[morzh]

I am on the fence about restoring this 38-14 I got during the last Kutztown.

http://www.nostalgiaair.org/PagesByModel...013238.pdf

I paid maybe $15 for it. A fairly durty backelite case, but the chassis is clean enough.
This will require lots of cleaning, especially the case.
No original caps, the tubulars all were replaced by the black beauties; the electrolytics are paper-cartridge axials.

Now the question: I decided to check the speaker first. The transformer is good, the cone is good, the FC is 23 kOhm. According to the sch it is 3 kOhm.
It is not open, it is 23K. I cannot imagine the failure of a wirewound coil where it does not get open, but develops 23K resistance.
The speaker seems to be original Philco; same cone assembly as in 38-12, and the FC looks the same, except the resistance value.
The coil in 38-14 is in parallel to the first rectifier cap and is not used as the filter; they have a separate choke for it. So, naturally, it is expected to be a higher resistance than in the 38-12 (3K vs 1.5K). But 23K?

So, my question is: has someone encounter this before and can 23 kOhm FC resistance be legit? If not, at this point starting this radio makes little sense for me.

PS> I thought, why not check my old idea of mounting a strong RE magnet to the centerpole and replace the FC as the magnet (it is not being utilized as the filter). Well, tomorrow is another day.

[morzh]

Well, I climbed down off of that fence and decided to proceed with restoration.
I did experiment with the speaker.
I never did that before.
I fed 10V AC (from the Isol. xfmer and Variac) to the outnput xfmer, and heard the hum.
Then I connected a 9V battery across that coil and the hum increased a bit.
Putting a Neodimium magnet inside the VC did not change the percepted loudness.

Anywho, I also found a 38-12 speaker with half cone in my stash. I might get it re-coned.
Or I might hunt down a 38-14 speaker. I have seen one listed as 38-12 one, but somehow sith the FC resistance of 2.5K, which tells me it might be 38-14 one. It is on that Play THings of the Past site, so this is why I was asking about it.


So. The radio was repaired before, and it was in 50s, because the parts (Black beauty and some other non-Philco tubulars) are mostly not original. The elctrolytics are also tubuilars: the originals were removed and discarded.
Because of that, after making sure all RF coils (other than the IF; I did not check it, never do), I decided to proceed with the restoration, but with a lighter version of it: I will be replaacing the parts directly, without trying to restuff and such. The reason is, I would have to find all original things, and I am not sure I want to do this. So, the radio will be flaunting bright yellow caps inside, should someone ever be interested enough to look there. As well as the aluminum radial Panasonic electrolytic caps.

Most resistors seem good as they are. All tubes are good and strong.
Maybe in Kutztown I might be able to get another junker with a good speaker.
Plus there is auction in Oct in my town; who knows what that might offer....

To be continued.

[morzh]

Other ailments are also present.
The lamp socket requires rebuilding as the insulation inside is all crumbling. I have done it but forgot I need a nylon washer. Will have to find one.

I have finished the recap. THis is how it looks now.

   

Also, a funny find: the 300 Ohm resistor happened to be a 27K one, measuring at 30K. Considering there is no obvious change in wiring, I wonder how it ever worked after that repair. The resistor is visible here. It is #31 in the sch below.

   







http://www.nostalgiaair.org/PagesByModel...013238.pdf

What else...all resistors are done. Now I need the speaker. For alignment I could get by a PM one (I cannot seem to locate my perm 8" speaker after that French drain disaster of a project...).

Obviously, all has to be done via an isolation xfmr, as this one is a "warm" chassis radio, the AC IN wire (I intend to make it N) separated from the chassis by 0.15uF cap, which is a true death cap.
Now I will pause a bit.

[morzh]

Strange thing about that speaker.
After I removed the transformer to try to use it with a PM speaker to align, the resistance of the FC became about 4.5K.
Then it sat awhile on the table, and the resistance increased to 6.5K.
I made an experiment: I put a 9V battery across the field coil, and then I measured the resistance, which became 4.5K again.
So I put the speaker back together and put it in the radio. It works fine.

