LA500A opto comp MAY 2021 BUILD THREAD 500/51X

[Everett]

Ok, the Threshold pot measures 93k and Makeup pot 9.7k. Similar on other comp.

[Joe Malone]

Ok, the Threshold pot measures 93k and Makeup pot 9.7k. Similar on other comp.

Ok either swap the BIO500T pcbs between comps and see if the high level swaps to the other comp or stays with the current faulty comp.

Or unplug the bio ribbon cable from the sc pcb on the faulty comp and do the 2 link test to see if the fault is on the BIO500 T or not.

Test BIO500/LA500 main pcb by itself by unplugging BIO500/LA500 main pcb ribbon cable from LA500SC pcb and fitting two cut-off component leg loops as shown below.
You should have a unity gain audio loop if BIO500/LA500 main pcb is working. This test proves BIO500/LA500 main PCB and output transformer are both working.
You can plug the relay green wire loop in and out to switch between bypass and BIO500 audio path to check the audio level is correct.

[Everett]

Ok, I swapped the BIO500T pcbs (not transformers) and the high level stayed with the faulty comp.

[Joe Malone]

Ok, I swapped the BIO500T pcbs (not transformers) and the high level stayed with the faulty comp.

Ok points to the problem being on the sidechain pcb in the faulty comp. Can you take some photos of the top of that pcb so I can see all the resistor colour codes

[Everett]

Great, here's some pics of the SC pcb.

[Everett]

SC pics

[Everett]

Resistor readings matched on SC pcbs for both comps.

[Joe Malone]

Resistor readings matched on SC pcbs for both comps.

Yes looks fine. How many dB louder is this comp compared to the off position with no makeup and no threshold?

With both comp on full makeup is there still the dB difference between the comps?

[Everett]

I think something may be grounded on the pot pcb on the faulty comp? When I disconnect the ribbon cable on the pot pcb and measure resistance from any lead to the case, I get readings. If I do the same on the good comp, I don't.
When the ribbon cable is connected, if I measure resistance from any point on resistor lead on any pcb to the case, I get a reading. On the good comp I don't.

[Joe Malone]

I think something may be grounded on the pot pcb on the faulty comp? When I disconnect the ribbon cable on the pot pcb and measure resistance from any lead to the case, I get readings. If I do the same on the good comp, I don't.
When the ribbon cable is connected, if I measure resistance from any point on resistor lead on any pcb to the case, I get a reading. On the good comp I don't.

Ok that will do it. Ok worth a tiny spray of caig pot cleaner or similair or even compressed air into the pot. If that doesn’t fix it you will need a new one of our Dual 20k reverse log pots with pull switch if you can get it if the pcb ok. Or one of our pot pcb kits.

[Everett]

Fantastic! A little spray of pot cleaner did the trick. Both comps are calibrated and ready to go. Thanks so much for the troubleshooting help! Now I'll dive into building my FREQ500 kits.

[Joe Malone]

Fantastic! A little spray of pot cleaner did the trick. Both comps are calibrated and ready to go. Thanks so much for the troubleshooting help! Now I'll dive into building my FREQ500 kits.

Cool Great to hear that solved the problem

[Alex1990]

Worth if you have any pot cleaner to give the inside of the pot a tiny spray and remeasure with pot fully CCW to see if it returns to 10k. Otherwise the pot will need to be removed from the pcb and replaced.

Okay so terribly slow with this over new year but I sourced some pot cleaner and the problem has remained unfortunately!

OK as I mentioned up the page. Can you get the pot of the pot PCB ok without damaging it? Otherwise you will need another pot kit.

Ok managed to get make-up gain pot off the PCB in order to be replaced.

[Joe Malone]

Okay so terribly slow with this over new year but I sourced some pot cleaner and the problem has remained unfortunately!

OK as I mentioned up the page. Can you get the pot of the pot PCB ok without damaging it? Otherwise you will need another pot kit.

Ok managed to get make-up gain pot off the PCB in order to be replaced.

So does the pot measure 2 x 20k or is there still a short in it? Short could be to the metal shaft to one terminal?

[Alex1990]

Ok so there still appears to be a short - in addition to this I had to cut a couple legs to get it out so not exactly workable. My desoldering is getting better but this was a tricky one