All things LA2a related

[Paul B]

OK, other Paul character is back.

[quote author="CJ"]Voltages look good on your 6AQ5 output!

Something is fishy on you input circuit. The Sowter has a 1:4 ratio, so you should be seeing an increase in ac volts as your source goes thru the input transformer. Also, if using an XLR jack, you measure your signal between pins 2 and 3. Pin one is usually the shield.

Try ditching the DI box and just hook up a cd signal straight into the LA2. Measure input vs output on bpth sides of the input transformer. Also make sure your lim/comp sw is wired right.

cj[/quote]

I wired the input straight into the bloo. The input signal measred an avg. of about 300-320 mV with peaks at about 450. This is about the strongest signal I can put into it without it distorting at the output, even at the lowest levels. By the time I turn the gain to 10 it is already about 3 times as loud as not running throught the bloo, and starts distorting at about 20.

With this 300-320 mv avg, I was getting a reading of about 1100mV average at the R5-R6 junction (w/ the blue sowter input tranny wire), with peaks at about 1640mv.

Using a 313mV strong 1k sine wave straight into the bloo, I got a reading of 1153mV at R5-R6. So it seems Im getting more like 3.7:1 ratio rather than 4:1.

with that 313mv sine wave I got these readings also:

495 mV at R6-R7 (insulated turret) and at top of R2
114VAC on both sides of C9 with PR at 100, 58.4V w/ PR at 50

as far as output, with these gain readings I got the following output readings at the output jack:


gain at 10: 215 mV
15: 712mv
20: 1268mV (reading 0db on the VU meter)
25: 1672mV
30: 2.6V
The VU meter was pegging by the time I got to 30 with the sine wave so I didnt test it any higher than that.

With the signal hooked straight into the bloo, it seems to only lower the distortion threshold on the gain knob.

thanks,

Paul

 

[wilebee]

hey there everyone la2a #2 is up and running passing signal and has a few quarks I doesnt seem to be getting a signal at the floresent panel optos ae working because when I take the housing off the t4b the gain goes down but nothing when its on at hitting it with +4 @1k. also It doesn't seem to be regestering at +4 or +10 setting.

1st what should I be seeing at the t4b feeding the floresent panel ?

any thoughts on the +4/+10 meter setting ?

Wil

Wilebee

 

[Scenaria]

hey wil,

im off to watch some football BUT

you have a good situation there.. in that you have one that works that you can take some measurments off of...

thats where I would start

 

[wilebee]

We'll that figures football

 

[wilebee]

ok Wilebees advice for the day dont trust that someone elses transformer jumpers are good ! that being said I've got my levels to +4 and +10 now that I'm using my entire Secondary. But I still don't have gain reduction meter deflection any thoughts anyone ? Wil

 

[AnalogPackrat]

Wil,

If you have two and one works, follow Scenaria's advice. You say you've got the output metering working, but are you getting any gain reduction on the output signal? Does your EL panel blink with the music? If it still doesn't then your meter isn't going to read anything GR since there is no GR happening if the EL panel doesn't light up.

If the problem is the EL panel, then you're going to have to start comparing voltages around V3 and V4. What are your DC voltages (pg 25 of the Bloo manual)?

A P

 

[AnalogPackrat]

[quote author="Paul B"]OK, other Paul character is back.

I wired the input straight into the bloo. The input signal measred an avg. of about 300-320 mV with peaks at about 450. This is about the strongest signal I can put into it without it distorting at the output, even at the lowest levels. By the time I turn the gain to 10 it is already about 3 times as loud as not running throught the bloo, and starts distorting at about 20.[/quote]

That's strange and seems to contradict your measurements with the sine wave input where you put in 300mV and got out 215mV with gain at 10. Out of curiousity, what pot (brand, type) did you use for gain? Is it linear or log (audio) taper?

Using a 313mV strong 1k sine wave straight into the bloo, I got a reading of 1153mV at R5-R6. So it seems Im getting more like 3.7:1 ratio rather than 4:1.

with that 313mv sine wave I got these readings also:

495 mV at R6-R7 (insulated turret) and at top of R2
114VAC on both sides of C9 with PR at 100, 58.4V w/ PR at 50

as far as output, with these gain readings I got the following output readings at the output jack:


gain at 10: 215 mV
15: 712mv
20: 1268mV (reading 0db on the VU meter)
25: 1672mV
30: 2.6V
The VU meter was pegging by the time I got to 30 with the sine wave so I didnt test it any higher than that.

With the signal hooked straight into the bloo, it seems to only lower the distortion threshold on the gain knob.

