Use of non-screened transformer in old monitor

Did you check to see if the regulator does have sufficient headroom? Maybe the replacement transformer has a lower output voltage or lower current capacity.

My first suspect after the power transformer would be a filter capacitor that had become dry or leaky.

Considering that signal levels in a monitor are much higher than in sensitive analog systems like audio amplifiers, I doubt that the symptoms are caused by stray fields from an unshielded transformer.

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fectly well the picture now sort of moves around but at a rate of 1-2Hz. T urning up the brightness shows a wavy left/right edge implying a few hundre d Hz effect. But the effect is also vertical.

nsufficient headroom after the rectifier before the 15v regulator. I am hap py this isn't happening.

th an extra grounding wire.

moved away and screened the transformer temporarily to no effect. Various t hings to try next include ferrites on primary/secondary wiring, a UPS and m aybe some extra caps and also jumpering across from a working monitor.

with a railway depot over the road.

our thoughts.

ened one.

Ferret beads aren't going to do anything at the frequency of interest. Have you checked all the psu rails for hum? Tried some grounded steel betwe en trafo & tube? Put grounded steel over the whole monitor to see if it's a n external field?

NT

I did try doubling up the 4700uF with another which yes I checked the capacitance of first.

Also what are R602 and D605 for ?

Jon

The 4700uF cap sits at 28V which given a measured consumption of 1.4A is fine giving a ripple of the order of a hundred millivolts. And as I said doubling it makes no difference.

There's a reason they put a shielded transformer in there initially.

-- "I am a river to my people." Jeff-1.0 WA6FWi http:foxsmercantile.com

Typically there are other smoothing/decoupling caps in there for low current rails.

NT

Most likely it is magnetic radiation from the transformer. osme transformers have steel shields, others have copper shorting straps to reduce the external magnetic field. It REALLY doesn't take a lot of external field to affect the picture. You might be able to help by changing the orientation of the transformer, or getting it farter away from the neck of the CRT. Mu-metal shields might be fashioned to help, but if you bend the mu-metal, you have to re-anneal it.

Jon

So, the 60 Hz magnetic field leakage from the new transformer is beating against the 62 Hz (or whatever) vertical sweep. Magnetic field from the transformer reaches through the glass into the electron beam path..

A conductive belt around the new transformer (can be made from copper tape, and soldered at the seam) would be a good start. Reorienting the transformer (if possible) might help. If the original transformer was shielded, now you know why...

Rewinding the original might be what's needed.

NT

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** That tranny has a very low external 60Hz field. Besides the steel box, it may have a copper flux strap around it and use more than usual primary turns.

The rest of your post is pure, wishful thinking.

Maybe the whole 15V supply can be placed at some distance from the monitor, in its own box.

..... Phil

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** They de-couple the two electros, so the voltage on C604 does not follow the ripple on C603, allowing it to hold a higher average DC value.

Using your 1.4A load figure for the PSU with a 4700uF smoothing cap - the p-p ripple voltage works out at 1.9 volts.

I = C dv/dt ( where dt = 6.5mS )

... Phil

We're 50Hz here as is the IBM MDA frame rate so beating is likely.

+1 you may have a magnetic field problem, not a circuit problem. Try moving and re -orienting the power xformer relative to the CRT. Often you can find a null point. Otherwise you need mu metal shield or copper strap bucking turn. m

This is not a complicated transformer. About 19 volt secondary ot thereabouts, not even center tapped.

Maybe you can find a toroid somewhere which should have less flux leakage.

Or maybe a wall wart.