Crackling and popping Blackstar Soloist HT60

[Stratfordade]

I'm having a heck of a job trying to track down a fault causing an HT60 Soloist to make crackling and popping noises as soon as it comes off standby, even with all controls turned down to zero.

A bit of history. I don't think it's a problem with conducting PCB, as the amp was working fine after a fashion and has only started making this noise after I 'fixed' it!

The amp had been blowing fuse after playing for 30 minutes or so. When I checked it out I found one of the valves was running away and 'red plating'. Suspected C123 was leaking and found that was the case, so I replaced both C122 and C123.

Was concerned that the high anode current might have damaged other parts of the PA, so I replaced the two IRF830 PI Mosfets at same time, along with the 2W R140 and R141 (someone in past had made a mess of replacing SMD R145 with a conventional resistor and a huge ball of glue encasing the Mosfets, so I wanted to put in correct SMD part and make sure the IRF830s were OK. Since reassembling the amp I've had the awful crackling noise that is something I must have done as it wasn't there before!

In checking the amp I find the gate-source voltage of each Mosfet fluctuates between around 12-18V and the highest readings coincide with loudest pops.

Weirdly the drain of of TR2 is at 375V, and drain of TR3 only 117V. This means about 240V is being dropped across R140 corresponding to about 1.3W. By comparison only 0.2V is dropped across R141 which is a current of just microamps! I'm baffled about these readings.

The bias test voltage across R217 can now be set to 50mV and remains stable (before it would climb up as one valve ran away). The bias switching all seems to work correctly (switched on by jack in input socket). The bias voltage on each EL34 with jack inserted is about -55V, and switches to -95V when jack removed which puts valves into hard cutoff.

Thanks in advance for any advice on where to go from here.

[mikeydee77]

Quite a few blackstars have problems with pcb material in between the phase splitter pins breaking down causes a crackling frying Bacon sound. The cure is to drill out a gap between the pins until there is a clear gap. Think it was a particular date range than precise model.

You need a small bit like 0.5mm and a steady hand and a dremel is handy for this kind of root canal work.

If you contact blackstar they may supply you with fix instructions. I have found them pretty helpful in the past.

[Stratfordade]

Thanks Mikeydee. This PCB already has slots cut between the mosfet legs. I think this was done at factory as they have coloured outline on the slots which are very neatly done.

Also just noticed I’ve uploaded the schematic of the pre-amp section! PA schematic below.

[Mick Bailey]

A leaky PCB would still be on my mind. If you check the IRF840 pin volages with respect to ground you can sometimes get a better idea of what's going on. This is because the leakage can cause an offset that isn't necessarily picked up measuring gate to source etc. Think of it this way; 0v gate/18v source would give the same reading as 100v/118v. Look for unusually high gate voltage and compare both sides.

The time to check for leakage is really when the Mosfets are removed and one end of R146/R147 lifted. This gives a clearer picture. You sometimes get 80v or more on the gates when the board is breaking down.

Edit: Additional information while I was typing made my reply redundant........

[xtian]

Failing stand by switches are an issue I've encountered many times that cause all kinds of ugly sputtering and static noises, and also cause fuses to blow. Just use an alligator clip to jumper the stand by switch out of the circuit to test.

[The Dude]

You might also pull the PI tube to see if the amp still crackles. The noise may be coming from the preamp somewhere.

[Stratfordade]

Failing stand by switches are an issue I've encountered many times that cause all kinds of ugly sputtering and static noises, and also cause fuses to blow. Just use an alligator clip to jumper the stand by switch out of the circuit to test.

I’ll try that but it’s unlikely in this case as there’s no HT via standby switch. Blackstar often use a design like this one where the standby switch just switches bias voltage to high -ve. The input jack is part of same switching circuit.

[Jon Snell]

Replace TR1 and TR2. All will be well after that, if the Drain load resistors haven't suffered.
An extremely common design fault that if taken to task, Blackstar are finacially liable for.

[Stratfordade]

Thanks Jon. I installed new TR1 and TR2 after the crackling issue started (sourced from Farnell so the real thing!) and no change. At same time I replaced both drain load resistors R140 and R141.

I have noticed that the valve sockets which have suffered from a fair amount of heat (the more burnt one on left of photo was for valve with runaway bias voltage) are alarmingly loose when waggling the EL34s, so perhaps one or more contact sockets in base are arcing. Problem is I’ve not seen ceramic PCB octal sockets available to replace the ones Blackstar use (the board connections are on a much bigger radius than the pin sockets). Are these sockets available anywhere? Searches so far unsuccessful.

[Mick Bailey]

Try cleaning and re-tensioning the sockets. You can remove each EL34 in turn and see which side is playing up. I rang Blackstar many years ago to tell them there was a problem with their PCBs and they denied it, despite me having numerous amps with the same failure. It's interesting that the PCB appears to be slotted from the factory. This is not a long-term fix; conductive boards eventually leak all over the place - between output tube pins, preamp tubes and other areas where there is significant voltage differential. When you cut into the PCB there's a darkened layer in the middle where it's conducting.

[mikeydee77]

Those valve sockets have some seen some action... If there are any scorch marks then they will be potentially conductive to HT.

Lots of good ideas above and one more I can think of is that resistors that touch the PCB can sometimes be problematic if running over a track where the solder-resist is weakened (age / heat etc). If the PCB is partially conductive then I guess that could be even more scope for occurring.

Also I had a problematic boogie amp doing this and by operating it in the dark I could actually see little sparks jumping around the PCB. Very pretty and utterly un-fixable from a long term reliability point of view.

[Stratfordade]

Thanks Mick I’ll attend to those sockets. Unfortunately can’t try EL34s individually as in their wisdom - adding to their conductive boards faux pas - they series the heaters for both the EL34 and ECC83 pair!

I echo your issue with their PCBs. Recently I had a long job finally fixing a Blackstar Artist 10W where the board was magically leaking massive bias voltage onto a resistor on the other side of the board.

[mikeydee77]

ALSO this is some notes I have from BS....

Advice from Blackstar on Crackling
I have seen the problem show up on the 60/100 pcb but it is very rare as it had a different substrate to the HT5

One thing to check on any of the amps built around 2010 -2013 is the anode stopper caps on the 2 preamp valves ( ECC83)

They are usually 22nf @400v, ( there may be a 47nf in there dependant on the model) we have changed a fair few over the years, the symptom is crackling and popping and volume drop off,

Changing all 4 will usually clear this type of problem up,

[Stratfordade]

Those valve sockets have some seen some action... If there are any scorch marks then they will be potentially conductive to HT.

Also I had a problematic boogie amp doing this and by operating it in the dark I could actually see little sparks jumping around the PCB. Very pretty and utterly un-fixable from a long term reliability point of view.

Thanks Mikey if all else fails I will be looking for fireflies in a pitch dark room tonight!

[Stratfordade]

Quite a few blackstars have problems with pcb material in between the phase splitter pins breaking down causes a crackling frying Bacon sound. The cure is to drill out a gap between the pins until there is a clear gap. Think it was a particular date range than precise model.

You need a small bit like 0.5mm and a steady hand and a dremel is handy for this kind of root canal work.

If you contact blackstar they may supply you with fix instructions. I have found them pretty helpful in the past.

My mistake Mikey I forgot I’d noted lines drawn between the PI pins on the solder side of the board, but they’ve not been slotted! I’ll get hold of a tiny dremel bit and try that if nothing else works, thanks.