Forming filter caps

[syscokid]

That is odd.

Did you remove all of the tubes? (sorry to ask but it has been done)

What you can do is desolder the caps and jumper them + to - with the last one - to ground on the chassis, that way you know they are isolated form the rest of the circuit.
Usually if the caps are no good they bottom out at 20v and then start to climb again.

Success... I think...

But first:
I must have misunderstood your suggestion in how to apply it. After setting up the chain of caps, and turning on the amp, the voltage drop and the B+ was identical at 256. I turned it off after a minute.
I thought about the wiring again, and I rewired all positive lugs in series after the dropping resistor. Then grounded individually the three caps to the chassis. 250+ at the start. One minute later, around 20. And after only 20 minutes: 4vdc. The Main F&T cap is about 7 years old. The other 2 new caps are ARS. All 50/50uF (500v).

I've formed new caps before at least 3 to 4 times, and never had a problem. I still don't know why, this time around, I ran into a conflict. Maybe a screwy connection somewhere?

[Derrick]

I will be going through some very nice vintage Marshall, Hiwatt, Orange, and VOX amplifiers that have not been turned on for quite some time. With the value of these amplifiers in original state, and not wanting to desolder several wires that risk easily melting their insulation, I was thinking about this old post and had an idea...

Couldn't I get a power transformer capable of 500v, make it variable voltage output with a simple put a rectifier, with leads having alligator clips that I could just clip to each capacitor terminal to eliminate the need to desolder anything (unless there is a bleed resistor)? Not many of the caps in these amps have a bleed resistor in them, so there would only be a few of those to contend with.

This way, I could set the voltage out to just above the amp's B+ rating but make sure to keep the voltage under the cap's max rating, and just clip the leads across each cap (or cap section for multi caps) that have no bleed resistor (or lift a resistor leg if it does). Am I missing something, or should applying an adjustable voltage power supply set to the correct voltage for the amp/cap, and with a 100 ohm resistor in series and a current meter across it, do this job without having to desolder (unless there is a bleed resistor)?

[danman]

That would work but once you clip the power supply across one capacitor, all of the capacitors in the amp will be seeing the same voltage. This is normally not a problem with new caps as they will all normally form at about the same rate. With an older amp that has sat for years, if you have one bad capacitor in the amp, the forming voltage will never fall to the desired range and you will have no way of knowing which capacitor in the amp is causing the problem. With older caps of unknown quality, it's best to do them one at a time by themselves so that you can see if they are forming properly.

Having an adjustable power supply is helpful but the main reason this method works so well is that the 100k resistor limits the current to a very small amount. Without the resistor, forming at a lower voltage would still allow excess current to flow if a capacitor was bad. By limiting the current you eliminate the chances of a bad cap heating up and exploding.

If your amps have a standby switch, you can easily solder the 100k across the switch terminals and then power up the amp. The standby switch must remain in the open position so that all of the current is forced to go through the resistor. Closing the switch would allow current to bypass the resistor and the forming process will not occur. As mentioned earlier, be sure to remove any bleeder resistor if they are soldered across the caps.

[Derrick]

Thanks for the reply. I have two questions:

• From your description to add the 100k resistor "across the switch terminals" (I assume you refer to the Standby switch?), this would place the resistor before the rectifier. Will that cause any issues with reforming? Can the 100k be before the rectifier to properly do this process?

• After looking at the schematics of the 50w and 100w, and looking at the process outlined in this long thread... I see that the center tap of the power transformer's secondary goes to ground on the 50w and to a filter on the 100w. It also looks like 50w examples didn't use bleeder resistors. I assume this is all why the 50w doesn't require disturbing any solder connections to perform this type of cap reforming? What if I left the center tap of the 100w on the filter during reforming? Would it have the same effect as leaving in the bleed resistors across the two filters?

[danman]

If your standby switch is wired into the circuit before the rectifier, installing the 100k resistor across the standby switch will not work. The resistor needs to be in the B+ line, after the rectifier and just before the first filter cap. In the 50 watt circuit, the standby switch makes for an easy place to add the resistor in because it meets these requirements. Since your standby switch is before the rectifier, you will need to break into the B+ line just before the first capacitor for this method to work. You could unsolder the B+ wire right at the first filter cap, solder one end of your resistor to the filter cap and then connect the B+ wire to the other end of the resistor.

