LA500 opto comp leveling amp BUILD THREAD 500/51X

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LA500 opto comp leveling amp BUILD THREAD 500/51X

Postby Joe Malone » Tue Oct 04, 2011 12:45 pm

The LA500 is a 500/51x version of our well know MAC opto comp rack version.
For an idea on what the LA500 / MAC comp can do read this AT mag review here

New LA500 slow release attack mod info is http://www.jlmaudio.com/forum/viewtopic.php?f=2&t=253

LA500 recall sheet here

PDF of this build thread here
(PDF made by Eric Sills http://www.stonesouprec.com :D 8) )

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LA500 Parts List
http://www.jlmaudio.com/LA500/LA500%20Complete%20500%2051X%20Module%20parts%20list.txt


Complete LA500 kit with every part down to the last nut and bolt.
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LA500 PCB schematic
http://www.jlmaudio.com/LA500/LA500%20Schematic%20120304.pdf

LA500 PCB kit
1. LA500 main pcb is a stripped down INX5 pcb so its overlay has the exact parts and values marked on it for the LA500 kit to make the build easy.
2. 51X and 500 compatible with 51X pins cut off. Kit fits all 500 and 51X racks and lunchboxes.
3. PCB automatically uses highest power available. +/-16v for 500 and +/-24v for 51X.
4. LA500 PCB also has mix buss out on pin 11 for Radial Workhorse racks.
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LA500 PCB. For 500 series racks cut and file 51X pins off)
Notes and Errata
Note 1. Replacing R19 51R with new 33R give slightly better low frequencies response when using JLM111DC and JLM111DC compact output transformer.
In future kits we will be replacing all 51R with 33R. So R19 and R20 will both be changed to 33R on new PCBs.
Note 2. R4 and R6 should be 5k1 NOT 10k as marked on the LA500 PCB. Edited correct overlay shown below as on all New LA500 kits

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If you are not 100% with resistor colour codes use a multimeter to check values as you place the resistors
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Fit all resistors at once bending the legs sightly outwards to hold them in place. This helps to make sure no resistors are put in the wrong position.
Notes and Errata
Note 1. Replacing R19 51R with new 33R give slightly better low frequencies response when using JLM111DC and JLM111DC compact output transformer.
In future kits we will be replacing all 51R with 33R. So R19 and R20 will both be changed to 33R on new PCBs.
Note 2. R4 and R6 should be 5k1 NOT 10k as marked on the LA500 PCB. Edited correct overlay shown below as on all New LA500 kits

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Solder all resistors while holding PCB firmly upside down on flat surface.
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Cut all resistor legs off at top of the solder joint and double check no solder joints missed soldering.
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Fit the 4 x 1N4007 diodes. Diodes are POLARIZED and must go in with there stripe matching the strip on the PCB overlay.
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Solder diodes in once you have checked there POLARITY is correct.
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Fit 6 x 1mm sockets for 99v opamp. Do NOT press the sockets in. Just let them sit in the holes. Use a piece of cardboard to hold them in while turning the PCB over.
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Solder 6 x 1mm sockets in place
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Fit and solder DIP8 socket in other opamp position. Make sure the POLARIZED socket matches the PCB overlay.
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Fit and solder 3 x 0.1uF MONO caps in place. The 100nF caps are NOT Polarized so can go in either way around.
Caps should be marked 104 or 0.1u or 100n.
27th Dec 2015 a batch of kits has been packed with wrong value 102 1nF caps these will work fine for C14 & C15 but C18 will need to be 104 or 100nF or 0.1uF or larger. Can be any type of mono ceramic or MKT cap as it is only for the meter.
If C18 is smaller than 100nF you will not be able to calibrate the VU meter.
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Fit 5 x 47pF ceramic caps in place. The 47pF caps are NOT Polarized so can go in either way around.
Fold 47pF caps down flat to PCB before soldering them.
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Fit 12v relay in the POLARITY shown in the photo.
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Bend 2 legs out to hold relay in place for soldering
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Solder relay in place once POLARITY has been checked
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Fit and solder jumper pins in place
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Fit and solder IDC headers in the POLARITY shown. Do not reverse. Triangle on header indicates pin 1 which is the square pad on the PCB.
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The front opening in the IDC headers should both now be facing as shown below.
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Fit and solder Electro caps to PCB. The 2 x 100uF caps are POLARIZED so must have there long positive leg fitted to + marked on PCB overlay.
2 x 10uF and 4 x 470uF caps are NON polar types so can go in either way around on the PCB.
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Fit OPA2604AP opamp in the correct POLARITY shown. 99v opamp can be fitted now or later. LA500 PCB is ready to go.
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Put nuts and bolts etc back into zip lock bag the LA500 PCB kit came in to make sure they are not lost while you build the Small MAC PCB next.
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MAC PCB Schematic with MAC alignment
http://www.jlmaudio.com/MAC/MACSCH.pdf

