Full Bridge Rectifier Output to Transformer

[metrologist]

I'm trying to repair my Tek 2430A o-scope. The main input is rectified and each +/- outputs go through windings of a transformer with a 10k resistor in parallel with each winding. Would they be using the transformer as an inductor, or what else?

[Galaxyrise]

The schematics are available in the service manual on Tek's web site:
http://www.tek.com/oscilloscope/2430-manual/2430

Page 409 for that part (diagram 23), around 3C on the schematic. There's also a nice picture of what waveform to expect on the positive side, which shows a much higher frequency component than just mains.

The part list calls it a "TRANSFORMER,RF:HIGH FREQUENCY COMM MODE". I'm also interested to hear more about what's going on with it; it's something to do with the switching supply that follows it, I guess?

[RobertHolcombe]

The transformer is acting as a common mode choke to suppress noise - inductors can be used for the same purpose but will produce more distortion on the output waveform. I'm not sure how critical that is in power supply design but I guess it just depends on the application, hopefully someone else can elaborate for me.

[metrologist]

Thanks! Legwork and insight  The two 2200 pf metalized paper caps there were replaced and I get about a minute of on time before the display goes off and the scope locks up. Back to the bad cap witch hunt.

[Kleinstein]

Those 2200 pF caps likely need to be Y-rated caps. This are high quality caps with a special safety rating, so they won't fail with a short.

The common mode choke acts as an high inductance for the common mode signal. The magnetic flields of the two coils compensate so the core will not saturate when used in the normal way. The normal load current will also see not much of the inductance. Usually it's used to prevent RF interference, not so much for normal function.