Replacing Marshall power switches?
JimmyB
The lights have gone in both standy and power switches and as a result
I'm constantly leaving the amp on when I thought I'd switched it off.
Seeing an online retailer selling replacement Marshall switches, I
bought a couple.
http://www.hotroxuk.com/marshall-amplifier-main-switch-11578-0.html
Both standby and power switch have four connectors, so imagine my
surprise when opening up the beast to find that the original power
switch has 5 connectors, each one with a wire connected.
From the bottom:
left 1 - blue
left 2 - orange
left 3 - black
right 1 - brown
right 2 - brown
Do I need to track down an original replacement, or can I wire up the
new one and if so, how?
BTW, it looks like I'm just too late with the new switches. I powered
it up today to make sure I drained the capacitors before removing the
chassis, and nothing happened.
Guess the next thing will be trying to track down replacement fuses. <sigh>
Jim
--
50th Anniversary Edition
http://www.jamesbisset.com/blog/
christofire
"JimmyB" <sp...@jazzrascals.com> wrote in message
news:2009091918073816807-spam@jazzrascalscom...
Looking at the circuit diagrams at
http://www.drtube.com/schematics/marshall/78614-3.gif and
http://www.schematicheaven.com/marshallamps/jcm800_clubncountry_100w_4140.pdf
it appears the mains power switch is a double pole single throw switch,
accounting for four of the connections, with the inbuilt neon lamp
internally wired between one of the switch contacts and, probably, the fifth
connection on the switch unit. In the circuit, the neon (with a series
current-limiting resistor, not shown) appears to be connected to the
lowest-voltage tap (120 V) on the mains transformer primary, presumably so
it will get the same voltage irrespective of the actual mains voltage as
long as the correct tap is chosen. However, this could be an international
version of the circuit - does your amp have the option of 120 V mains
voltage?
If your replacement switch has its neon connected internally to two of the
connections, and if you're only intending to use the amp on UK and European
mains (220/230 V) then you could probably dispense with the detail of
feeding the neon from the 110 V tap. If you understand this then you'll be
able to work out which one of the five wires to insulate or remove and how
to connect the remaining four, checking the switch first using an ohm-meter
or a battery and bulb. The colours of the wires might suggest mains line
(brown to brown) and neutral (blue to black), leaving orange for the neon,
but I wouldn't rely on colours - you'd need to correlate the actual wiring
with the circuit diagram to be sure.
The standby switch and inbuilt neon should be a simpler deal since it only
has to deal with one voltage, but it doesn't help that one of the two
diagrams linked to above shows it as SPST and the other DPST.
If you don't understand all this you would be advised to seek professional
help - you could easily end up wiring a switch across the mains (via your
mains fuse).
Chris
JimmyB
Thanks for that Chris,
It does have switchable voltage, but it also has a sticker on the back
on which someone has scribbled 'wired for 240v internally'. The standby
has no wires on left 1 and 2 and two wires each on right 1 and 2.
All the wires running from the power switch and standby switch
disappear under the fuse board, so it's not straightforward to see if
one any of them have a resistor in series.
One of the 500ma fuses looks like it has blown and I was measuring it
to confirm it was 32mm when I got a mild shock. This probably explains
the high voltage warning on the fuse board which I've just noticed.
Having read some of the scary stories about valve amps on the Marshall
forum while researching, I'm kinda inclined to take it to a
professional!
BTW, thanks for the links to the schematics. And to think I paid a
fiver over the internet to get a photocopy of something very similar a
few years ago.
Trevor Ridney
"JimmyB" <sp...@jazzrascals.com> wrote >
Both standby and power switch have four connectors, so imagine my
> surprise when opening up the beast to find that the original power switch
> has 5 connectors, each one with a wire connected.
>
The five contact switches are now discontinued.
The fifth contact is there to supply 120V, whatever the local mains supply,
to power the neon indicator from a tap on the circuit board.
Just disconnect this wire from the switch, shrink wrap the end and use a
cable tie to tuck it out of the way.
If you use the amp abroad, the neon will still work, albeit dimly.
HTH,
JimmyB
<trid...@hotmailSPAMBEGONE.com> said:
I'm not likely to be using it abroad, I can barely get the bugger down
the stairs and out of the house.
Thanks for the information though. I had the amp 'customised' not long
after I got it in the nineties, to make it just a little dirtier. If I
can track down Class A Amps I might get him to fix it.
I'd love to have a go myself, but at my age, I seem to be stuck with
the 'If anything can go wrong, it will' mantra. I've just wired up two
battey powered stomp box kits to accept DC jacks, and although my
multimeter says all connections are right, the pedals promptly switch
off when I plug in the DC jack.
Applying the same technological skill to a vintage Marshall seems to be
asking for trouble!
jim