All things LA2a related

[AnalogPackrat]

Jp,

Something in your sidechain is oscillating for sure. The EL panel only responds to AC, so the sidechain circuit has to be making it. I suggest you print out the circuit diagram and recheck your wiring on related to V3, V4, and the T4b. Get a hilighter and mark every component and wire as you check it in your circuit. That way you'll know you checked it all and it will be obvious if anything is missing.

If you want a quick sanity check, turn the GR all the way down and post your DC voltages on V3 and V4. Maybe something obvious will pop out, but it may be that it's a wire placement issue that's generating positive feedback. You could try CJ's technique of using a wooden chopstick to physically move wires around while the unit is on (and oscillating) to see if you can change the oscillation or make it stop. If you move something and it changes the oscillation, you've found part of the problem.

Is your wiring pretty neat? Did you use shielded wire (or twisted pairs) for the front panel controls that carry audio? Got an o'scope?

Good luck,
A P

 

[AnalogPackrat]

[quote author="Bluzzi"]I didn't get a chance to try the 12AY7 sub yet but I have a double question about R7.

This is the resistor marked 1k to 2.7k. So what determines its value and what effect does it have?[/quote]

Read the note at the bottom left on the schematic. It was selected to help match the CdS cell response. I can't recall what I used there. maybe pick something in the middle 1k5 or 2k or something. If you're not trying to make a matched pair, it probably doesn't matter much.

A P

 

[John Peacock]

[quote author="AnalogPackrat"]Jp,

Is your wiring pretty neat? Did you use shielded wire (or twisted pairs) for the front panel controls that carry audio? Got an o'scope?

Good luck,
A P[/quote]

Thanks for the reply and suggestions. My wiring is pretty neat, and I did use sheilded wire for the front panel controls. No scope currently, though I might be able to track one down if it comes to that.

Meanwhile, I'll retrace the wiring once again and try the chopsticks.

Scenaria thought perhaps a bad tube. I might have 12ax7 lying around, but no extra 12bh7a. Any thoughts on that idea?

Thanks.

 

[AnalogPackrat]

Maybe on the bad tube. It would be the second 12AX7 or the 6AQ5, though. I think we can safely rule out the 12BH7 since it passes nice audio when the sidechain is out of the picture (GR = 0). Got another 6AQ5?

 

[John Peacock]

[quote author="AnalogPackrat"]Maybe on the bad tube. It would be the second 12AX7 or the 6AQ5, though. I think we can safely rule out the 12BH7 since it passes nice audio when the sidechain is out of the picture (GR = 0). Got another 6AQ5?[/quote]

I can do you one better. I've got a working LA2A compressor! Chalk another one up on the board, Packrat. Your solution was actually last page. I had the "cross" on the TB4 circuit, all soldered correctly, but no ground. I went back and looked all over for that in the BLOO manual. Couldn't find it... Checked it on the schematic, and saw it there. Thought what the hell, and I tried it. Fixed. (Though not before miss wiring the unit it into my patchbay and thinking the output was now no longer working... Argh!)

THANK YOU!!

I still am measuring LOW voltage coming from the back side of R34, but it's working (I'm getting around 81VDC and it should be 131VDC). If all of the other voltages are fine and the unit is working, should I be concerned?

 

[CJ]

Good going!

Just make R7 2.7 K and don't worry about it.

whats your B+ like on the other side of the 81 volt plate resistor?

 

[AnalogPackrat]

[quote author="John Peacock"][quote author="AnalogPackrat"]Maybe on the bad tube. It would be the second 12AX7 or the 6AQ5, though. I think we can safely rule out the 12BH7 since it passes nice audio when the sidechain is out of the picture (GR = 0). Got another 6AQ5?[/quote]

I can do you one better. I've got a working LA2A compressor! Chalk another one up on the board, Packrat. Your solution was actually last page. I had the "cross" on the TB4 circuit, all soldered correctly, but no ground. I went back and looked all over for that in the BLOO manual. Couldn't find it... Checked it on the schematic, and saw it there. Thought what the hell, and I tried it. Fixed. (Though not before miss wiring the unit it into my patchbay and thinking the output was now no longer working... Argh!)

THANK YOU!!
[/quote]

Great! You're welcome! :thumb: :thumb:

I still am measuring LOW voltage coming from the back side of R34, but it's working (I'm getting around 81VDC and it should be 131VDC). If all of the other voltages are fine and the unit is working, should I be concerned?

So, did you build the early version or the late version of the V4 circuit? I think the Bloo manual is a little vague on this part. The schematic in the manual is the early version, but parts of the manual test refer to the later one. It gets confusing. Basically, do you have an R38 (22k) in your build? And a C13 (510pF)? If so, that's the later version and you should have R34 of 10k and R36 of 470R. The early version has no R38 and no C13 and uses 22k for R34 and 1k for R36. Check that...

And if you get the voltage CJ requested we can figure the current draw of V4 and see if it makes sense.

A P

 

[John Peacock]

[quote author="AnalogPackrat"]The early version has no R38 and no C13 and uses 22k for R34 and 1k for R36. Check that...[/quote]

I'm an early version with R34 as a 22k. But I do have a C13 in there... Hmmm.

[quote author="AnalogPackrat"]And if you get the voltage CJ requested we can figure the current draw of V4 and see if it makes sense.

A P[/quote]

Which pins do I measure to get that? Is B+ the front side of R34 (or Turret Board B20 in "BLOO speak")? I get 267VDC there.