MAC PCB parts kit
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MAC PCB. PCB overlay has all values and there are no options.
so PCB can be assembled directly from overlay.
Notes and Errata (NEW RED COLOURED MAC PCB has all the below errors corrected)
JLM 34mm round VU meter needs D5 to D8 fitted.
LED D9 which is the meter led has A and K marking on MAC PCB are reversed . Rectangle Pad is A (+meter led)
R14 should be changed from 2k7 to 1k8 in all 500 series LA500 versions.
MAC kit now comes with all 1k trim pots replacing the 20k which makes setting the Meter 0VU easier

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Fit all 8 x BAT85 diodes with there Cathode POLARIZED black stripe matching the white stripe on the MAC PCB overlay
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Solder all BAT85 diodes while holding PCB firmly upside down on flat surface. Make sure none of the center close pads are NOT shorted together.
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Cut all diodes legs off at top of the solder joint and double check no solder joints missed soldering or are shorted together.
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If you are not 100% with resistor colour codes use a multimeter to check values as you place the resistors
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Fit all resistors at once bending the legs sightly outwards to hold them in place. This helps to make sure no resistors are put in the wrong position.
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Solder all resistors while holding PCB firmly upside down on flat surface.
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Cut all resistor legs off at top of the solder joint and double check no solder joints missed soldering.
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Fit and solder DIP8 socket in other opamp position. Make sure the POLARIZED socket matches the PCB overlay.
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Fit and solder 2 x 0.1uF MONO caps in place.
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Fit 1 x 100pF ceramic caps in place.
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Newer builds Fit and solder NSL32 optos as shown below. Short legs are LED & Long legs are LDR on NSL-32.
The NSL32 Cathode has a dot marked on the case which goes to -K. Be gentle with the wire legs on the opto when shaping them for the PCB.
Note: If you are building a stereo matched pair. Fit the 2 optos in the plastic bag marked U1 into the U1 position on each separate PCB and same for the bag with the U2 pair.
If building just one comp use the 2 optos in the bag and either opto can go into U1 or U2.

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Older builds Fit and solder 2 x 5C2 optos in place.
Note: If you are building a stereo matched pair. Fit the 2 optos in the plastic bag marked U1 into the U1 position on each separate PCB and same for the bag with the U2 pair.
If building just one comp use the 2 optos in the bag and either opto can go into U1 or U2.

