telefunken: 9U (Export f. USA); Gavotte

hello all. i just acquired a nice telefunken gavotte 9u which powered up safely, but would not hold any fm (ukw) station without unpleasant static sibilance. this went away when i replaced the EABC80 detector valve, and i got the customary "big" gavotte sound. however, lately i'm getting a very loud hum (no station signal) when i power it up. once, while listening to a station, it reverted to this loud hum and i had to power down. what is the likely cause of such a noise? thank you.

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Dear Charles,

try to replace the selenium bridge rectifier with a silicon bridge.

Best wishes, Emilio

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...Along with Emilio, I will entertain the possibility of something having gone amiss with the rectifier causing the loud hum. But the greater possibility by far is that one or both of the HT (or, to us Americans, B+) filtering capacitors have failed. 

...I have a schematic for an Opus 7U (probably very similar, but a slightly older model).  It shows two of these filter capacitors: two 50µF electrolytics; the first comes right off the output of the rectifier, then there is a choke, then the second one.  Most likely these are in a two-section 'can' which will probably be labelled something like '50+50µF 385V'.  These are always the 'prime suspect' in cases of loud hum in radios, especially as radios get older (electrolytics, especially those designed for high voltages, do not age well.)  In radios of this age (in your case, about 50 years), most restorers will replace these straightway as a matter of course.  Old electrolytics can also quickly progress from merely causing a loud hum to catastrophic failure (they have been known to explode), which can make quite a mess of the insides of the radio and turn a relatively easy repair into a nightmare.

...The first thing you should do is to replace those electrolytics before you try to power up the radio again.  If you don't want to do away with the old 'can' (many people don't, to preserve an 'all-original' appearance), you have two options:

1.  Obtain two 47µF 450V electrolytics (which you need to do in any case).  Disconnect the wires from the 'can', one at a time, from wherever they are connected underneath the chassis.  After disconnecting each wire, connect the positive lead of one of your new 47µF caps to the terminal you removed the wire from, and connect the negative lead to the chassis.  (Alternatively, you can cut the wires close to where they come out of the 'can' and wire them to separate lugs on a terminal strip, then wire your new caps to that strip (one positive lead to each separate wire, the two negative leads to the chassis.)  This way your original 'can' is in its original place, but is completely removed from the circuit and the new caps are discreetly 'hidden' under the chassis.

2.  Try to 'restuff' the original 'can' (which involves somehow opening up the 'can', completely cleaning out the original contents, actually mounting your new caps inside the 'can', connecting the positive leads to the wires coming out of the can and the negative leads to the inside of the 'can', and finally resealing the 'can').  Many restorers have done this successfully using different techniques, but it is considerably more work, and somewhat delicate, and is usually not something for a beginner to try.  (It is also possible to 'restuff' the rectifier in a similar manner, if it is in the form of a black 'can' that mounts onto the chassis and is usually marked 'AEG'.)

...If you already know all of this, my apologies.  You appear to be relatively new here, and I don't know your level of expertise with radios/electronics. 

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wow! thanks, emilio and todd, for your very helpful responses. todd, i will start with replacing the electrolytics, and i'll do it under the chassis, leaving the disconnected "cans" attached on top. that'll be a fun repair, and i look forward to finding if this will do the fix. it'll be awhile (the holidays) before i can report back on this. you've sure left very little room for error in this repair. thanks again for your very helpful responses. i'm very grateful.

--charles jones

colorado

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i replaced the filter caps with new electrolytics under the chassis and powered the radio on safely, only to learn that the ECC85 preamp valve was blown. the AM (MW) and shortwave bands seemed fine, but i got no FM (UKW) reception. after replacing the tube, the radio played beautifully on all bands, and even the EM80 indicator valve burns brighter green. thanks again for your valuable assistance.

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