Blackstar 40 weird behavior - randomly blows Main fuse

[spunko]

Hello there!

Amp is Blackstar HT40 MKII

Like the tittle says, this amp blows the main fuse randomly. It only happens with the "power" swicth on / standby off. Most of the time works pretty well.
No other fuses blown.

These are the symptoms:
- When it fails, just with power on swicth/standby off, the amp makes some loud noises (pops, scratchs) starts to hum for a couple of seconds and then blows the main fuse. This could happen between 20 sec. to a minute of being on. Sometimes it doesn't make any noise, and the fuse blows just after I flip the power swith on.

- When it hums, or make noises, if I switch it off quickly, it doesn't blow the fuse. And if I inmidiately turn it on again, works fine or it blow the fuse (random behavior).

I took out the power tubes (EL34) and tried again several times just with the power swicth on and off, but this time it wouldn't blow the fuse, so I thought maybe a new set of tubes will fix it, but no cigar, it blew the main fuse with a set of good working tubes. I also tested the original power tubes on another amp, and they work perfectly.

With the amp power switch on / standby off, I took some readings (without power tubes), and I got plate volatge (470v) at pin3 on the socket, so I guess the standby cuts the signal, not the voltages. 

The readings were really odd:
- Negative voltage both EL34 on pin5: -70VDC
- Heater voltage is 6.3VDC. On one tube just pin2 reads 6v, pin7 reads 0v. But on the other tube is the other way arround, pin2 reads 0v and pin7 reads 6v.

- Using a bulb limiter, amp on/standby on, working condition, no blown fuse, amp sounding great:
  *Bias probes read 0mA which is odd. I even thought my probes were bad, tested them on another amp and they work fine.
  *Negative voltage on pin5 keeps at -70VDC.
  *Heaters same as explained above.
  *With the bias probes installed, switching off the amp (standby off, then power off), it reads 12mA for a second and goes to 0mA as expected.

This is the info I've got for now, I'm still searching the web for the schematics.
Dunno where to start 

[sluckey]

Pin 5 at -70 volts will cut off most EL34 and cause the bias probe to read zero. Most EL34s run with pin 5 at -35v to -40v.

[Latole]

Schematic ;

https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/30029-asking-the-impossible-blackstar-ht-club-40-schematic

One of the few pages ;

[Latole]

Good luck for working in this amp