Neotek Elite weird PSU issue

[Studio Mollan]

Hi,
I have an issue with one of he power supplies to my Neotek Elite desk.
+18v, -18v and -25v dips radically when i power on the desk. Sometimes they stay on for half a minute but the goes down again. They seem to work when disconnected from the desk, without load.
I have just have both PSUs serviced as they were blowing fuses, replace CR111, cr112 (just after the fuses in +/-18v section)

The weird thing is that now the voltages drop but all the fuses are intact. I have measured the voltages in the desk when it dipps to:
+18v reads +0,5v
-18v reads -0,5v
-25 reads -7v

-18v and -25v have a connection within the PSU as you can see inte the schematics. Probably for some sort of sensing or a voltage reference for the -25v. Some of the logic circuits actually use the potential between -25v and -18v (-7v) so it makes sense.
Anyone have an idea on how to solve this? Anything really, i need to get this fixed ASAP.
/
Emil

 

[gyraf]

..have you tried powering up with fewer modules?

..removed all channels and tried powering up?

..mounted one at a time then powering up - to find the channel that shorts out?

in the PSU, is all mounted correctly to heat sinks - over-heat power down seems likely

 

[Studio Mollan]

Hi Jacob, and thanks for a quick reply.
No i haven't done that yet. What's weird is that three voltages drops at the same time, and only in one psu. For some reason i suspect PSU failure. All is mounted as it should and it all started this morning when the desk had been turned of all night. Heat may absolutely be an issue but probably not "directly" related to this specific issue. 
Also, neither the Current limit or voltage limit leds turn on when it happens,

 

[JohnRoberts]

To expand upon what Gyraf suggested do the individual power supplies work with external loads, one at a time or together?

Does it get hot before folding back..?

JR

 

[Studio Mollan]

The voltages dip immediately so not directly heat related, i guess? Could definitely have been caused by heat over some time but heat has no direct cause on the voltage drop.
I have managed to get my spare PSU up and running and so far it works. The spare has not been modified to eliminate transformer hum so it is a bit annoying but given the circumstances id rather be able to work.

I'll have to send the broken one back to my tech for further investigation.
It is hard for him to measure correctly without the actual load from the console though. Do you really need an expensive dummy load for this or is there a cheaper shortcut to fake the load from the console on the supply?
Thanks for your replies!
/
Emil

 

[Studio Mollan]

Didn't quite answer this.
One PSU is working fine. the faulty one gives the correct voltages when NOT connected to the desk. When connected it dips +/-18v and as the -25v uses the -18v a reference that goes down to -7v.
Could be the rectifier for +/-18v perhaps? Or something that the bipolar voltages share as it happens instantly to both. There is no short between the +/-18v. Neither to themselves nor ground. I've read on the notes forum that the LM399 has caused similar issues fro a couple of other notes users. That seems to only be directly connected with the negative supply though.

To expand upon what Gyraf suggested do the individual power supplies work with external loads, one at a time or together?

Does it get hot before folding back..?

JR