All things LA2a related

[imo]

I am using the allied power transformer supplied with the Bloo kit(6K88VG). I have the whole unit hooked up, have gone through the wiring a couple of times and I am pretty confident that everything is where it should go. Upon powering up I noticed a funny smell, neon didn't light, so I turned everything off and checked connections again. I noticed that I was getting continuity between ground and the B+, measuring between turrets a 22 or 23 and ground. I disconnected the transformer and found that the continuity was coming from the transformer itself, between both (250v)red's and the red/yellow center tap, as well as between the greens(6.3) and the green yellow center tap. I had the power transformer hooked up as I think it was supposed to be, one black to turret and switch, other to neutral on the power inlet. One red to turret a22, other red to turret a23, green to a24 and a25, center taps of both to ground terminal between the 2 33u power caps.
Once I took the power transformer out everything else tested right for contintuity throughout the unit. I hooked the transformer up, this time only to the inlet and switch, and tried measuring AC volts between center tap of the 250 and either of the red wires. I got a negligible reading. Also the smell that I had smelled the first time was there, definitely coming from the trannie. It was really hot to the touch. I am thinking that somehow the transformer fried. Does this sound copasetic? I have built quite a bit of equipment, but generally non tube, using small toroidal transformers. I am a bit psyched out by this and wanted to make sure that I proceeded carefully. Any ideas or directions would be taken quite gladly
Peace
Ian

 

[Bluzzi]

I am not sure but I think continuety in a finished circuit doesn't indicate much other than ...well continuety. Great for simple wiring but not where current can flow all over.

Ian, its usually hard to fry a transformer unless you seriously short the outputs.

What are the voltages between the center tap of the 250v (red/yellow) and either red wire?

Also same between filament heater voltage (green/yellow) and each green?

Hope it checks out.

Also for better safety make sure you wire the HOT ac into the fuse. I made that mistake and had the neutral going to fuse. Meaning HOT was always there with potential (no pun) to cause problems with other gear.

I am also at the end as you can see. Hang in there.

jim

 

[imo]

Jim,
Thanks for the response, i should have been more clear. I was trying to look at the power transformer, as i am pretty sure that is the problem. I was getting continuity between the 250 leads and the center tap, as well as between the 6.3 leads and the respective center tap, even out of circuit.
I am pretty sure that there is no polarity on the primary side of the Allied transformer i am using. If there is, there is no marking at all, simply two black wires. I pulled the transformer completely out of circuit, only running it through a switch and IEC, tried to measure voltage, and was only able to get about 1-2 V AC between the two hot 250v leads. I was about to measure between the hot and the center tap when the transformer started smoking again. I am pretty sure it has thermalled or something, just not sure how it happened? It gets really hot really quickly. I used the Allied 6K88VG, which has two black primaries, a grey shield, two red 250v, two green 6.3's, a red/yellow center tap, and a green/yellow center tap. I tied both center taps to the ground buss, the two red's to the back side of the diodes, and the two greens attached to the heater supply.
I tested for continuity between both the B+ power and ground and the heaters and ground. Everything seemed to be properly isolated
Ian

 

[Bluzzi]

Ian, did you get your problem solved? I have the same transformer on my second LA2A that is not hooked up yet and I could test it to see if I get same readings.

jim

 

[imo]

Jim,
I am waiting on another power transformer. Short of a component failure i think everything in the unit is wired correctly and hopefully will work with the new tranformer. I appreciate your help and will post, hopefully tonight with some actual working voltage.
Ian

BTw, lost $50 to Albert Collins playing blackjack when i was 17<g>

 

[Bluzzi]

Ian,

I missed Albert Collins two years in a row thinking I'd catch him. He passed away the third. What a fool I was. One of the most underrated Blues guitar stylists that was. You are indeed a lucky man to have lost $50 to him. I would have lost a lot more just to be able to jam with him, or play cards.

Have you checked any of the components for failure? You are sure the diodes are on the right way? I'm sure they are but you never know as you suspect transformer failure which I still think is highly unlikely (unless you had a real solid short before the fuse).

I am going to try posting a seperate message for my problem as I'm not getting any responses (my meter needle is oscillating in -10 and +4 mode).

jim

 

[Bluzzi]

Ok, here I go again, somebody p l e a s e.....HELP! :sad:

I'm totally baffled. Probably something totally idiotic on my part but I can't find it. Checked my wiring and it all looks good and solid. At least nothing blew up or burst into flames. Even .5A fuse never blew.

Problem -
Meter is still showing same symptoms (needle wobbles when in -10 or +4 mode).

I've switched to another meter and same thing occurs. I've taken some voltage readings as follows.

All tubes are in. T4B out.

Here are voltages so far.

Heaters: 6.4

Turret board:

A14 = 250
A19 = 123
A20 = 132
B20 = 265
B21 = 354

V1:
1 = 102
3 = 1
6 = 102.6
8 = 1.23

V2:
1 = 58
3 = 3.6
6 = 248
7 = 58
8 = 100

V3:
1 = 72.2
3 = .834

V4:
2 = 6.41
5 = 124.3
6 = 116.6

Pin 1 of V2 seems low and pin 6 of V2 looks high. The other voltages seem close enough to Dave Jahnsen's book. I used Cayocosta's layout.

Any help would really, really , really be appreciated.

Thanks.

jim

 

[AnalogPackrat]

Maybe it's oscillating or motorboating. Do you have a scope? Can you take a look at the output? Did you use a Sowter output transformer? If so, you probably need 1k grid stoppers (in series) on the 12BH7 grids.

Good luck,
A P

 

[Bluzzi]

Motorboating is what it looks like. I haven't tried to put a signal into it yet and that was my next step after trying another meter.

I don't have a scope yet but I can get hold of one as a last resort.

I am using A10 and A24 transformers.

Thanks for the good luck wishes. I know its not easy to help in this way and I appreciate you trying.

jim

 

[Bluzzi]

I changed all the tubes to rule out any bad tubes and still the same effect.

I tried plugging in a microphone (I heard the LA2A has enough gain to drive one) and the meter is not affected at all whatever level I set.

I think at this point I will have to re-verify my wiring entirely again as well as parts values. I took it for granted that resistor and capacitor values were as per label.

From what I've read so far about motorboating it can be caused by insuficient power supply filtering. I'll start by verifying the 30uf and 40uf caps in the power supply were wired correctly, although voltages check out so I doubt they are bad.

My front panel is detached and the wires leading up to it a bit longer, could that be the problem? I've tried moving all the wires around without any effect.

Anyone have any ideas here? Would pictures help? Thanks.

jim

 

[AnalogPackrat]

A couple of photos might help. Some of the eagle eyed might spot something. What is the frequency of the meter movement? Does it just move evenly and smoothly (like a sinewave thing) or does it jump up and down (like a square wave)? Does anything you do change the frequency of the movement?

I wonder if you have a node somewhere that's supposed to be grounded that is floating. Did you use star washers under your ground lugs to the chassis?

A P

 

[Bluzzi]

The meter moves about 2 or 3 swings per second mostly but...yes it does change when I start to increase or decrease the Gain.

I have a from panel that is seperate from the back panel. They are both aluninum and All ground points have star washers. But I will check my grounds again, you never know.

I am going to be re-checking all my wiring again when I have some uninterupted time. If that checks out then I will check out all component values just in case I received a wrong resistor.

Thanks AnalogPackrat your help is much appreciated.

There is a lesson here for me to learn. I just can't see it by myself.

jim

 

[John Peacock]

Hello all.

I am a total noob. I just completed my BLOO, and using this forum I have managed to change it from a fuse blowing machine into something that actually powers up and passes audio. Still some issues though...

1. I have a Hoyt meter installed and am trying to hook up the lamp. It looks like I just take some wires off A24 and A25 (tube heaters). But how do I mount the lamp for the Hoyt? I also have a lot of other wires that pass on top of the VU. Do I need to be concerned about noise from the VU bulb and its wires?

2. With tubes in:
A24 A25 = 6.53V AC
B21 = 346V
B20 = 267V
A1 = 56V
A20 = 97V
A14 = 222V
A17 = 95V
A19 = 122V

The only one of major concern for me is 97V for A20 (back side of R34). However with my intial testing I was getting 81V there, so I replaced the resistor I was using (it said it was 22K, but it was measuring almost 25, so I replaced it with a Radio Crack Resistor that measures around 21.7) and got A20 up to 97V. However, this is still low. Also, when I changed that resistor, A19 went from 112V to 122V. This doesn't concern me too much, but A20 is still quite a bit off from the recommended 131V. Thoughts?

3. Since things seemed "close enough," I popped in the TB4 and ran some audio. The good news is that audio passes. And until I crank peak reduction up to about ninety, it sounds very clean. Then around ninety it begins to distort just a bit. The problem is that it is NOT compressing. At all. As I increase the peak reduction, nothing changes except for that distortion around 90. TB4 problem? Wiring?

4. And finally, when I turn peak reduction up to 90, I get distortion when in compress mode but not in limit mode. This happens whether I am in +10, +4 or Gain Reduction. Could this possibly be normal?

Sorry for the long post, and thanks so much for any help.

-Jp

 

[John Peacock]

Just to update, I did a check on my TB4 using a flashlight in a dark room, and I get tons of gain reduction that way. So I am guessing I either have a bad electro-fluorescent panel, or I have some bad wiring going to (or inside) the TB4.

