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Build help and troubleshooting => LA502 Optical Compressor => Topic started by: bizzlebot on January 24, 2024, 12:32:53 AM

Title: LA502 - Only 12V output during Setup Step 5
Post by: bizzlebot on January 24, 2024, 12:32:53 AM

Trying to setup my newly built LA502 and getting stuck on Step 5 - 
I just finished building a LA502 (I've built about 15 500 series projects so have a little bit of experience). When I was doing the setup, I verified I'm getting 16V and -16V, and I'm getting 1V exact VAC when bypassed. When I flip the switch to in, the range of the gain is about 0.3V to ~12V. I cannot get it to go over 20V per the instructions and am stuck.

I've re-flowed every joint, ensured there are no bridges, and validated that the gain increases after I add a single OPA2134 1 by 1 in each of the 4 slots.

Any suggestions on where to troubleshoot to see why its not able to get over 20V? 

One other note, parts list still shows P5 as part number 502/5K vs 103/10K (pictures/instructions are correct).

Step 5 instructions:
Set the Bypass switch to IN. Increase the GAIN pot and check that you can see the voltage on output increase up to 20VAC at least. Adjust the gain pot to get 15VAC Now turn the PEAK pot clockwise and check that it makes the output voltage go down to 1VAC or less.

Title: Re: LA502 - Only 12V output during Setup Step 5
Post by: bizzlebot on January 24, 2024, 12:57:22 AM

One more note, I followed all the steps in the troubleshooting guide and got the expected values but it still won’t go over 12v: https://www.soundskulptor.com/forum/index.php?topic=19.0


R2=0.7
R5 = 0.49
Tp1 = 13/14V (something like that)

Since its in circuit I got the in-circuit values (5ohm etc).

thanks again!

Title: Re: LA502 - Only 12V output during Setup Step 5
Post by: JPK on January 24, 2024, 09:30:27 AM

You get the good value on TP1 so the issue is in the output transformer or after.
Is R17 correct (1k: brown, black, black, brown, brown)?
Can you measure the secondary DC resistance with the module unplugged between fingers 2 and 4 (~23 ohms)?
If OK you should remove the output transformer and check the primary and secondary resistances.

Title: Re: LA502 - Only 12V output during Setup Step 5
Post by: bizzlebot on January 24, 2024, 06:38:37 PM

22ohms between pins 2 and 4. R17 is the 1 watt large resistor and is brown black black brown brown. 

To confirm, I should desolder the output transformer and test the pins?

Thanks for your help you’re the best!

Title: Re: LA502 - Only 12V output during Setup Step 5
Post by: bizzlebot on January 24, 2024, 07:08:22 PM

Desoldered it. 
1-3 - 9.17
6-8 - 9.3
9-12 - 11.33
13-16 - 11.42

Let me know if I should check anything else. 

Title: Re: LA502 - Only 12V output during Setup Step 5
Post by: JPK on January 25, 2024, 11:04:57 AM

The transformer is good.
Do you confirm you get 13/14V on TP1, against 0V?
Resolder the transformer and measure the output voltage you get across R17 (one probe on each side of R17).
If the voltage is OK (20 VAC) then the issue must be in the relay RLY1.

Title: Re: LA502 - Only 12V output during Setup Step 5
Post by: bizzlebot on January 25, 2024, 09:07:20 PM

I’m getting 14.33 at tp1 vac
I’m getting 27.5vac across r17

It sounds like it must be the relay then?

Title: Re: LA502 - Only 12V output during Setup Step 5
Post by: JPK on January 26, 2024, 04:12:09 PM

First check your connections to the output. Can you measure the output voltage on the finger connector (between pins 2 & pin 4).
Then, if not good, you should try replacing the relay.

Title: Re: LA502 - Only 12V output during Setup Step 5
Post by: bizzlebot on January 26, 2024, 09:34:25 PM

I’m not sure if my capi rack limits output or what but direct on the module pins 2 and 4 give me the correct voltages. I just took my previously built la502 and it had the same exact scenario (I had calibrated it with a different lunchbox). Anyways it probably worked the whole time and I apologize for wasting your time!

Title: Re: LA502 - Only 12V output during Setup Step 5
Post by: JPK on January 27, 2024, 08:50:27 AM

(https://fonts.gstatic.com/s/e/notoemoji/15.0/1f44d/32.png)