Blackstar HT20 MKII standby problem (probably)

[The Dude]

Here's a forum link from the WayBackMachine. You won't be able to post, but if it helps....... Maybe there is existing information that would be useful.

https://web.archive.org/web/20190910...mps.com/forum/

[mraggy]

Delta, do you think you could possibly share or send (mraggy at gmail) me the front end input section of the newer HT20 amp? Troubleshooting a strange signal gain reduction issue that follows throughout all channels and appears to be right at the front somewhere. Any help is very appreciated!

[Delta362]

mraggy , please start a new thread for your issue. I've also snipped the front end of the schematic and attached it per your request.

[Jon Snell]

Check D13, 14 & 17 plus the speaker jack ground switching and the ground switching on the input jack.
When a speaker is plugged in, pins 1 & 2 of IC12 go high and when there is an input jack inserted pin 5 also goes high. If you look at a logic table for this chip the output pin of IC 11 pin 1 goes low, allowing the bias to stabilize through D15.
Is all that happening? If it isn't TR4 may be at fault.