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Re: Help! Marshall 4203 blowing fuses, not PT

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2/1/2006 2:59 PM

Re: Help! Marshall 4203 blowing fuses, not PT

Hi,
OK Heres what happened. I powered on, indicator light came on ok, hit my standby on, everything good. I put the first power tube in, no problem. I put second power amp tube in , no problem, I then put in the 12ax7 phase inverter, no problem. No blown fuse anymore. I plug in my guitar and everything works as it should. Then I turn off the amp. I leave the tubes in this time. I turn back on the power, everything is ok, indicator light is on, BUT THEN... when I turn the standby back on, the fuse blows. It seems to be something related to the standby switch, but I don't know. Any Ideas??? Thanks. John L.

2/1/2006 3:39 PM
Peter	The circuit shows a Time Lag or "slo blow" fuse either T 2 Amp for 120volts or T 1 Amp 240 volts. In normal use for 120 volts the amplifier draws less than 1 amp but on switch on there is a sudden rush or surge thus the fuse has to accomodate this.

2/1/2006 9:37 PM
John L.	The fuses that were blowing are Radio Shack 2- Amp Fast-Acting Fuses. I take it this means I need a Slow acting 2 amp fuse? Maybe my tubes are in ok shape then? And maybe the power transformer just burnt out from old age? Any further thoughts? Thanks. John L.

2/1/2006 9:56 PM
Enzo	Most definitely you need slow blow fuses. The T means "timed." Fast blow fuses will blow all the time, and mostly when you flip the switch.

2/2/2006 5:52 PM

I meant to say
In normal use for 120 volts the amplifier draws less than 2 amps
I would fit new output valves if a transformer has been replaced.
Its not worth the risk to keep using the ones that were in it when it blew.
I have seen one with a tiny bit of metal rattling
around inside and another with a loose getter.
Although in your case this may not be so I would
sleep better at night knowing the risk of frying another transformer was minimised .

2/2/2006 6:39 PM
John L.	Hi, Makes good sense to me. I got the slo blo fuses and they work fine. I am going to get some new tubes. Thanks to everyone for your invaluable help and assistance. Best regards, John L.

2/3/2006 7:19 AM

My brain finally dredged up what may be an important bit of info for you.

I dimly remember that some Marshalls have the bias supply connected so that when the standby switch is turned to the sound-off position, the bias supply is turned off as well. When the standby switch is turned to the play position, the main B+ comes up faster than the bias supply, and so there is a short time when the tubes are unbiased with the heaters hot. This makes for a big slug of current that diminishes as the bias supply comes up. That kind of thing could definitely pop a non-slow-blow fuse.

I don't remember the model number, or where I saw it, unfortunately - sorry. But you may be seeing that.

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