One (right) preamp blow! I've located the problem to the right C160 module. When disconnected everything seems fine. Diodes and transistors are apparantly ok measured onboard as are all resistors and nothing seems "burnt". Power on and the output is nearly 33 volts - the healthy channel app. 0 volts

Dave , If that fuse is blowing , there is a problem in the power supply or output section. If you have a meter that can test diodes / transistors. please check the output transistors first. Most likely one or more are shorted. If they check ok , you should check the power supply diodes .You can order new ones from a number of online stores by simply putting the part number into google. Hope this helps ?

If you are pulling the transistor that drives the final and the problem goes away, is the driver bad? Is the power supply a multi channel isolated unit? Could the channel for that side be bad?

Well you are on the right course. I suspect you think the problem is in the power supply and it's probably not. I think that what's gone is either a transistor (or more) or an IC amp - if it uses one - on the main power amp stage. Look at the devices on the heat sink. I think that's where you will find your shorted device.

I would also replace 2sa992 (2pcs) and 2sc1845 on preamp board- sometimes they look ok when you test them by regular meter. Check 1.2 kOm and 4.7ohm resistors at preamp pcb. Check diodes on output pcb (could be smd type, connected from base of output transistors to emitter i think)

The fault may not be in the power supply area! The other area of blame is the main amp section. Suspect any of the devices on the heat sink (also any device in the power supply with an heat sink). Transistors can be tested easily with this problem. An Ohm Meter on the resistance setting, on the terminals (even in circuit) of the transistor, will act like you have touched the test probes together. Just like a piece of wire, because that's what it has become and why they blow the fuse!

When a fuse blows suspect an overload in the power supply or power amp section.
For culprits in the power supply check the transformer, large capacitors, rectifier, voltage control semi-conductors, diodes.
In the amp section any semi-conductor on the heat-sink.

Semi-conductors - transistors and IC's are the biggest cause of fuse blowing. They become short-circuit acting like a piece of wire!
Because of this transistors and diodes are easy to spot with an Ohm Meter, as it will be like you have just touched the probes together.
Very often they have holes, or damage, or have burnt up with this kind of fault.
If it is a fault on the main amp section, check your speaker wiring as the + & - wires could have touched to cause it.

Fuse blowing is due to excessive current drain and the the first clue is the blown fuse itself. If the fuse blows with a flash a bang and splatters copper across the inside of the glass you have a hard blow caused by a dead short on the power supply, check for power switch flashing over internaly. Short circuit main filter capacitors. Shorted turns in power transformer primary winding. If fuse just separates check for short on output transistors C to E. Try isolating fault by disconnecting LT pos/neg supplies to output stage. Also check for partial shorts on speakers and speaker wires using an analog ohm meter on low ohms setting.

Between the ground and the "PRD" pin of the CPU, there are three diodes: D503, D504 and D505. The first two seem to be useful to protect the CPU itself against any over-current. In case of short circuit or similar, these 2 diodes can easily blow out, and force to "0" the pin 139 voltage of the CPU ("PRD" pin, dedicated to detect any abnormal DC voltage from output transistors)
This is the reason why the protection circuit shuts-off the unit, even if the faulty power transistors have been replaced

Diodes are normal surface-mounted 1SS355: I've replaced them with two 1N4146, and everything works perfectly now.
Diodes are located in the "Function" board, at the left side of the unit: they are easily accessible

I've realized that the problems seems to be extremely common for this model, so I hope that this hint will be useful in several cases

If the output module(s) has/have not failed (you need to know a bit to check this), then the power supply problem is probably traceable to a failed capacitor that caused the further failure of a switching transistor.
Locating this problem requires some electronic knowledge and at least a (digital) multimeter.

About the only thing you can do by eye is look for cans, normally mounted vertically on the board, mostly with the aluminum of the case visible at the top and shrink-wrapped sides that have values printed on them.
If you see any that have cracked tops or deposits on the board nearby, these may have caused the problem but the major component that is causing the fuse to blow is/are diodes or switching transistors on the AC side or shorted diodes on the DC side.

Because the failure might be related to bad capacitors, not having any way to check them pretty much precludes do-it-yourself repairs.

Replacing a blown fuse is sensible; replacing it twice is futile.

I can hardly even tell what you're talking about because you don't use any punctuation.