Tag Archives: capacitor

Cap job

Discharging the old Davy capacitors

Discharging the old Davy capacitors with my new tool. Note the horrible quality soldering, courtesy of Marshall.

This past weekend I was able to replace the larger 50-50 caps in my amp. It meant I got to use my capacitor discharge tool which I’d been toying with for a while. Soldered between two alligator clips, I have a 5W 5k Ohm wire-wound resistor. I left them clipped on each positive cap terminal for a few minutes. That seemed to do the trick; my test was to do “the screwdriver trick” and short them to ground. There was no “loud pop and sparks” so my tool was successful.

Original Davy 50-50 cap being removed

Original Davy 50-50 cap being removed

So I de-soldered the old wiring and removed the old clamps. It was easy to put the new ones in and re-attach the wiring. I had taken pictures along the way,so it was trivial to get the wiring back in the right place. History has shown that my memory is close to useless, so I didn’t even attempt to hold it all in my head.

You can see the underside of the main PCB through the hole in the chassis (see left). I left this cap, the one sitting next to the choke,  sitting quite high in its clamp (see the picture at the end of this post) as I was paranoid about shorting the wires against the PCB.

Two of the new caps in place

Two of the new caps in place.

You can also see in the above pictures that Marshall wasn’t too picky about employing people that could solder particularly well. Their preparation seemed to be the key…i.e. when they stripped insulation off wires they did not bother to spend half a second to twist the strands together before tinning it. I had to replace a few wires as the original wire was not going to hold up too well.

Once completed, I fired her up and checked the bias. All was normal, and it appeared to have greatly reduced the amount of hum…especially when cranked. Success!

Next step will be replacing the electrolytic caps in the grid bias circuit.

New filter caps installed

New filter caps installed

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Replacing/discharging capacitors?

While searching for stuff about my Marshall 2204, I happened upon a fantastic source of information; The Hoffman Amplifier forum, hosted by EL34 World. I saw that there was an ‘introduce yourself section’ so I posted this: Hello from Chicagoland

A forum member was kind enough to point me in the right direction (the Hoffman Amp store) after suggesting I replace the ‘electrolytic capacitors’ rather than replace the output tubes.

So from Hoffman, I’ve ordered 3 50-50 500v ‘can’ capacitors. The logic being, the Daly capacitors on the amp are original and therefore very old. They might be the source of all the noise/hum when the amp is even slightly cranked (in the sweet spot, so to speak). I’ve also ordered parts to make a capacitor discharge probe. It seems there’s no consensus on how to do this. Techniques vary from taking a screw driver and shorting the caps to the chassis. This can’t be particularly safe, although it’d be dramatic. Other methods involve essentially the same thing, but using a resistor in series to slow the rate of discharge. But nobody can agree on the value of such a resistor. I have found these variants:

  • 100k Ohm, 5W
  • 470 Ohm – 2k Ohm, 2 Watt

I have ordered a 5k, 5W wirewound resistor. And I already have some clips and heat shrink with which to make the tool.

I might eventually make one of these though: Capacitor Discharge indicator. It implies that “finishing it off with the ol’ screwdriver” is still a good idea once voltage drops below lethal levels.

UPDATE!

My good friend Tom Reid has some insights on this:

My standard discharge tool is a very large Craftsman screwdriver. It worked
well for picture tubes over the years. The added resistor to a discharge probe creates a current through to discharge procedure. For best results, discharge through a standard incandescent light bulb, as the filament is both resistive, and inductive so as to absorb the shock.

UPDATE AGAIN!!

Finally I found a page where someone quantifies the value of the resistor one might use:

It’s smart, then, to discharge the unit deliberately, through a resistor equal to about one ohm per volt of charge.

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