Day 1: Remove chassis from cabinet
Sounds easy, but rarely is when a unit is 90 years or more of age. Manual tells you to remove front controls first. The band selector knob (dead center) had a corroded brass screw in center holding it on. I carefully removed corrosion to reveal screw slot almost gone but using a nice tight fitting driver (only one chance at this - brass screws shear easily) I gently undid the bolt (6ba thread)
The 2 Bakelite knobs either side came undone straight away, I was expecting to drill or force them away, but was fine. Chassis bolts came next underneath, 4 at each side close together and 2 at rear. I noticed while underneath the left side of cabinet had split, looked fresh - possibly while in post, it was well packed but no real buffer zone inside (polystyrene corner pieces are best.) I glued that later.
Day 2: Vacuum and clean cabinet & chassis
Another job that sounds simple, but rarely is. It's also a good time to inspect cabinet & chassis for signs of previous repairs or damage. I noted 8uf HT capacitor on isolated earth rail had been replaced. Next remove valves and clean them checking the numbers, heater continuity and cleaning pins.
While out I can remove dust with vacuum and small brush, taking care not to break any wires. (I sometime photograph wires in place, just as a back up if the fall loose) The IF or TRF can cleaned up wonderfully like new. All wires in this model are solid and housed in fiber sleazing apart from speaker wires and Selenium connecting cables.
If no access to switch I carefully drill a tiny hole in the housing to spay into. Check no short on secondary winding, this read around 75 ohms. So I switched the set on and with test meter on HT rail. This showed only 20v DC. So I looked for shorts that could be dragging the rail down, non found. Checked Selenium/Westinghouse diodes, 2 off them in series wiring, both read very high in both directions. So I decided to fit 2 modern types. The power supply is a little strange in this set using a voltage step-up arrangement. Done via 2x4uf paper capacitors, stepping up 75v AC to 240v AC.
Day 4/5: Check sound output components and circuit.
Really to fault find on any unfamiliar circuits, you need to understand how it all works, that can be explained in accompanying manuals most time if a new circuit arrangement has been employed. The variations on this model are in the step-up HT arrangement and the isolated negative earth, basically supply's negative bias for audio valves. Check speakers for continuity, both have o/c HT magnetic coils, one also m used to smooth HT supply. In some early speakers they used an electro magnet rather than a large magnet.
fitted 2 Sony speakers I had been given my a neighbor when he was having a clear out. I also fitted a tweeter to brighten up the high end frequency sound. I was now missing a LF choke, so I found one of those in my 'donor box' another padded box I have full of old transformers from scrapped radios. The 2nd speaker choke was left un wired as not needed now. I had to re-mound audio output transformer on side of chassis as this had been attached to old speaker. I also drilled another hole in front to hold LF choke in place. I stripped old speaker down (curious to why o/c) and found the the copper wire inside had shorted to outer housing and burnt out wiring and probably the cause of mains rectifiers going o/c too! (it's nice when you can find the fault trail) Now I checked all the voltages on audio output valve and all is well but still now sound.
 Period advert for 1933 |
 LF Coupling circuit stripped down and re-soldered wires, 0.5uF on rear was also o/c. rear hides HT resistors using a wire wound center tapped coil |
Day 6 More work on audio stages
Still working my way from right to left looking at wiring diagram. Found some lose wires, re soldered them to audio pre amp (triode) Replaced 2nd smoother capacitor 8uf 450v (it read 150 uF ?) HT now at 250v on input. Still no audio? Checked continuity of LF unit. Read o/c, it's an alternative to the more used inter stage transformer as it only uses one center tapped coil (autotransformer) and a feed capacitor. Both were o/c and I had to break open the unit to repair lose wires and o/c 0.5uF capacitor. The unit also houses 2 feed resistors for the HT (labeled HT1, HT2, Power, Grid and Earth) similar but different to earlier types of transformers. I made a new base for the unit, glued and re-fitted. We now have radio signals. By the way I also had replaced 4 paper capacitors in screen decoupling and feedback decoupling wiring as insurance against more fault arriving.
Day 7 final checks and cabinet repairs
While testing smoke started pouring from underneath and a giant 1uF paper capacitor decided to go short circuit and cook it's feed resistor, replaced that and turned back on. Did alignment using a signal generator with procedure outlined in the manual (simple just 4 trimmers for max output) Dial light a little dim, so re-spayed inside area white again to reflect light better to the translucent dial. I thought when I purchased to radio that the wood finish on the cabinet was painted, but on close inspection the front is composed of 3 triangle shaped sections of very fine grain mahogany veneer. I touched up some areas with brown water colour paint (mainly outer edges) and lacquered them when dry. Finishing off cabinet with a bees wax shine, re-covering speaker grilles and cabinet rear vents with material I found using a gold silk pillow case (original cloth is £35 a meter, ...heck! )
No comments:
Post a Comment