Please Help Me Salvage a Half Gutted LA-4a

I managed to get an LA-4a for free.
However, it's missing all switches, all front panel pots (the trim pots on the board are still there), and the Optocoupler.

From looking at this schematic (http://www.waltzingbear.com/Schematics/Urei/LA_4A.htm) it looks as if it should pass signal even without the above connections--Did I miss something? I could definitely use a fresh pair of eyes.

I'm trying to make a minimal amount of connections/component additions, just to get it to pass signal. Currently it is not.

Thanks in advance!!!
-E

The signal goes through the Output Level pot. You need to fake that much. Power-up, connect a amp-speaker to the output. Put your finger on the wires where the Out Level pot was. The one that was "wiper" should cause a great buzz when you touch it.

Another OutLevel pot wire should be pretty clearly ground/shield.

Then take the third wire and tack it to the wiper wire. Now touching the Input screws should make a great buzz, same as if OutLevel control was all the way up.

If still no joy: start with basics. Are the power rails live and about the right DC voltage? What is the DC voltage on the output transformer primary (should be zero)? Attach alligator clips to a guitar cord. Plug into a practice amp. Ground to LS4 case. Feed a line level at the input screws. Touch the hot clip of the practice-amp to each stage's output: IC1A, IC1B, IC1C, IC1D, IC2D, two 15 ohm resistors driving output transformer. With luck, you have signal in early stages and not in later stages, you have narrowed the problem.

Look at my opto compressors.
Both the single and the dual are sprung from the LA4.

There is also a manual at my site in PDF form.

It should pass a signal without the opto ...

I had a dumb attack there.
Duh! :roll: if you don't have the output pot that varies the resistance to the output connected ya won't get signal! I just shorted the wiper to the other terminal, and audio doth come forth! O-scope says its clean as a whistle.

Thanks PRR and Kev (as always!)
...It's always the simple stuff that you overlook that makes ya look dumb :grin:

I do have one question though...Why did it buzz with my finger on it, and not when I touched it with something else?

I would guess your body is a big antenna. so its just amplifying stuff.... :grin:

> not when I touched it with something else?

What "else"? A feather? a screwdriver? a 120 pound dog's nose?

The feather will do nothing. Screwdriver usually a small tic. While I do not advocate, and do not use, the nose of my large dog for signal injection, I bet he'd work as well as me and my finger. (The picture is my small dog. She would not work so well. {Especially now that she is perpetually grounded.})

It is capacitance to the general power-line field, which is about size. Screwdriver has a surface area of maybe 2 square inches. Humans (and large dogs) more like 2,000 square inches. 1,000 times as much area, capacitance, and hum injection.

And feathers, pencils, chopsticks, box-kites etc have such low conductivity that, even if they are of large area, they won't pass much current into a circuit.

OK so I shorted the threshold wiper to the ouput pot terminal with a 2.2K (R11) resistor. I get signal, which is great, but it is much lower than the input signal. Please correct me if I'm wrong, but since there is no resistance from the output pot shouldn't the output signal be amplified?

Thanks
-E

Ethan,

I might actually have a NOS LA-4 dual-concentric output pot/rotary switch lying round somewhere... Let mw see what I can dig up!

Keith

That would be awesome Keith!

Does the above description of what's happeneing look normal for the circuit? It just looks like the output should be fully open at full gain, so I don't understand why the output is attenuated... :?

Shamless Bump

Use a voltmeter and trace the signal.

Ok,

I've tracked down that problem, and now I have near unity gain (off by -2dB) but I've got this low level Bhzzzz. It doesn't just sound like normal 60Hz oscillation. I disconnected the internal power supply section and used my bench supply to minimize troubleshooting variables. Any tips on how best to go about tracking down what's causing the noise?

Thanks as always!
-E

> now I have near unity gain

It isn't unity gain. With the "Output Level" pot all the way up (or jumpered), gain is 7.5X or 75X depending on the rear "Input Range" switch position. (Default, switch missing, is 7.5X.)

SH*T!

Just when I thought I was getting somewhere.

I very much appreciate all the time you take PRR. Thank you. I'll check out what's going on between 1X and 7.5X

YES!

I somehow totally missed the connection from pin 12 of the opamp to the wiper. I only concentrated on the wiper and R11. Output is amplified :grin:

I suspect the slight noise that remains should disappear when I rewire it with shielded wire...right now it's temporarily just connected with bare buss wire.

PRR, what would be your best educated approximation on the specs of that output TX?

600Ω:600Ω, +23dBM, size to fit, frequency response to taste.

You are a beautiful man! :guinness: