Cant get it to move at all

The lights and heaters are on...the bulb lights up with the zero adjust, but nothing else happens...what do I do?  I have gone back threw and have not found any errors...the only thing I can contribute is that it has to have something to do with the trannys...I got Sowter's 1009 input and the 1010 output...Do I need to float the heaters with "2" 100k res?

 

[college101]

anybody? ???

 

[simonsez]

OK guys..I am having a lot of problems with my DIY la-2a...no signal threw the input and the meter needle is stuck on 20! Cant get it to move at all

The lights and heaters are on...the bulb lights up with the zero adjust, but nothing else happens...what do I do?  I have gone back threw and have not found any errors...the only thing I can contribute is that it has to have something to do with the trannys...I got Sowter's 1009 input and the 1010 output...Do I need to float the heaters with "2" 100k res?

Try it without i/o transformer.

 

[college101]

without the transformers?  OK..so I went back threw..and I used a 470k res in stead of a 4.7k 2 watt

 

[simonsez]

without the transformers?  OK..so I went back threw..and I used a 470k res in stead of a 4.7k 2 watt

check every component value, 4k7 to 470k is away too far ... ;D, check voltage with DMM, i think i saw voltage chart somewhere here...power on without T4B and input output iron first. Be carefull, high voltage!

 

[college101]

ok..will be doing the power up without the tranny's and t4b today

 

[college101]

OK Guys..IM not convinced my wiring is messed up...I did change R36 to a 470 Ohm resis...then I realized there is a little adjust knob on the back of the sifam vu meter...(Is that the zero adjustment control for aligning the needle?

I started to play with it today...and by adjusting it...I can move it off of 20...but with only a few turns to move it off 20...Im geussing everything is correct and Im getting high freq oscillation? Does this sound right? If its oscillating, no matter what signal I put into the compressor, it wont register cuz of the frew oscillation..am I right?

Next question... Since I have a Sowter 1009 input transformer...It has the wire leads instead of pins like the A10...I need to ground # 10...which is the bottow wire off of the secondary? Right guys?

Im going to change R7 to a 2K7 since I have seen in other people's posts about using the higher value resis to improve the compression...

Im open to comments...help me out people


Thanks Simonsez

 

[kazper]

Take some pictures of your build and post them..

As far as the VU do you have your 3.9K in line or did you adjust your zero adjust pot AKA R4 or a 1M pot? I think you could remove the VU from your equation and just focus on the compressor. Verify it works a little piece at a time otherwise it looks real complicated as a whole.

The actual manual has a whole troubleshooting guide and adjustments procedures, have you looked there?

 

[college101]

I have the manual I believe from an email..have to go check real quick...Im wondering if the switch#2 has anything to do with it? I bought it from studioelectronics...the exact replacement part...I had to take out the two screws holding it all together, and replace them with longer screws to attach it threw the faceplate..I was having some trouble with the switch and knob sliding...Once I mounted it, the problem has stopped...

 

[college101]

O..sorry..I have the 3.9k in line with the negative side of the vu meter...I have the out put tranny and the input tranny set up correctly...I have pin3 pos side and pin1 neg side of sw#2 correct to cayacosta's wiring

Still lost..I cant get the needle to move off of 20...

Pics tommorow morning sometime

 

[college101]

Im going to remove the 3.9k resis and pop in a 3.6 resis...then re check the vu meter

 

[kazper]

http://www.recproaudio.com/diy_pro_audio/diy_files/la2a/la2a_layout.jpg
http://www.recproaudio.com/diy_pro_audio/diy_files/la2a/la2a_layout.jpg

Both show the VU Meter with a 3.9K inline (to - side of VU)

It's also shown as the pole of the VU switch. (Switch 2)

Does it stay at 20 when your on the +4/+10 outputs?

 

[simonsez]

Does it stay at 20 when your on the +4/+10 outputs?

Yes, and when in GR mode it should can be trim to 0 with zero adjust trim(R4)
Does it pass audio? did u check the voltage? did u try to put grid stoper? use shield wire in front of V1?

 

[college101]

yes I put in the grid stoppers...the two 100k off the green heater lines

Yes for the shielded cables infront of v1

last night it would not pass audio..but I have not switched out the 3.9 for the 3.6k resis...thats my next step

Kazper...yes it stays at 20 when in +4  or +10 and in GR mode

 

[simonsez]

I think u should try to get the audio pass first, and take care everything else later. That 3k9 resistor has nothin to do with audio signal.
You must check the DC voltage using DMM to locate the problem, did you?

 

[college101]

Ok..I wrote down some of those test points and will be checking..just got some resis to change out the 3k9... check this out from page 96 of this thread

"After doing some reading, I've found that my meter, in order to read 0dBVU, needs to see 1.228VAC across the terminals with a 3.6K resistor inline.  "

Now I have the Sifam al39 meter...brand new ..so im gonna try this out and see what happens...

 

[Biasrocks]

I think you have bigger problems than the +4db resistor. You need to check your AC and DC voltages and do a signal trace to see where it disappears, or you might have built a giant oscillator. All have nothing to do with the meter.

Mark

 

[college101]

Ok..There is no dc voltages coming into the R6-R7 area...I kept reading .003 AC on my DMM  and If i went to DC on my Dmm, then it would just zero out

 

[kazper]

On the top of the voltage chart is the configuration of the controls to get the voltage readings. Don't miss that step or there not going to be close.

Now that I re looked at the chart/schematic... it's all AC that is derived from the supplied input signal.