Marshall JTM60 Standby Switch

[jets]

Has anyone ever had a problem when you turn on the standby switch it does not work ? Power light is on and when the standby switch is turned on it does not lite and no sound from the amp ??? Any Ideas what could cause this ?

[TD_Madden]

speaker cable loose?

[jets]

Yep, the speaker cable is plugged in. Souldn't the standby switch light anyways when switch to the on position ?

[Alex R]

Is the HT fuse ok?

[jets]

I checked the main fuse and it was ok, I took the head out of the amp and there are 2 other fuses, one of which was hidden behind some capacitors.
When I checked that one and it was blown...
Thanks alot I did not realize the last on was hidden like that...
I'm not sure what amp rating the fuse is but I will get a acouple of replacements...
Thanks a million...

[trevorus]

I checked the main fuse and it was ok, I took the head out of the amp and there are 2 other fuses, one of which was hidden behind some capacitors.
When I checked that one and it was blown...
Thanks alot I did not realize the last on was hidden like that...
I'm not sure what amp rating the fuse is but I will get a acouple of replacements...
Thanks a million...

Might check the casing of the fuse. The metal ends should have the rating stamped on them.

[jets]

Thanks trevorus, I would if I could... my eyes arn't what they used to be even with glasses.. I'll check with someone at the electric store and have them match it up..
Thanks Again..

[trevorus]

Thanks trevorus, I would if I could... my eyes arn't what they used to be even with glasses.. I'll check with someone at the electric store and have them match it up..
Thanks Again..

I just checked the schematic. It's 500 mA, or half an amp. (That is, if it's the fuse on the high voltage supply coming from the standby switch.)

[Enzo]

If it is 500ma (1/2 amp) make sure it is a slow blow if that is what is needed. If it says "T500ma" by the fuse clips, T means time delay - slow blow. Often Marshall prints the fuse data on the circuit board near the clips.

And another warning: while it might just be a fuse that aged and failed, much of the time a blow high voltage fuse means a power tube failed.

[trevorus]

Enzo is right. It is a slo-blo.

Here's the link to the schematic: http://www.drtube.com/schematics/mar...00-61-02-1.gif

It's about in the center. The standby switch is labeled, and the fuse is right next to it, marked T500mA

[TD_Madden]

Question: Why would you want a slo-blow as the HT fuse? Didn't Marshall used to use a regular fuse there?

[trevorus]

Question: Why would you want a slo-blow as the HT fuse? Didn't Marshall used to use a regular fuse there?

A slo-blo is better for that duty because it is near a switch. The turn-on draw there can peak sometimes. Slo-blo gives just a bit of wiggle room on it, so it doesn't blow every time the switch happens to arc inside.

[Enzo]

When you flip the standby switch to on, the caps all of a sudden have to charge, and that is a brief surge in current. With fast blow fuses, yuo need to have a fuse large enough to handle that surge. But then it is too large to really work as intended during operation. By using a slow blow fuse, we can use a smaller value fuse which is more likely to work as we desire.