DBX 160VU - No Output.

[smilan]

Hi,
I have a DBX 160VU on the bench and I need some help with troubleshooting.
The problem is that as soon as the ratio is other than 1:1 there's no output.
Even with no signal at the input the "ABOVE" LED is bright.
While checking voltages the +24V measured +21.6V while the -24V measured -24V.
While tracing the signal I found out that the first half of OA 3 works fine, but there's no signal coming to the first half of OA 4 (after the VCA board.
I disassembled the RMS board and now there's a signal coming from the VCA board.
I tested the components of the RMS board and replaced the ML1458 with a 4558 but as soon as I connected the RMS board the +/- 15V on the RMS boards drops down to +/-11.5V, the "ABOVE" LED is turned on and there's no output.
One more thing is that the RMS schematic shows 6 transistors while on the RMS card there are only 3. I have no idea what is the "domino block" with the 6 holes?
There's a way of testing the RMS board outside the main circuit?
If so, what should I look for?
Other things I should test beside the RMS board?
By the way, I was servicing this unit a year ago and replaced all the electrolitycs and it worked well until this Sunday.

 

[beatnik]

The RMS module is prone to failure and IIRC with the symptoms you describing, I don't remember if the weak link is the OTA or the LM301 but I would say start with the 301 and if that doesn't fix it get a new OTA. Maybe keep a few spares because the original OTA is obsolete but some ebay sellers stock genuine vintage ICs if you are in Europe I also recommend electronicpool.de for vintage semiconductors
BTW also IIRC I don't think you can use IC sockets on the RMS module because they would make the internals too tall and the module wouldn't sit flush with the pcb anymore
The domino block is for thermal coupling some of the VCA transistors