Blackstar Artist 10 high voltage collapsing

[Helmholtz]

Standby switch does not always carry high voltage. Sometimes it makes a ground connection (at CT of HV winding, or grounding the (-) terminal of a bridge rectifier).

I think in those cases you should have high DCV across the open switch (except if the reservoir cap is fully charged).

[Helmholtz]

With amp powered up one of terminal measures 0V and the other 0.63V.

Did you check for both DCV and ACV?

[J M Fahey]

Famous for its complexity brand?
Mystery unexplainable failure?
SMT build?
No schematics?
Gut it and build an eyelet board Champ or an AX84
Serious.

I would.

[Stratfordade]

Famous for its complexity brand?
Mystery unexplainable failure?
SMT build?
No schematics?
Gut it and build an eyelet board Champ or an AX84
Serious.

I would.

Agree there are better alternatives, but I picked this up very cheap because of fault, so will be selling it in if I can sort the problem(s).

I have confirmed the pre-amp section is working so getting closer!

[Stratfordade]

Did you check for both DCV and ACV?

Doh only DC! I’ll recheck

[Mick Bailey]

Blackstar amps are pretty bad for defective solder joints and when I got one in for repair I'd always check the solder side of the PCB for telltale circular cracks, especially around the legs of ceramic resistors. I've had amps within warranty where you could pull a power resistor straight off the board with no effort. As the PSU most likely uses RC filtering for the HT, I'd look at the closest ceramic/higher power resistors close to the main filter caps and take some voltage measurements. Of course, you could just have a straightforward component failure. look for any resistor that has an excessive voltage drop, though if a joint is high resistance you'd need to take measurements off the pads, or nearest connected components, as the voltage drop will occur in the joint itself.

[Helmholtz]

Does anybody know how the standby switch is wired in these amps?
Does it switch AC HT a la Marshall?

[g1]

Does anybody know how the standby switch is wired in these amps?
Does it switch AC HT a la Marshall?

In the HT40 Club, they toggle the bias control of the PIC.

[Helmholtz]

In the HT40 Club, they toggle the bias control of the PIC.

But this amp would use cathode bias.
PIC?

[g1]

But this amp would use cathode bias.
PIC?

PIC is basically a programmable micro-controller. For the bigger Blackstars, both the input jack or the standby switch disable the speaker output by shutting down the bias line out of the PIC.
As this unit also is doing odd things when the input jack is connected, I assume it has some similar feature though it may not be a bias line if it is cathode biased. You are supposed to be able to use this amp silently in standby mode (headphones or spkr. emulated out).
As this one is having issues with the HV when input jack connected, I would guess some solid state circuitry controlled by the standby/input jack is faulty and pulling down the HV.

[Stratfordade]

Some progress today but no fix yet.

Pulled all the boards from the Artist 10 including the power amp daughter board hidden under the main board. No sign of cold joints anywhere though I reflowed all the valve pin joints.

Checked all ceramic and big power resistors and they were all OK too. For good measure I compared resistance readings with a near identical but working Studio 100 I have (KT88 version) and found no differences.

Put everything back together and the Artist behaved just as before — HT voltages collapsing to less than half their correct level as soon as I insert input jack.

Now it gets more interesting. I checked Artist cathode current with a bias probe and it was 5mA even with no input jack inserted. Compared this with the Studio which was 0mA with ni input jack inserted.

When I inserted jack into the Artist (uses an EL34) the bias current shot up to 175mA so I pulled the jack out after observing this. For comparison the Studio (KT88) bias current goes to 72mA with jack inserted which sounds much more reasonable considering it has a KT88.

I can't measure grid voltages with power valve inserted in either as no access to bottom of power board when everything is connected up. But I can measure voltages at the socket with the valve removed. With the Studio (KT88) the voltage is 0V between grid and cathode whether jack inserted or not. But for the Artist which I'm troubleshooting, the grid is at 275V whether the jack is inserted or not!

So it looks like there's something way wrong with a huge +ve grid voltage making the valve run very high current when jack inserted and whatever is switched to turn the power stage on. This high current is too much for the power supply which sags under this abnormal load.

If only I had a schematic I could quickly see how the grid bias voltage is established and I'm sure I could find problem, but I'm in the dark without that schematic. I'll have to try to follow connections from the grid by trial and error or take the boards apart again and follow the connections directly.

[Greg_L]

More reasons to hate Blacksatar amps. Besides simply not sounding all that good, they're stupidly designed and a pain to mess with.

[g1]

When I inserted jack into the Artist (uses an EL34) the bias current shot up to 175mA so I pulled the jack out after observing this. For comparison the Studio (KT88) bias current goes to 72mA with jack inserted which sounds much more reasonable considering it has a KT88.

I can't measure grid voltages with power valve inserted in either as no access to bottom of power board when everything is connected up. But I can measure voltages at the socket with the valve removed. With the Studio (KT88) the voltage is 0V between grid and cathode whether jack inserted or not. But for the Artist which I'm troubleshooting, the grid is at 275V whether the jack is inserted or not!

So it looks like there's something way wrong with a huge +ve grid voltage making the valve run very high current when jack inserted and whatever is switched to turn the power stage on. This high current is too much for the power supply which sags under this abnormal load.

Agree with your assessment of power tube over-conduction pulling down the B+. As it is likely cathode biased, the high voltage getting into the grid could be from a leaky coupling cap. Disconnect it or replace it and re-check grid voltage, should be zero.

[Stratfordade]

Agree with your assessment of power tube over-conduction pulling down the B+. As it is likely cathode biased, the high voltage getting into the grid could be from a leaky coupling cap. Disconnect it or replace it and re-check grid voltage, should be zero.

Hey I think you’re onto something. There is a cap C1 near R35 180K that has 275V across, and one end goes to connector that goes off to power amp board. R35 gets very hot as is dissipating nearly 1/2W.

If I measure resistance across the points that look like they go to C1 then on the ‘good amp’ it rises then overflows 20M ohm as C1 charges. The ‘bad amp’ sticks at 1.24M ohm.

I will need to pull the board again to remove C1 (which means pulling the socket board again too, but I’m getting pretty fast!). If it is leaky, what kind of cap could I replace it with?

Edited to say C1 looks ok now removed and it’s more likely one of the SMD electrolytics perhaps C20. Problem is they are hard to remove without special tools and my desoldering gun doesn’t cut it!

[Jazz P Bass]

Most of those solder joints (at least in the attached pic) should be touched up.