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Fit and solder IDC headers in the POLARITY shown. Do not reverse. Triangle on header indicates pin 1 which is the square pad on the PCB.
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The front opening in the IDC headers should both now be facing as shown below.
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Fit and solder Electro caps to PCB. The 4 x 100uF caps are POLARIZED so must have there long positive leg fitted to + marked on PCB overlay.
2 x 3.3uF acaps are NON polar types so can go in either way around on the PCB.
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Fit and solder 3 trim pots as shown 20k = 203(replaced with 1k), 1k = 102 code. Place pot so trim screw is in corner shown below
Note MAC kit now comes with all 1k trim pots replacing the 20k which makes setting the Meter 0VU easier
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The three switches for the MAC when used with the LA500 have to have there 6 legs straightened with a set of small long nose pliers like below.
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Align 3 switches in MAC PCB as shown with roughly 2mm of the short legs showing.
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Solder one short leg on each switch to hold them in place.
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Wind the back nut fully down on to the 3 switches. Fit the flat washers on next.
Cut the sides of the middle flat washers with sudecutters so it does not overlap the other washers.
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Fit the 3 shake proof washers on next.
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Fit the 3 switches into the holes in stainless steel bracket and do up the 3 nuts hand tight to hold the switches in place.
Push the bottom edge of the MAC PCB back under you have about 45mm from the front of the bracket to the bottom edge of the PCB.
This is to leave enough room for the Stereo LINK pull switch version of the threshold pot which is now standard in LA500 kit.
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Solder all the switch legs you can reach easily while the switches are still fitted to the bracket.
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Remove MAC PCB from bracket and finish soldering the switches and cut the switch legs down to the solder joint.
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Cut the 4 wire colours provided into 75mm, 65mm and 45mm lengths and strip 5mm insulation of all wire ends.
Twist wire strands together and tin with solder. Place colours as shown for meter in PCB and solder in place.
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Wires for the meter go through the top of the PCB and are solder underneath the PCB.
Wires for the pots go though the bottom of the PCB and solder on the top of the PCB.
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Cut off locate tabs on each pot with sidecutters
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Fold the pot legs 90 degrees with log nose pliers and solder and link as shown below. Fit the 2 pots to the bracket and do up nuts hand tight.
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Fit the MAC PCB to the stainless bracket as before and do up the 3 nuts tight.
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Fit the meter to the bracket using 2 of the black screws hand tight to hold it in place.
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Use a piece of insulation tape to cover the opening in the pots so no solder can fall into them while soldering.
Tin the 6 pot terminals with solder. Yellow wire on makeup pot connects to both center and end terminal.
Twisting the coloured wires in there 3 groups and solder the wires as shown on to the meter and pots.
Remove insulating tape from pots.
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Fit the 4 x 12mm countersunk screws found in the LA500 PCB kit to the bracket and screw 6mm nylon spacers to them.
Nylon spacers have countersink on one side that should go down on to the bracket to take the protruding part of screws countersink.
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Last chance to cut off 51X pins if converting the PCB to a 500 version.
Fit LA500 PCB and hold down with shakeproof washers and nuts provided in the LA500 PCB kit.
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Fit ribbon cable provided to join the MAC PCB header to the JLMIO header as shown below.
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JLM111DC with wiring and hardware pack. Be warned do NOT touch the 3 yellow and black wires while handling the transformer as these are very thin wires and can easily be broken
(LATEST LA500 kits ship with the NEW JLM111DC COMPACT transformer with red top PCB to make it easier to slide into 500 racks.)
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Remove link jumper if fitted and fit JLM111DC in place and bolt to the bracket.
(NEW JLM111DC COMPACT transformer with red top PCB needs 2 or 3 flat washers fitted between the LA500 support bracket and two transformer mounts)
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Fit ribbon cable between JLM111DC transformer and JLM111DC header on LA500 PCB.
Use shallow ribbon cable end with no cable relief clip fitted at the transformer end.
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If you have a extender cable you can now test and align the LA500 before fitting the front panel and knobs.
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If you are going to test in the rack remove the pot nuts and washers and any screws holding the meter and fit the front panel.
Do up the pot nuts tight and screw the 4 meter screws in GENTLY and do NOT over-tighten them or you may strip the bracket thread. Lock-tight can be useful in a always traveling lunchbox to stop them shacking loose.
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Best NOT to fit the chicken head knobs until you have fully aligned and tested the LA500.
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Compressor setup procedure
1. Set LA to Threshold pot fully CCW, Makeup pot to CCW. Ratio switch to 3:1, Meter switch to VU, HPF switch to FLAT.
2. Apply 1kHz tone to LA500 input at the reference level (usually +4dBM) you are going to use for 0VU.
3. Check output of the LA500A between pins 2 and 3 of the rack XLR (or pin 2 and 4 of the edge connector) for 1.23vac with multimeter.
If voltage not correct adjust 1kHz generator until correct. Or plug LA500A output into VU or PPM meter that is calibrated to a known level.
4. Adjust meter 0VU trim pot for desired level for 0VU on the meter.
5. Switch Meter switch to GR and adjust Zero GR trim pot for 0VU on meter.
6. Turn up Threshold pot until you have 5dB of compression on meter.
7. Switch Meter switch between GR and VU and adjust Balance trim pot until the same reading is seen in both GR and VU.
8. Your ready to rock.
Joe :-)
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LA500 opto comp threshold pot with link switch mod.