As for the centertap in the 100 watter, it must be grounded for the power supply to work correctly. Leave the centertap in place but remove any bleeder resistors. The bleeders will allow current to make it's way to "ground" and the voltage across your 100k will never fall into the single digits like it should when good caps are fully reformed. I would try reforming all of the caps at once using the 100k resistor. If the voltage falls into the single digits then you know that every cap in the amp has reformed properly. If the voltage never falls, you will then need to separate the capacitors and do one at a time so that you can figure out which one is bad.

[Derrick]

If your standby switch is wired into the circuit before the rectifier, installing the 100k resistor across the standby switch will not work. The resistor needs to be in the B+ line, after the rectifier and just before the first filter cap. In the 50 watt circuit, the standby switch makes for an easy place to add the resistor in because it meets these requirements. Since your standby switch is before the rectifier, you will need to break into the B+ line just before the first capacitor for this method to work. You could unsolder the B+ wire right at the first filter cap, solder one end of your resistor to the filter cap and then connect the B+ wire to the other end of the resistor.

As for the centertap in the 100 watter, it must be grounded for the power supply to work correctly. Leave the centertap in place but remove any bleeder resistors. The bleeders will allow current to make it's way to "ground" and the voltage across your 100k will never fall into the single digits like it should when good caps are fully reformed. I would try reforming all of the caps at once using the 100k resistor. If the voltage falls into the single digits then you know that every cap in the amp has reformed properly. If the voltage never falls, you will then need to separate the capacitors and do one at a time so that you can figure out which one is bad.

I'm confused then... what it the yellow/red wire labeled here (see link below) marked as "1" in the yellow arrow with the instruction to remove on this 100w Marshall 1959 build? Isn't that the center tap of the power transformer?

http://i661.photobucket.com/albums/uu33 ... g-caps.jpg

Also, the majority of schematics I see for both the 1987 and the 1959 have the standby switch before the rectifier after about '67. How are people putting the 100k resistor on the standby then?

Thanks again!!

[danman]

You are correct about the red/yellow centertap. Larry does state that it needs to be disconnected in the 100 watt models. All of my amps are 50 watters so I'm use to leaving the centertap connected. The 100 watters with SS rectification use a different rectifier setup than the 50's so you will need to disconnect the CT.

All of my 50 watt builds had the SB switch in the B+ line after the rectifier diodes. Some amps do place the standby switch in the AC line before the rectifier so it's not possible to solder the resistor across the switch with this setup. The resistor must be in the B+ line (DC voltage) for it to work properly so choose the easiest and most convenient spot to solder it in.

Keep in mind that the circuits changed throughout the years so it's really difficult to give a "one size fits all" answer. You will need to evaluate each amp and determine the best spot to solder in the 100k resistor.

[Derrick]

Thank you. It appears that all of the solid state rectified Marshalls in this big old pile all have the standby switch before the rectifier. BUMMER!! I wish there was a way to form these caps using a 100k resistor without desoldering anything... That's why I was trying to think if I could build a variable power supply that I could just apply to each cap separately with a 100k resistor... I could use an alligator clip to keep from having to desolder, but I guess parts of the circuit could still be an issue that way (like the center tap on the 100w and bleed resistors. I could to it on the 50w though I guess. Hmmm...

[novosibir]

The HT secondary CT immediately has to be disconnected while forming filter caps of the 100W Marshall models (EL34 models only, not the Super100JH).
Follow the steps which I've described in the 1st post of this thread.
There's no more simple way, to accomplish the forming process.
If there'd be a more simple method, then I'd have described it.
Don't use variacs, light bulbs in line or other funny ideas, if you'd like to reach the optimal result.

The method with the 100K across the Standby switch i.e. is working on Fender amps, where DC is across the switch.

Larry

[Derrick]

The HT secondary CT immediately has to be disconnected while forming filter caps of the 100W Marshall models (EL34 models only, not the Super100JH).
Follow the steps which I've described in the 1st post of this thread.
There's no more simple way, to accomplish the forming process.
If there'd be a more simple method, then I'd have described it.
Don't use variacs, light bulbs in line or other funny ideas, if you'd like to reach the optimal result.

The method with the 100K across the Standby switch i.e. is working on Fender amps, where DC is across the switch.

Larry

Thank you, Larry. Rather than look for an more simple method, I am actually exploring if there is a more difficult method using the 100k resistor that would reduce or eliminate the need for desoldering connections. It is only because many of these amps still have all the original dye/solder connections and are quite valuable examples. I have access to all the parts to required to build a custom power supply and was brainstorming if it might be possible to build a power supply with 100k resistor that I could just inject separately to each cap section with alligator clips... even if it takes doing it one at a time. The time it takes and resources to build such a device are a non-issue.