Posts: 597
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Joined: Apr 2018
City: S. Dartmouth
State, Province, Country: MA
Lets assume the radio has the original transformer, for a 115vac primary. To prove the transformer: Since the wiring has be refurbished, there could be a compromise in the filament connections. A shorted or partially shorted filament circuits, all could be an avoidable disaster for this test. So, If you feel confident that all filament circuits are without fault, proceed, if not, lift the all filament leads, except the power rectifier, tape the ends off so there will be no possibility of a short. Ya, I know brittle rubber covered wire or crunchy cloth covered wire. A little wire trick: Heat the wire with a heat gun, the hardened rubber will soften ad allow movement so long as it is hot.
Now, remove the power rectifier or any other device installed that is supposed to be the power rectifier. Energize the radio, since filaments are not connected, pilot lights will remain out. Watch and listen, faint crackling sounds inside the transformer are a short, heated turns like that of the Weller soldering gun are burning, in 5 minutes this should increase. This is clearly a bad transformer, but if there is no evidence of something wrong, stay with the radio, still alert, for half an hour. The transformer should remain cool, often cold. But slightly warm is O.K. If all is well so is the transformer... Shut off power and remove plug from line.
Pull all the tubes and pilot lights, take a discontinuity check of the filament circuits, all.. Insert the pilot lights and check again, looking for a failure in the pilot light sockets. If all is well re-install tubes and re-connect filaments circuits to the transformer...
Carefully review the filter capacitor installation especially the bias circuits and the bias resistors, review any coupling capacitors to the last audio tube, Check any chokes, output transformers, field coils and tone caps for leakage to frame or chassis. Resistors like Candohms can leak to the metal casing...
Instrument Notes: Many digital volt-ohmmeters will give strange readings when attempting to measure an inductance for resistance.
Use of a 10kohm/volt analog meter is highly recommend for voltage readings, ideally a VOM with a load resistance as specified on the OEM schematic should be used. If not spec, 5K to 20k/volt is O.K. with knowledge the readings will be somewhat lower with a 5K meter.
..Plating on the chassis reacts with rivets and even screwed electrical connections, even a sensitive meter will not disclose a poor riveted ground joint to chassis. For such poor ground joints, loosen hardware and wiggle the parts, then re-tighten. If in doubt add serrated washer to improve bite to chassis. Same for what appear to be solid soldered ground joints, simply re-solder. Tube sockets that have ground rings, either solder the side of the ring to the chassis or drill out the rivet and replace with screw, nut and serrated washer.
Chassis solder joints will require a soldering iron that has a large mass of copper for a tip. That mass of copper holds calories that get transferred to the joint faster then the chassis joint can disperse the heat. Using a 250 watt soldering gun will still suck out the heat, solder will not flow. The chassis joint may resist rosin flux, clean the joint with 91% alcohol to remove any flux. Get some plumbers liquid tinning flux, this is an acid! It is a pale yellow color, using a cotton swab moisten with the flux flow the smallest amount into the chassis joint, apply the soldering iron, flow the solder as soon as the boiling stops. The solder can be 60/40 rosin core. Use a second cotton swab moistened with water to remove any residual plumbers flux. As long as the plumbers flux is used very sparing and only to places where it can be remove by the soldering heat and a water swab there will be no future issue. This is not a grease base flux!
Faulty grounds to the chassis are often a hidden problem, at MF and HF frequencies even though a meter may say the joint is good at high frequencies there are problems. Noises, poor alignment, weak or no signals, improper operating voltages.
One example I had is a Raytheon Belmont, refurbish, but no video. poking, probing around the video amp live would show a signal fading in/out. Pressing hard on a tube socket ring showed the problem to be the riveted joint. All tube socket rings were re-soldered using the plumbers flux. The video signal returned.
I had always kept acid flux away from electronics, however, there were no resolutions to the corrosion in older chassis short of a tear-down, simply not a time conservative project. The flux works if used with caution in mind.
Looking at the grounding in the antenna, oscillator and mixer may be worthwhile.
This same grounding issue can also effect the IF circuits via the AVC. So long as the DC resistance of the IF coils is within range IF's should be OK, exceptions are the IF types with open, fixed, silver mica caps.
Do refer to the schematic as to the circuit of which the return from the meter is connected for voltage measurements to comply with OEM recorded data. Some returns are to chassis, other is to B-.
YMMV
Chas
Pliny the younger
“nihil novum nihil varium nihil quod non semel spectasse sufficiat”
(This post was last modified: 03-12-2019, 11:33 AM by
Chas.
Edit Reason: More thoughts
)
