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EQ573 Equaliser / Re: EQ573-II ran in my lab

« Last post by Slembones on July 15, 2026, 10:18:42 PM »

This sweep has about 6dB boost and cut you can see the roll off. I also included a flat setting 

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EQ573 Equaliser / Re: EQ573-II ran in my lab

« Last post by Slembones on July 15, 2026, 10:02:09 PM »

 the red Line is switched out and the blue Lin is switch in but with all bands "off"

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EQ573 Equaliser / Re: EQ573-II ran in my lab

« Last post by Slembones on July 15, 2026, 09:41:51 PM »

the unit is beginning rolling off at 10K even with the band switches set to "off"

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EQ573 Equaliser / Re: EQ573-II ran in my lab

« Last post by Slembones on July 15, 2026, 09:40:06 PM »

can I mod the unit to be flat to 50K and restore the shelf

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EQ573 Equaliser / Re: EQ573-II ran in my lab

« Last post by JPK on July 15, 2026, 06:34:38 PM »

It is a shelf but it is followed by a high frequency attenuation after 20kHz caused by the compensation caps in the circuit.
Can you make another diagram with a +/-6dB correction?

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LA502 Optical Compressor / Re: 502 Compressor not working - test results.

« Last post by JPK on July 15, 2026, 06:26:40 PM »

Remove U1 and check again R5 right pin.
You can keep U1 removed until this is fixed.
Check the value of R5. It will be lower than 100k because of the other components around.
Checl the optocoupler is in the right direction.
Look for solder bridges or bad joints around R2, R3, R4 and the ratio switch.

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EQ573 Equaliser / EQ573-II ran in my lab

« Last post by Slembones on July 15, 2026, 04:39:16 PM »

 this is the hi freq eq tests. the boost is a bell when it should be a shelf. the cut is a shelf. Did I do something wrong?  please advise. the unit works fine but thought I would lab test. the plots for mid and low are as expected

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LA502 Optical Compressor / Re: 502 Compressor not working - test results.

« Last post by noeqplease on July 14, 2026, 07:09:10 AM »

pic of the OPA2134 tests...

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LA502 Optical Compressor / Re: 502 Compressor not working - test results.

« Last post by noeqplease on July 14, 2026, 04:41:11 AM »

Just tested all 6 OPA 2134 ICs, all passed the test, both amps on each IC.

Will continue testing...

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LA502 Optical Compressor / 502 Compressor not working - test results.

« Last post by noeqplease on July 13, 2026, 04:16:31 AM »

Hi,

So one of my 4 LA 502 compressors did not work when first turned on. today I tested all the items on your handy troubleshooting list.

1. Checked for bad / cold solder joints, did not find any visible ones. Tested some different places on the circuit, and was able to get readings everywhere pretty much.
All ICs, transistors, and capacitors are in their correct orientations. No leads are near the chassis plate at all.

2. Checked DC voltages: + voltage is 16.14VDC, - voltage is -16.60 VDC. On TP1 I do get 1.105 VDC with the circuit bypassed, 1.04 VDC with the circuit engaged. So, there is some issue.

3. Applied a 1 VAC 1kHz signal to the input to follow it: Black probe on 0 V point.
R2 right leg reported 0.69 VAC
R 5 Right leg reported 0.025VAC with circuit in bypass, 0 VAC when engaged. REV 1.4
TP1 Reported 0 VAC, and no change when turning the GAIN knob.

So, something is definitely up. Please let me know what else I should test and how, so I can pinpoint the issue.

Thanks!!!

PS the other 3 compressors work beautifully.