You mean that changing PR made the distortion worse? What happens to your output levels at PR 50 and PR 100 for 300mV sine in and gain set at 10?

A P

p.s. What pot did you use for PR?

 

[wilebee]

nope voltages are al right on and the t4b works in the other la2a I'm quite stumped !

 

[AnalogPackrat]

Wil,
You compared DC voltages at all nodes around V3 and V4 and at the T4B socket? They ALL match between the two units? Carefully compare all of the wiring in those areas. Try swapping T4Bs between them. Something is different or they would both behave.

A P

 

[wilebee]

I've done that I think ill give it the night off re attack it tommorrow

 

[Paul B]

Hi AP,

the pots for R1 and R2 are the alpha 100k log. they "ohmed out" just fine before I put them in - about 102k for R1&2 and 998 for R3, 4 and 37. they are all grounded fine. let me know if you think I should change them.

with the 313mV sine wave in and the gain set at 10 and getting the 215 mV at the output, the VU meter is barely registering. And it is already about 2 times as loud as compared to bypassing the Bloo.

 

[AnalogPackrat]

Strange...what does the output measure with your DMM on ACV? Does the output change at all when you turn up the PR?

 

[Paul B]

gain at 10: 215 mV
15: 712mv
20: 1268mV (reading 0db on the VU meter)
25: 1672mV
30: 2.6V
The VU meter was pegging by the time I got to 30 with the sine wave so I didnt test it any higher than that.


thanks,

Paul[/quote]

these were all AC readings at the output jack.

 

[AnalogPackrat]

Right. So hook up your meter at the output and set the gain at 10 or 15 and fiddle with the PR knob. Does anything change when you do that? It seems like things are working pretty well for the most part.

 

[Paul B]

Ok now I understand - Ive been in "no t4b mode" for a while. With my 313 sine wave in, and gain at 15, I get 795mV at the output and -4db on the VU. With t4b in, when I raise PR to 61 I get half voltage, about 655mV. Sure enough, VU was down 6db at -10.

Raising the gain to 20 where I get 0db on the VU, I get 1295mV at the output. Turning PR to 61, I get half voltage (655) and -6 on the VU. At PR = 71 I get another half in voltage (325) and about -12 on the VU. With PR at 100 I get 172mV at the output, so the total gain reduction is less than 18db.

Problem is, that 313 mv input gives me a very distorted signal on the output. Everything seems to be fine except that I can' t put in a strong enough input signal to both drive the t4b and not get a distorted sound at the output.

I find myself suspecting C5. I will run out later this morning and replace that sucker.

thanks again for your help.

 

[AnalogPackrat]

I'll try to dig some of my test equipment out this week and reproduce your set up for comparison. Can you describe the distortion you're hearing? Does it sound like an overdriven guitar amp or is more like a ring modulator? Is it buzzy? Just trying to figure out if it's really the audio path being overdriven or something else.

Have you ever used an oscilloscope? Do you think you could find a way to borrow one (and maybe some of the owner's help, too)? I think it would really help to have a look at the signal to see where things are going wrong...

A P

 

[CJ]

I will do a complet ac and dc voltage chart on an LA2 schematic with a given input and GR on 5. This would be a good referance to have here and might be all that you need to troubleshoot.
Paul, time to get out the highliter pen as a last resort. It's not that tough, give it a shot.
cj

 

[Paul B]

AP,
I would describe the distortion as "puffy" and "splatty".
I will see what I can do about the O-scope.

CJ,
I've already traced out the schematic with a highlighter. I didnt find anything wrong.

I replaced C5 with a 22uf 450V. The output is cleaner with less distortion at higher levels. My other problems remain, tho. I will wait to see what readings you and AP come up with. If that doesnt narrow it down, then I'm looking at a complete re-work on the thing.

 

[CJ]

You can also ohm out the circuit to make sure there isn't a bad joint somewhere. (Remember when a joint was a bad place to hang out?)
You are esentially tracing the schematic with the ohmeter instead of the highliter. Sometimes I have turned up wires that I forgot to solder on tube sockets and things.
At least you will know every nook and cranny on this thing. So the next one will take ten minutes to fix.

 

[AnalogPackrat]

Paul,

So the distortion sounds similar both with the T4b in and with it out? That is, the T4b doesn't change the character of the distortion, right? If so, it seems like something is awry in your audio path (maybe a bad joint [seeds?] as CJ pointed out). What kind of caps did you use for C1, C2, and C3? Can you make the distortion go away with really low gain settings or is it always there?

A P