Tektronix 2213 - terrible humming

[DmitryL]

I've recently acquired Tektronix 2213 on a car boot sale in a pretty good condition (apart from blown 250v-rated X-capacitors in PSU). Seems to work well, but it produces terrible 50Hz hum, very loud. After taking it apart and finding a service manual it appeared that I am unlucky owner of an old version of the device, equipped with so called "Current Limit" preregulator.  The hum comes from a big inductor that looks like a mains transformer; it seems to produce quite powerful magnetic field that I can feel with a steel scredriver if I put it close to the choke (after removing a metallic screen).
It seems that this magnetic field causes screen vibration and very loud sound.

Questios:

1. Is is normal ?
After about an hour of working everything seems to be fine, nothing burns, PSU is just a little bit hot. Just in case I've checked electrolytic capacitors in PSU, they are well in specs, including ESR. Dumb checking of semiconductors doesn't look attractive to me...
If I power the scope from 110V AC from autotransformer, the noise is much less audible.

My guess is that version of "preregulator" I have (based on IRF730), chops part of the sine wave to provide stable voltage output amplitude independent from input (100-240V AC).

2. What shall I do then ? 

2.1 Currently I incline to removing screen from the inductor (it vibrates) and mounting choke to the rear panel with some soft padding. This will increase 50Hz magnetic field near it. On the other hand, the CRT is well screened already and is quite far away.

2.2 I couldn't find the schematics of this scope's PSU with the preregulators.. What is the role of this inductor (it seems to be in series with mains) ? Just filtering the noise from preregulator and SMPS ? or something more complicated, like AC current source (it seems to have quite large inductance, about 13mH... ok, less than 1 Ohm @ 50Hz ).  Don't want to experiment with shorting it

2.3 powering the scope via 230->110v autotransformer, it's another brick under the table...

It would be interesting to see the shape of the current the scope consumes from mains, but it would involve play with isolating transformers..

Any ideas ?
Thanks in advance !

[amspire]

I've recently acquired Tektronix 2213 on a car boot sale in a pretty good condition (apart from blown 250v-rated X-capacitors in PSU). Seems to work well, but it produces terrible 50Hz hum, very loud. After taking it apart and finding a service manual it appeared that I am unlucky owner of an old version of the device, equipped with so called "Current Limit" preregulator.  The hum comes from a big inductor that looks like a mains transformer; it seems to produce quite powerful magnetic field that I can feel with a steel scredriver if I put it close to the choke (after removing a metallic screen).
It seems that this magnetic field causes screen vibration and very loud sound.

If a transformer or inductor is overloaded to the point the core goes into saturation, the core will almost certainly hum. It would also get hot if you leave it running.

So the hum could be an indication of an overload. If you compare voltages on the supply to the voltages marked on the schematic, look for any voltages significantly lower then specified. Be careful of course near the high voltage secondary.

Richard.

[grumpydoc]

It's certainly worth checking whether any of the rails are overloaded - shorted tantalums are notorious in pretty much all Tek 'scopes.

However loud hum from the early power supplies on the 2213/2215/(and ?)2225 is "normal" in the sense that they almost all do it after a while - in fact it was one of the reasons that Tek redesigned the power supply.

Also, I think 2213's are amongst the 'scopes that suffered from a design flaw that ran the heater at 8v instead of 6.3V - this lead to a lot of early CRT failures. There's a modification which involves removing one turn from the transformer.

If you do go looking at the heater voltage remember that although the heater itself is only 6.3V it sits at -2kV relative to the chassis so take extreme care.

Join the Tekscopes group on Yahoo and look through the message archive and files section - I'm pretty sure there's info on the PSU in these 'scopes there.

[G7PSK]

Does the unit use a bridge rectifier I have come across similar problems where one diode in a bridge has gone short.