LarBal 4/25/2018 11:52 AM
Blackstar HT60 soloist oops!
The own said his son took the speaker out from another amp, and plugged into the input of the Blackstar. Oops.
He says the amp worked fine before that, and now does not. Output is very low and grainy.

Upon my first powerup, I see he is correct. Amp is putting out sound, but very low volume, and not quite right.

schematic: [ATTACH]48630[/ATTACH]

First thing I do is check voltages at preamp tubes. Everything normal there.
I then check EL34 voltages with tubes out. Plates and screens at 525VDC, and there's -55V at the grid.
Pop some known good tubes in, and I am able to adjust the negative voltage to the grids.
I pop in a bias probe, and now all I'm getting is -95V on the grids, and turning the bias adjust pot does nothing.
I have no idea what happened there.

So the first thing I'm looking to do is get an appropriate amount of neg voltage to the grids. I'm thinking something isn't grounded, somewhere, but this is a complicated bias circuit for me, balance adjust aside. What does TR4 do? Any ideas on what to look for? There are so many small surface mount components it's driving me a little crazy.

Bias circuit is top right page 4
 
alexradium 4/25/2018 12:14 PM
Quote Originally Posted by LarBal View Post
The own said his son took the speaker out from another amp, and plugged into the input of the Blackstar. Oops.
He says the amp worked fine before that, and now does not. Output is very low and grainy.

Upon my first powerup, I see he is correct. Amp is putting out sound, but very low volume, and not quite right.

schematic: [ATTACH]48630[/ATTACH]

First thing I do is check voltages at preamp tubes. Everything normal there.
I then check EL34 voltages with tubes out. Plates and screens at 525VDC, and there's -55V at the grid.
Pop some known good tubes in, and I am able to adjust the negative voltage to the grids.
I pop in a bias probe, and now all I'm getting is -95V on the grids, and turning the bias adjust pot does nothing.
I have no idea what happened there.

So the first thing I'm looking to do is get an appropriate amount of neg voltage to the grids. I'm thinking something isn't grounded, somewhere, but this is a complicated bias circuit for me, balance adjust aside. What does TR4 do? Any ideas on what to look for? There are so many small surface mount components it's driving me a little crazy.

Bias circuit is top right page 4
this line of amps has that negative voltage when in standby,which happens when your input and output cables are not plugged in.
Your problem is probably in the input opamps,although they should have protection diodes,just scope out the signal flow and you will find out easily.
 
LarBal 4/25/2018 1:21 PM
oh wow that is correct. Thanks.
 
Jazz P Bass 4/25/2018 4:06 PM
If I recall, the input plug enables full bias voltage. (-45Vdc)

The speaker output plug enables the high voltage for the output tubes.

Also, the output tube heaters are in series, so all heaters must work.
 
mikeskory 8/2/2018 9:20 AM
I have a similar problem with the Blackstar HT soloists. i can see the signal through the ECC83s pre-amp tubes up to the plate resister. Ithere are 2 ECC83s, one for the clean channel and the other for the hi gain channel. Should I start a new thread or continue here since the model is the same and problem is similar.
 
Enzo 8/2/2018 9:54 AM
Hi Mike.

Always start a new thread for a new repair, even if it is similar. Otherwise we can get confused as to who checked what.
 
Jazz P Bass 8/2/2018 10:04 AM
Yes, I would suggest that you start a new thread.
 
Dave H 8/2/2018 12:56 PM
Quote Originally Posted by LarBal View Post
What does TR4 do? Any ideas on what to look for? There are so many small surface mount components it's driving me a little crazy.

Bias circuit is top right page 4
The bias supply is bottom left of page 4. It's a bridge rectifier followed by a pi filter. It connects to the circuit at the top right of page 4 via the V_BIAS signal net. TR4 looks like it switches the bias between the normal bias voltage and the max bias level (-95V?) The switching signal is BIAS_ON. Check the BIAS_ON signal level with and without the bias probe.
 
Jazz P Bass 8/3/2018 12:24 AM
Having worked on a number of these since the original post, I see now that a statement that I made is incorrect.

"The speaker output plug enables the high voltage for the output tubes."

Plugging in a speaker connects the 'Spkr On' signal to the front panel PIC microcontroller.
 
mikeskory 8/3/2018 10:14 AM
Quote Originally Posted by Dave H View Post
The bias supply is bottom left of page 4. It's a bridge rectifier followed by a pi filter. It connects to the circuit at the top right of page 4 via the V_BIAS signal net. TR4 looks like it switches the bias between the normal bias voltage and the max bias level (-95V?) The switching signal is BIAS_ON. Check the BIAS_ON signal level with and without the bias probe.
There is no audio signal at the junction called bias_on. Bias_on connects with C163, C164 and D15. There is no audio signal on any of those 3 components. So that means there is no audio signal to control the TR4. Is that correct? That means it is muted all the time.
 
Dave H 8/3/2018 1:05 PM
Quote Originally Posted by mikeskory View Post
There is no audio signal at the junction called bias_on. Bias_on connects with C163, C164 and D15. There is no audio signal on any of those 3 components. So that means there is no audio signal to control the TR4. Is that correct? That means it is muted all the time.
Enzo has got this one in the other thread.