Thanks.

-Jp

 

[AnalogPackrat]

Just measure the other side of R34. I can't remember where it is on the Bloo.

 

[John Peacock]

[quote author="AnalogPackrat"]Just measure the other side of R34. I can't remember where it is on the Bloo.[/quote]

267VDC

 

[AnalogPackrat]

Also, be sure you've got R3 and R37 wired right--easy to get them backwards. You've got them turned all the way, uh, which ever way the manual says, correct?

 

[John Peacock]

[quote author="CJ"]Good going!

Just make R7 2.7 K and don't worry about it. [/quote]

Cool. That's what I've got.

[quote author="CJ"]whats your B+ like on the other side of the 81 volt plate resistor?[/quote]

267VDC.

Thanks, CJ. Your previous posts in this thread were a big help with the trouble shooting I did before I started posting.

-Jp

 

[John Peacock]

[quote author="AnalogPackrat"]Also, be sure you've got R3 and R37 wired right--easy to get them backwards. You've got them turned all the way, uh, which ever way the manual says, correct?[/quote]

No mix up with the wiring of R3 and R37.

I originally had them both set where ever it was recommended (I think counter clockwise). Then I read earlier in this thread that someone was having similar problems with their R34 voltage and I think Scenaria recommended that R3 be moved more towards the middle. I tried this and it maybe gave me a volt or two.

My R34 measures rather high (it's supposed to be 22 but is pretty close to 25k), and when I swapped it out with a better 22k (only a .5 watt resistor), it got me up to about 95VDC. But that is still no 131VDC. I eventually went back to the original high resistor because it was the only one I had that was rated for 2W.

Also my C13 is actually 560pf instead of 510. Why? Because it is what came in the box. I figured close enough.

So should I get online and get these values corrected? I probably will anyway, but I can't figure that they are making that much of a difference in the operation as is.

-Jp

 

[AnalogPackrat]

Well, technically, if you're making the early type you shouldn't have C13 and R38 at all. But I think CJ has said that it doesn't really matter.

Here's the deal on V4. You're supposed to get about 135V at the plate. Recalling Mr. Ohm's law:

V = IR or I = V/R

We know the voltage across R34 is 267-81 = 186V and it's resistance is, you say, 25k, so I = 186V/25000 = 7.44mA.

You're supposed to get something closer to 275-135 = 140V / 22000 = 6.4mA.

So yours is drawing a little more current than it should. Is R36 1k? If so, I would close that thing up and use it. I think you're done.

A P

 

[John Peacock]

[quote author="AnalogPackrat"]
So yours is drawing a little more current than it should. Is R36 1k? If so, I would close that thing up and use it. I think you're done.

A P[/quote]

Now that is the best advice I've gotten yet!

R36 is 1k. So...

Done and done.

Thanks so much.

Pictures soon. Same as everyone else's, except this time it will be mine.

-Jp

 

[The Kid]

it got me up to about 95k. But that is still no 131k. I eventually went back to the original high resistor because it was the only one I had that was rated for 2W.

Umm, you mean 95 VOLTS and 131 VOLTS, right? :wink:

You're supposed to get something closer to 275-135 = 140V / 22000 = 6.4mA.

You have the same problem as Happy Tom (starting on page 88 of this thread). As I said before, that 131~135 volts only applies to the 10K/470, not the 22K/1K. Basic electronics. Bigger resistor = lower current = bigger voltage drop. Sounds like that kit might need a revision! (Not a good idea to combine schemos!)

 

[John Peacock]

[quote author="The Kid"]
Umm, you mean 95 VOLTS and 131 VOLTS, right? :wink:
[/quote]

Yeah... It was getting late.

 

[John Peacock]

I've created sort of a photo journal and put it online:

http://www.flickr.com/photos/35001164@N00/192632697/in/set-72157594203518828/

Let me know if the link doesn't work for everyone.

NOTE TO ANY FUTURE NEWBIES: THERE ARE MISTAKES IN SOME OF THESE PICTURES!!! :shock: If you scroll through the pictures one by one, it will give you a caption for each (this is not there if you select slide show), and I tried to point out any mistakes that are in each photo (there are only a few). Extra credit for the old timers who can catch them all. :thumb:

Also, for those who are only interested in mocking my wiring, jump right to the last six photos. :wink:

-Jp

 

[AnalogPackrat]

Looks good, John. The only thing I saw that made me nervous was that you didn't use any grommets or heatshrink to protect your audio transformer wires where they pass through the mounting plates into the chassis. I would consider going back and protecting those somehow. The Sowter wire is small and has very soft insulation if I recall correctly. Other than that--very nice job!

A P

p.s. Your bench is way too neat, though. How can you get anything done on such a clean work surface? :razz:

 

[John Peacock]

[quote author="AnalogPackrat"]Looks good, John. The only thing I saw that made me nervous was that you didn't use any grommets or heatshrink to protect your audio transformer wires where they pass through the mounting plates into the chassis.:[/quote]

I actually did that, but not until after the "solo" pictures of the transformers were taken. And it's clear shrink tube that I used, so it may not even show up well in the later pictures.

[quote author="AnalogPackrat"]Your bench is way too neat, though. How can you get anything done on such a clean work surface? :razz:[/quote]

Yeah... I'm still new to all of this. Give me some time. :wink: Actually, it was a mess before each shot, and I would just push everything over to the side right before each shot. So not actually neat, just very anal.

Thanks for the comments (and getting it working!).

-Jp