Post by trombone on Dec 11, 2021 14:19:56 GMT 12
Goodie Twoshoes Fixes the Wireless
Or “ Ultimate Virtue Rewarded “
Goodie has invented a new game called, “I’m more virtuous than you”. Here’s Goodie….
Hello everyone .There’s a new game. Its called “I’m more virtuous than you”. We both parade our servicing virtues before a panel of radio servicemen and they decide who is the more virtuous. Want to play? I’ll start.
Round one : “At auction I found a radio with a missing valve so I put in a very low absentee bid ,thinking no other punters would bid because they saw there was a missing valve - an 80 . I was correct . I won the auction .”
Panel’s reply : While your efforts to save an old radio from becoming scrap iron and firewood are commendable your actions are not strictly a servicing procedure . So – no score.
Round two: ” Oh , um okay but whenever I come across an intact Candohm I always measure and write the ohms across each section on the chassis in felt tip pen in case a section goes open later and the next bloke’s servicing blind .”
Panel’s reply: Who’s a goodie two shoes then? Alright – two points.
Round three: “Whenever I replace a component I always slip a piece of spaghetti over the exposed leads before soldering it into place to help avoid any shorting”.
Panel’s reply: O.K. Two points
Round four : ”After initial static tests and observation I always power up the radio in series with a light bulb to avoid overwhelming any leaky transformers and electrolytics.”
Panel’s reply: Yes. Very commendable . Three points.
Round five: “ I always turn off the workshop radio when I’m powering up a new radio so as to listen for hisses squeaks pops and boiling noises -also explosions -from electrolytics and transformers.”
Panel’s reply: Commendable but not on its own. You mentioned “Observation”. Isn’t it true that the reason the 80 was “missing” was because a previous servicer had soldered in two tiny semiconductor diodes across the 80 socket ? So that a ferocious D.C. voltage was immediately unleashed on the elderly and pretty much un- prepared electrolytics every time the set was turned on ,with virtually no rise time whatsoever?
“Well …er …yes but…”
Panel continues: And the elderly and unfit electrolytic heated markedly and began to make expanding noises , especially when you had rushed to run the set on full mains voltage? Shouldn’t the unusually bright flash from the series light bulb on switch- on when the radio WAS in series with the bulb have alerted you to there being something amiss? And the continuing bright glow? Okay – carry on .
“Er , well , I think I’ll stop now ,um playing this game. Perhaps someone has a pack of cards ?Um.. Happy … Families anyone …?”
Yes, this is the story of an Ultimate EB6 of 1940. Some kind person had serviced it previously ,replacing most of the wax paper capacitors and a few resistors and also the electrolytics but quite some time ago. Originally the second detector was a 6B8 diode pentode but the screen grid had been disconnected and a 6Q7 fitted in its place. A new volume pot had replaced the tapped original. And there was a dim but still discernibly greenish tinged magic eye.
As received it had a thicket of wires poking out the back to facilitate the previous owner’s use for it as an early entertainment centre. One added switch on the back took the original voice coil out of circuit to allow connection to a bigger(better ? ) external speaker. There was an odd two pin mains plug also switched on the back presumably for a turntable and a bit of modern co-ax tipped with a small jackplug probably for the turntable input. But most of this wiring – speaker voice coil and mains for the turntable was single strand double cotton covered wire (!) Because I had the circuit courtesy NZVRS Library I snipped all this growth off promptly and that’s where the virtue , if there had been any at the start evaporated completely.
One voice coil wire from transformer to voice coil now terminated in a flying lead, as did its mate from the other side of the voice coil. So the voice coil circuit was essentially open circuit .I’d checked field and output transformer for continuity so I was perplexed once I’d ditched the diodes for an 80 and replaced the pressure cooked electrolytic. All the voltages around the 6V6 were now correct but not a sound from the speaker. An “Aha!” moment and some embarrassed twisting together of flying leads later and reassuring hum and crackles from the speaker signified continuity restored.
So after all this mucking about what was actually wrong with this set?
This is a two band set -shortwave and broadcast– and the aerial shortwave and broadcast coils and the oscillator shortwave and broadcast coils are continuous. They are not on the same former of course, but are electrically continuous. The wave change switch , on shortwave, simply shorts the broadcast aerial coils to earth via a capacitor and the broadcast oscillator coils simply to earth, if I’m reading the circuit correctly.
On broadcast and shortwave the oscillator grid voltage on the 6K8 was virtually zero. Then I noticed that the shortwave oscillator coil ,wound in very fine green wire had no terminals as such on the former. The winding terminated simply in being looped several times through a hole in the end of the former. From there the connection to the broadcast oscillator coil was by a continuation of this fine green shortwave oscillator coil wire. But this fine green wire was broken!
Cigarette lighter ignited, a brief red glow of fine wire but greenness and varnish removed ,cleaned wire tinned and a new lead joined up and soon ten volts negative on the triode grid and I’m listening initially to something on shortwave and then to broadcast.
Perhaps the mouse , anxious to gnaw on one of the few remaining wax caps , pushed past the oscillator connecting wire and broke it. His teeth marks are there on the capacitor. Would that all our radio woes were as readily resolved . And I guess the mouse probably knew more of hunger than he did of virtue ,so he was never really in the game.
Post Script : There subsequently appeared another problem which I have called “The Case of the Bi-metallic 6K7 “.Once the set was going satisfactorily and I was tidying up a few loose ends the set suddenly stopped playing. The dial lights still glowed, voltages mostly were in order.
The signal tracer was deployed. Signals stopped at the grid cap of the 6K7 i.f.valve. And that valve, a metal tube ,was cold. It appeared that the filament in this valve opened and closed as it heated up and cooled down to produce a very confusing intermittent fault. Tested cold the filament is continuous. No such problem with its replacement.
