LA500A opto comp leveling amp BUILD THREAD 500/51X

[MattM]

Hi Joe -- bumping this question, since I think it got overlooked:

I'm curious what the advantage/disadvantage is to having the VU and GR handled on different pots rather than in standard, "balance" mode? Took me quite a while to get the hang of the balance pot. Even if you're not matching for stereo, is there an advantage to "balance matching" the GR and VU values rather than just throwing the MATCH jumper to the other pin and tweaking the values separately?

Thanks!

[Joe Malone]

Hi Joe -- bumping this question, since I think it got overlooked:

I'm curious what the advantage/disadvantage is to having the VU and GR handled on different pots rather than in standard, "balance" mode? Took me quite a while to get the hang of the balance pot. Even if you're not matching for stereo, is there an advantage to "balance matching" the GR and VU values rather than just throwing the MATCH jumper to the other pin and tweaking the values separately?

Thanks!

The balance only lets you Balance the 2 optos (audio opto and GR opto) so you can match GR and VU but not usually perfectly maintain -5dB. Where as the separate pots allow each opto to be backed off to match another set of optos exactly.

As I mentioned before this will only be needed if you ordered LA500A kits at separate times and even then if you remove U2 from the original comp and move it to U1 in the new comp and split the new optos into U2 in both comps balance pot should still be all that is needed.

If you link two comps after doing the alignment at least twice to both comps and one comp when linked shows more GR and -VU. Say when linked one comp stays at -5dB and the other shows -7dB. Then by moving the jumper on the -7dB comp to ST Match you can back of Balance/GR pot on that comp to show -5dB in GR and switch to VU and back up or turn up ST Match VU pot (as this pot has never been set yet) until VU shows -5dB as well. Now both comps will track better.

BUT if you ordered a pair at one time or moved the optos as mentioned above and have fitted the optos to the correct place you should never have to move the jumper to stereo matched.

[knadles]

I realize this has most likely been asked before, but I've tried searching the answer and haven't found it. I apologize if I've overlooked something fundamental.

Built the LA500A (500 variant) and I seem to be having some issues. First order of business: is there a difference in any components when building the 500 variant (as opposed to the 51x)? Because I trimmed off the 51x tab but otherwise followed the standard build instructions.

If there are no changes besides removing the tab, I'll try to detail my symptoms, but I wanted to check that first.

Thanks,
Peter

[Joe Malone]

I realize this has most likely been asked before, but I've tried searching the answer and haven't found it. I apologize if I've overlooked something fundamental.

Built the LA500A (500 variant) and I seem to be having some issues. First order of business: is there a difference in any components when building the 500 variant (as opposed to the 51x)? Because I trimmed off the 51x tab but otherwise followed the standard build instructions.

If there are no changes besides removing the tab, I'll try to detail my symptoms, but I wanted to check that first.

Thanks,
Peter

There are No component or other changes between 500/51X. The LA500A will just use the highest rails available automatically.

[greenote]

I just completed the new version. It passes audio and compresses. The threshold/makeup pots work as does the meter. However I cannot get either VU or GR trimmers to budge the needle. The needle is pinned to the Right (RED).

I've read and re-read this thread and the old one thread.
I've re-heated the joints on both boards
I've setup the 1K test tone at 1.23v andI'm getting 16v at OpAmp pins 4,8, passing audio.

Any ideas on my calibration problems.
T

[Joe Malone]

I just completed the new version. It passes audio and compresses. The threshold/makeup pots work as does the meter. However I cannot get either VU or GR trimmers to budge the needle. The needle is pinned to the Right (RED).

I've read and re-read this thread and the old one thread.
I've re-heated the joints on both boards
I've setup the 1K test tone at 1.23v andI'm getting 16v at OpAmp pins 4,8, passing audio.

Any ideas on my calibration problems.
T

If the meter pins without audio in you have a solder short usually between two solder pads on the LA500SC or another fault on that PCB.