Thoughts on how to confirm this? Other things I should look for?

-Jp

 

[John Peacock]

Okay... I'm feeling very close now. So I found at least one of the problems. The wire that eventually goes to pin three on the TB4 socket was connected to pin 5 instead of pin 6 of V4 (6AQ5A).

I think that should ideally fix the "lack of compression" problem, but now whenever I turn my gain reduction past 20 (or basically whenever the unit begins to compress the audio) I get this aweful sizzling distortion with lots of HF content. This is true whether I am monitoring +10, +4 or in gain reduction mode. This also pins my meters. This isn't what the grid resistors are supposed to correct? Seems more like it might be a bad tube... But then it wouldn't only do this at higher gain reductions, right? I really don't know... This is all very new to me.

Did I possible fry something by having that wire from pin 3 of the TB4 connected to the heater wire?

Oh, and this correction did not correct my low voltage at A20. It has actually now gone down to 95.5V instead of the recommended 131V. I'm still not overly worried about this one, but maybe I should be.

Anyone?

-Jp

 

[John Peacock]

I've added the grid stoppers (1K Resistors) to pins two and seven of the 12BH7A. No difference. Still getting aweful HF distortion when the unit starts to compress.

I also tried running audio while leaving the lid off of the TB4 unit. Using a flashlight I was able to get the meter to show -20dB, yet the audio running through the unit did not drop at all. That doesn't seem right...

I feel like I've done a pretty good job of trouble shooting on my own, but I'm running out of ideas.

Help.

Please.

-Jp

 

[John Peacock]

It's been a couple days. I did manage to find one other small mistake with the placement of R6 around the input transformer, but this correction has had no impact on the unit's performance. Still lots of HF noise whenever I try to compress. Also using a flashlight to trigger the TB4, I was able to detect gain reduction using an external metering source, though nowhere near the 15-20dB displayed on the LA2A meter.

My A20 voltage is still very low, and I suspect this may be part of my problem, but I don't know. That's why I am asking for your help.

Please!

-Jp

 

[Bluzzi]

John, I feel for you my friend. As you see I too have some problems so I am not in a perfect position to help you but I can try as best I can.

I'll be borrowing an oscilloscope this coming week and I am sure I'll pin down the problem. So hopefully I'll be in a position to help you.

I built it acording to Cayocosta's layout which I believe is very close to your Blue kit.

Have you tried to reach Scenaria privately? Or CJ.

Its also summer and so maybe people are taking a break, or maybe they are just sick of answering questions. Not an easy thing to do this thing of trying to pinpoint problems through words.

In the meantime if there is a wy for me to help you let me know. I'll do my best.

Hang in there.

jim

 

[AnalogPackrat]

[quote author="Bluzzi"]The meter moves about 2 or 3 swings per second mostly but...yes it does change when I start to increase or decrease the Gain.[/quote]

Does the meter movement change frequency or amplitude when you change the gain? It sounds like your problem is in the audio amp--V1 and V2 somewhere. Carefully check the feedback stuff: R11, R21, R22, and C4. What did you use for C4?

I have a from panel that is seperate from the back panel. They are both aluninum and All ground points have star washers.

Is there continuity between the front panel and the rest of the chassis?

If that checks out then I will check out all component values just in case I received a wrong resistor.

I don't know if you did this, but try to get in the habit of checking component values as you build (just before soldering the thing, measure it). This forces you to think one more time about what you are doing as well as giving you that sanity check in case of a bad component.

Thanks AnalogPackrat your help is much appreciated.

No problem, Jim. I hope you can get your build up and running soon.

A P

 

[AnalogPackrat]

[quote author="John Peacock"]It's been a couple days. I did manage to find one other small mistake with the placement of R6 around the input transformer, but this correction has had no impact on the unit's performance. Still lots of HF noise whenever I try to compress. Also using a flashlight to trigger the TB4, I was able to detect gain reduction using an external metering source, though nowhere near the 15-20dB displayed on the LA2A meter.

My A20 voltage is still very low, and I suspect this may be part of my problem, but I don't know. That's why I am asking for your help.

Please!

-Jp[/quote]

If you can pass audio without GR, it sounds like your problem is not related to V1 and V2 circuitry.

Pop the cover off the T4B, hook up a signal of some sort to the input, turn off the lights and look at the EL panel. Does it change brightness with the input signal strength (it's not really all that bright, so make sure you're room is dark)? If it's pulsing in brightness with the music/signal, then your sidechain is probably working. Perhaps the wiring on your T4B or its socket has a small mistake. Did you wire the "X" and solder the point where the two wires that form the "X" cross? Is the X grounded at some point?

A P

A P