Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Default Marshall JCM 2000, DSL of 2003

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


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C13 100V 1uF at V1(A) cathode , of all things, is highly sensitive to hot
air.
Monitoring the hum over the output load. Once force warmed and hum level
risen it is reluctant to go down again from natural cooling or from freezer
spray - what process is going on with in it?
Now to replace by fudge fitting to in situ component leads or take the whole
thing apart to replace properly?
Marshall saw fit to bodge 3 resistors by cutting off the originals and
flying replacement ones to the cut wires - so whats good for the goose ....


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On 3/15/2012 8:16 PM, N_Cook wrote:
C13 100V 1uF at V1(A) cathode , of all things, is highly sensitive to hot
air.
Monitoring the hum over the output load. Once force warmed and hum level
risen it is reluctant to go down again from natural cooling or from freezer
spray - what process is going on with in it?
Now to replace by fudge fitting to in situ component leads or take the whole
thing apart to replace properly?
Marshall saw fit to bodge 3 resistors by cutting off the originals and
flying replacement ones to the cut wires - so whats good for the goose ....



Fudge it to verify the fault then do a proper number.

Rheilly P
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Rheilly Phoull wrote in message
. au...
On 3/15/2012 8:16 PM, N_Cook wrote:
C13 100V 1uF at V1(A) cathode , of all things, is highly sensitive to

hot
air.
Monitoring the hum over the output load. Once force warmed and hum level
risen it is reluctant to go down again from natural cooling or from

freezer
spray - what process is going on with in it?
Now to replace by fudge fitting to in situ component leads or take the

whole
thing apart to replace properly?
Marshall saw fit to bodge 3 resistors by cutting off the originals and
flying replacement ones to the cut wires - so whats good for the goose

.....



Fudge it to verify the fault then do a proper number.

Rheilly P


Any ideas about the origin of the "heat ramping" effect?
Allowing the whole amp to cool for 1/4 hour obviously resets the effect


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"N_Cook" wrote in message
...
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




This might be the infamous output PCB fault.
Keep metering the bias voltage from the 3 pin connector provided, (Should be
90mV both sides).
If it starts to climb/runaway in sync with the hum increasing, then that's
what the problem is.

You will need to buy another PCB from Marshall if so.



Gareth.




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Default Marshall JCM 2000, DSL of 2003

If it is the output PCB, it can very quickly destroy the output tubes by
putting HT where it shouldn't, so do not put any new ones in there til you
have eliminated this possibility.


Gareth.


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Gareth Magennis wrote in message
...

"N_Cook" wrote in message
...
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




This might be the infamous output PCB fault.
Keep metering the bias voltage from the 3 pin connector provided, (Should

be
90mV both sides).
If it starts to climb/runaway in sync with the hum increasing, then that's
what the problem is.

You will need to buy another PCB from Marshall if so.



Gareth.



I will check that, I could understand with that old lino or whale hide they
used to use on Fenders.
Marshall fault , some chemical getting capilliary fashion down the glass
fibres of the composite?


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This might be the infamous output PCB fault.
Keep metering the bias voltage from the 3 pin connector provided, (Should

be
90mV both sides).
If it starts to climb/runaway in sync with the hum increasing, then
that's
what the problem is.

You will need to buy another PCB from Marshall if so.



Gareth.



I will check that, I could understand with that old lino or whale hide
they
used to use on Fenders.
Marshall fault , some chemical getting capilliary fashion down the glass
fibres of the composite?





It is the board material itself that goes faulty. I do not know the exact
mechanism, but clearly parts of it become conductive, causing absolute havoc
with the bias.
The output tubes often get red hot and are destroyed.

Unfortunately the first thing the owner tends to do is replace the valves.
Which very soon get destroyed again.

(Expensive business when it eventually gets fixed properly - that would be
12 x EL34's, the output PCB, and a fair bit of labour).



Keep a careful eye on the bias as it warms up, adjusting all the time to
keep it less than 90mV each side. The moment you run out of bias adjustment
is the moment you have pretty much demonstrated the faulty PCB, and you want
to turn it off immediately.



Gareth.










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"N_Cook" wrote in message
...
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





Couple of other tips on the DSL and TSL amps:

Resolder (and preferably reinforce with wire) the DIN footswitch socket. It
is only held in place by the solder joints which often (always) fail in
regular use.

The back panel PCB mounted mains and HT fuseholders also tend to suffer from
dry joints. Whip off the PCB and resolder them. 3 minutes tops.

