Soundcraft 1600 PSU Voltage issue

[portersage]

Hey all, I had the PSU for my Soundcraft Series 1600 console partially fail recently and have been trying to get it working again. Only the +24v rail failed, so after a recap and a new LM338K regulator I’m getting voltage again, but it’s showing to be just over +40vdc instead of +24. I have a hard time believing that it’s that much over just due to testing with no load. The top portion of the schematic is the circuit for the +24 section if anybody has any ideas. Thanks!

 

[mjrippe]

Sounds like your regulator isn't regulating. Make sure it is installed correctly, and insulated if necessary. Check for AC in case the bridge rectifier has failed. Also, add protection diodes (D1, D2) as shown in this sample circuit:
https://images.app.goo.gl/8uyW1EfgSsbMWXN39

Mike

 

[Brian Roth]

What supplier sold the replacement 338K? IE, a reputable place like Mouser/Digikey/etc or Allibooboo <g>?

Bri

 

[portersage]

What supplier sold the replacement 338K? IE, a reputable place like Mouser/Digikey/etc or Allibooboo <g>?

Bri

Pulled out of a different soundcraft PSU. I’m aware of the fakes out there

 

[portersage]

Sounds like your regulator isn't regulating. Make sure it is installed correctly, and insulated if necessary. Check for AC in case the bridge rectifier has failed. Also, add protection diodes (D1, D2) as shown in this sample circuit:
https://images.app.goo.gl/8uyW1EfgSsbMWXN39

Mike

Ok thanks! I’ll check that. I replaced the bridge rectifier as well when it failed initially but I’ll check for AC now. I’ll try the protection diodes as well

 

[portersage]

Sounds like your regulator isn't regulating. Make sure it is installed correctly, and insulated if necessary. Check for AC in case the bridge rectifier has failed. Also, add protection diodes (D1, D2) as shown in this sample circuit:
https://images.app.goo.gl/8uyW1EfgSsbMWXN39

Mike

Now that I’m thinking about it, I thought the heat sink was non conductive (weird purple stone feeling material), so I wasn’t super careful about making sure the leads weren’t touching the through holes. Could that be the issue?

 

[Brian Roth]

Do you have a pic of the purple heat sink? The description doesn't ring any bells.

Bri

 

[Harpo]

Any load connected (min.load current 5mA). A temporary 4k7 will get you there.
C5 und C6 with a 25VDC rating won't be happy with 40V kicking their butt.
Is the current setting R2 still 100R (min.rating 0.25W)?
Is the voltage setting R1 still 1k8 (min.rating 0.25W)?
Double Check the correct operation of the rectifier.
..Just some ideas..

 

[Riley Casey]

C6 specced at 25V seems a bit odd on a 24 VDC supply quite aside from any other issues

 

[Johnny1234]

Only two active parts. My bet replace the regulator and insulator.

 

[Matt Syson]

Black anodised heatsinks do go a bit purple when too hot for too long. Technically the anodising is an insulator but can NOT be relied to insulate correctly all the time as it is easy too puncture.