You don't need Youtube video to repair a Radiola 28! It will only mess you up!
I have done several 28's, 25's and the 812, 24, 26 series. All before there was WWW/Internet.
This IS NOT a shotgun replacement project!!! There are NO paper caps inside and only one resistor.
There are NO vintage schematics with internal part values! Each catacomb was custom tuned and aligned on jigs when built then potted. Meaning any values found were for that cat that was measured...
A change a cap and the IF or oscillator will be off and the cat may oscillate or fail to work.
There are no replacement tubes that will substitute and not have oscillation, volume, distortion problems.
The RCA Service notes have the schematic but the schematic in Greenbacks is more user friendly...
Follow the RCA guide on how to make the resistance/continuity measurements Do that first. . Use a VOM not a digital as readings will be deceptive with a digital.
Do all the checks and keep a list of errors.
Then melt out the wax, it is a blend of bees wax and spruce rosin as a potting.
Can is suspended by the cat body with end hardware and tube top plate removed. Oven set for 265F, after 90 minutes can will begin to slide off to a foil tray. Increase temp to 285, What happens is the volatiles will evaporate causing the melt point to increase, may raise again but do not go over 295, the compound will then drip off of the components, Pour off potting compound into a mold for later use if desired. Do NOT return the compound as the heat has set a new pour point that is too high for the coils AND the result will also shrink much more and that is what breaks the internal wires.
There are two audio transformer, they are 3:1 do not use the cheapie transformers, these have been reported to cause audio oscillation. Verified by an ARF member...
There are several failure modes in the cat... Broken RF/IF coil wires, open transformers and "open" grid leak. Leak is 8.2 or so megs, use a 2 watt carbon so leads will not have to be spliced.
If a coil wire is broke at the coil start, game over, get another cat... Pressing apart the common lamination stack for the IF's will destroy the IF performance despite replacing the coil.
Pay careful attention to the broken wires and the coils they are associated with, there may be cloth tape tags with numbers but these were not recorded by RCA to any data sheets and may not be consistent with anyone's recording of these numbers.
It is
very rare, some 8-tube cats actually have IF adjustment mica compression caps. There is no recorded data why or how, can be seen as tiny holes in between the tube sockets.
All caps are precision clinched open mica, built to be temperature compensated of brass and steel.
THEY ARE FRAGILE and must not be pushed or prodded or de-soldered as the leaves c
an be broken and the capacitance decreased. They never fail unless messed with...
There is also a neutralizing cap, it is a wire within a sleeve and
mimics the capacity of a '99, do not mess with it...
The resulting rosin left on components is fine, do not attempt to remove it only what is needed to repair broken wires.
Much of the wiring terminates to 4-40 long brass screws, these are often loose in the condensite tube socket array, re-tight, some have solder in the nut, re heat carefully and tighten. Leads that solder to the screw heads often break away, re-solder as needed.
Do Not cut away the whiskers from the terminal strip, extract solder from terminal and unwrap the stranded wire.
Whisker numbers and terminal numbers jump around OEM shows numbers in round circles and and squares each respective of terminals and whiskers. Be aware not to mix oscillator coil wires. There is only one combination of the four wires that works, that 15 times its wrong...
Rheostats often need a cleaning as well as jacks, the jack is removed with a hex key into the from of the jack, there is teeth that bite the wood in the jack frame.
Tighten all tube socket contacts re-solder wires and replace transformers if need be. Put the cover on retest as to the check list. dozen. Stray rosin can be cleaned with 91% alcohol.
Follow the rest of the neutralizing as in the manual. The radio MUST have the correct loop antenna or the loop coupler. Do not directly connect an outside antenna or ground unless a loop coupler is used or several extra turns around the existing loop. Will need a loop link or coupler if radio is in a steel building or a cellar/basement.
When this radio is correctly working, it receives down to the local noise level. Even in DAYTIME I was copying NYC some 300 miles from my home just the loop...
Beware, be sure you have a battery model and not an AC model. If it is an AC model it can be converted back to battery and operated from an ARBE, if not the RCA 104 speaker/SPU will operate.
The AC model will have a capacitor bank in the battery box these will ALWAYS leak and can be replaced with poly or electrolytic. On AC all filaments are in series and there is a resistor strip which re-arranges filament connections and creates bias. The meter jacks are disconnected and rheostats different. On the DC model it is a jumper strip puts all fils in parallel. The two rheostats have different resistances, no external capacitor blocks.
I have several spare catacombs and three skeltonized radios that are used to re-align a cat if I encounter one that has failed from a "restorer", fortunately, I have not needed to use them as I have only serviced cats that still have OEM potting.
May, have to mechanically re-align the tuning condensers, they can warp from the condensite side plates shrinking. Alignment consists of "egging" out the condenser mounting holes in the steel face plate, behind the wooden slant front.
If you need more info post here to get my attention.
if you want OEM cat audio transformers I may have a spare cat for salvage...
You can get new reproduction antenna wire (a close copy) from an ebay vendor if all the rayon has fallen off...
https://www.ebay.com/itm/333181108573
GL
Chas
