la2a - oh gawd!

[Ethan]

take the lid off the T4b and see if the EL panel is flashing when you run music thru the compressor and have the compress knob at full tilt boogie.

I already suggested that CJ! ;D
YOU practically held my hand through my first LA2a build (wayyyy back when).  Just trying to pay it forward...
Little do the newer guys know, CJ's investigative hacking was vital to realizing that project!

 

[studmuffin]

test t4's in a dark room and see if they light at all when you feed it a +4 signal build looks real solid.  My only negative comment would be the t4 itself... A clear see through cover looks nice and all but will cause the t4 to not function porperly or worse in this case not at all. Find a way to replace that with a non see through black cover or something. Paint it anything to keep the light out. also check the 12bh7 tube. Had to replace those with in a year on some new LA2A reissues we got at work

Will do!  Thanks guys

 

[DaveP]

Studmuffin,

It worries me that someone would make an LA-2A with the T4's in a clear case, I have made one myself and its imperative its totally blacked out.  If they did that then other stuff might be wrong too.

There is a DC feedback loop running from the WCF 12BH7 to the first tube's cathode, has that broken anywhere like a dry joint?  This would stop the feedback and increase the gain to unuseable proportions.

If you have a DMM then you could check the voltages on the schematic. There should be 18~19V on the junction of the 68k and 100K resistors in the feedback loop.

If all else fails I could fix it for you, but you'd have to get it to either Stevenage or mid-wales and collect it afterwards.

best
DaveP

 

[gemini86]

Can you get a better pic of the opto element?

That's no t4, I don't see an EL panel, looks like a simple vactrol or LED/LDR combo, there should be two in there.

 

[Winston OBoogie]

If it's a copy of the original LA-2A then that thing in the clear octal case could just be an input attenuator network.  There was an octal socket on the rear of the old ones and you inserted an octal plug with either some kind of network (pad, filter or?) installed or you had jumpered the appropriate pins on the plug for a straight through signal.

Anyway, I HOPE that's what it is! 

 

[alexc]

Check the setting of the trim potentiometers at the back.

Make sure the 'stereo adjust' one is turned all the way up.
It sets the level of peak reduction.

Having it set low makes for very little peak reduction.

I've done it before!

Also, the guy who does these does quite a few of them - I'd be very surprised
if he got anything basic wrong.

That clear box does look like an attenuator plug.

 

[CJ]

Winston O Boogie, your not dead?

wtf over?

cue the twilight zone theme...

???

holy cow, good to see ya if you are not reincarnated,

 

[shabtek]

CJ!
we love you!




W O' B!!!
we are still waiting for you to square up with us.

 

[kato]

For what it's worth, it looks well built. All except that T4B which looks mighty suspicious. Without a T4B, you'd get what you describe - passes signal but no compression. With so much care taken in building the unit, there's no reason to cheap out on the opto cell - the heart of the LA2.

:-[ (daughter entered smiley)

 

[studmuffin]

Oh dear!

Thanks for th input everyone.  I'm in over my head now so I'll do three things:

1.  Delete the pic that identifies the brand of this build.  Thinking to spare the maker's blushes if it's a genuine mistake.

2.  I'll pass the thread to my tech and hope that he can perhaps interact with you guys to get the clues he needs.

3.  And because I'm clearly trying to pay it foward, I'm sent a donation to your site as a kind of thank you in the hope that you guys will check back and babysit my tech through a satisfactory repair.

Maybe I should also let the manufacturer see the posts too.  He's now been in touch via ebay.  He was at a trade show - hence the initial silence.

Thanks everyone!

 

[gemini86]

I didn't realize that company made anything but guitar amps...  but we pick apart all sorts of manufacturer's gear, that's what the internet it for, getting the knitty-gritty. I also doubt that this thing was produced DOA and then sold without anyone noticing. Something is either broken or defective, rather by incident than by manufacture. So there's really nothing for the manufacturer to be embarrassed about.

Anyway, sure hope you get it working!

 

[studmuffin]

Hi again all

My techie still has the unit and will look at it shortly.  In the meantime I've done some digging around and established that the so-called T4 in there is obviously not a T4 but looks like some kind of design based on a couple of cheap Vactrol VTL5C4/2 optocouplers.  Obviously we don't yet know if that is causing the fault.



If I was looking to replace the cheap and cheerful inaccurate "T4" design that's in there with something pucker, can anyone say what the options are please?  UK would be best.

Many thanks
Many thanks

 

[kato]

Nothing that I know of in the UK.

But Joe Electro sells a highly regarded copy of the vintage ones for $100.

http://www.gearslut.com/store.php/products/kenetek-t4b-opto-attenuator-module

 

[studmuffin]

Nothing that I know of in the UK.

But Joe Electro sells a highly regarded copy of the vintage ones for $100.

http://www.gearslut.com/store.php/products/kenetek-t4b-opto-attenuator-module

Thanks Kato - but it looks like he's out of stock at the moment due to QC issues.

 

[Kingston]

http://www.dripelectronics.com/index.php?option=com_content&view=article&id=9&Itemid=30

also highly regarded. You want the slow for "vintage".

 

[studmuffin]

http://www.dripelectronics.com/index.php?option=com_content&view=article&id=9&Itemid=30

also highly regarded. You want the slow for "vintage".

Thank you Kingston - but they have a 2 to 3 week lead time....

 

[bkbirge]

http://www.dripelectronics.com/index.php?option=com_content&view=article&id=9&Itemid=30

also highly regarded. You want the slow for "vintage".

Thank you Kingston - but they have a 2 to 3 week lead time....

If you order now you'll have it in 3 weeks instead of wishing you had ordered it when 3 weeks from now is now.

 

[kato]

http://www.dripelectronics.com/index.php?option=com_content&view=article&id=9&Itemid=30

also highly regarded. You want the slow for "vintage".

Thank you Kingston - but they have a 2 to 3 week lead time....

If you order now you'll have it in 3 weeks instead of wishing you had ordered it when 3 weeks from now is now.

Also it says 3 days to 2 weeks.

 

[Hasse]

Check the "R36" The 270 Ohm resistor. I made mine with a 270 K Ohm... -No GR as a result.
R36 as in:

 

[college101]

UMHH isnt- R36 -  470 ohm?