All things LA2a related

[JW]

Anybody know if I can use a Yamaha PM1000 meter for the LA2A. I assume I can because I can use it for the 1176, and I don't think they're that different. (?)

How do I go about making sure it will work? I can't even remember what I did in my 1176, but I'm using one there and I have a resistor in series to the meter. I assume this was to make sure it was exactly 0VU at +4dB.

Can someone remind remind me what I need to do to make sure it will work?

 

[weiss]

Hey guys, finally i am coming closer to finish my build  8)

a while back i bought a couple of parts for the la2a from a member here on gdiy.. does anyone have a clue on how to wire this can cap for the p2p build? (https://www.tubesandmore.com/products/capacitor-ce-mfg-450v-40403030-f-electrolytic)

second question regarding my 1009 sowter wiring, is it correct that the corresponding pins 2 + 5 of the actual a-10 are just left out?

thanks in advance!
weiss

 

[kosi]

I guess, you know the cayacosta layout ?
Follow this one for the plus poles (the 4 pins), make sure, that you connect the housing of the capacitor to ground. That's it.

1009: , pins 2 and 5 are not connected.
Left is HA100X, right 1009
1 = 1
6 = 3
7 = 4
g/10 = 6

 

[weiss]

I guess, you know the cayacosta layout ?
Follow this one for the plus poles (the 4 pins), make sure, that you connect the housing of the capacitor to ground. That's it.

1009: , pins 2 and 5 are not connected.
Left is HA100X, right 1009
1 = 1
6 = 3
7 = 4
g/10 = 6

thx for your explanation, kosi! what i was looking for 

 

[sr1200]

I'm gonna start losing some hair over this...  one of my LA2A p2p builds all of a sudden has a pretty annoying 60 cycle hum.  Compressor works perfectly as far as passing audio, compressing.  All voltages seem to be within range.  The only thing i havent tried is to replace the big caps in the power section.  (the compressor build isn't that old, but I guess things breakdown some times). 

Any other suggestions as far as maybe trying to ground anything out or move anything around?

 

[dbonin]

power filter caps - first place to look for 60 cycle hum?

 

[sr1200]

Figured as much  :-[

A guy can hope....  as in I hope i have some in the shop to replace them. Hate having to order $10 worth of parts with $7 for shipping.  such is life.

 

[andyfromdenver]

I'm gonna start losing some hair over this...  one of my LA2A p2p builds all of a sudden has a pretty annoying 60 cycle hum.  Compressor works perfectly as far as passing audio, compressing.  All voltages seem to be within range.  The only thing i havent tried is to replace the big caps in the power section.  (the compressor build isn't that old, but I guess things breakdown some times). 

Any other suggestions as far as maybe trying to ground anything out or move anything around?

"all of a sudden" makes me think in no particular order: some flux-line problem with another unit's power transformer, tube heater center tap ground is loose, perhaps a wire sagged close to a noisy heater wire, perhaps a tube went noisy.

450V new-ish pwr caps should be good, but that's an easy check too, supposing you have stock.

have you isolated the unit from other gear and tested? that would help with the first point. can you give a "what the heck" tightening down of all your critical ground points? can you move your heater wires or the sensitive signal wires going to V1, like the wiper of the gain control to 12ax7 pin 2? can you sub out the tubes, start by simply pulling V1 and listen if the hum vanishes.

Andy

 

[sr1200]

Andy,
thanks for the reply.  I just ordered replacements for the filter caps (thought i had em... i dont).  While i was taking them out of the unit one of the legs felt a little loose, heres to hoping that its a quick fix. 
As far as the heater wires go, everything is in tight (no wiggle room on the twisted pairs).  I did re-tighten down the star ground.

Once i get the new filter caps, if i still have an issue, I'll try and pull the tube.  I did notice the 12BH7 was getting exceptionally hot.  I did recently replace that tube (within a few months).

 

[sr1200]

Replaced the caps... didnt fix it.

