Blackstar HT 60

[mikeskory]

I have a similar problem to another recent Blackstar thread but let's start anew. My project is a Blackstar HT soloists. I can trace the signal through either of the 2 the two ECC83s. (One is for clean channel the other for distorted channel) My test signal disappears the other side of the ECC83 plate resisters in each case. It has new EL34s and speaker and input are plugged.

I believe this is the right schematic or close to it.

I've been reading about the complicated standby circuit in these amps but I don't understand how to isolate the symptoms.

[g1]

No schematic is showing. But 'other side of plate resistors' is usually connected to the power supply where we should not see signal.
How about after the coupling cap from the plate?

[mikeskory]

I'm sorry I was unclear. The signal ends at the plate of the ECC83s. I'll attempt to upload the schematic again so we can be on the same page. There is no signal at the grid of the EL34s. The control grid is also very low. -90

[mikeskory]

http://music-electronics-forum.com/s...ad.php?t=39730

[eschertron]

Ch1 and Ch2 are each on their own tube. I'm thinking that if they behave the same way, it is because they have something in common. What are the voltages (DC first, then AC) on each pin of the two tubes? First lets find out about the operating conditions.
Next, where are you measuring 'other side of the plates'? Please name the junction of components where you are taking the readings. "junction of C29 and R37" for example.

[nickb]

I'm sorry I was unclear. The signal ends at the plate of the ECC83s. I'll attempt to upload the schematic again so we can be on the same page. There is no signal at the grid of the EL34s. The control grid is also very low. -90

The -90V is a symptom that the auto mute/standby is on. When a you plug in to the input the signal IP_JACK goes to the processor IC1 on sheet 3. This then outputs BIAS_CNTRL. This is turn is amplified by IC17B and then on the TR4 on sheet 4. There are plenty of places for that to go wrong in that path for you to check.

[mikeskory]

I went through it again this morning with a speaker plugged in and a 300 hz tone in the input jack. I can see the tone at TP3,TP4 on page 1. I can see the tone at TP8 on page 2. After that my schematic is different then the unit. This particular Soloist 60 has 2 EL34s. There is no signal on any of the grids of the EL34s. The voltages on the EL34s are the same. Pin 1, pin2 are 0. Pin 3 is 513, pin 4 is 511v, pin 5 is -92 and pins 6,7,8 ar 0. We have a new clue now unless I didn't notice. When I flip the power switch on/0ff I hear a slight thump in the speaker or a brief tone. Sometimes neither.

@nickb: When the voltage is -90v, that means the standbymute is "on" therefore it's turnong "off" the power tubes. I get and will check those sections now. Thanks

[mikeskory]

The -90V is a symptom that the auto mute/standby is on. When a you plug in to the input the signal IP_JACK goes to the processor IC1 on sheet 3. This then outputs BIAS_CNTRL. This is turn is amplified by IC17B and then on the TR4 on sheet 4. There are plenty of places for that to go wrong in that path for you to check.

There is no audio signal at the junction called bias_on. Bias_on connects with C163, C164 and D15 all on page 4. 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. I'm back at IC1 like you suggested a while ago. I'll let you know on Monday what I've found. Thank you.

[Enzo]

You have signal at TP8, which is the preamp out/power amp in.

So looking at the schematic, even if there are differences, do you have IC12? Pin 7 is the signal feeding the tube stages. I am thinking FET21 is not turning on?

Bias on is a DC control signal, it controls FET21 amongst others, so no signal should appear.. TR4 is the bias transistor, it is not signal driven either.

Let me try it this way. Look on page 6 all the speaker jacks. Note the sleeve has a cutout contact, each jack grounds the 12v pullup. Plug into any speaker jack, and it opens that ground shunt. That allows the particular pullup to put +12 into the diodes. ANy one of them will make SPKR_OK to rise, I guess to +12. PAge 3, TR5, SPKR_OK turns on TR5 to bring down SPK_JK from 3.3v to zero.

IC11 is the PIC controller IC. SPK_JK inputs to that IC on pin 5. On the other side of IC11 is BIAS_CNTRL on pin 23. Also page 3 lower right is IC17, which inverts BIAS_CNTRL into BIAS_ON.

If you short FET21 source to drain, does it let audio through?

[mikeskory]

1)Yes. Shorting FET21 S to D lets the audio through.
2)The IC11 PIC control is labelled IC10 on my chassis HT Soloist 60. The schematic IC11 and my chassis IC10 are the same PIC16F57.
3)The schemo has 5 output jacks, mine has 3. D13,D14 & D17 present voltage when a spkr is plugged in BUT only rises to .6v from 0.
4)There are 3 39K resisters on my chassis to D13,D14 & 17 with 12v on the common side of these resistors.

[mikeskory]

No schematic is showing. But 'other side of plate resistors' is usually connected to the power supply where we should not see signal.
How about after the coupling cap from the plate?

I found the schematic for the Blackstar CLUB 40. It's closer to the Soloist

[mikeskory]

HT Club 40 closer to Soloist 60

Enzo, This is a closer schematic. I'm home so I can't check everything but it's similar output section and pre-amps,

[mikeskory]

Update 8/7/18 Enzo and whoever else is interested. So far the schematic I posted for the HT 40 STUDIO is the same as HT60 solosist. Makes it easier to follow. So speaking using the HT 40 schemo page 3, I have audio signal from IC9B pin 7, have it through C114 and have it at the FET21 pin 3 Drain. As Enzo suggested, I shorted the Drain to source and heard the tone through the speaker. So to simplify this problem, is the loss of 12v from the speaker jack switches stopping the FET21 from working?

[Enzo]

I described the basic control signal path, I think you need to go through that step by step to see if it breaks down anywhere. As I see it here from afar, either that control path has an issue, the PiC is bad, or the JFET is bad.

[mikeskory]

I believe it to be the control path. The speaker switches are working properly and 12v present where the R24,25,26 resistors start the 12v control. But the 12 v dies at the 1st SPKR_OK. Let me work on it a while and Ill let you know what I find. Thanks!