dslreports logo
site
 
    All Forums Hot Topics Gallery
spc

spacer




how-to block ads


Search Topic:
uniqs
567
share rss forum feed


shortckt
Watchen Das Blinken Lights
Premium
join:2000-12-05
Tenant Hell

Restoring old EICO pwr supply

I've had this thing for years and use it primarily as a battery charger. One of the selenium rectifiers has finally shorted, but in troubleshooting that, I found that the transformer is shorted to the case. It's 60 VAC case to ground, and not microamps either, it will light up a 40W bulb.

This transformer has the primaries on the inside of each coil and secondaries on the outside. The side of the transformer that faces the front of the case has exposed windings much like a variac, and a knob on the front panel operates a wiper that makes the connection across from one secondary to the other.

It's about 60 years old and the paper and varnish holding the windings together is getting brittle with heat and age. Rewinding is likely to be prohibitively expensive, so it's out of the question, but I'd still like to restore the power supply. I have a hunch though that since the leakage is 60 VAC and the transformer is wound as two identical halves on a square form that the short is near the inside of one of the primary halves, electrically near the center of the primary and putting it up against the core.

My question is does anyone have experience repairing power transformers and can give me an idea what would be involved in opening the core, taking the windings off the core as a unit to check the center of the windings, put it back together and protect the windings from further damage with varnish or whatever is used now?

I read somewhere that today transformers are not just sprayed with varnish and dried in an oven, but put in a vacuum to get the air out so the varnish will seep into all the air spaces. One of the things that comes to mind is if I open the core but do not separate the laminations from each other will I need to re-varnish the laminations at the joints where the two parts of the core come together?

In this case it's not a question of ability but is this transformer something I can tackle on a workbench in an apartment.


EICO 1050 pwr sup


EICO 1050 xfmr


doitnot

@68.63.161.x

from experience, my advice is, don't do it, but do it.

chances of success under 50%.
value of experience 200%

a lot depends on if the cores laminations are separable, or welded.
if welded, you have to grind out the weld to get them apart, made with uniform E section to an I section.
if just varnished, the will come apart easily. usually alternating E sections.

either way, they never go back packed as tightly, so the core never fits back over them either.

as you mention, with crackly paper and insulation, disassembly and work can cause more problems than you started with.

but go ahead and do it all anyway, you will learn another skill you will never need again...

----

plan b is to just gut it, and replace the transformer with one from a mid-size stereo amp/receiver, and a modern bridge rectifier or diode pair, with giant heat sink.

pick the parts base on the wattage of your existing supply:
if your supply does say 10 volts at 10 amps, that's 10 x 10, or 100 Watts. so pick a stereo that's rated about that wattage on the label where the power cord enters.

that's just rough rule of thumb, but will get you close enough for playing testing.

oh, since you were wondering, the power rating of any transformer is conventionally set at the current it supplies when its unloaded voltage drops by 10%.



cowboyro
Premium
join:2000-10-11
Shelton, CT
reply to shortckt

Why repair it? Really, why? A new electronic charger will charge the battery properly and switch to float charge when done. This kind of chargers only destroy batteries if left for too long and there is no control for the charging current.
Opening the core is no easy task especially if it was vacuum varnished. You won't be able to take out the pieces without seriously bending/ripping them.
Now if you insist on doing it, your best bet is cutting out the windings so that you can open the core with a knife, piece by piece. Make a new support for the windings out of a 0.03" FR4 sheet (some fine cutting/finishing is involved), you'll have to make it *exactly* the size of the core. Calculate the fill factor before you start, the transformer may already be at the limit and you won't be able to do the windings if the fill factor is over 0.9.
I have the formula for calculating transformers somewhere, if you decide to go on this path let me know and I'll dig it... However the proper way is the way around, decide what you want and then choose the core that suits.



tschmidt
Premium,MVM
join:2000-11-12
Milford, NH
kudos:9
Reviews:
·G4 Communications
·Fairpoint Commun..
·Hollis Hosting
reply to shortckt

Never rewound a transformer so can't help there but before you start are you sure it is the transformer with the short to the case and not one of the other components?