The 16 ohm speaker output is a switched socket that disables the 4 and 8 ohm
sockets when used. These often get damaged, becoming a problem when the
other sockets are used instead. Check, repair or replace.

The speaker ohmage selector switch also tends to suffer from dry joints.

(Either of the above 2 problems can destroy the output TX, output valves and
other stuff, so best do it)

The output mute switch is stupidly relying on the switch contact to feed the
signal to the power amp, rather than being a mute. Clean it.




Gareth.

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It would be nice to find a schematic for this specific variant , but only
one Google Ref for the number on the end face label
JCM2-20-06

I'm wondering if there is any significance in the bodged 1/3 W R
replacements
R70 of 5K6
R7 of 5K6
R48 of 100K

unfortunately none of these numbers agree with any JCM2000/ DSL schemas that
I have.
Similarly I cannot find the 2K2 HT line dropper for (a) half of V1 mentioned
on these schema




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"N_Cook" wrote in message
...
It would be nice to find a schematic for this specific variant , but only
one Google Ref for the number on the end face label
JCM2-20-06

I'm wondering if there is any significance in the bodged 1/3 W R
replacements
R70 of 5K6
R7 of 5K6
R48 of 100K

unfortunately none of these numbers agree with any JCM2000/ DSL schemas
that
I have.
Similarly I cannot find the 2K2 HT line dropper for (a) half of V1
mentioned
on these schema





This site mentions people finding 220k grid resistors (R70, R7 etc) instead
of the 5k6 resistors they expected, along with other details with regards to
this board problem.

Looks like someone may have tried to sort this problem out before, but
didn't replace the PCB?

http://www.lynx.bc.ca/~jc/TSL122.html



Gareth.




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"Gareth Magennis"

Looks like someone may have tried to sort this problem out before, but
didn't replace the PCB?

http://www.lynx.bc.ca/~jc/TSL122.html



** The PCB material is definitely the cause of the problem.

It is made from UTTER CRAP !!

However - uber clever fixes involving a Dremel or mounting resistors in
mid air or any similar hare brained ideas are all just re-arranging the deck
chairs on the SS Titanic.

If no new PCB is available from Marshall at a reasonable price - just add
a bloody fan !!!!!!!!!!!!!!!!!

A single 120mm, 240VAC fan blowing directly on the OP tubes from below cures
the overheating / bias runaway problem completely.

BTW:

Marshall are a total bunch of arseholes.

The Pox of the audio industry.

Just like their ****ty amps.



.... Phil


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"Phil Allison" wrote in message
...

"Gareth Magennis"

Looks like someone may have tried to sort this problem out before, but
didn't replace the PCB?

http://www.lynx.bc.ca/~jc/TSL122.html



** The PCB material is definitely the cause of the problem.

It is made from UTTER CRAP !!

However - uber clever fixes involving a Dremel or mounting resistors in
mid air or any similar hare brained ideas are all just re-arranging the
deck chairs on the SS Titanic.

If no new PCB is available from Marshall at a reasonable price - just
add a bloody fan !!!!!!!!!!!!!!!!!

A single 120mm, 240VAC fan blowing directly on the OP tubes from below
cures the overheating / bias runaway problem completely.





New PCB's ARE available from Marshall, at about 60 to the customer if I buy
and fit one. Marshall tell me they sell a lot of them.
I understand costs are much higher outside the UK, however.

I totally agree the bodges and drilling malarky - the problem IS the PCB
material.

Charging the customer a whole bunch of labour to bodge a way around this
problem is, IMHO, unethical, as he STILL has a faulty PCB, and has been
charged loads of money not to fix the real problem.

Replacing the PCB is relatively cheap (compared with the alternative labour
costs) quick and easy, and is the only logical solution here.



Incidentally, 2 old faulty boards I have here and have kept, both have 220k
grid stopper resistors instead of the 5k6 the schematic shows.



Cheers,


Gareth.


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On Fri, 16 Mar 2012 20:56:53 +1100 "Phil Allison"
wrote in Message id: :

just re-arranging the deck
chairs on the SS Titanic.


*guffaw*

Gotta remember that one!
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"Gareth Magennis"
"Phil Allison"
"Gareth Magennis"

Looks like someone may have tried to sort this problem out before, but
didn't replace the PCB?

http://www.lynx.bc.ca/~jc/TSL122.html



** The PCB material is definitely the cause of the problem.

It is made from UTTER CRAP !!

However - uber clever fixes involving a Dremel or mounting resistors in
mid air or any similar hare brained ideas are all just re-arranging the
deck chairs on the SS Titanic.

