Blackstar HT20 MKII standby problem (probably)

[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

[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]

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.

[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]

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

[Delta362]

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

[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]

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]

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]

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]

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]

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.

[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.

[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]

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.