Marshall JCM 2000, DSL of 2003

Well, that's where you will find the problem, almost for sure. Hum that increases over time is very likely to be bad electrolytic caps.

Jon

a

they

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Why did the mod-bods choose to isolate the grid pins rather than the anode pins?

Anyone know if they use mineral filler (cheaper) in the epoxy of epoxy pcb manufacture. Like the use of calcium carbonate in the epoxy bulking/fixing of toroidal transformers or even car body repair filler. This morning I made a test cell of some calcium carbonate and water to thick paste consistency in an inch wide plastic bottle cap. Resistance across a diameter about 60K. Waft a low-setting hot air gun over it and resistance drops to about 2K, now rising again. So moisture/condensation can get into the edges and component holes of such a pcb and the glass dutifully conduct it capilliary fashion. Perhaps a cure might be a low oven bake of a day at 105 deg C of a populated board , assuming nothing comes to grief at that temp and then some sprayed on conformal coatinf along all edges and component leads, but that would not get to the prime source of problems , inside /under the valve bases and those pcb holes

I can see run away bias , less neagative grids with temperature rise being caused by conductive epoxy. But as I say (for the moment) this amp PA seems ok, but where is the hum coming from around V1? some conductive path from the heaters?

For an owner who knows he has such an amp, this would be worth doing. Obtaining or making an amp sized heavy duty polythene bag that can be resealed easily and some sachets of activated silica gel crystals and a large jam/pickled gherkin jar to keep the oven acivated ones in, until use. After each use of the amp , place amp while still warm prefereably ,in the bag with a fresh sachet from the storage jar.

I'd be willing to donate one of my faulty PCBs if you sincerely thought it might help you to further establish the mechanisms involved here.

My email reply address is valid.

Gareth.

intermediary

chemistry

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bulking/fixing

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A day on of drying out and my test cell showed 150K, warming easily brought it down to 10K. Tried a sample of calcium carbonate as it is , powder straight from the open bag a decade old at least , and no response to a megger.

My chemistry failed. 30 percent HCl showed no fizzing with the powder. Warming up and a lighted taper extinguished in the tube. Ground off a sample from this Marshall board and the same. But then tried a tube of HCl on its own and same extinguishing of taper. I'll stick with the day job.

My test cell now measures >500M cold or warm , so perhaps not calcium carbonate

Now trying a test cell mix of NaCl and calcium carbonate, as no convenient source of calcium chloride to see if a mix of those , via deliquescence, will go thermally conductive without addition of liquid water. Until anyone gets more info I will go with chloride contaminated calcium carbonate as the suspect pcb filler

As nothing is learnt by handing over money to Mr Marshall, modded this main board. As this 06 variant is almost a google-whack I doubt an exact replacement board is available ,off the shelf, anyways. Missing opto devices etc Now I've worked out how to make a 5mm diameter hollow end face cutter , to go in a Dremmel. Once the main board is disconnected ( how many connections?) it is now a simple matter of cutting away a neat hole of pcb around the grid socket pins. And while at it, did so to all 12 grids ECC83 and EL34 and hard wired with silicone sleeving , so all grids are now isolated from the pcb. Now no trace of any hum over normal background even over an hour and also no sensitivity at all from blowing hot air around V1 or the other valves. Just leaves the mystery of what the exact hum intrusion mechanism was , something to do with DC+AC to V1 and V2 heaters? Now all that is left is to get more history from the owner, why and what Marshall had farted about with when returned to them before, other than the all too obvious cut and fly 1/3 W resistor replacements.

Diamond-tipped hollow drill bits/saws are commonly available (they're used for punching holes in ceramics, gemstones, tiles, etc.) and they work quite nicely on PCB material... they make nice pad-cutters.

--
Dave Platt                                    AE6EO
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to

Nothing so grand as that, I will look out for those, but I was unaware of them.

Hollow cutting tool Particularly for Marshall conductive epoxy pcb boards To isolate valve base pins by cutting into the board around a pin. A steel sleeve , this one dimensions internal 4mm and external 5.3mm . Cut eight small cuts on one end to make a castellated form with Dremmel and .6mm disc. Grind a rake angle behind each cutting edge. Place a rod inside the other end to mount in a drill chuck. Start at an off axis angle and bring up to axial after first cutting. Leave soldered joint in place. As this was actually perhaps a roll pin rathe rthan a sleeve, not a solid ring in plan, an axial join line along its length. Found a use for a Dremmel mandrel where the screw is sheared off inside the stem. With free hand grinding with the mandrel rotating in a sleeve and grinding against a disc brought the diameter of the mandrel support down to a tight fit inside this sleeve. Then a plastic filler for the other end and fitted in a Dremmel. Perhaps more reliable , repeat with a good mandrel , find a longer screw, and some compressible silicone sleeving to grip the inside of the sleeve like those Dremmel sanding cylinder holders. Not necessarily punch through as will be ragged anyway. With a dart point excavate around while desoldering the pin .

Owner had previoiusly returned the amp to Marshall for changing those resistors and general maintainence only. He has exactly the same amp and components back , only difference is a few holes in the pcb and wiring path a bit different.

I just realised the ideal starting form for such a tool would be one of thise jeweller's screwdriver nut spinners. Grind back the internal hex form, castellation cuts and cut through the stem to mount in a chuck

20 percent NaCl with calcium carbonate seems to show the undesired affect over a few days so maybe the Marshall contaminated boards have 1 or 2 percent contamination. Pitty no Chinese? E number on their boards so we could tell if they turn up with other makers products, but perhaps not with valve voltages.

So after a few days the dry test cell showed about 500M and over a few hours dropped to 80K or so ,maybe the Megger high V across the probes accelerating this effect , and light heating with hot air , drops to 40K or so, returning to 80K or so soon after. Much the same today as yesterday

I knew you wouldn't buy one. ;)

Good luck.

Gareth.

Marshall didn't tell the owner about changing those resistors . Recent hum problem developed over about 10 hours total of gigs until it became too intrusivce and embarrassing