Postby Joe Malone » Fri Nov 25, 2011 8:41 am

Cut off locate tab with sidecutters
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Link pot and switch pins as shown and solder all joints.
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Bend comp link pins to 45 degrees and solder a wire to each pin. Twist wires (remove output transformer if modding a built LA500)
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Solder 3 pots wires back to the new pot in the same order as the old type pot and solder 2 wires from comp link terminals to the switch terminals as shown.
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LA500 now with threshold with pull to link switch fitted. (refit output transformer if modding a built LA500)
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Only 1 in every 2 LA500 need the pull switch pot to link. The LA500 without pull switch will need to have the jumper fitted to the comp link terminals to link.
If both LA500 have pull switch pot both will have to be pull out for the link to work.

Note on link in different 500 racks.
All API 10 racks and API lunch-boxes will need a wire soldered between the 2 link solder pads on the back plane PCB joining the 2 slots the LA500 are fitted into for the link to work.
(This connects pin 6 on the back edge connectors together. Loop this wire up and around the edge connector so it does not block any edge connector.

On Radial workhorse racks click the link switch between the 2 slots for the LA500 pair to on.

Purple TEN racks you will need a TRS to TRS patch lead insert into the back panel as explained on there web site.
Joe :-)

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LA500 opto comp stereo matching mod.

Postby Joe Malone » Thu Feb 02, 2012 10:46 am

1. Check Both LA500 comp alignments are correct.
2. With 1k tone of same level into both LA500 comps set 0VU on meter switched to VU and turn up Threshold pot to get -5dB on meter and switch to GR to check you have -5dB showing on both meters.
3. Link the comps. If meters move away from each other by more than a +/-1dB do mod below to compressor which shows more gain reduction on the meter.

Remove center 1k trim pot by heating all 3 pins at once and pull trim pot out.
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Cut off left leg on removed 1k trim pot and same for new extra 1k trim pot.
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Refit original 1k trim pot now with only 2 legs. Resolder 2 pads with pot legs.
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Trim legs of second 1k trim pot as shown.
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Solder second 1k trim pot into place as shown.
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Original trim pot will now adjust what meter shows when switched to VU (U1 audio opto) and new trim pot will adjust what meter shows when switched to GR (U2 meter opto)
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1. Setup both comp with 1k tone and set 0VU on meter switched to VU and turn up Threshold pot to get -5dB on meter.
2. Turn threshold pot off on modified comp (fully CCW)
3. Link the comps. Adjust modified VU trim pot until both comps show the same VU meter readings.
4. Switch both comps to GR and adjust modified comp new GR pot until both comps show same GR meter readings.
5. Your comps should be matched.

To check match
1. With 1k tone of same level into both LA500 comps set 0VU on meter switched to VU and turn up Threshold pot to get -5dB on meter and switch to GR to check you have -5dB showing on both meters.
2. Link the comps. If meters move away from each other by more than a +/-0.5dB trim modified comps New GR & New VU trim pot until perfect match when linked.
Joe :-)

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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Postby wthrelfall » Sat Feb 04, 2012 12:05 am

Thanks for the info Joe, i've finished building my 2nd LA500 but there's a problem. Compression works, but with compression 'in', the low freqs rolls off around 1khz and there's a level drop around 7db. In bypass the signal is normal. I've swapped over the red pcb with my good unit and the problem is still there. I also swapped the op-amp for a known working one on the green mac pcb and the problem persists. Had a very good check for shorts and bad soldering and can't see anything unusual. So I figure something on the green Mac pcb or one of the pots/switches is causing the problem.. What does this suggest to you?
thanks.

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LA500 opto comp audio path testing without MAC PCB

Postby Joe Malone » Sat Feb 04, 2012 8:47 am

wthrelfall wrote:Thanks for the info Joe, i've finished building my 2nd LA500 but there's a problem. Compression works, but with compression 'in', the low freqs rolls off around 1khz and there's a level drop around 7db. In bypass the signal is normal. I've swapped over the red pcb with my good unit and the problem is still there. I also swapped the op-amp for a known working one on the green mac pcb and the problem persists. Had a very good check for shorts and bad soldering and can't see anything unusual. So I figure something on the green Mac pcb or one of the pots/switches is causing the problem.. What does this suggest to you?
thanks.


