SA-3A (LA-3A Clone) Support Thread

[dtrax]

Good call. Looks like this guy is going to be on my bench for another week.

But, it'll all be worth it in the end.  8)

 

[jordan s]

Well, I replaced the parts, no smoke but more disappointment. They seem to be working fine as channel amplifiers but I can't get either channel to compress. The meter works and when I switch it to GR I can adjust it to stay at 0 but no matter how much I turn up each peak reduction pot, no compression. I've checked the voltages, I made sure there were no shorts in the wiring, I fiddled with it until it could no longer be fiddled. Any suggestions on where to look for a problem?

Update: more problems. As I was trying to adjust the bias by measuring TP1, I accidentally set my meter to test continuity and immediately channel 2 became a distortion unit. I was unable to get the voltage down the 12v (it sits around 16.8 no matter what I do) and now the channel has stopped outputting signal all together. Now it's kinda going back and forth between no sound and distorted sound while I adjust VR1 but it's super unpredictable and keeps rising and falling. Le sigh. Sometimes when I get to this point, I think it would be easier to just start over completely.

 

[jordan s]

Any suggestions guys?

 

[ruckus328]

Jordan, just saw your post, will get to it in the morning.

 

[jordan s]

Thanks! I'll be waiting patiently while twiddling my thumbs.

 

[ruckus328]

Jordan, ok so first thing you need to do is stare at the schematic some and get familiar with it.  I can point you in the right directions but only you'll be able to trace the signal to see where it stops.  Circuit is fairly simple so narrowing down where your problem is shouldn't be too difficult.  And good news is you have a working audio path to compare with.

1) Regarding the problem audio path, obviously you fried some things by grounding TP1.  Hard to say what to be honest, inspect all resistors but take a real good look at R18/R19 (4R3) resistors.  Q5/Q6 probably shot, hopefully you ordered extras like I said    Only really safe bet is replace all 5 transistors in the audio path (including matched pair), but at bare minimal replace Q5/Q6 and give it a try.  If you need another matched pair let me know, will send you another set, on the house.  An unmatched pair is fine to use in the meantime, just means higher distortion figures is all.

2) Compression - the fact both units exhibit the same issue leads me to believe it's an assembly or miswiring issue.  If you want to try to troubleshoot this issue in your working channel, feed it a decent signal (at least 4dbu or so), get the gain going nice, and crank peak reduction to the max.  Measure the AC voltage at TP2 and note whatever it says.  This is where the sidechain branches off fromt he audio path.  Now measure the voltage at C6, it should read the same.  Now measure the voltage at TP3 (this is right before the sidechain signal gets stepped up by the EL transformer to go to the T4B).  The AC voltage might be more or less depending on the rotation of VR4, but you should see a decent amount of AC here.  If you don't then your problem lies somewhere in the sidechain between C6 and TP3(refer to schematic to see which components it could be).  If you do see AC, then check the AC voltage after the transformer (TP4).  It should be significantly higher and about 10x's more than what you saw at TP3.

By the way, how do you have your stereo link wired?

 

[jordan s]

These are the AC readings I got from sending it a 1K tone from Pro Tools at -20dBFS. I measure 0.27V on + and - pins of the input xlr.

TP2 0.14V AC
C6 same
TP3 .001V
TP4 .017V

This is with gain and peak reduction maxed and Mod and 50dB mode both on. I was able to hear a very small audio signal at each of these points. What voltage readings should I expect? Following a schematic and understanding what the circuit is doing is not my strong suite. I'm taking a good look at it and trying to figure this out but so far I have nothing. My link switch is wired just how the illustration earlier in this thread is shown. It doesn't make a difference when I engage it or unplug the connector from the switch.

Right now all I can think of is that maybe I fried something in the sidechain when I put 2N4037 in Q9. Maybe Q7, Q8 or Q10?

