Blackstar HT20 MKII standby problem (probably)

[Delta362]

I have the schematic, but since it's dated 2018, I don't want to get into legal issue with publishing it. But, I'll help as much as I can on this.

In your schematic, the HT20 input switch is tied to the 3.3V thru the R1 resistor, but the MKii does it differently. Instead it comes to the attached switching control scheme. I've also snipped the bias circuit and attached that section. Hope it helps. If not, let me know.

[glebert]

Plugging instrument in does not change the -60V bias. I figure if nothing else I can crowbar a jumper (or maybe a pot) in to give a path to ground for the bias pot.

[christarak]

These amps should be called Black Hole, not Black Star. I was sucked into the black hole recently in the 5w and 100w version. After hours bumping into the black walls I discovered that with no instrument plugged in, there is a shut-down level of negative bias of something like -100v. Plug something in and see if that helps.
Its too late for me, but please save yourself, Glebert. Take a wooden spike and drive it through the main PCB.

[glebert]

PIcked up this amp for super cheap, absolutely no sound out of the speaker but the emulated/headphone out works. Looks brand new otherwise. I have seen posts on forums of others having this problem. Problem seems to be the bias on pin 2 of the EL84s is at -60V, which I believe is basically its standby mode (may also happen if there is nothing plugged into speaker jack). I believe the voltage at pin 2 is supposed to go to something like -18 or -7V (there is a test point that says -18V (-7V) but measures -60V) when an instrument is plugged in, but it doesn't. I don't have a schematic for this model, only the HT20 Studio, which has some similarities but also differences. The MKII can switch down to 2 watts (Studio can't), maybe that is the lower bias voltage? MKII has +/-15V and 5V regulators (which all look good) instead of +/-12V and 3.3V. When I measure voltage at the shield of the input Cliff jack I get 0V when nothing plugged in (makes sense) but only about 300mV when plug inserted. I would think it would be something like 5V. I don't see any logic IC like the NAND chip on the Studio that looks at the input jack and speaker jack for plugs.

Anyone worked on one of these? Any thoughts? I wish they hadn't gooped a thick coat of black conformal coat over everything, makes it hard to follow traces.

Here is the schematic for the HT20 Studio, but like I said it is not exactly the same, but I think the basic approach to the bias control is similar. Picture is of the bias part of the board on my amp.

TIA,
Greg