Rainy Day Monday - Old Radio Restoration - Philco 52-940

Memorial Day, 2020 -'Twas dark, rainy and dreary as compared with the last 10 days of nice weather.   A good day to retreat to the Covid-19 Proof Shelter in Place Bunker / Workshop for a "Now for something entirely different" project.   This old entertainment radio was picked off the shelf for restoration:

Philco 52-940 acquired at the Seaside Hamfest more than 10 years ago.

Blog Ver 2 Edit Note:  The 52-940 radio is a newer version of the "Hippo" radio from Philco:

Above:  Philco Hippo radio from 1948 (photo from Ebay)

The rainy day restoration radio- was missing its back which contains the loop antenna.  On initial power-up, using Isolation Transformer and Variac, revealed FULL AUDIO level AC hum noise from the speaker.  The volume control had no effect.  This was hopeful news that told me the series string tubes are not burned out, the rectifier and audio output tubes are working.

A sacrificial scrap wood 'cradle' was fabricated in the Acme Woodworks shop to protect the vulnerable components on top of the chassis.  Did the uncontrollably extremely loud hum cause the speaker to shatter apart?  or was the radio attacked by Speaker Bugs?  Under the chassis- the first organ removed for transplant was the DC power supply Filter Capacitor- shown under speaker above.

Under the chassis the usual paper+wax capacitors were replaced with Yellow Polystyrene caps.  Red arrow points to 3 electrolytic caps, taped together, sitting in a bed of silastic silicone compound which secures and isolates them from the chassis.  The original filter cap wires were reused to connect to the new caps.

Looking closely at the IF cans- their base is made from clear plastic- which means that they contain mica wafers  separating silver plates that create the capacitors in the IF can's 455 KC tuned circuit.  Almost always- these caps develop Silver Mica disease which causes scratchy sounding pops in the radios audio.  Since this radio plays without noise- the IF cans did not undergo the SM Cure procedure.   

At first I thought this radio was in the "Hot Chassis" category- where one side of the AC Line Cord attaches to the metal chassis- which was popular back then.  Analyzing and experimenting reveals that the NEG power supply has a separate GND path- not connected to the chassis.  Some items like the speaker common wire and tuning capacitor are GND'd to the chassis.  It was confusing where to connect the polarized AC line cord NEUT wire- which, BTW connects to the AC ON/OFF switch (backwards in my thinking).   The radio had a wax paper .047 mfd cap across the AC line to bypass noise from getting into the RF sections of this radio.  Two .022 mfd X safety caps were transplanted that now perform this function.  Note:  experimenting has demonstrated the necessity of AC line bypass caps.  Man made noise is drastically reduced in most cases.

In the foreground is the new radio back panel and loop antenna.  A piece of 2.3 mm plywood was carved, little by little on the table saw to fit into the rear of the radios case.  1.125" holes were hole-sawed.  The loop antenna organs- the dowel-bobbins and the 40 ft of cloth covered wire- were harvested from an old battery powered set from the RCT Free table. 

A same sized speaker was not found in the workshops acres of junk spare parts. The best sounding speaker that fit is from a Motorola Golden Voice radio- which sounds pretty well.  Experiments were conducted with 'tone control' capacitors- from the Audio Output Tube Plate to the Negative supply- to make sure the selected speaker didn't sound too treble-y or too bass-y.

The original crappy speaker was screwed to the radio chassis- which made it more difficult to replace.  The Motorola Golden Voice speaker is mounted to a plywood adapter plate- in the same place as a cardboard gasket resided to isolate the original speaker from the bakelite radio case.  The speaker was moved 3 times on the plywood- to get it to fit just right with the chassis placed inside of the cabinet.  The front of the plywood adapter plate is painted black to hide this modification from outside view.

The rear cover in place.  More vent holes were later drilled after it was noticed that the power supply side of the radio seemed extra hot. 

Here the radio is on it's shelf in the Bunker Workshop- not being a Shelf Queen- but playing KIXI, KOMO, KVI on a regular basis.  

The bakelite case received a bath in Dawn Dishwashing suds, then polished with automotive plastic headlight cleaner.  The brass trim got an overnight soak in vinegar then precision polished with Brasso.

After careful alignment the radio plays well, especially the Hispanic Music stations at the top of the AM band.

End of Blog Report.  KR7W