Marshall JCM 2000, DSL of 2003

[N_Cook]

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

 

[N_Cook]

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 ....

 

[Rheilly Phoull]

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

 

[N_Cook]

Rheilly Phoull <rheilly@bigslong.com> wrote in message
news:fKydnQmj09_6Q_zSnZ2dnUVZ_gKdnZ2d@westnet.com.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

 

[Gareth Magennis]

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:jjskk4$n2e$1@dont-email.me...

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.

 

[Gareth Magennis]

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.

 

[N_Cook]

Gareth Magennis <sound.service@btconnect.com> wrote in message
news:YoWdnTjOt5FakP_SnZ2dnUVZ8vqdnZ2d@bt.com...

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:jjskk4$n2e$1@dont-email.me...
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?

 

[Gareth Magennis]

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.









>

 

[Gareth Magennis]

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:jjskk4$n2e$1@dont-email.me...

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.

 

[N_Cook]

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

 

[Gareth Magennis]

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:jjuugr$a8i$1@dont-email.me...

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.

 

[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.

BTW:

Marshall are a total bunch of arseholes.

The Pox of the audio industry.

Just like their shitty amps.



.... Phil

 

[Gareth Magennis]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:9sgh38F8s9U1@mid.individual.net...

"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 Ł60 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 re: 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.

 

[JW]

On Fri, 16 Mar 2012 20:56:53 +1100 "Phil Allison" <phil_a@tpg.com.au>
wrote in Message id: <9sgh38F8s9U1@mid.individual.net>:

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

*guffaw*

Gotta remember that one!

 

[Phil Allison]

"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 Ł60 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 shitty 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 re: 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

 

[Gareth Magennis]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:9sgl57F88aU1@mid.individual.net...

"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 Ł60 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 shitty 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 re: 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.

 

[N_Cook]

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

 

[Gareth Magennis]

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:jjvb1c$989$1@dont-email.me...

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.

 

[N_Cook]

Gareth Magennis <sound.service@btconnect.com> wrote in message
news:fJGdnerpc6b8p_7SnZ2dnUVZ8rSdnZ2d@bt.com...

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:jjvb1c$989$1@dont-email.me...
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

 

[N_Cook]

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