Blackstar HT20 MKII standby problem (probably)

[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.

[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.

[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!

[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/

[Delta362]

Thanks, Steve! While I was able to offer a solution to this problem, I'm far the guru that many others are on here. But, I'm here to learn and to help as much as I can.

I've fixed several of them (Stage, Studio and Club series of various wattages and various Mark series) these last few years. Now the ID Core series, those are disposable/non-field serviceable due to how cheap the boards are. You'd spend more time than it's worth trying to fix it rather than to just swap a board on those.

Regarding their forum, I thought they had one, too, but looks to be gone.

[Steve Blackdog]

I have two Blackstars in for repair at the moment. One is an HT Stage 100. Seems one of the Molex connector pins on the PT was shorting out. Parts due next week. The other is a Club 40 that powers on, but has no output. I'll start on that Monday.

blackstar gets a bad press and I don’t know why. I am rebuilding an HT60 soloist from parts. I am due to test it tomorrow. I would never have found the solution to glebert’s issue.. If I have a problem, I know who to come to!

btw- I thought Blackstar had its own forum, but I can’t find it anymore.

It is great that you are so happy to help others!

[Delta362]

I have two Blackstars in for repair at the moment. One is an HT Stage 100. Seems one of the Molex connector pins on the PT was shorting out. Parts due next week. The other is a Club 40 that powers on, but has no output. I'll start on that Monday.

[Delta362]

You're quite welcome, glebert! I'm guessing TR26 failed as you found, but your fix will take care of it

I'm hoping to be a little more active and contribute more since I've learned quite a bit from here over the years.

[glebert]

I'd try shorting pins 2 and 3 of TR26.

This fixes it. Bent a piece of cutoff resistor leg into a U shape and jumped around from pin 2 to 3. Tubes start conducting and the bias voltage goes to either -18 or -7V depending on the 20W / 2W switch. Really nice that this can be done without removing the board, would be a 10 minute fix if you have the right info going in.

Interesting sounding little amp. Wouldn't say it is my absolute favorite, but it is pretty cool.

Thanks Delta for the support, doubt I would have figured it out without your help.

[Delta362]

I'd try shorting pins 2 and 3 of TR26. I assume you have a speaker cab connected to enable the speaker jack. What happens to the PA_ON_LL (power amp on) voltage when you short TR26? That signal heads over to the bias circuit.

Edit: did you happen to see the three new attachments as they tie into the circuity?

[glebert]

R168 and R169 both test fine. Is there any reason not to just pop off TR26 and short the three terminals together? Or maybe wire in a switch from pins 2 to 3 to be a proper standby? I think the input jack mute thing is kind of useless.

[Delta362]

Are R168 and R169 reading good? I've attached a couple more snippets of the power supply and bias scheme for additional troubleshooting. Also note that PA_ON_L links into the bias thru D25. Check TR32, FET13-FET16, FET6, and TR29 and TR30.

[glebert]

Seems like it is definitely a problem with TR26. If I understand this right TR26 and TR27 have to be fully turned on (saturation) which keeps the base of TR45 low and therefore in cutoff.This keeps the collector of TR45 high, which keeps TR31 turned off. This keeps PA_ON_L a negative voltage. There is a thing with the SW_BIAS_HI which I assume means TR31 might turn on more in one of the bias modes.

In my amp TR27 is turned on fully with plug inserted. Base voltage about 0.65V, collector voltage of 12mV. TR26 base voltage is only at 0.3V with plug inserted, and collector voltage at 0.65V instead of being something like 24mV.

[Delta362]

You're quite welcome, glebert! Let us know what you find.

[glebert]

Thanks Delta! I think that is exactly the info I need.