So my 6aq5a pin 5 im seeing high voltage. The only thing in there is the 10k 2w r34 thats dropping my B+ from 278 to 178.

On my 12bh on pins 6,7,8 im seeing about 20v higher than what is supposed to be there so approx 220, 85 and 120 respectively.

Pin 6 on the first 12ax7 (below t4b) is about 128v which is kind of high also.

Is my b+ too high?  Pin 6 on the 12bh pin5 on the  6aq both only have one component between them and b+.

 

[andyfromdenver]

Replaced the caps... didnt fix it.

So my 6aq5a pin 5 im seeing high voltage. The only thing in there is the 10k 2w r34 thats dropping my B+ from 278 to 178.

On my 12bh on pins 6,7,8 im seeing about 20v higher than what is supposed to be there so approx 220, 85 and 120 respectively.

Pin 6 on the first 12ax7 (below t4b) is about 128v which is kind of high also.

Is my b+ too high?  Pin 6 on the 12bh pin5 on the  6aq both only have one component between them and b+.

voltages seem ok to me. what's pin 1 of the 12BH7? R15 and R18 are definitely correct values? If you have clip leads, ground pin 7 of V1 (the second gain stage before the output tube), see if the hum goes away. Think maybe a shield on either in/out tx is disconnected? did you pull V1 and see if it quiets, too?

 

[sr1200]

Pin 2 on the 12BH is around 105 (trying to remember before it was slightly higher than the reference sheet i have says 92)

Grounded pin 1 on the 12ax7 noise got worse, took the tube out altogether and the hum was still there.

EDIT:  The plot thickens... im not getting a lot of compression.  It IS compressing but not like it should be...  I normally just barely touch it with GR so I don't know if this has been ongoing or just started with the 60hz issue.

ALSO... i noticed with the unit powered off, im still seeing 60hz (way lower) on the output.  Bad ground somewhere or could this be the transformer?

After looking at it further, its passing signal when its off as well.

 

[andyfromdenver]

Pin 2 on the 12BH is around 105 (trying to remember before it was slightly higher than the reference sheet i have says 92)

Grounded pin 1 on the 12ax7 noise got worse, took the tube out altogether and the hum was still there.

EDIT:  The plot thickens... im not getting a lot of compression.  It IS compressing but not like it should be...  I normally just barely touch it with GR so I don't know if this has been ongoing or just started with the 60hz issue.

ALSO... i noticed with the unit powered off, im still seeing 60hz (way lower) on the output.  Bad ground somewhere or could this be the transformer?

After looking at it further, its passing signal when its off as well.

I promise I am not trying to be mean, but you grounded pin one of your 12ax7? the plate resistor? I am guessing you meant to type 7. neither of those pins are the ones I mentioned. 
Hope you solve the hum keep at it and you'll find it out, or take it to someone, it's an amazing box as you know and should operate as a quiet and mega boutique piece of studio gear.
ps pin two of the 12BH7 shouldn't read any voltage except perhaps a tiny fraction.
If you're picking up hum and the unit is off, it HAS to be interference from something reaching your output transformer.  Did you use a shielded OT? I am really not into the Edcor little unshielded guy, it takes really carefully positioning for it to not get induced by the power transformer (PT).
I wish you the best with it!
Sincerely,
Andy

and I'll edit perhaps the schematic you are going by swapped 678 and 123 of your 12bh7 from every schematic I look at. My 12bh7 pin 7 follows coupling cap C2

 

[sr1200]

you're correct, i fat fingered it.  was pin 7 with the cap strapped from it to pin 1 (C1 i believe) as well as a 470k resistor heading to ground.

  I have 2 of these built around the same time from the same parts list... only major difference is the case.  Transformer wise i used the hammond 369jx.  For i/o transformers i used Cinemags.  Unit 1 is quiet and one of the best vocal compressors i've ever owned/heard.  Unit 2 used to be a little noiser but has a little thicker tone on the bottom end. (by noise, not a 60 cycle thing but just a little more hiss if you cranked the hay out of the output)

my 12bh7 pin 7 is going to C2 and R15

thanks for the help i appreciate it... hope i dont have to break this thing down and start again, I thought i did a pretty decent job on the wiring.