With power on and no audio give me the dc voltages +/- at all pins of the opamp on the LA500SC measured to 0v?

As usual if not sure attach here a hi res in focus well lit photo of the top and bottom of the PCB so I can check it over.

[greenote]

Thanks Joe,

If the meter pins without audio in you have a solder short usually between two solder pads on the LA500SC or another fault on that PCB.
Don't see any shorts, photo's attached. Perhaps I should check the value on the blue film caps as I saw there minor shipping error in previous post.

With power on and no audio give me the dc voltages +/- at all pins of the opamp on the LA500SC measured to 0v?
I'm assuming you mean the OPA. I may be blowing this as I had +/- 16 V at Pins 4 and 8. I'll replace and report voltages on all pins. I can measure at Discrete pins as well.

As usual if not sure attach here a hi res in focus well lit photo of the top and bottom of the PCB so I can check it over.

IMG_5279.JPG

IMG_5281.JPG

IMG_5282.JPG

Cheers Todd

[greenote]

I'getting the following voltages

Edge Connector
12. +15.4 volts
14 -15.7 v

Chip
1. 0
2. 0
3. 0
4. -14.9 volts
5. 0
6. 0
7. 0
8. +14.6 volts

JLM 99v
-14.9 v
+14.6v

Todd

[Joe Malone]

I'getting the following voltages

Edge Connector
12. +15.4 volts
14 -15.7 v

Chip
1. 0
2. 0
3. 0
4. -14.9 volts
5. 0
6. 0
7. 0
8. +14.6 volts

JLM 99v
-14.9 v
+14.6v

Todd

HI Todd the photos and voltage measurements I need are on the LA500SC PCB which you didn't send any pictures or voltages ?

Please clean the underside of the pcbs with a old toothbrush and some isocol or tiny bit of pure alcohol if possible first.

[Joe Malone]

I'getting the following voltages

Edge Connector
12. +15.4 volts
14 -15.7 v

Chip
1. 0
2. 0
3. 0
4. -14.9 volts
5. 0
6. 0
7. 0
8. +14.6 volts

JLM 99v
-14.9 v
+14.6v

Todd

HI Todd the photos and voltage measurements I need are on the LA500SC PCB which you didn't send any pictures or voltages ?

Please clean the underside of the pcbs with a old toothbrush and some isocol or tiny bit of alcohol rub if possible first.

[greenote]

Okay, sorry I refer that was as the Mac.
Had to de-solder the meter to remove PCB and get shots of the board.
I've attached photos and will send the voltages from the op amp on the LA500SC PCB after re-assemby.

IMG_5285.JPG

IMG_5286.JPG

[Joe Malone]

Okay, sorry I refer that was as the Mac.
Had to de-solder the meter to remove PCB and get shots of the board.
I've attached photos and will send the voltages from the op amp on the LA500SC PCB after re-assemby.

IMG_5285.JPG

IMG_5286.JPG

As you can see both 10R protection resistors are fried due to some dead short so need to be replaced first.

Was the opamp ever plugged in backwards as this and a dead solder short between some pads are the only 2 things likely to burn both 10R resistors?

[greenote]

I believe this op amp was initially plugged in backwards.

You're referencing R23 and R24 back on the Edge Board. I saw these darkened, but they still measure 10 Ohms. I'll rip them out and report back.

I did get voltages on the LA500SC PCB opamp. Perhaps it's moot, until I replace the 10 Ohm-ers
1. 13.84 v
2. 0.49
3. 0
4. 1.04
5. 13.62
6. 2.37
7. 2.37
8. 14.5

All were positive voltage.
Thanks
Todd

[greenote]

Okay replaced the 2 10 Ohm resistors. VU calibrated, GR now on zero, no motion on trimmer. It's working great and the VU gives me relative compression indicator. Beefy

[Joe Malone]

Okay replaced the 2 10 Ohm resistors. VU calibrated, GR now on zero, no motion on trimmer. It's working great and the VU gives me relative compression indicator. Beefy

Cool great to hear you got it working

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