OK first thing to check when low end is missing and there is a level drop is the output transformer windings.
With a ohmmeter you should be able to measure roughly 15ohm per winding between +1 to -1, +2 to -2, +3 to -3.
If any winding is open the wire would usually be broken right at the transformer pcb solder joint if any of the wires had pressure on them during handling.

Even if transformer tests ok since you have 2 LA500 can be worth swapping the transformers between to see if the problem follows the output transformer.
Also a common problem is the IDC10 header on the LA500 has had a pin that is not soldered.
If the transformer tests ok swap transformer ribbon cables between the 2 LA500 to check them.

If both of those test ok email me a couple of large macro on photos of the top and bottom of the MAC PCB and your pot wiring for me .

How to loopback link the LA500 PCB to test the audio path is correct. One link operate the bypass really and the other loops the unbalanced in and out together.
Unplug ribbon cable caming from MAC pcb and add wire links. With everything in the audio path correct you should have 6dB of fixed gain.
If audio is down in level and sounds thin with no low end remove output transformer ribbon cable and link P+ to S+ and P- to 0-.
If audio is now ok and unity in and out check transformer and IDC header on transformer for broken solder joint or broken wire.
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Joe :-)

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wthrelfall
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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Postby wthrelfall » Sat Feb 04, 2012 9:22 pm

Yes, it's the transformer. the Ohmmeter readings are ok but swapping the transformers over between units results in the problem moving also. Switched ribbon cables as well but that seems fine.

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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Postby Joe Malone » Mon Feb 06, 2012 4:56 pm

wthrelfall wrote:Yes, it's the transformer. the Ohmmeter readings are ok but swapping the transformers over between units results in the problem moving also. Switched ribbon cables as well but that seems fine.


If the meter reading are ok for all 3 windings then the transformer is ok and it can only be a solder short or solder joint missing on the IDC connector on top of the transformer.

Only the 7 outside pins need to be soldered on the IDC header on the transformer to work. You can see the tracks going from each +/-1 to 3 going to the pins. So it is easy to ohmmeter them out.

If you cannot spot anything send me a clear photo of the top of the transformer so I can have a look.
Joe :-)

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wthrelfall
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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Postby wthrelfall » Tue Feb 07, 2012 7:46 am

got it sorted. one of the solder joints had been pulled up off the board - probably when I pushed the cable in with too much force :oops:

on another note, the 2 compressors aren't compressing the same amount when linked.. Pretty sure they are calibrated correctly but I will check.

wthrelfall
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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Postby wthrelfall » Mon Feb 13, 2012 4:32 am

Hi Joe.
So I've just got around to checking that the 2 units are tracking correctly when linked. I decided to re-calibrate first, but came across a strange problem - I'm sending a 1khz tone through my desk, meters are hitting 0db on the desk, patching the compressors to the master bus. I'm unable to get the meters to hit 0VU on the compressors, the pots will only let me get to -2db on both units. So I just increase the signal by 2db so that I can continue calibrating and check the link. Now I have another strange issue.. with the 2 units calibrated exactly (with 5db GR/VU showing and balanced as described in the instructions), it seems that to achieve the same amount of GR, one unit must have the threshold turned up much higher. I've attached a photo showing the 2 units, unlinked, attenuating the same signal by 10db to show the pot positions. When i then link the units, the units jump further apart, ie. the one compressing less compresses even less, the other compresses even more. I've attached another photo showing the difference in GR between the units when linked (right hand unit is the master with the new linking pot).
any thoughts?
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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Postby Joe Malone » Sun Feb 19, 2012 9:20 pm

wthrelfall wrote:Hi Joe.
So I've just got around to checking that the 2 units are tracking correctly when linked. I decided to re-calibrate first, but came across a strange problem - I'm sending a 1khz tone through my desk, meters are hitting 0db on the desk, patching the compressors to the master bus. I'm unable to get the meters to hit 0VU on the compressors, the pots will only let me get to -2db on both units. So I just increase the signal by 2db so that I can continue calibrating and check the link. Now I have another strange issue.. with the 2 units calibrated exactly (with 5db GR/VU showing and balanced as described in the instructions), it seems that to achieve the same amount of GR, one unit must have the threshold turned up much higher. I've attached a photo showing the 2 units, unlinked, attenuating the same signal by 10db to show the pot positions. When i then link the units, the units jump further apart, ie. the one compressing less compresses even less, the other compresses even more. I've attached another photo showing the difference in GR between the units when linked (right hand unit is the master with the new linking pot).
any thoughts?