If no new PCB is available from Marshall at a reasonable price - just
add a bloody fan !!!!!!!!!!!!!!!!!

A single 120mm, 240VAC fan blowing directly on the OP tubes from below
cures the overheating / bias runaway problem completely.



New PCB's ARE available from Marshall, at about 60 to the customer if I
buy and fit one. Marshall tell me they sell a lot of them.
I understand costs are much higher outside the UK, however.



** It's not just the cost in dollars or whatever - availability is crucial
too.

Even common Marshall power and output transformers ( the ****ty Indian made
one ones that sit the lams right on the chassis) are regularly out of stock
here in Australia for months on end.

New output stage PCBs are not even on the radar !!!!!!!!!



I totally agree the bodges and drilling malarky - the problem IS the
PCB material.

Charging the customer a whole bunch of labour to bodge a way around this
problem is, IMHO, unethical, as he STILL has a faulty PCB, and has been
charged loads of money not to fix the real problem.

Replacing the PCB is relatively cheap (compared with the alternative
labour costs) quick and easy, and is the only logical solution here.



** See above.

The logical solution is the use a bloody FAN !!!!!!!!

Takes less than 1 hour to fit one to any combo model and the job is DONE !!

An IEC inlet mounted on the lower back panel supplies AC power to the fan
and it is up to the owner to use an extra IEC lead to run it.



..... Phil





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"Phil Allison" wrote in message
...

"Gareth Magennis"
"Phil Allison"
"Gareth Magennis"

Looks like someone may have tried to sort this problem out before, but
didn't replace the PCB?

http://www.lynx.bc.ca/~jc/TSL122.html


** The PCB material is definitely the cause of the problem.

It is made from UTTER CRAP !!

However - uber clever fixes involving a Dremel or mounting resistors
in mid air or any similar hare brained ideas are all just re-arranging
the deck chairs on the SS Titanic.

If no new PCB is available from Marshall at a reasonable price - just
add a bloody fan !!!!!!!!!!!!!!!!!

A single 120mm, 240VAC fan blowing directly on the OP tubes from below
cures the overheating / bias runaway problem completely.



New PCB's ARE available from Marshall, at about 60 to the customer if I
buy and fit one. Marshall tell me they sell a lot of them.
I understand costs are much higher outside the UK, however.



** It's not just the cost in dollars or whatever - availability is
crucial too.

Even common Marshall power and output transformers ( the ****ty Indian
made one ones that sit the lams right on the chassis) are regularly out of
stock here in Australia for months on end.

New output stage PCBs are not even on the radar !!!!!!!!!



I totally agree the bodges and drilling malarky - the problem IS the
PCB material.

Charging the customer a whole bunch of labour to bodge a way around this
problem is, IMHO, unethical, as he STILL has a faulty PCB, and has been
charged loads of money not to fix the real problem.

Replacing the PCB is relatively cheap (compared with the alternative
labour costs) quick and easy, and is the only logical solution here.



** See above.

The logical solution is the use a bloody FAN !!!!!!!!

Takes less than 1 hour to fit one to any combo model and the job is DONE
!!

An IEC inlet mounted on the lower back panel supplies AC power to the fan
and it is up to the owner to use an extra IEC lead to run it.



.... Phil




OK, I see that in your situation that makes a lot more sense than in mine.


Any thoughts on the 220k resistor anomaly? Someone stuffed the component
insert machine with the wrong resistors?
Bit of a disaster, this PCB.



Gareth.


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I should be able to get back to it in a couple of hours. Only just got
around to building a Dexion support frame so I can toss these sorts of amps
around with gay abandon, and not risk the bottles.

The valve set that are in here are all 2009 , he took out the new ones and
replaced these used ones. They look brand new except for simple dragon's
teeth marks, ie not repeatedly moved around.
The Russian markings are quite visible and there is no sign of overheating
of the bases , still light brown , and the pcb around looks as new.
But the first thing I will do is check for o/p bias drift, then heating the
pcb with hot air and a 2Gohm megger and then remove V1 and see if hum
returns then replace and monitor DCs around V1 on heating ,

No mention of that Marshall Forum page about hum but it could be these pcb
problems around V1 the immediate problem


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"N_Cook" wrote in message
...
I should be able to get back to it in a couple of hours. Only just got
around to building a Dexion support frame so I can toss these sorts of
amps
around with gay abandon, and not risk the bottles.

