Looking for some input on La3a PCB markings

[sonicrevolution]

Good evening, I wanted to see about some help with identifying some of the markings on the PCB. The PCB is pretty close to the original, with one of the main exceptions being that it differs from the Urei original by including the mod with a 24v relay, as well as xlr's instead of pigtails, more-so closer to the UA reissues from circa 05'. I've highlighted the letters in question along with a picture of the actual PCB and the schematic I am using. The schematic provided has green highlighter markings on the letters in question. I have hunted down many interior pictures during my quest until I get a better grip on making actual sense via more experience and study, and the originals have the markings, as well as the reissues I believe. However the PCB I am using does not. I have been working on this, but I still have a lot to learn. Thank you for any and all help with this project.

 

[gyraf]

Are you referring to a (non-existing) picture?

green highlighter markings on the letters in question

 

[sonicrevolution]

Are you referring to a (non-existing) picture?

My apologies, I did not realize they were not posted. I've attached the related photos here. I've also added a photo of the UA reissue board sparsely populated to provide some subtle differences as well. Thank you in advance for any help, as this has been a fun (and frustrating) journey.

Matt

 

[dmp]

So you want to find the nodes on the pcb that correspond to the highlighted letters on the schematic?
(it's always helpful if you have a "?" in your post because it's not always clear what someone is asking)

The way to do this is find the components and find the nodes (connections) between them.
For example:
I: the connection (node) between the 68R and 1k3 resistor, which you can see on the lower left side of the PCB.
D,E,F: the "C/L SW" switch on the lower left of the PCB
F,J,K: "R4 GAIN"
etc
Can you do the rest?

Post a picture of the backside of the PCB would be helpful

 

[sonicrevolution]

So you want to find the nodes on the pcb that correspond to the highlighted letters on the schematic?
(it's always helpful if you have a "?" in your post because it's not always clear what someone is asking)

The way to do this is find the components and find the nodes (connections) between them.
For example:
I: the connection (node) between the 68R and 1k3 resistor, which you can see on the lower left side of the PCB.
D,E,L: the "C/L SW" switch on the lower left of the PCB
F,J,K: "R4 GAIN"
etc
Can you do the rest?

Post a picture of the backside of the PCB would be helpful

Thank you for this info, yes this is helpful. I will try and be more clear with my specific questions. I am following the schematic and logically trying to deduce what is happening, while also being on the learning curve, so some of this is lost on me, for example, some insight on the designation of square solder pads vs circular, for say xls in, R4-GAIN, C/L SW etc, and why this is? The left ST ADJ connection just to the right of the output Trans. going to LINK to the left of the power caps has eluded me as well. Again, I am using this to learn to think properly and build better fundamentals. One challenge has been mapping the letters on the original to some of the subtle differences in this schematic design, being that I'm still building my circuit understanding and so have gaps. My previous projects were PTP Pultecs and the likes. I was not familiar with node, so thanks for this insight. Also, if you look at the last pic above, there is a chasis ground at the edge of the PCB to the right of the output trans, I understand chasis grounding, however wondering why it's not on this particular design. Here are some pics of the back side, as well as one with the light showing through to see the traces more clearly if this is helpful. Thank you again for your help, it is greatly appreciated!

 

[dmp]

some insight on the designation of square solder pads vs circular, for say xls in, R4-GAIN, C/L SW etc, and why this is?

That really depends on what the PCB designer was doing. It usually would be a convention (i.e. square pad = ground) to know how to hook up the pot so the clockwise rotation would give the intended behavior. But especially with a DIY board, you don't know what the designer did, so it is best to check. Or just hook the pot up and if the rotation is wrong, come back and swap - haha
Now with the backlit picture, I can see the:
square pad: D
round pads are: E,F
The L is a chassis ground to shield the cabling going to that switch. The closest ground to that is the other side of the 39k resistor (which has a node with the 69k. But there seems to be something weird going on at the gain pot connection below there. The ground looks like it connects to both sides of the gain pot. Can you check that with the PCB in hand?

 

[sonicrevolution]

Thanks for the info! I've definitely wired a few pots backwards in my time, ha! And this is interesting, I just verified with a meter that they both are connecting to ground. I'm looking on the schematic and seeing D and L both eventually going to a ground point. Perhaps the unlabeled square pad just above the left R4-GAIN is not supposed to connect to this side of the Gain pot? It looks that possibly that may have been a ground point for something, but I am not clear as to what is going on here either.

 

[dmp]

Looks like a mistake in the PCB there.
The top of the gain pot will have to connect up at the top right pad of the C/L SW

 

[sonicrevolution]

Just for clarity, the top right of the R4-Gain pot will no longer go to where the mistake is (left side R4 Gain) and instead rerouting to the 1.3k 68k node side of C/L-SW?

 

[sonicrevolution]

Just for clarity, the top right of the R4-Gain pot will no longer go to where the mistake is (left side R4 Gain) and instead rerouting to the 1.3k 68k node side of C/L-SW?

I think I have got it, thanks again for the help.