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1951 Pontiac Chieftain - Back up and running

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Well, I was doing a little reading and research between circuit phase inversion and transformer phase inversion.


Seems there's two schools of thought going on here. Some people prefer a transformer design, claiming superiority because it has a very low input impedance and means you can run in class AB2 ("grid current") mode and make the very most of the capabilities of the power output tubes.

Using a circuit to do the same is a cheaper alternative because it uses a few electronic components. It however recreates a more honest representation of the original signal with less distortion, class AB1.




Blah blah blah electronics. The '56 radio is louder and uses a more reliable method of phase inversion.


Phase whatnow?


I am going to make the assumption most of you know how a loudspeaker recreates sounds from an electrical current. Connect a battery across a speaker and it pushes out, turn it around and it pulls down the opposite way, right? Okay good.

So the principle of the audio output stage is the same. A single tube (or indeed transistor) can only push the speaker in one direction, much like connecting a battery one way can make the speaker push out and then return under the spring of its suspension to it's resting position.

While this works (single-ended operation) it is far from hi-fi audio. A lot of cheap transistor radios use this design (see: cheap) because it's simple and doesn't require many electronic components.

So, what's done is to take the audio signal, split it into two paths, and by some mechanism turn the signal on one path "upside down"- that is, alter it's phase by 180 degrees.


That is then fed into two tubes (or transistors) and that means then that the circuit can both push AND pull on the loudspeaker, meaning that instead of the speaker cone having to bounce back under it's suspension when it's in a negative position, it's actually being put there by an electrical current (push-pull configuration).


Okay, right. Uhm.


Imagine a swing in a park. You stand on one side and give it a push. It swings away from you, then back again. At the point where it comes back to you is the only time you can push it and control how high it swings.

Now add someone on the other side- you push to them, they push to you. Between you, there is much greater control over the swing's position. That's open-ended versus push-pull operation.

The person on the other side needs to be doing exactly the opposite to you (push when you would be pulling) and the way to do that is what's being decided on here.


Got it? Good.



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So the chuckle brothers were in fact, phase invertion pioneers!!!! Whodathunkit!


I am simply gobsmacked every time I see this car……. I can't help going through the photo's over and over again. I am a properly jealous guy. You are a lucky fella.

I also learn - which is a bonus for a thicko like myself. Appreciated



EDIT - Meant to say - apart from THIS




This gives me the cold sweats……. fuck that for a game of soldiers.

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Ahem, point of order from the audio-nerd at the back.


Pushing and pulling is quite ok with just one tube or transistor. Just don't let it ever turn right off, so keep it in class A and the speaker will be pushed and pulled just fine.

All those cranky audiophiles spending car$$$$ on amplifiers like these post-17481-0-08320700-1555346673_thumb.jpeg aren't wrong, because such amps are lovely to listen to.

Despite measuring badly and being very inefficient. The straight-eight flathead of amplifiers so to speak.

It's not what it does but how it does it that matters.

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Yeeeessss, but I wanted a bit of clarity for the description.


"Oh but this guy is always pushing just a bit and halfway up is actually zero and that's why your HiFi goes POP when you turn it on. Etc


Figured I'd already lost half of the crowd :P

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You can just about see that through the window in the plate when driving EL34's hard.


It's all good but if the glass isn't blocking the UV it's a quick way to get arceye/a suntan



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You can just about see that through the window in the plate when driving EL34's hard.


It's all good but if the glass isn't blocking the UV it's a quick way to get arceye/a suntan





tubes are 99% of the time made with fairly regular glass


which does not transmit down to the UV-C region so you wont get arceye, but if they are made of hard glass they would transmit a nice bit of UV-B for a good sun tan if they are powerful enough :)


(However in that regard you would need something like a 2000W Mercury lamp to get a sun tan in a meaningful amount of time, as one of my friends found out the hard way!)




(I wonder if any particularly powerful/compact vacuum tubes/valves are made of Quartz aka fused silica?)

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Tonight's achievement was to unpick the lid off the power transformer and discover the thing is potted in pitch. Lovely. At least I should be able to replace the capacitor.




like 90% of all US Fluorescent ballasts then making them a bitch to repair LOL (they also have fixed wires, rather then terminals, so if a wire breaks off your quite screwed)


luckily with a heat lamp/gun you can melt the tar out to get to their built in capacitor....


(and in theory get to the actual ballast part to resolder on new wire on)


they also leak tar spectacularly when burning out

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I had a moment this morning so sat and looked at the schematics of the two chassis again.


I have made a decision, definitely rebuild the '51, mainly due to the coupling between the IF, detector and AF preamp stages.


In the '56 the detector stages are doubled up in one tube, the IF is directly coupled through a self contained circuit to the AF preamp. As such there's a +7.0V bias on the volume control, feeding into the grid of the AF preamp.


