draining caps in a JTM45

[passerby3141]

Hello!

I am relatively new here, this forum started after I began to frequent these boards.

Simple question, is pulling the rectifier tube in a JTM45 and powering up for about 5 minutes a sure fire way to drain the filter caps. I have read on the net that it is, but thought I'd ask here. I have a meter to test afterward. Just got a JTM45 board kit for my reissue, this is my first attempt at this kind of work on an amp.

[thousandshirts]

If you're here

You might as well learn to do it properly

Construct a cable to drain the voltage

Using two alligator clips, some wire, a bit of shrink tube, and a resistor

http://forum.metroamp.com/viewtopic.php ... or#p248268" onclick="window.open(this.href);return false;

Ask any further questions if you are in doubt

[Drifting]

The method on George's Wiki doesn't use the resistor between the two gator clips(but does lead to a resistor in the first gain stage), but I guess that doesn't apply to the method you just showed, right? Either way, I'll go down to Radioshack to pickup a resistor.

[Structo]

First of all, always be sure the amp is unplugged. When working on a cluttered bench it is sometimes easy to get mixed up where what cord goes where.
So cleanliness can be a good thing for safety.

Depending on the amp, switch the standby switch to play, because some amps have a filter cap before the switch which can stay charged even though the other caps are drained.
Turning the standby switch to play ensures that all the filter caps are connected.

The first method works good if you want to hook up one lead to the actual filter cap.

The way I normally do it is to use a jumper cable with an insulated alligator clip at each end.
I clip one end to a plate load resistor between the preamp tube socket and the board.
This will generally be a 100K-220K resistor.
The other end gets clipped to a bare area on the chassis. (ground)

This method uses the plate resistor to drain the filter caps down.
It may take a minute or two depending on the dropping resistor string in the power supply.

Always confirm that the filter caps are indeed discharged with a meter before exposing yourself to the interior of the amp.

I like to leave the jumper in place while I work to prevent any residual build up in the caps.

REMEMBER TO REMOVE THE JUMPER BEFORE POWERING BACK UP!