OK is the master buss insert fully balanced +4dBM or unbalanced -10. As it should be easy to get meters to 0VU = +4dBM if balanced.

The difference in threshold pots could be down to one threshold pot being linear and the other being log as some kits originally shipped with 100kB linear pots. All pull switch pots are 200k log and need both the front and back half of the pot paralleled to make a 100k log pot.

There is no master compressor when linking old style compressors. You must adjust both comps for the roughly same amount of compression on music before linking the 2 devices. With 1k tone into both comps and 5dB Gain reduction the meters should only deflect away from each other by a 1 or 2 db on each meter when linked which is fine and will not show on music signal. If error is greater first check mechanical -20 point on both meters is set the same with meter front adjustment before calibration then check calibration is not out of adjustment if both are ok then one of the optos may have settled to far out of range.

Since the release of the pull link switch version of the LA500 kit at the end of January this year 2012 we have started adding a 3 digital number to the side of the opto cells when we test them to make stereo pair matching better than it really needs to be. Even if the second unit is ordered at a later date with the 3 digit number we can match units.
Joe :-)

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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Postby no2 » Thu Feb 23, 2012 12:57 am

Here's my thoughts on being a complete beginner to DIY electronics and choosing the LA500 to be your first project. This was a very enjoyable project and I still have a little trouble shooting to do but I am sure glad that I didn't try to build an 1176 right off the bat. If I was at the same place with one of those as I am with the LA500 I would be so frustrated and discouraged by all the possibilities of problems and trouble shooting I would have before me. With this opto comp I have to figure out a few things but it's not too crowded and it's small. And there's help offered here. The pictures and instructions are nearly perfect for making it as easy as possible for a novice. There's a couple minor things I wasn't sure about. I did get a few extra resistors and someone else pointed out an extra red LED. Those didn't cause me any problems, though.

After building this what I experienced confusion with was the non-stereo linked threshold pot. Since the 1st picture on the top of the thread has a different threshold pot, I was wondering what do I do with the additional parts that I have?
For example: do I pinch all 6 tabs together and solder them. In a picture further down the thread you see 2 of the pairs of tabs bridged for the stereo link. In another picture you see all 6 soldered together. I guess for a beginner that doesn't know what any of it does, it can be a tad bit confusing.

Also, the makeup pot has a bridging across an outside terminal (right word?) and the center one. I didn't catch this in the instructions but see it in the picture, so I just soldered a little end-piece of a resistor wire across it. Will that work?

At this point I am able to calibrate(right word?) the VU meter but not the GR part. The light comes on!!!!! I did something right! I tested all the resistors after installation to PCB on the front and back sides of the PCB and they appear to be working. Though some of them give me an instant reading and others I have to move the leads around a bit to get it. Does this indicate poor soldering for some of them?

I also have no idea how to test the opto cells. I tried to check for continuity after I accidently thought it was a near by resistor and checked for resistance. I don't know if sending a current thru opto cells can damage them or not.

I checked my capacitors with an ohm meter and they just kept increasing in resistance value. I guess this is them charging? Don't know how to do my capacitor test.

I checked the diodes too and got a 0.2 volt reading on all of them.

I also burned one of the resistors when trying to solder one of the switches. I read that it will be fine and it still gives the correct reading.

I have a million questions if anyone would like to educate me or have any valuable comments please do so.

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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Postby Joe Malone » Thu Feb 23, 2012 8:25 am

no2 wrote:After building this what I experienced confusion with was the non-stereo linked threshold pot. Since the 1st picture on the top of the thread has a different threshold pot, I was wondering what do I do with the additional parts that I have?
For example: do I pinch all 6 tabs together and solder them. In a picture further down the thread you see 2 of the pairs of tabs bridged for the stereo link. In another picture you see all 6 soldered together. I guess for a beginner that doesn't know what any of it does, it can be a tad bit confusing.

The 2 x 200k pot sections need to be bridged to make a single 100k log pot as the comp needs 100k load to align properly. But the 6 switch contact can be bridged or not. We use both sides of the switch for longer reliable life. Only the contacts used need to be bridged together. All kits now ship with the new 2 x 200k log threshold pot with link pull switch as standard.