 

[kosi]

I once had a 12AX7 in my LA2A, which caused 50Hz hum.  It was a JJ. I replaced it and that solved the problem. Can't remember wether it was V1 or V3... Maybe swap them both with the working one ?

 

[sr1200]

still no luck on this... 

I did notice that my heaters have continuity to ground.  Is this correct?  I have the hammond 369jx in there  the two solid greens are going to heaters and my CT for the 6.3v (green/yellow) is going to ground.

I've swapped tubes.  I fixed what i thought was an error near r30 and c6 but turns out it was right, just wound up cleaning it up a bit.

 

[sr1200]

Anyone in the NY area care to take a crack at this?  Out of the dozens of projects i've built, this has to be the one to get me.

 

[sr1200]

did some more tracing... found that at v1, pin 1, clean signal, pin 6 i have the 60hz hum introduced.  tried swapping out tubes, no good.  Anyone have thoughts on the 6.3v having continuity to ground?

 

[sonicrevolution]

Good evening everybody,

Where to begin,

First of all, I have read through this forum about 2, oh, maybe 3 times, definitely 2 solid times, and a 3rd skimming.  I am very grateful for the wealth of information that has been contributed throughout this thread over the years, and hence, why I am a bit reluctant that my first post is one in need.  Thank you to all who have made it possible for me to get this far.

Now, for the fun stuff,

I am building a PTP la2a based on the 68' schematic.  I am using original UTC A10 and A24 transformers, as well as a reissue R4A power transformer.  I made sure to tirelessly go back over every connection before making my way to asking for help on this thread.  I have quadruple checked components and routing.  Here is the state of my unit.

The unit powers on, and thankfully no smoke the first go.  The Neon is lighting, and the meter is functional, being that I was able to calibrate it to zero on GR mode, and the 6.3v heaters are functional due to the meter bulb lighting, as well as all tubes lighting up.  The unit does not pass audio, and the peak reduction knob or the gain knob do not affect the signal, not in a visual way on the meter, nor an audible way, except for one thing.

  -  There was a point where I put a -18 500hz sine wave through the unit, and i can very faintly hear it, and it became a bit louder as I placed chopsticks on the right side of the R5 that connects to ground and pin 7 on the A10 T.  However, the current state of the unit no longer does this.  more below. -

I ruled out the tubes being bad by having a few extra sets to swap in and out.

I took the T4B out, took the cover off, and verified that the meter. and the T4 does, in fact, attenuate in the light of my room, as well as was responsive to my hand covering it to dim the light, thus changing the state of the meter.  ( none of this was audible)

I tried out 2 separate C5 capacitors, just to rule out the first being faulty, due to audio not passing, still, no avail.

I am suspicious of my R1 and R2 wiring, although I followed what I thought was correct using both recpro, as well as Cayacosta's layout, and now checking against Martin's layout (rainton).  (all while keeping the R34 R29 R32 and R38 relationships in check, I chose 10k 2w 4.7k 2w and 1k and 22k for R38.)  I fear I may be missing something with the way the shield works, as well as grounding properly.  I did unscrew the front bolts and make sure I cleared away the chassis paint to ensure proper metal-metal contact for the ground tab. 

One big note to make here, I am hesitant to checking voltage readings at various stages due to not quite knowing what to look out for.  Am I simply clipping the negative alligator clip to chassis for ground, and probing around with the red probe to take readings?  Thank you tons in advance.  I will continue digging, and checking, I've fooled myself more than once with this things.  My biggest thing now is knowing how to properly check continuity, and where the unit is failing while the unit is live.   


here is a link for anyone who wants to have a look, thanks again.

https://www.dropbox.com/sh/hqk9ejehtctijnu/AABqjmkpMrGWV7zfKYykayXta?dl=0

Matt

 

[sr1200]

U sure your transformers are good?