Hey guys,
Long time no see.
This week I picked up several radios for my future projects, and the first one to be subjected to the merciless restoration is the Philco 80.
It has a cabinet in a decent enough shape, though eventually will need to be re-finished, but structurally it is fine.
Missing the 42 tube (I figured, since both the 36s and the 80 are in place).
The chassis is very dusty, but no rust detected.
So, any good advice as to how to start?
Mostly, procedural - electrically I will figure things out, though if anyone lists the typical "gotchas" - great.
First things first - I intend to remove the chassis, and the speaker seems to not be connectorized.
The bolts holding the speaker board are decorative, and I don't want to damage their finish.
So I probably should only remove the speaker.
Bottom line - any good advice as to how to start, and not to damage anything - the radio was obviously built on the cheap and was not intended to ever be repaired
, or to easily lend itself to a repair.
Regards,
Mike.
Hi Mike,
Common problems bad paper caps, bad resistors 10k and 1Meg, open ant coil pri, open oscillator coil feedback winding, poor grounds though rivets hum, and filter caps.
Fun set work well when all the bugs are out. Coils are easy to rewind.
Terry
7estatdef Wrote:Hi Mike,
Common problems bad paper caps, bad resistors 10k and 1Meg, open ant coil pri, open oscillator coil feedback winding, poor grounds though rivets hum, and filter caps.
Fun set work well when all the bugs are out. Coils are easy to rewind.
Terry
Hey Terry,
so as for taking it apart - any particular advice? I just need to pull the chassis out and I don't want to damage anything while doing it.
Nah just remove the knobs, remove the nuts and washers from the speaker and screws from the bottom.
Terry
Hey guys,
Proceeding with reviving the Jr.
Washed the tuning cap in dishwashing liquid - sparks like it has just come off the assembly line.
Now, the two electrolythic caps need to be replaced.
Actually, one cap measures about 8uF (yeah....I know....have to still replace it, too old) and another, exact same cap, measures at 16nF (dried up).
Here's the question #1:
In the sch I can see that the first cap, right off the rectifier, is 8uF, and the second one (after the speaker coil acting as a choke filter...wow! They really saved money on this one!) is 4uF.
Now the caps that are actually in the chassis are 16uF.
Q: should I leave them at the same value, 16uF, or follow the sch?
Question #2:
Directly from the Cathode of the First detector (detector-oscillator) there is a wire that ends nowhere and is wrapped around the central contact of the volume regulating potmeter. I first thought it was a broken wire, but them from the sch I saw all connections are intact, and this wire is just there.
I think it is a small improvised capacitor, of a few picofarads.
Does anyone have the same thing in their Philco 80s? What's the purpose of this "capacitor"? (I think they found out they needed a small cap, and it was cheaper to do it this way than to put in a mica one; this was supposed to be the "el cheapo" of them all).
#1 Not too critical I'd keep the input cap the same value the other 16mf should be fine.
#2 Is a gimmick to make the circuit work better. Just easier to twist the wires than solder in a mica. Also it may vary from set to set.
Terry
Terry
Same value as in the sch (it is 8uF), or as in the real chassis (both caps are the same and are 16uF)?
As I remember, the rectified voltage depended on the primary cap value in the 20. Although there it was a smaller cap, and they might've relied upon lower average value.
here, if they actually maxed out the possible voltage, then increasing it any further won't affect the value anymore.
Don't think it's going to matter much.
Terry
Well, today I tested the power XFMR, first with light bulb in series, then straight from the 110V outlet. It seems to be in a good health: the full-wave rectifier winding gives me 2x347.5V x 2 = 695V, and the filament voltage on the 80 rectifier gives me 5.6V, and the filament voltage for the rest of the tubes is 6.9V, which is all good.
The radio is missing the 80, but then I have one in my Philco 20, so I will probably use it for testing.
First I want to rebuild the electrolytic caps, then the bakelite ones. Especially important is one cap, dual 0.15uF, as one half of it is used as the Line filter, and those caps are not Y-rated, their age notwithstanding.
So, since I have no way right now to judge how much the rectified voltage will drop under load, I have to assume the worst case and buy 520V capacitors (this is, after all, the max. voltage of the orig. caps, their working voltage being 450V).
Actually, what I have is several good caps, from the samples I used for developing that switching supply, 56uF 400V. Two of them in series will make an excellent 28uF 800V cap, and it should do the trick.
Wish me luck.
So, no question, Full speed ahead, adamn the torpedos! Eh?
Me I would get some 8 or 10 uF 500V caps.
28uF seems like a lot, I don't know if the inrush current when it fires up might be too much for the 80??
Just my 2 cents and it may only be worth 1 cent.
Always good to have a nos 36 around as the front end can be "fussy"
LASJayhawk Wrote:Me I would get some 8 or 10 uF 500V caps.
28uF seems like a lot, I don't know if the inrush current when it fires up might be too much for the 80??
Just my 2 cents and it may only be worth 1 cent.
Always good to have a nos 36 around as the front end can be "fussy"
I thought of that, but then the 80 sch calls for 8uF and 4uF caps, and in the real chassis I have there are two 16uF caps, factory-installed, so I thought they did not see much difference at the factory (I assume they tested that arrangement before releasing it to the unsuspecting public
.
Also, the tube inrush is different from the solid-state diode inrush. With solid-state it is the voltage that exists at the moment you plug it in the outlet, that may be at its peak for all we know, that gets rectified immediately and applied to a cap, getting effectively shorted and limited by the ESR/ESL of the cap itself for the peak instantaneous value, since the dV/dT is huge.
With the tube, it takes it some time to get fully conductive. It rubs its palms, blows into them few times, jumps a bit, warms up and then slowly starts charging the cap.
So maybe this will save my behind.
Then, nothing prevents me from serializing three of the caps and making an 18uF cap out of them. They will fit just fine. And I have lots of samples
I doono, somewhere in the back of my beer soaked brain, it seems to me that to big could be bad, but for the life of me I don't remember why. Issue with the frequency of the pi filter made by the caps and field coil....na that wasn't it.
I almost think by making the supply 'harder' it upped the peak current in the field coil or something dumb like that. 30 years ago I could give you a better answer.
I looked in the back of my 80 (listening to it atm) I have a mallory 8uF and a philco 4uF
And looking at later philcos (from 1937-39) when they used 16uF off the rect. they used 2 8uF caps ..note most of these had 5U4 or 5X4 tubes. I suspect the 16uF caps are a 60 year old repair more likely than an 80 year old oem part.
FWIW: type 80 rect tube at 350 vrms on the plates at 10mA you will have 440 VDC at the filter cap, by 135mA it will have dropped to 310 VDC Don't know it that has anything to do with it or not but I thought I'd throw it out there.
Transient peak plate current!
As the source voltage goes up or as you increase the capacitance in a capacitive input filter you have to increase the plate source impedance or you risk exceeding the transient peak plate current and the tube striking an arc and making a bad mess of things. (end of tube, cap and xformer)
I now recall a Majestic cathedral and an experiment with a 47uF 450 V cap that ended quite badly.
Now I'm gonna have a beer and kill off that brain cell for good.
If you use too large of a capacitor as the first electrolytic then you risk stressing the rectifier and transformer (higher B+). As
LASJayhawk mentioned, I don't think those 16uF caps are original to your set. The other issue you have to consider is the line voltage is higher today, also contributing to higher B+ (stress on the rectifier and transformer).
Capacitors are fairly cheap and I'd just buy a handful of 10uF's to have in stock.
http://www.oldradioparts.net/caps_lytic.html