Agree with cowboyro See Profile that modern smart chargers are better in that they are less likely to overcharge and cook the battery.

As mentioned an option is to simply find a replacement transformer so the exterior remains unchanged.

Just for fun took a quick look on ebay, there are lots of vintage Eico chargers available, even the 1050.
»www.ebay.com/sch/i.html?_from=R4···_sacat=0

/tom


b10010011
Whats a Posting tag?

join:2004-09-07
Bellingham, WA
Reviews:
·Comcast Formerl..
reply to shortckt

How about isolating the transformer from the chassis?

Mount it on a couple glastic insulators.

»www.glastic.com/en/products/glas···ors.html

BTW: When you exchange a selenium rectifier for a silicon diode you will find that the DC output voltage to your equipment has increased by 5 to 15 volts. In order to maintain the originally specified output voltage, you will want to add a series resistor between the cathode and the first filter capacitor. Sometimes the increase in output voltage is not a particular problem, but a resistor should be added in all cases to reduce the surge current.

Selenium rectifiers have a naturally higher internal resistance, so they self-limit the surge current. Silicon diodes have a lower forward resistance, so you need a resistor in series just to reduce the surge. The old rule-of-thumb is to size the resistor to drop about 10 volts, simulating the drop of the old selenium rectifier.

Word of warning selenium rectifiers release some very dangerous gasses when they burn out so you might was well replace the other one too.
--
Netflix is not "pushing data" onto Comcast's network, Comcast customers are "pulling data" from Netflix.

Bellingham Scanner Kicks Ass! »bhamscanner.kicks-ass.org/


TheMG
Premium
join:2007-09-04
Canada
kudos:3
Reviews:
·NorthWest Tel
reply to shortckt

Are you totally certain it's the transformer leaking to chassis and not some other part?

I'd take the transformer out completely and take resistance measurements from its core to primary, core to secondary, and primary to secondary. All should read infinity ohms.

If it's a short from the primary winding to the core, slipping some insulating material in there should be able to isolate it and give it a bit of extra life (but probably not for long). If it's an inter-winding short, that will necessitate a full re-wind, which would be much more trouble than it's worth and quite a lot of work.



shdesigns
Powered By Infinite Improbabilty Drive
Premium
join:2000-12-01
Stone Mountain, GA
reply to shortckt

As there are two primaries, I bet it is in the wire that goes from one side to the other. You might be able to get to it without taking the cores out.



aurgathor

join:2002-12-01
Lynnwood, WA
kudos:1
Reviews:
·Frontier Communi..
reply to shortckt

I mostly agree with:

quote:
chances of success under 50%.
value of experience 200%
except if you try to repair the transformer, the chance of success is probably under 25%. However, if you just replace the transformer, that should go up to at least 75%.

But other than gaining experience, I see very little reason to fix an old power supply / battery charger when you have issues with the transformer.
--
.sig


tschmidt
Premium,MVM
join:2000-11-12
Milford, NH
kudos:9
Reviews:
·G4 Communications
·Fairpoint Commun..
·Hollis Hosting
reply to shdesigns

said by shdesigns:

As there are two primaries,


Looks like it only has a single primary with center tapped secondary and some sort of wiper arrangement to vary voltage.

Found a schematic on the interwebs. How did we survive before the Internet.

Switching between 6V or 12V is accomplished by selecting full wave bridge and full wave center-tapped rectification. The only current control is the resettable circuit breaker listed as K1

»www.nostalgickitscentral.com/eic···1050.pdf

/tom


shdesigns
Powered By Infinite Improbabilty Drive
Premium
join:2000-12-01
Stone Mountain, GA

Does look like two primaries, if you follow the power leads, one goes to each half winding.

The crossover point is probably on the far side of side pic.