LA-3A Design Thread [Autotransformer with Pics]

[Rybow]

Good to hear that someone has made some progress on this thing.

 

[PATAPETER]

hi .... folks !!!! anybody have made the original pcb's ? I'm looking for 2 with the original urei schematics.
Thanks.Regards.

 

[PATAPETER]

anyone ???????

 

[TheGuitarist]

still interested too.

 

[rrs]

I really don't know why no one have picked up this project.
I'd definitely be in on it. I'm sure many would - how many more GSSL's and 1176's do we need?

 

[Rob Flinn]

Did any of you guys try the search function

http://www.groupdiy.com/index.php?action=printpage;topic=31057.0

 

[rrs]

Yes ......

Not after files to make my own PCB that's not known to be working.
I think a project with some support like the GSSL's and 1176's is more so what many would go for.

 

[Rybow]

Thanks for the link Rob. Have you made any progress on tracking down your level issue? How about the noise issue? I am guessing you used the PCB etch file in the link.

rrs- I don't think we can get this project moving forward and verified until people start working with the information that's available. This project looks like one that needs to be moved forward by the community. The 1176 and GSSL projects have one person who developed and did all the leg work. This could be really interesting to develop this project as a collaborative effort.

 

[Rob Flinn]

Yes ......

Not after files to make my own PCB that's not known to be working.
I think a project with some support like the GSSL's and 1176's is more so what many would go for.

I'm just wondering how you think a project turns into one that is known working with all erratas solved & has a massive support thread ?  It takes people taking chances on a project & contributing to threads.

The reaility is that these boards do work. 

Insomnia
I cured the noise problem which was just a cooked transistor. 

I still haven't worked out the level issue, mainly due to being very busy with work.  I did check the board values against the diagram, but couldn't see any issues.  It seems that the sidechain just isn't creating a big enough signal, since this pretty much just comes straight from the input.  I have subbed in a Sowter transformer for the input & was suspecting that the ration may not be high enough.  All the other transformers are proper spare parts so should be ok. I just need someone to check the ratio of an original for me.
I don't have the input pad fitted& I have the low noise mod done to the make up amp.

 

[Rybow]

Glad its kinda working out for you. I just looked back over some of the posts in this thread, and I found that Terby used a 1:10 cinemag 75101 APC for his build. Its unfortunate that both the guys that have built working prototypes have dropped off the map. You may want to be careful Rob, someone might be picking them off!

I was thinking I would try to do a couple of home etch boards soon, and start working this thing out. My experience is limited, but I have 2 LA 4's under my belt, and the LA 3 has some similar ciruit paths and what not. Seems very possible, and there is a lot of good information in this thread. I have been craving some "down and dirty" DIY lately. This could be the ticket!

Ryan

EDIT: I have started putting together a BOM for this project. I am having trouble locating C16 on the schematic, but it is present on the component layout sheet from kenetek.

 

[Rob Flinn]

For the autoformer & the output transformer I used spare parts from David Kulka studio electronics https://www.studioelectronics.biz/sunshop/index.php?l=product_list&c=61

I didn't buy the input transformer becasue it was a lot more expensive than the autoformer & o/p transformer which were quite reasonably priced with the exchange rate when i bought them.

 

[Rybow]

Thanks Rob. That input transformer is very pricey. Custom Cinemag though. Too bad they don't give a ratio for it, but the guys at Cinemag might be able to divulge that information. Maybe.

Any thoughts on the value of C16? Its the only part I don't have a value for in my BOM. Everything else is mapped out according to the waltzing bear schematic, JBL manual schematic, and the kenetek component layout sheet.

 

[Rob Flinn]

Any thoughts on the value of C16? Its the only part I don't have a value for in my BOM. Everything else is mapped out according to the waltzing bear schematic, JBL manual schematic, and the kenetek component layout sheet.

C16 looks to be across pin 6 & 7 of the T4B & looks like some sort of stabilisation cap, but I never found out what it value was.  I didn't  install it for that reason, & have never noticed any adverse effects, & don't think it effects the side chain gain !  We could do with the help of someone with an original LA3a.

 

[Rybow]

Any thoughts on the value of C16? Its the only part I don't have a value for in my BOM. Everything else is mapped out according to the waltzing bear schematic, JBL manual schematic, and the kenetek component layout sheet.

  We could do with the help of someone with an original LA3a.

Agreed.

 

[edanderson]

the original input transformer is 1+1:10, wired as a 1:5, IIRC.  i don't think the cmmi-75101 is the same transformer, but it does have the right ratio.

you could ask bill (aka kenetek) who posts here as joe-electro.  he has done quite a bit of work on T4Bs and could probably help you with the C16 issue.

ed

 

[Rybow]

Thanks Ed! Just sent Bill/joe-electro a PM.

EDIT: Just got the word back from Bill. C16 is not required. Pins 7 and 8 are the audio photocell of the T4B.

 

[joe-electro]

Rob,

Check the orientation of all the signal transistors in the sidechain to make sure the collector and base are in the correct holes. Since the transistors haven't gone up in flames yet the emitters are probably in the correct places. This is the first thing I always check when I run into gain issues because a lot of times modern transistors don't have the same pinouts as their vintage counterparts.

Joe

Yes ......

Not after files to make my own PCB that's not known to be working.
I think a project with some support like the GSSL's and 1176's is more so what many would go for.

I'm just wondering how you think a project turns into one that is known working with all erratas solved & has a massive support thread ?  It takes people taking chances on a project & contributing to threads.

The reaility is that these boards do work.   

Insomnia
I cured the noise problem which was just a cooked transistor. 

I still haven't worked out the level issue, mainly due to being very busy with work.  I did check the board values against the diagram, but couldn't see any issues.   It seems that the sidechain just isn't creating a big enough signal, since this pretty much just comes straight from the input.  I have subbed in a Sowter transformer for the input & was suspecting that the ration may not be high enough.  All the other transformers are proper spare parts so should be ok. I just need someone to check the ratio of an original for me.
I don't have the input pad fitted& I have the low noise mod done to the make up amp.

 

[Rob Flinn]

Thanks for the tip Joe.

It's been a while since I looked at the ones I built, so I can't remember whether I did actually check them as I put them in.

By the way thanks for doing the board design !!

 

[joe-electro]

Thanks for taking it to the next level!

 

[TheGuitarist]

if joe here doesn't have all the answers we need (doubtful ) I can get my hands on a pair... I'm not an electrical tech by trade but i could probably convince the owners to let me have a poke around the insides of one if noone else who has no knowhow can.