Marshall JCM 600 oscillating

[N_Cook]

On 23/12/2015 12:52, John Heath wrote:

On Tuesday, December 15, 2015 at 7:00:52 PM UTC-5, Phil Allison wrote:
Gareth Magennis wrote:



** Found this pic of the insides of one:

http://gitaradiy.pl/uploads/2_974_JCM600_6.jpg

The OT is on the far left ( right behind the input jacks ) and primary wires
run next to all the pre-amp valve wiring on their way to the valve sockets
on the right.

That is a *really* poor layout for a valve guitar amp and sitting duck for
oscillation.

Be worthwhile twisting the three primary wires and moving them further back.





The multitude of grey interconnects are not actually screened cable, just a
signal wire lying beside an earth wire.
I don't know what level of protection that actually offers.


** Close to none.

The layout relies on the two plate wires being in close proximity so the out of phase electric fields cancel at a distance.



... Phil

There is a third option. Stop being a hero. You can not fix everything. Phone the customer and say this is a tricky problem and that they are better off taking it to the manufacturer for service. You will lose money but gain some cookie points for being honest. Better still the manufacturer is now spinning their wheels trying to fix it not you which the way it should be if it is a design problem.

When an owner finds out how much it costs in carrier fees plus insurance
to a main agent for repair sometime in the next 3 months(even if
manufacturer pays the return cost), then finds out a good chance of said
item being repaired locally for much the same cost , in half a week,
they always tend to one option, as they have a gig next week.

 

[John Heath]

On Wednesday, December 23, 2015 at 8:14:17 PM UTC-5, Phil Allison wrote:

John Heath wrote:



The layout relies on the two plate wires being in close
proximity so the out of phase electric fields cancel at a distance.




There is a third option. Stop being a hero.
You can not fix everything. Phone the customer
and say this is a tricky problem and that they
are better off taking it to the manufacturer for
service.



** The OP fixed the oscillation problem, after acting on my advice about cable dress. A previous repairer had been careless.


.... Phil

I can see how wire dressing would cause this with high gain of a guitar amp.. However this is Monday night quarterbacking. One does not know it is wire dressing , flat electrolytic , cold solder. Oscillation problems are almost as bad as intermittent problems. You can go round and round all day with no clear path to success. I say do not be a hero. If it walks like a dog and it talks like a dog get out of it while you can. Once that repair is billed you have entered a contract for 90 days warranty where the the window of opportunity to get out of it is lost. It can put you out of business.

Related to wire dressing I have noticed in some preamps the power transformer is mounted in an odd orientation. Often at 45 degrees to the chassis frame. I imagine an engineer when prototyping move the power transformer here and there finding the sweet null spot where stray 60 CPS would not couple into the inputs. I have a bench frequency counter that is the same with the power transformer mounted at 45 degrees relative to the chassis frame.

 

[Phil Allison]

John Heath wrote:

Phil Allison wrote:



The layout relies on the two plate wires being in close
proximity so the out of phase electric fields cancel at a distance.




There is a third option. Stop being a hero.
You can not fix everything. Phone the customer
and say this is a tricky problem and that they
are better off taking it to the manufacturer for
service.



** The OP fixed the oscillation problem, after acting on my
advice about cable dress. A previous repairer had been careless.




I can see how wire dressing would cause this with high gain of a guitar
amp. However this is Monday night quarterbacking. One does not know it
is wire dressing ,

** Moving the input signal and output plate wires BACK to where they were originally and having all oscillation stop is enough proof for anyone familiar with tube guitar amps - but apparently not you.

AC supply transformer orientation can be critical in sensitive devices, graphic equalisers and tape machines are good examples - plus CRT based scopes.

In tube amps, AC and output transformers are normally mounted close together with the cores at right angles to prevent direct injection of 50/60 Hz hum.

In case of the OP's JCM600, Marshall elected to mount the two at opposite ends of the chassis, maybe to balance the weight, but doing so can easily lead to instability.


..... Phil