nordmende: Othello - Hot Transformer

First off, I replaced every resistor that was out of tolerance.  But the transformer got excessively hot after a moment so I turned off the power.  I then proceeded to replace all of the remaining resistors, but the transformer continues to overheat.  Does anyone have some suggestions as to what might be the problem here?   I found none of the capacitors or electrolytics to be neither open nor shorted.

Thank you,

Bob Isaac

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If you unplug the EL84 does the transformer get hot?

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Here are many defects possible:

• rectifier cell shorted inside (black stick with"AEG" print?)

Red wire (+), blue wire (-) and 2x yellow for the AC-input

• power transformer primary/secondary windings can be shorted inside
• EMC shilding inside transformer can be shorted (not all transformers)

Desolder the secondary connections and check the open circuit primary input current and temperature development.

Rgds,

Detlef

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Hello Michael,  it still overheats with the EL84 removed.  Thank you for the suggestion.

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I guess do what Detlef Boeder suggests. Disconnect everything from the transformer. If it still gets hot then there must unfortunately be an internal short. Sometimes this can be fixed.

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Hello Detlef,

 It appears your first suggestion was the problem.  The selenium rectifier appears to be shorted (if I am testing the correct circuit - red and black wires).  I assume this should be replaced by some modern equivilent, but I do not know.  Could you tell me?

Thank you,

Bob

I stand corrected, I tested the rectifier once again (while disconnected from the circuit) and it is not shorted.  I will disconnect the secondaries and see what happens.

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Crap. After de-soldering the secondaries, the transformer continues to overheat.

Unfortunate.

This is the first radio that my wife purchased for me, so doesn't it figure that THIS set would have a shorted transformer which is most likely beyond my novice means of repair?  Fixing this set would help ensure her continued support for me addic... er..  hobby.  Ha ha

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Dear Bob,

even if a shorted transformer is not a common fault, it can be readily replaced. Probably the seller caused the burning of insulating layers inside its windings when he tried to operate the radio even with the shorted Graetz bridge. I faced the same problem with a Loewe Opta Luna Stereo 6761W bought on e-bay, where the dealer in the description said: 'It lights on'. Unfortunately he forgot to add that it smoked too.

Anyway you can easily replace the roasted transformer with a new one. Here you can find a suitable replacement, as the TRA801, toroidal, or the TRA200, conventional EI core. Of course you must replace the bridge rectifier, you can use any silicon bridge type, 1 amp or more at 400V or more. And of course you should spend a little time to fasten the new transformer to the chassis.

Best wishes, Emilio

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If a replacement isn't available, or you don't want to manually re-wind (time consuming) or pay for re-wind (moderately costly), here is a faint possible repair.

Obtain some Yacht Varnish or other Polyurethane varnish that isn't water based.

Remove as much outer covering as possible from windings carefully to avoid snapping any wires.

Let the transformer heat up on mains, but not too hot. In a plastic dish (maybe a milk carton cut in half) pour varnish into the windings periodically as the transformer cools, tapping the iron vigorously between each pour. Leave for 3hrs. Pour a last amount and agitate/tap. Leave perhaps for 6 hours.

Then try transformer.

It may not work and would make taking off the old wire to re-wind much harder.

If it now is dramatically cooler, replace any coverings and enjoy.

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Hello again,

if the windings inside transformer are defective, the best way is a spare part complete radiochassis from ebay. The list in rm.org shows 19 entries (!!!) in 2011. I think this is the best way to get another (good) original transformer.

Detlef

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Thank you gentlemen, you are all most kind.  I will report back here after I have obtained and installed the replacement transformer. 

Bob

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Hello Friends,

 I received a good used tranformer and am preparing to replace the roasted one, but I am a bit confused about the rectifier.  I have a universal selenium rectifier 6500v DC output, 1 amp, which I believe to be sufficient (if I understood Emilio's guidance correctly).

The issue I have is that I cannot determine this component's location in the schematic.  The rectifier has four connections, the blue is negative and the red positive, which is straight forward enough. Negative (blue) gounds at the base of the EL84, red (positive) to the 50uF electrolytic, and then there are tow yellow/brown wires coming from the rectifier.  both of which connect to the low voltage secondary of the transformer.  My question is what to do with the two connections (yellow wires) after I've bypassed the plus and minus of the original rectifier with my new one?

Thank you,

Bob

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Dear Bob,

first of all, I gave approximate voltage and current values referred to silicon bridge rectifier replacements, since you cannot find silicon rectifiers rated under 1A. If you are going to use a selenium rectifier, a type rated for 250V (rms) and 100mA, as a B250C100, will be fine.

Next you must connect the transformer high-voltage secondary, about 220 V rms, to the AC lugs or wires of the selenium rectifier. The + lug goes to the plus of the aluminum fiter capacitor and the - lug must be wired to the chassis in the same place of the old rectifier.

You have to cut away the wires from the old rectifier.

Best wishes,

Emilio

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A 6500V Selenium rectifier may have too high a series resistance. I can't really tell without the data sheet.

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Hello again, I want to thank all of you for your vast knowledge and willingness to help the novis (me!)

I've been reading a bit on the use of diodes in place of the bridge rectifier, and I am at a crossroads for a decision.  I'm torn between purchasing a NOS (or potentially a brand-new) seleium bridge rectifier, or attempting the diode replacement technique.  The direct replacement method is appealing both for its ease and for the maintenance of visual originality, while the diode method seems a good, permanent solution. 

Which do you gentlemen recommend?

Many Thanks,

Bob

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Dear Isaac,

selenium rectifiers were reliable enough on radio and television sets when they were in daily use. If you foresee to leave your set inoperative for long periods, two months or more, the selenium rectifier could fail. This can be due either to an oxidation of the iron plates, or even to a deterioration of the rectifying selenium barrier during long storage periods. Selenium rectifiers usually ask for a reforming cycle at reduced voltage, more or less as required by aluminum capacitors, to prevent high reverse current build up. After few minutes of reforming, the reverse current usually drops to its nominal value.

Silicon rectifiers are then more reliable, even if a preforming after a long storage period is always required by aluminum caps.

Emilio

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I vote for 1N4007 and a series resistor on the + output. However I don't automatically replace Selenium rectifiers.

It depends on repair & use vs "museum collection". in 1970s   BY127 or similar silicon diodes would have replace metal rectifiers.

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