 

[ruckus328]

Jordan, feed it a bigger signal (1V or more).  Makes it easier to trace the signal when there's more of a signal to trace.  Then check again at TP3, if you still get no AC there then problem lies in one of the sidechain transistors.  (.001V you got at TP3 means no signal)

 

[jordan s]

Finally some satisfaction. On channel one, I replaced the sidechain transistors (Q7, Q8, Q10) and now I can finally get it to compress. I'm a little confused about how the mod and 50dB modes work together. Sounds like the mod is just lowering the output gain and thus the noise level. I notice that 50dB mode lowers the output signal AND the input to the sidechain. I can hardly get any compression unless the input signal is cranked and the peak reduction is turned all the way up. I don't find that to be particularly useful for my needs. Looking at the schematic suggests that the default is to have the switch up but for +35v not to reach R48. Perhaps I orientated it incorrectly?

Well, I guess I have to replace the same transistors on channel two and all the transistors in the output path... basically all of them. Luckily I followed your advice and ordered extra plus others, just in case. Maybe I'll have this up and running soon.

 

[jordan s]

SUCCESS!
So after I got channel one working, on channel two I replaced all transistors in the output path (besides the matched pair) and all sidechain transistors (besides Q9 and Q11) and sweet sassy molassy, she's up and running and sounding great. Some of those traces were none too happy with me but I made it work. I got the stereo link calibrated and the HF adjusted just where I like it and I'm a happy man. Switching those transistors from the get go really messed things up and added weeks to my build. As simply laid out as this board was, I'll let this be a lesson to not only double check, but triple check my stuffing and wiring. Thanks so much to ruckus for making this project available and providing such helpful support, you rock!

 

[ruckus328]

Awesome Jordan, I knew you'd get it. 

FYI - Mod mode does lower the output signal, but lowers the noise floor even more so, so increases signal to noise ratio and puts the unit in a better position for studio use.  This is a very common mod done to vintage units as well as already incorporated on the commercial reissues.  It needs to be used in conjunction with 50db mode, because you will not have enough gain now with it in 30db mode (which puts a 20db pad on the input signal).  From the sounds of it, you've wired the 30/50 switch backwards.  You should easily be able to get 15-20db of gain reduction with a +4dbu input signal with mod/50db enabled.

 

[craigmorris74]

What type of fuse do we need to use?  Original schematic says 1/8 amp  slo-blo.

Thanks,
Craig

 

[craigmorris74]

Also on page 1, Mike says don't connect the shield of the link cables to both boards, but on the diagram on page 2, it look like the shields of both cables are connected to the boards.  I think I'm misunderstanding something.

 

[Grooveteer]

Also on page 1, Mike says don't connect the shield of the link cables to both boards, but on the diagram on page 2, it look like the shields of both cables are connected to the boards.  I think I'm misunderstanding something.

Yes. Connect them to the boards, but not to the switch. That way there's no continuity, and therefore no ground loop.  (Gurus, please correct me if I'm wrong)

Fuse: It's mentioned in the BOM on the first page of this thread.

Hope this helps. 

 

[craigmorris74]

Thanks, Grooveteer, just wanted to make sure.

 

[craigmorris74]

Before I fire mine up, am I reading the schematic correctly in that the shield should not be connected to your input XLR's?

Thanks,
Craig

 

[ruckus328]

Before I fire mine up, am I reading the schematic correctly in that the shield should not be connected to your input XLR's?

Thanks,
Craig

This is correct.  Pin 1 on the XLR's should go only to chassis or star ground.

Grooveteer - thank you for helping, you are correct.

 

[craigmorris74]

Darn it!  Reading the schematic closely and noticed I was supposed to pre-trim VR1 and VR2, but I've already got the soldered to the board.  Any tips as what to do there?

Also, on the schematic, it says to set the stereo adjustment fully counterclockwise, but the instructions for stereo calibration say to start with it fully clockwise.  Which should I start with when powering up?

Thanks,
Craig

 

[craigmorris74]

Just wanted to bump this thread to see if I can get an awswer to my last questions.  Thanks!
Craig

 

[ruckus328]

Darn it!  Reading the schematic closely and noticed I was supposed to pre-trim VR1 and VR2, but I've already got the soldered to the board.  Any tips as what to do there?

You should be fine, the Pots should come centered by default (or fairly close to centered anyways).

Also, on the schematic, it says to set the stereo adjustment fully counterclockwise, but the instructions for stereo calibration say to start with it fully clockwise.  Which should I start with when powering up?

Doesn't matter.