Marshall JCM 2000, DSL of 2003

For those thru-board valve bases it looks as though you would have to
replace with standoff types to retain structural integrity but with
electrical isolation , as well as all that hard wiring and intermediary
isolation/ mounting points.
Looks like too much of a work up in comparison to paying Mr Marshall for a
replacement board.
I'm just wondering if this particular board , only 1 google ref, that they
will not have a replacement
 
"N_Cook" <diverse@tcp.co.uk> wrote in message
news:jjvqrc$9h2$1@dont-email.me...
For those thru-board valve bases it looks as though you would have to
replace with standoff types to retain structural integrity but with
electrical isolation , as well as all that hard wiring and intermediary
isolation/ mounting points.
Looks like too much of a work up in comparison to paying Mr Marshall for a
replacement board.
I'm just wondering if this particular board , only 1 google ref, that they
will not have a replacement
Click to expand...

I think replacements are generic, i.e. many different models share the same
output PCB.
Its the number of channels and other such stuff that are the variants.


Problem with repair, is that you might repair your V1 symptoms, but 6 months
later it comes back with the massively more disappointing bias destruction
problem.
Or vice versa.

The whole PCB is possessed, it needs to be exorcised.


It would be interesting if you could establish the physics and chemistry
behind all this though!



Cheers,

Gareth.
 
N_Cook wrote:

Hum particularly on clean channel builds up from nothing ,over half an
hour, and very intrusive 0.3V rms of hum over 8R speaker load and still
rising. Putting a signal in Return, for PA only, is fine but opening
guitar input hum returns.
Owner had replaced all the valves and exactly the same hum.
Hum is negligible on the downstream HTs from HT1. What sort of grounding
problem increases with warmth?
I've not started exploring the low voltage electros around V1 yet, I'm
letting the amp cool down
Well, that's where you will find the problem, almost for sure.
Click to expand...
Hum that increases over time is very likely to be bad electrolytic
caps.

Jon
 
Gareth Magennis <sound.service@btconnect.com> wrote in message
news:9SL8r.77866$ir4.13429@newsfe13.ams2...
"N_Cook" <diverse@tcp.co.uk> wrote in message
news:jjvqrc$9h2$1@dont-email.me...
For those thru-board valve bases it looks as though you would have to
replace with standoff types to retain structural integrity but with
electrical isolation , as well as all that hard wiring and intermediary
isolation/ mounting points.
Looks like too much of a work up in comparison to paying Mr Marshall for
a
replacement board.
I'm just wondering if this particular board , only 1 google ref, that
they
will not have a replacement





I think replacements are generic, i.e. many different models share the
same
output PCB.
Its the number of channels and other such stuff that are the variants.


Problem with repair, is that you might repair your V1 symptoms, but 6
months
later it comes back with the massively more disappointing bias destruction
problem.
Or vice versa.

The whole PCB is possessed, it needs to be exorcised.


It would be interesting if you could establish the physics and chemistry
behind all this though!



Cheers,

Gareth.
Click to expand...

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
 
Jon Elson <jmelson@wustl.edu> wrote in message
news:haednc-QOI11LP7SnZ2dnUVZ_tydnZ2d@giganews.com...
N_Cook wrote:

Hum particularly on clean channel builds up from nothing ,over half an
hour, and very intrusive 0.3V rms of hum over 8R speaker load and still
rising. Putting a signal in Return, for PA only, is fine but opening
guitar input hum returns.
Owner had replaced all the valves and exactly the same hum.
Hum is negligible on the downstream HTs from HT1. What sort of grounding
problem increases with warmth?
I've not started exploring the low voltage electros around V1 yet, I'm
letting the amp cool down
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
Click to expand...

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.
 
"N_Cook" <diverse@tcp.co.uk> wrote in message
news:jk1i9o$i9k$1@dont-email.me...
Gareth Magennis <sound.service@btconnect.com> wrote in message
news:9SL8r.77866$ir4.13429@newsfe13.ams2...


"N_Cook" <diverse@tcp.co.uk> wrote in message
news:jjvqrc$9h2$1@dont-email.me...
For those thru-board valve bases it looks as though you would have to
replace with standoff types to retain structural integrity but with
electrical isolation , as well as all that hard wiring and intermediary
isolation/ mounting points.
Looks like too much of a work up in comparison to paying Mr Marshall
for
a
replacement board.
I'm just wondering if this particular board , only 1 google ref, that
they
will not have a replacement





I think replacements are generic, i.e. many different models share the
same
output PCB.
Its the number of channels and other such stuff that are the variants.


Problem with repair, is that you might repair your V1 symptoms, but 6
months
later it comes back with the massively more disappointing bias
destruction
problem.
Or vice versa.

The whole PCB is possessed, it needs to be exorcised.


It would be interesting if you could establish the physics and chemistry
behind all this though!



Cheers,

Gareth.





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
Click to expand...

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.
 
Gareth Magennis <sound.service@btconnect.com> wrote in message
news:%d69r.4692$z23.1030@newsfe03.ams2...
"N_Cook" <diverse@tcp.co.uk> wrote in message
news:jk1i9o$i9k$1@dont-email.me...
Gareth Magennis <sound.service@btconnect.com> wrote in message
news:9SL8r.77866$ir4.13429@newsfe13.ams2...


"N_Cook" <diverse@tcp.co.uk> wrote in message
news:jjvqrc$9h2$1@dont-email.me...
For those thru-board valve bases it looks as though you would have to
replace with standoff types to retain structural integrity but with
electrical isolation , as well as all that hard wiring and
intermediary
isolation/ mounting points.
Looks like too much of a work up in comparison to paying Mr Marshall
for
a
replacement board.
I'm just wondering if this particular board , only 1 google ref, that
they
will not have a replacement





I think replacements are generic, i.e. many different models share the
same
output PCB.
Its the number of channels and other such stuff that are the variants.


Problem with repair, is that you might repair your V1 symptoms, but 6
months
later it comes back with the massively more disappointing bias
destruction
problem.
Or vice versa.

The whole PCB is possessed, it needs to be exorcised.


It would be interesting if you could establish the physics and
chemistry
behind all this though!



Cheers,

Gareth.





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'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.
Click to expand...
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.
 
In article <jki28p$u6g$1@dont-email.me>, N_Cook <diverse@tcp.co.uk> wrote:

Now I've worked out how to make a 5mm diameter hollow end face cutter , to
go in a Dremmel.
Click to expand...
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 <dplatt@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
 
Dave Platt <dplatt@radagast.org> wrote in message
news:5kqu39-lnk.ln1@radagast.org...
In article <jki28p$u6g$1@dont-email.me>, N_Cook <diverse@tcp.co.uk> wrote:

Now I've worked out how to make a 5mm diameter hollow end face cutter ,
to
go in a Dremmel.

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 <dplatt@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!
Click to expand...
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
 
"N_Cook" <diverse@tcp.co.uk> wrote in message
news:jki28p$u6g$1@dont-email.me...
As nothing is learnt by handing over money to Mr Marshall ..............
Click to expand...


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
 
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