Blackstar HT40 constant noise

[gandaralf]

Hello!, here my message in the bottle

My amp sadly start doing constant noise, -independent of any volume level -, just a low freq hum that stops when I unplug the cable.

The only clue I have is a strange behaviour that came with the failure, is that now by turning OFF-ON the power switch it does a terrible noise in speaker, as if power output were active, but the stand by switch is OFF!.

If anyone had the same problem or knows what to think about this.

[Jon Snell]

The main smoothing capacitors fall off the board or unsolder them selves or in the worst case, lose their capacitance.
Check C138 - 143.

[g1]

This should be the complete schematic:

[Mick Bailey]

How far you can get with this depends on whether you have any electronics experience, can use a DMM and are able to take voltage measurements safely with the amp under power. Other than that, you're limited to tube swaps and general observation.

These amps suffer from a number of issues highlighted by John, as well as the PI MOSFETS failing either on their own, or more commonly, due to voltage leakage between the mounting pads due to the PCB breaking down and becoming conductive.

[gandaralf]

I have tested sound ok from Send Output and from Line Out/Phones output and - the noise is Not there -, only in the speaker, so I assume the problem is only at the output stage.

Very sad to read things like mosfets failing, capacitors unsolder themselves, this amp is only a year old! it's suppose to endure decades...

Yes, I can get to do measurements with multimeters and even oscilloscope if needed, but to know where to start, is very useful to read any experiency you all have on this

[Mick Bailey]

The first thing to eliminate is any problem with the output tubes. The input socket mutes the output, but HT is still applied to the output transformer centre-tap (and therefore the output tube plates) and under certain conditions a faulty output tube could give rise to noise even when the amp is muted or in standby. The standby switch doesn't operate in the same way as most amps - it controls the logical state of one of the PIC inputs - it doesn't merely switch the HT on/off as you may expect.

Where are you located? With an amp this old you may still have some warranty under locally applicable laws. Even so, you would need to eliminate the output tubes as a possibility by fitting a replacement matched pair. and re-biasing.

[gandaralf]

Thank you Mick for the detailed response about the stand by switch, so it seems plausible the valves be the cause, I'd already thought of buying a replacement just in case, so worth a try.

I still haven't disassembled it because the amp is still under warranty, I am located in Argentina, but 400km away the technical service, they told me to send the amp but that it can take months to solve because is the only service (thing is, I could buy valves in the US and have them delivered home the same week.) so I'm still wondering what to do.

[Mick Bailey]

I've had customers send their Blackstar amps back under warranty only to find it was a tube issue and were charged as a non-warranty repair. A tube swap is the easiest thing to do at this stage and you always need to eliminate this possibility. If it turns out not to be the tubes it leaves you with a spare pair for the future. My initial thought is that you may have a power tube arcing internally, so it would not be a good to run the amp if this is the case.

[Jazz P Bass]

Not too sure what the engineers where thinking when they came up with that goofy mosfet phase inverter.
The tube after it is a buffer.
I worked on a newer 100 watter & they changed the whole drive circuit. (sorry, I did not document it)
Goody goody.

[Helmholtz]

[QUOTE=Jazz P Bass;n910987]Not too sure what the engineers where thinking when they came up with that goofy mosfet phase inverter.
The tube after it is a buffer.QUOTE]

Sorry, what tube?

[Jazz P Bass]

[QUOTE=Helmholtz;n910990]

Not too sure what the engineers where thinking when they came up with that goofy mosfet phase inverter.
The tube after it is a buffer.QUOTE]

Sorry, what tube?

Good catch,

[gandaralf]

I have not experience with valves so perhaps it's normal, but I have measure temperature with an infrared thermometer in the two EL34 valves, one of them gave approximately 135ºC an the other 145ºC, in multiple measures 10ºC of difference, and more, so I swapped them and temperature also swapped, the valve with high temperature also shines brighter, so I think it is not normal and that the valve can be faulty, although amps works, and at times the amp does not make the noise, temperature keep being this different.

What do you think?

[Helmholtz]

I have not experience with valves so perhaps it's normal, but I have measure temperature with an infrared thermometer in the two EL34 valves, one of them gave approximately 135ºC an the other 145ºC, in multiple measures 10ºC of difference, and more, so I swapped them and temperature also swapped, the valve with high temperature also shines brighter, so I think it is not normal and that the valve can be faulty, although amps works, and at times the amp does not make the noise, temperature keep being this different.

What do you think?

I think your IR thermometer measures approximate bulb temperature. The difference shows that one of the tubes has a higher idle current than the other. Different idle currents are likely to cause output ripple noise. The amp has a bias (PR2) and a bias balance (PR1) pot. The bias bal pot (PR1) can be used to adjust for minimum hum/ripple noise. The bias pot PR2 should be carefully adjusted so that the voltage at test point TP10 measures around 70mV wrt ground (TP18).

Just thought as you are in Argentina you might try to contact MEF member J.M.Fahey in Buenos Aires, who runs his own amplifier company. Contact data contained here: https://www.ssguitar.com/index.php?topic=3827.0

[gandaralf]

I was able to fix it by replacing the power tubes, but I have still not determined the underlying cause.

[Helmholtz]

I was able to fix it by replacing the power tubes, but I have still not determined the underlying cause.

Well balanced power tubes in PP amps mutually compensate power supply ripple in the output signal.