The valve set that are in here are all 2009 , he took out the new ones and
replaced these used ones. They look brand new except for simple dragon's
teeth marks, ie not repeatedly moved around.
The Russian markings are quite visible and there is no sign of overheating
of the bases , still light brown , and the pcb around looks as new.
But the first thing I will do is check for o/p bias drift, then heating
the
pcb with hot air and a 2Gohm megger and then remove V1 and see if hum
returns then replace and monitor DCs around V1 on heating ,

No mention of that Marshall Forum page about hum but it could be these pcb
problems around V1 the immediate problem




The "Hum" I have associated with the PCB problem is ripple due to the huge
currents the output valves are taking from the PSU.
One pair of valves inevitably draws way more current than the other pair,
which again manifests as a hum. The more current the valves draw, the
hummier it gets, until things start glowing red, then things start to break
and the hum is gone.

Yours may not actually have this PCB problem, but the bodged in grid
stoppers make me suspicious.



Gareth.


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Gareth Magennis wrote in message
...

"N_Cook" wrote in message
...
I should be able to get back to it in a couple of hours. Only just got
around to building a Dexion support frame so I can toss these sorts of
amps
around with gay abandon, and not risk the bottles.

The valve set that are in here are all 2009 , he took out the new ones

and
replaced these used ones. They look brand new except for simple dragon's
teeth marks, ie not repeatedly moved around.
The Russian markings are quite visible and there is no sign of

overheating
of the bases , still light brown , and the pcb around looks as new.
But the first thing I will do is check for o/p bias drift, then heating
the
pcb with hot air and a 2Gohm megger and then remove V1 and see if hum
returns then replace and monitor DCs around V1 on heating ,

No mention of that Marshall Forum page about hum but it could be these

pcb
problems around V1 the immediate problem




The "Hum" I have associated with the PCB problem is ripple due to the huge
currents the output valves are taking from the PSU.
One pair of valves inevitably draws way more current than the other pair,
which again manifests as a hum. The more current the valves draw, the
hummier it gets, until things start glowing red, then things start to

break
and the hum is gone.

Yours may not actually have this PCB problem, but the bodged in grid
stoppers make me suspicious.



Gareth.




main bias results
monitoring every 5 minutes for the first 15 minutes
rate of increase is falling
passing 10 to 15 minutes in
one side rising 1.0mV per 5 minutes and the other side 1.5mV per 5 minutes
starting from that 15 min reading of 76.9mV and 68.2mV should be about 90mV
over an hour .
locally heating with hot air (low setting) for 20 seconds the voltages shot
up
5 or so mV but soon dropped back to where they were , unlike the problem
around V1 it would seem.

now for the other preliminary tests


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No E number of board supplier found but I would say, by trying to press a
needle into the board, it is epoxy rather than polyester composite.
Megger showed nothing untoward
With no V1 in place , the grid socket pins measure about 10 or 20mV DC, wave
hot air over the valve base and the readings shoot up to 100 to 200mV, just
like applying a magic wand.
The cathode lines have 2/3 orders of magnitude lower resistance to ground so
any such effect not so obvious there

These are the small bottles not the hotter big bottles, something to do with
that metal shield plate for them? I'm assuming the effect is at the valve
base through board , hot pins rather than where the anode dropper leads pass
through the pcb, and passing through rather than along the surface . I will
explore this, as hard wiring the HTs to isolated valve bases ,only, is quite
different to hard-wiring all HT traces.
Now what is the physics /chemistry of all this ? I assume something
hygroscopic is grabbing moisture then forming a conductive salt that stays
within the micropores of the surface of the glass fibres, but why
temperature increasing the conductivity , what salt has highly temp
dependent conductivity, we're only talking 50 degree C or so




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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


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"N_Cook" wrote in message
...
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.



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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
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Gareth Magennis wrote in message
...


"N_Cook" wrote in message
...
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


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Jon Elson wrote in message
...
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



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?




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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.


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Default Marshall JCM 2000, DSL of 2003



"N_Cook" wrote in message
...
Gareth Magennis wrote in message
...


"N_Cook" wrote in message
...
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.

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Default Marshall JCM 2000, DSL of 2003

Gareth Magennis wrote in message
...


"N_Cook" wrote in message
...
Gareth Magennis wrote in message
...


"N_Cook" wrote in message
...
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.


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.


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


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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.






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In article , N_Cook 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 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!
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Default Marshall JCM 2000, DSL of 2003

Dave Platt wrote in message
...
In article , N_Cook 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 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!


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.


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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


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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


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"N_Cook" wrote in message
...

As nothing is learnt by handing over money to Mr Marshall ..............




I knew you wouldn't buy one.


Good luck.


Gareth.




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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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