LA500A opto comp MAY 2021 BUILD THREAD 500/51X

[slb1959]

Hi...

Great great work Joe.

[Alex1990]

[quote=Alex1990 post_id=5212 time=<a href="tel:1701388593">1701388593</a> user_id=2368]
Hi- So with both SC and/or BIO boards unplugged it’s still 300ohm

Pull out the pot pcb out and if it measures 10k try measuring from the pot legs to the pot metal frame to see if there is a short internally in the pot.

Let me know what you find.
[/quote]

So with the pot PCB out it is still reading 300ohm. I can’t seem to get a reading between any of the pot legs vs the metal frame, so I’m not sure about that.

[Joe Malone]

[quote=Alex1990 post_id=5212 time=<a href="tel:1701388593">1701388593</a> user_id=2368]
Hi- So with both SC and/or BIO boards unplugged it’s still 300ohm

Pull out the pot pcb out and if it measures 10k try measuring from the pot legs to the pot metal frame to see if there is a short internally in the pot.

Let me know what you find.

So with the pot PCB out it is still reading 300ohm. I can’t seem to get a reading between any of the pot legs vs the metal frame, so I’m not sure about that.
[/quote]

Can you send me a couple of photos of the makeup pot soldering? Can you get the makeup pot of the pcb ok?

[Alex1990]

Yep just desoldered meter so it’s all good.

Hope this helps.

[Joe Malone]

Yep just desoldered meter so it’s all good.

Hope this helps.

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.

[PeterBregman]

This might be a dumb question, but why is the ratio switch a DPDT rather than a SPDT? It looks like it could have been done with a single pole.

[Joe Malone]

This might be a dumb question, but why is the ratio switch a DPDT rather than a SPDT? It looks like it could have been done with a single pole.

It is to use the same switch for all toggles and make mounting easier as it stops the toggle twisting when done up. I also often dual contact switches and parellel pots like in LA500A to get a 20+ year life out of heavily used moving parts as I come from a background of redundant design for critical industrial electronics where failure would be very bad

[Alex1990]

Yep just desoldered meter so it’s all good.

Hope this helps.

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!

[Joe Malone]

Yep just desoldered meter so it’s all good.

Hope this helps.

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.

[Everett]

Hello Joe,
I recently went to check calibration on my two LA500A comps and discovered an input level issue. They were built from kits over 5 years ago. I connected to an extender and from my DAW, setup a signal generator plugin to send 1k tone at -18db and set the send level at -4db. On one unit I had to adjust the signal to -19.25db to get 1.23vac (measured on pins 2 and 4 on edge connector) and the other unit had to adjust the signal level to -24.5 db to get 1.23vac (about a 4.7db difference). Wondering what could be causing the level difference?

[Joe Malone]

Hello Joe,
I recently went to check calibration on my two LA500A comps and discovered an input level issue. They were built from kits over 5 years ago. I connected to an extender and from my DAW, setup a signal generator plugin to send 1k tone at -18db and set the send level at -4db. On one unit I had to adjust the signal to -19.25db to get 1.23vac (measured on pins 2 and 4 on edge connector) and the other unit had to adjust the signal level to -24.5 db to get 1.23vac (about a 4.7db difference). Wondering what could be causing the level difference?

OK first switch both comps to off and measure the level on the output to check it is the same as the input level you are sending in ? Y / N

Then with Threshold and Makeup fully counterclockwise switch both comps to VU/OFF/GR. Does the output level stay the same (within +/-1dB) when switched from GR/OFF/VU? Y / N

If No does one of the comps have less output in VU or GR or more output in VU / GR?

[Everett]

Yes, both comps output level is the same as input level when switched to off.

No, one of the comps output level does not stay the same when switched to VU/GR. It has more output when switched to VU and GR.

[Joe Malone]

Yes, both comps output level is the same as input level when switched to off.

No, one of the comps output level does not stay the same when switched to VU/GR. It has more output when switched to VU and GR.

Ok remove the jlm99v opamp from comp that goes higher output and send me a couple of clear well lit photos of that pcb top do I can check the component values. Also photo of the underside of the pot pcb so I can check soldering.

[Everett]

Ok, here are the pics. Hopefully, lighting is OK.

[Joe Malone]

Ok, here are the pics. Hopefully, lighting is OK.

Ok that looks built fine. Unplug the ribbon cable going to the pot pcb and measure the threshold and makeup pots outside solder pads while pots fully CCW. Threshold should measure 100k and makeup 10k. Let me know what you measure.