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Antique Radio Technical Forum
paper vs electrolytic filters

Posted by Edd on 04/15/2009 22:25

Sir Scotty:

Might I pose question or two and give some info on that unit which might prove to be an impressive receiver for you. . . . .after all . . . .GREAT SCOTT. . . . .its a SCOTT.

I think that I remember that unit sort of being up on a slight pedestal base and being finished in a black crackle, not the typical Naval battleship grey, and being for AC power shore use.

You also might confirm if the 4 ufd caps are having a rolled seam at the tops. . . .like a tin can. . . and some used two bolts for mounting while others used a round clamp for mounting. The rolled seam units were oil filled paper capacitors for sure, while the other configurations were conventional paper caps. WHICHEVER type used, I usually expect a lifetime of “ ‘til the twelfth of never” from them. . . unless proven otherwise.

I enclosed a thumbnail of the shema of portions of interest:

Firstly, there is the power supply, which you can see did not skimp on its capabilities, initially there is the output of raw DC taken from the 5Y3 and then the first filter section and then a series choke filter in the order of a hefty 12 henry choke and then the intermediate 4 ufd filter capacitor.
At that point, the B+ is pure enough for being the main B+ supply for primarily the audio output stage.
That initial B+ level is then being passing on to the even heftier 32 henry final choke and being followed
by the final 4 ufd filter, that results in the production of the sub B+.
Now here is the kicker to check out, note that the center tap winding of the HV winding is NOT going to
ground, instead, it is routed to a 50 ohm resistor before it can complete its ground loop thru that resistor. That will result in a negative voltage being developed across that resistor, which you can see at the top that is being routed to other circuitry in the set for biasing uses.

Also take note of capacitor C119 and note that it is the filtering capacitor for that bias supply, and also take note of its polarity of installation.

Looking at the parts list for that receiver I am seeing that C119 capacitor, along with C117 and C118 as being 25 ufd at 25VDC electrolytic capacitors, the other , C101-102-103 being paper type capacitors.
The C117 and C118 are cathode bypass capacitors and with their failure mode, the effect is mainly the loss
of audio gain and a peripheral altering of overall audio tonal response.

If you have a failure of the C119 . . . .WHOAAAA. . . .somewhat of a different effect, note that it is shunting the
50 ohm resistor , so that means that the DC component of the power supply is going to pass thru it and be complete, less that small voltage drop for the bias supply.

Now what you also need to see that if that C119 deteriorates in capacitance or opens up, that opens up the, otherwise, very low impedance path of the AC component of the first and second 4 ufd caps on getting to ground, just visualize them trying to copmlete a path tru an electrolytic that has deteriorated to .1 ufd. . . . .Hummmmm...city .

Look over to the third and final 4 ufd and you can see, it just doesn’t care, since it IS connected to ground, BUT the two prior stages were NOT providing their normally, fully filtered B+ to it.

Soooooo. . . . check out the three electrolytic and see what their conditions are. I think that they were bathtub units and mounted near the chassis lips edges.

In accordance to supplier, some are stamped with their polarity, some have it almost engraved into the metal, while others have a splash of red and black paint on the solder blobs of the connector terminals.

If this does happen to be your filter situation, I would sort of hate to see you casting your three 4 ufd diamonds aside and replacing them with Cubic Zirconias.

Thumbnail Schemas:

73's de Edd

paper vs electrolytic filters 
SCOTTY 04/14/2009 19:56 
av1 04/14/2009 20:22 
SCOTTY 04/14/2009 22:53 
Peter G. Balazsy 04/15/2009 01:47 
SCOTTY 04/15/2009 14:35 
Edd 04/15/2009 22:25 
SCOTTY 04/16/2009 13:45 

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