The '51 is decoupled at the output of the IF/detector stage, and that is fed into a voltage divider circuit that's referenced to chassis for the volume and tone controls. As such the grid of the AF preamp is at 0 Volts with ground reference. That makes it perfect to switch input at that point, as a Bluetooth module will be chassis reference also. It also makes it good for Aux-in, too.


Good deal!



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FFS, this is serious restoration! Most people would have fitted a modern unit behind the old radio front and said fuck it.

You sure you're not after winning something at Pebble Beach ?

That's what the previous owners did.


It was ugly.





I want to have the original radio, natively running 12V (converted properly, none of this step down voltage converter nonsense) with the capacity to accept audio in that's not just crackly AM radio.



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Zinc pest in action. It broke like trying to lever soft slate. Crumble crack splinter.



Thankfully there is Replacement Tuner, which I need to carefully liberate from that chassis. That one is in much better shape.


I knew I bought a Dremel for a reason.


I may also change the clutch friction material when I'm in there also. It's slipping a little.



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Tonight's car time was taken up by lawn maintenance, throwing leaves all over the Renault with the mower and generally getting covered in grass. Wanted to get it done because the weather tomorrow is meant to be really bad.



I did make time to bring the spares radio tuner chassis out to the garage and attack the peened lugs with my Dremel.



Carefully ground back, then gently tapped with a drift on the more solid parts of the back to loosen it.



Gently tapped the thing onto the pegs.



Hammer time! I removed all the glass bits from the frame and peened the lugs to secure the frame.



Finagled the slugs back into the coil cores without breaking anything.



Finally I was able to attach the board, straighten it out and secure it correctly. The mechanism is now much less tight (funny, that) and the clutch doesn't slip any more.






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I cleaned up the tip of my slightly bigger iron- my fine work one wasn't cutting it for some of the bigger solder tabs.



This meant I was able to un-solder and remove the power transformer. I've decided I'm just going to shotgun the thing with new metal film resistors and new wire also, on top of the capacitors.


In short, it's gonna be a whole new radio set.




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The piece I removed works like the leverage of gears; the old one is wound to provide 250 volts output from an input of 6.

If I hooked it up to 12V then it would output 500V.

The new one generates 250V from 12V (would be 125V from 6V input).


Putting 500V into that radio would be a bad thing, so that's what's happening here.



The waxy yellow pieces have a design life of about 10 years and are well in excess of being due replacement- they are just two metal sheets sandwiching a piece of paper, rolled up. As they operate, one of the metal sheets is slowly dissolved by the electricity and ends up attached to the paper, which eventually becomes metalized paper, which is a short circuit, which is bad.


The vacuum tubes have a heater inside that works the same as a light bulb. 12V into a 6V one leads to a very short lifespan.


The little brown resistors are basically a lump of crushed coal inside, which has a particular resistance. Those have a tendency to "drift" and change resistance as they get older, due to the chemical breakdown of the stuff they're rolled in to stop them from falling apart. The resistances need to be quite accurate else the radio stops playing properly (or at all).


I'm replacing the bits that it needs to work on 12V, and the rest that are just really old and no more good.


In short, radioshite.



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Well.... If 'old = ruined part' then a refresh to AsNew spec is unavoidable Shirley!


The old 'factory air' in the tyres argument lol



Look Ma... A 50's radio, playing songs like in Christine, wow!!

*yes... surprise surprise... no chinkk solidstate radio guts inside.


Jeeze... Its a RADIO, like they used to be :)

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That's the thing, I'll be able to play that music from my phone.


Looking at Bluetooth receivers, there are a few with "call mute" (remember that wire on the back of your car radio that never got connected?) which I plan on using to operate a relay, which will switch from AM reception to Bluetooth when something is connected to the Bluetooth and attempting to play audio.



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I'm rebuilding to what would be original spec if the thing was 12V, just with more stable components- this would bump up the price of the radio significantly back in the day so that's why they didn't do it...




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Buggeration. I cracked the socket for the 12V vibrator while trying to drill out the rivets.


I think rivets are designed just to spin until you die of old age when you try drill them out.



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I mixed up some epoxy and stank up the house doing so. Wife unimpressed.



Stuck the two sandwich parts of the socket together and held it with screws and the clips of my test leads.



Added a flying lead to where the capacitor inside should go, going to mount that externally. Hammered the lid back on.



Took the smoothing capacitor apart. Dug the contents out (stinky).




Drilled holes for some new leads to attach new capacitors inside with.



Better, can solder onto the lugs with that.



The insides were full of the remnants of the capacitor and some tar that was used to hold it all in place.



Combination of heat gun and thinners later...



Finally, I might have become slightly carried away with the metal polish.



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