Crackling and popping Blackstar Soloist HT60

[mikeydee77]

Ah cool... maybe it got missed in the manufacturing rework?

I have done loads of them... think it will high chance of success

[Mick Bailey]

I think those screen printed lines are standard - I've often seen them, but never seen a slotted board as stock. I always cut an additional slot vertically midway between the two MOSFETs as there's usual leakage developing there which causes puzzling faults.

To cut slots in the narrow space between the pads I drill a row of three small holes; one central between the pads then another either side. Then gently cut away between them with a scalpel to leave a neat slot. keep the slot as narrow as you can - it only needs an air gap.

You can test each EL34 individually with series heaters by clip- eading to the heater pins of the removed tube. I have an old tube with just the heater pins and the rest cut off with a Dremel which makes things easier. Similarly, I have some dual triodes with one section's pins clipped off for testing circuits where I want to isolate one section, such as trem oscillarors.

[Stratfordade]

Thanks I’m going to source the right bit for that.

Have retensioned and cleaned EL34 sockets and put board back but no change. In fact I’ve found that even putting in a signal at FX return makes no difference - all I get is crackle.

Thought perhaps the runaway valve had caused damage to OT but the DC primary resistances seem ok between centre tap and each anode.

[Stratfordade]

I think those screen printed lines are standard - I've often seen them, but never seen a slotted board as stock. I always cut an additional slot vertically midway between the two MOSFETs as there's usual leakage developing there which causes puzzling faults.

To cut slots in the narrow space between the pads I drill a row of three small holes; one central between the pads then another either side. Then gently cut away between them with a scalpel to leave a neat slot. keep the slot as narrow as you can - it only needs an air gap.

You can test each EL34 individually with series heaters by clip- eading to the heater pins of the removed tube. I have an old tube with just the heater pins and the rest cut off with a Dremel which makes things easier. Similarly, I have some dual triodes with one section's pins clipped off for testing circuits where I want to isolate one section, such as trem oscillarors.

Good idea on the heaters and thanks for advice on cutting those slots. I’ve tried different pairs of known good EL34s though and am pretty confident it’s not an issue with a valve.

[Stratfordade]

Interesting. If I inject a signal at grid of V3 then I hear a fairly loud tone and the crackling is very muted and at much lower level.

Injecting same signal at grid of V3 at the more charred socket where the valve had runaway in past, the tone is very faint and the crackling volume level which is much higher doesn’t change.

Perhaps this is evidence that the V3 socket is the problem? Problem I have though is finding one of those sockets. Not seen one with similar outer radius for the contacts soldered onto PCB.

Edit: have found the last remaining socket at the only supplier in the USA (Parts Is Parts). They list the socket as discontinued but claimed to have one remaining in stock. There must be another source somewhere but have ordered this in case they’re unobtainium.

[Mick Bailey]

I’ve tried different pairs of known good EL34s though and am pretty confident it’s not an issue with a valve.

I wasn't suggestiing this to establish if it was a tube problem. The reason for eliminating either EL34 is to narrow down which side is affected, or whether it's both.

[Stratfordade]

I wasn't suggestiing this to establish if it was a tube problem. The reason for eliminating either EL34 is to narrow down which side is affected, or whether it's both.

Sorry I misunderstood. I’ll try your trick with the heater, especially as I’ve got some evidence that one socket may be conducting.

[Stratfordade]

Thanks for suggestion to connect heater via links.

I’ve tried different valves on both sides. Whichever of the two EL34s I place in socket and whichever socket I get crackling, though one side (actually the least charred socket) is quieter.

If I input music via the FX return (with jack plugged into input) I don’t hear a thing except crackle.

So my interpretation is that it looks more like a PI problem. That was working before I replaced parts of the PI (both MOSFETS, drain resistors, one of the gate resistors) so I’ll go back to focus on that part of circuit. I have a drill bit ordered to drill slots between mosfet legs but with Xmas it won’t be here for a week or so.

Thanks all for suggestions so far. It’s been invaluable in narrowing down the problem.

[Stratfordade]

Earlier I’d reconstructed some parts of the PI where in past someone had replaced the 100 ohm R145 SMD with a conventional resistor, attached to TR2 in mid-air and encapsulated in glue.

I’ve concluded this may have been a solution to get round board leakage, so by returning the circuit to original I have the leakage problem back!

I’ll try to correct it first by cutting those slots which should be more reliable than a bunch of parts mounted off the board!

[Stratfordade]

Just to close this thread all working great after cutting slots between the MOSFET legs and between the MOSFETs. For good measure I also removed C122, C123 PI electrolytics and drilled hole between mounting holes, cut slot between MOSFET ends of R140, 141 and also installed a new SMD R145 100ohm after cutting hole between its mounting tabs (this resistor had been replaced by a conventional resistor mounted off board by someone in past do I suspected a conductive problem was there originally).

Not sure where the problem conductive area of board was (and may have been more than one), but sounds great now!

Thanks all for your help on fixing this.

[mikeydee77]

Great news and thanks for closing the feeding loop. Will help many others no doubt.