Also, the makeup pot has a bridging across an outside terminal (right word?) and the center one. I didn't catch this in the instructions but see it in the picture, so I just soldered a little end-piece of a resistor wire across it. Will that work?

Again the bridge to the outside terminal is just good practice encase the pot ever goes open wiper but leaving it out makes no difference.

At this point I am able to calibrate(right word?) the VU meter but not the GR part. The light comes on!!!!! I did something right! I tested all the resistors after installation to PCB on the front and back sides of the PCB and they appear to be working. Though some of them give me an instant reading and others I have to move the leads around a bit to get it. Does this indicate poor soldering for some of them?

Ok I looked over your photo's and the wiring and top of the MAC pcb looks ok but I need a couple of photos around the switch soldering on top of the mac to check there for shorts and a photo of the underside of the MAC pcb to check the soldering for shorts there. Most faults I see are solder shorts between solder pads and wrong wiring. Meter reading of resistors in circuit is difficult as any capacitance in the circuit near by will have to charge or may have another resistor across it so meter reading may seem wrong or keep changing. Turn the switch to GR and measure the voltage across the meter (inside terminals)?

I also have no idea how to test the opto cells. I tried to check for continuity after I accidentally thought it was a near by resistor and checked for resistance. I don't know if sending a current thru opto cells can damage them or not.

Opto cells are fully tested here before shipping and are fairly hard to destroy

I checked my capacitors with an ohm meter and they just kept increasing in resistance value. I guess this is them charging? Don't know how to do my capacitor test.

yes they are charging. Again we are not looking for a faulty part but usually a short between to solder joints. Need to use a magnifying lens to check all soldering very carefully.

I checked the diodes too and got a 0.2 volt reading on all of them.

Ok correct. But because you could get the meter to 0VU this proves the diodes already.

I also burned one of the resistors when trying to solder one of the switches. I read that it will be fine and it still gives the correct reading.

Yes as long as it is not shorting to the switch soldering.

Let me know the voltage you get across the meter when switched to GR?
And if you are getting audio out of the LA500 when switched to GR or VU?
And email me clear photos of switch soldering on top of MAC PCB and photo of bottom of MAC PCB.
Joe :-)

JLM Audio

Capturing Audio without Injury

no2
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Joined: Mon Feb 20, 2012 12:42 am

Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Postby no2 » Thu Feb 23, 2012 3:25 pm

Thanks Joe for your efforts and time. I sent those photos and I'll get back to you on the other stuff soon.

no2
Posts: 9
Joined: Mon Feb 20, 2012 12:42 am

Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Postby no2 » Fri Feb 24, 2012 12:37 am

Let me know the voltage you get across the meter when switched to GR?
And if you are getting audio out of the LA500 when switched to GR or VU?


I am getting an audio signal through the comp in both GR and VU. I am able to move the meter a lot in VU and just a smidge in GR
I'm not sure how to read voltage across the meter. Am I sticking leads inside the lunchbox while it is powered on?

I also removed some solder on the switches and nudged the resistors nearby, away from the switch pads (pads? that's the term for where you solder a component to the PCB, right?)
I didn't get any change from doing so, however.

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Re: LA500 opto comp leveling amp BUILD THREAD 500/51X

Postby Joe Malone » Fri Feb 24, 2012 6:49 am

no2 wrote:
Let me know the voltage you get across the meter when switched to GR?
And if you are getting audio out of the LA500 when switched to GR or VU?


I am getting an audio signal through the comp in both GR and VU. I am able to move the meter a lot in VU and just a smidge in GR
I'm not sure how to read voltage across the meter. Am I sticking leads inside the lunchbox while it is powered on?

I also removed some solder on the switches and nudged the resistors nearby, away from the switch pads (pads? that's the term for where you solder a component to the PCB, right?)
I didn't get any change from doing so, however.


OK you should have about 4v on the outside terminals of the zero trim pot? As there is only a 2k7 resistor in series with the pot and the 16v rail.
How did you set +4dBM (1.23v rms) on output XLR to be 0VU on meter? If this is set wrong (to high level) you will only get a small movement on the GR meter from the zero pot. Also are you using the full 20 turns on the GR zero meter pot ?

I am away until Tuesday
Joe :-)

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