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The giveaway is a high voltage on the PI mosfet gates. I find it safer and easier to check with them removed - space is fairly tight.
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You're talking about TR1 and TR2, right? But what kind of voltage would prove a defect?The giveaway is a high voltage on the PI mosfet gates. I find it safer and easier to check with them removed - space is fairly tight.Comment
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Yes, TR1 and TR2. I've never checked the gate voltage on a good amp, because I've never had one for repair that wasn't showing some sign of leakage. You're looking at tens of volts - some 30v to 80v on the gates. Usually one side is worse. Check the power dissipation of R140 and R140 (measure the voltage across them at idle and use Ohm's law). Over-dissipation is another sign of board leakage. They run hot anyhow, but with a faulty board they will go over the 2W rating.Comment
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The Gate-to-Source voltage must not be greater than 20V. Otherwise, the transistor will emit magic smoke and go to the next world.Originally posted by Mick Bailey View Post
This one is just for reference.
Of course, in normal mode the voltage at Gate will not exceed even 12.6V. IC12 power supply will not allow it.Comment
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When the gate voltage rises due to board leakage, so does the source voltage and Vgs can still be within the maximum rating of the device. The leakage is usually unstable due to the properties of the PCB breaking down and causes random noises. It also causes other symptoms - low output, and the 'voice' switches can also kill the sound completely.Comment
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You are apparently talking about low-signal Jfet field-effect transistors, in the signal path circuits( switches, mute elements).Comment
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OK, I'm beggining to lose hope, here... I've changed the 3 caps and the noise remains.
Let's consider now it's a fawlty board : what should I look for? Or even worse : if it's the case, then what? Will it be repairable without having to change the whole PCB? (which as I understand is not produced anymore).Comment
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I've already suggested twice that you check the mosfet gates (TR1 and TR2 to be clear)
I said the problem causes other symptoms. I'm talking about TR1 and TR2 being the primary indicator of a leaking board. When the board gets really bad, the preamp stage gets affected too due to where HT traces run. The input grids to the preamp tubes also get a hefty positive bias. The first step is to identify if the board has a problem around TR1 and TR2, as this is where it starts and is at its worst. Often, fixing this can be enough to give the amp many years of additional serviceable life. Most amps only have a problem in this area. The board conductivity is very similar to the early-series Marshall DSL and TSL boards.Originally posted by x-pro View Post
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Is this a surface contamination effect or a bulk property of the board material?Originally posted by Mick Bailey View Post
I mean, does thorough cleaning of the board help?
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For TR1 and TR2 I have around 3V between gate and source. R140 to R143 seem to be operating within their limits (between 0.3 and 1.2W).
That was also my question, can such defect be visible on the board? What kind of cleaning can be done?Is this a surface contamination effect or a bulk property of the board material?
I mean, does thorough cleaning of the board help?Comment
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When the boards become conductive there are grey threads within the affected area that can be seen when the board is cut through. Otherwise there's no visible sign of anything being wrong. Check the gates to ground - when the board conducts the source voltage can also rise. I've just checked my old notes and I wrote that the source as well as the gate voltages increased on affected amps. I'm not saying this is the problem with this amp, but conductive boards are so common with Blackstar amps that it has to be eliminated as a possibility.
The other major problem with older Blackstars is poorly soldered boards, to the extent that components could be pulled off the board with little effort - particularly the ceramic power resistors. It's important to meticulously check the board under magnification. Also the spade terminals can be loose where they're soldered into the board.Comment
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Hey everyone. Sorry about the time without posting but I was away from home for some time.
I finally admitted that I couldn't fix it by myself and after all these components changed, I brought it to a pro. He quickly located the issue around the mosfets, and didn't find any DC leakage on the board.
He changed one of the new power resistors I put (R140 I think) and boom it worked, no more crackling noise. The crackling noise did come back when putting back this previous resistor. Let it running for several hours and the noise didn't come back.
Maybe the issue was fixed in the various components changes I did (including the capacitors), but the resistor, which was replaced before those changes, probably suffered and had to be replaced afterwards. But without a oscillometer, I couldn't locate the issue...
Anyway, the issue is fixed, and I'd like to thank you all guys for taking some time to help a rookie from the other side of the world. I sure learned a lot through your advice and almost fixed it by myself ;-)
Happy new year to all and keep being great.Comment
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