Reforming capacitors in consumer electronics?

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Rich Feldman
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Reforming capacitors in consumer electronics?

Post by Rich Feldman »

We all know that with ancient vacuum tube gear, to power up without attention to capacitor reforming (or replacing) is to invite damage far beyond the old electrolytic capacitors.

So how about solid-state consumer electronics, like a surround-sound (tm) receiver that has just reached drinking age, and whose power amplifier rails run at +/- 50 or 60 volts DC? Any hints about practical methodologies?
DSCN1550.JPG
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I'm inclined toward a conservative approach, after reading about some industrial (up to 200 HP) Variable Frequency Drives.

Suppose a VFD has failed in service, shutting down your process. You have a New-In-Box spare unit waiting on the shelf for that contingency. At least two vendors say that if it's been on the shelf for more than 2 years, in fact if its date of _manufacture_ is more than two years ago, you seriously need to follow the capacitor re-forming protocol. One poster on a machining forum spoke of reforming caps in over 200 drives, in the course of service calls and spares maintenance contracts. Jerry Biehler might have some experience with that stuff. I expect these units have solidly engineered capacitor banks, without much fat.
reforming_time.JPG
from
https://library.e.abb.com/public/cf2c9b ... _Rev_G.pdf

Here's the only previous fusor.net thread I've found about reforming:
viewtopic.php?f=45&t=1564&p=10177&hilit ... ing#p10176
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Jerry Biehler
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Re: Reforming capacitors in consumer electronics?

Post by Jerry Biehler »

About the only thing I have worried about reforming is my big yag laser welder. It sat for about 15 years before I powered it up. To reform the caps on that I used a 300v power supply and charged the bank up over a period. Seemed to work fine, nothing blew up when I connected it to mains.

Im not sure I would worry much about reforming caps on new stuff like a VFD. They should be able to set on a shelf for years and not have an issue. There was only one time where I can think of that I actually had something odd happen. I picked up a Miller XMT-304 off craigslist. It had been sitting in a shed for several years. Took to to my shop at work and powered it up. Tripped the breaker when the contactors switched to operating position. Worked fine after that and ever since, and sometimes it will be over a year between welding stuff, same for my tig welder. Back when I worked on CNC machines I never once had an issue with electrolytics.

But the general method is to use a variac to slowly bring the voltage up on the cap over a few hours allowing the layer to reform in the cap. Assuming the device will tolerate being on in an under voltage state.
Cai Arcos
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Re: Reforming capacitors in consumer electronics?

Post by Cai Arcos »

Rich:

I'm not expert in pretty much anything, but why take the risk? I have been watching this channel for a while now (https://www.youtube.com/user/MrCarlsonsLab), which seems quite an expert on the matter, and he always seems to straight out discard the possibility of reforming caps, preferring replacing them to avoid "reforming also the transformer". Why not buy another capacitors, which are cheap nowadays, and protect the transformer?

Please, tell me if I'm missing something. This is an interesting topic in which I would like to know more.
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Richard Hull
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Re: Reforming capacitors in consumer electronics?

Post by Richard Hull »

When restoring the huge counter that I posted in images du jour, I did, indeed, replace all the major filter capacitors with modern ones about 1/5 the size. I did leave the old tall can capacitors in the counter. (unconnected, of course) This preserved the original appearance as all the replacements went under the chassis, out of sight.

Reforming can be done for sure, provided you are will to break a line and monitor the current from the rectifiers to the filters as you bring the old device up to line voltage on a variac. It is normal for those old electrolytics to warm a bit as they attempt to reform.

It is just far easier to just replace old caps.

Richard Hull
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Rich Feldman
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Re: Reforming capacitors in consumer electronics?

Post by Rich Feldman »

Thanks for your insights.

Here's the background. Audio amp pictured in OP is one of four or five that I want to test. How much power can one deliver at well above 20 kHz? Continuously, given that their power input transformers are obviously undersized for the nominal audio output powers.

I think I will plug all of them in at the same time, on a variac, and probe to see if the reduced voltage is making it to all the DC bus capacitors. The unit in OP has relays in that path.

Here are three snips from one thread in another forum. The last poster, bringing contradictory advice, seems to have the most experience.
https://www.practicalmachinist.com/vb/t ... -d-331798/
reforming_3.JPG
reforming_4.JPG
reforming_5.JPG
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Ordinary Internet searching finds, right up front, very conservative rules from VFD makers. Perhaps directed by their lawyers. Maybe it reduces their liability when "new" equipment a few years old fails soon after power-up, in a few percent of the cases.
reforming_2.JPG
I bet those words were written by Marketing staff, not Engineering, but that doesn't make them wrong. :-)
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Chris Giles
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Re: Reforming capacitors in consumer electronics?

Post by Chris Giles »

Hi Rich,

there are two types of caps which might need replacing:
1. the 'X' and 'Y' caps which are on the AC line input as a filter.
2. the electrolytic smoothing caps on the DC lines to the amp

If #1 fails they can explode, see here for instance. However, that mainly seems to affect the common types which are cased in yellow epoxy and labelled "RIFA". If you see any of those - replace them.

For #2 I would just check visually to see if they have bulged or leaked. If not just try it out. If they have lost capacitance you will hear it as 50Hz hum on the audio output under load.

I see that someone has been fiddling with the power transistors as the heatsink is scratched up. Check the insulators between the transistors and the heatsink are not damaged - if it looks like it has been disassembled by an amateur then you may need to replace the insulators and silicone grease.
How much power can one deliver at well above 20 kHz?
If you plan to hack them to use as an HF power source (?) then i suggest:
- the speaker protection circuitry may need to be modified or removed
- there will probably be filter components as described here which you would want to remove
- the electrolytic caps in #2 would need to be replaced with modern low-ESR types designed for a switch mode PSU, also you would need to add modern 0.1uF film capacitors in parallel. This is to give low impedance at HF, which is not required in an audio amp since most of the power is in the bass.
- other capacitors around the power transistors may need to be upgraded - see if you can find a schematic.
- would be better if those transistors are MOSFETs but they may be bipolar/darlington ones in an audio amp.
- measuring the power would depend very much on the type of load you use, particularly how its impedance behaves at HF and how similar the test load (?) is to the real load.
- be prepared to melt / explode some things :)

cheers
Chris
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