Now about the radio.
Once the speaker was in, I powered it on and it hummed and motorboated, quite a bit.
Then I realized the shield for the 38 tube was missing. I have a spare from the sacrificial chassis, so I put it in, and the motorboating stopped, but lots of hum and noise remained.
TRhen I remembered to turn off my bench lighting, and the hum went down quite a bit.

Eventually I was able to receive one station (local), but the reception is very noisy.
I think I need to do two things:

1. Try to align it. As I use my isolation transformer, I think the Philco transformer for their generator (which they recommend) is not needed.
2. Buy a polarized AC plug. I am not comfy with the chassis being across 0.15uF cap[ from the Hot Line.
And I only have the non-pols.

Will continue....

[morzh]

I also found in the service bulletin, that in order to reduce hum modulation the 16uF cap across the rectifier tube's output is increased to 40uF. Will do that too.

[morzh]

What has made things better is an X-cap before the switch with bleeding 2M resistor. I chose it to be before the switch to save the switch from sparking.
But there is still some whining between stations, and the receprion is not as good, though I got a few stations.
Also, to align I need to put the dial back, and it is in rough shape plus a bulging paper on top the dial plate. I cut a slit in it, put some glue inside and left it between two pieces of wood in a vice overnight. I only need the dial for alignment; ot will be eventually replaced.

I also had trouble peaking the IF output from the 78 tube.
As I realized, the screw was stripped and they used two nuts to tighten it. I took the screw from the donor chassis, but it wouldn't peak until I soldered 30pF mica cap across the trimmer; then it peaked.

I never worked on a 38-14, so the challenges are new to me.

[morzh]

So, first, the additional whining was caused by ... my rear power strip.
It has a USB converter in it (I need it like a rabbit needs a stop light), and it was causing that. In addition to my lighting causing the rumbling. Without those two the radio is quiet.
A hum really depends on the polarity of the plug when plugged in. This is why I ordered polarized cords.
But with the isolation transformer it hums always: no Ground.

So, my shortwave works fine, and my BC does not.
Strange, as all coils are good, the switch is good....the capacitors (micas and a padder) are left to check.

Tomorrow I hope to find out what it is.

[radiorich]

Hello morzh,
Yes , that dreaded hum in my room where i do alighment I have my cable modem and router plus desktop plus old school 1950s Fluorescent light .

So puttin the safety cap before the swith and addinf that resistor helped then ?

I am currently making a few replace reproduction resistors for my Philco 38-10-T .

Sincerely Richard

[morzh]

Richard,

It helped some, but I still had the whining when engaging the rear power strip (I did not realize it was from it; plus I did not have it in other radios; I guess transformer vs AC/DC makes some difference in reaction). So when aligning now this specific radio, I turn off the rear strip and plug my generator in the front one, that has no active devices inside. I might yank that converter from the rear strip: I do not need it. All I wanted was the strip.

Oh, and this is an X-cap, so even though people call them "safety", I don't; for me the safety is the Y-cap.
And the resistor is just for bleeding, so I don't get zapped after I pull the plug and touch it.

[radiorich]

hello morzh ,
That sounds like a plan !

Sincerely Richard

[RodB]

Mike, I'm curious about a specific use of he Y cap. Lately I've seen a single cap across the power transformer primary. Do you think it can be replaced with just one Y cap or the standard two. I know effectively the two together in series is equivalent to one across the primary but the configuration with one may work just as well since the neutral side of the mains is grounded. My grandson is getting a 1950 Siemens German radio to repair in a couple weeks and that's how they did it in those days. What's your opinion?

[morzh]

Rod

Two Y-caps make sense if you use them from L/N to the chassis; this is only makes sense in the transformer radios with the chassis being common GND.
Especially if the chassis is grounded to the actual Earth GND.
Y-caps are used to filter out the Common Mode type of noise. This is typically RF type, when both L/N act as one antenna (have the same voltage on both). They are usually smaller value types (there are large ones, in hundreds of nF, but used in other applications).
A Common mode chokes are used for the same purpose, often in conjunction with the Y-caps.
Because the noise is CM, putting a cap of whatever value between the L and N will not help with this part of noise, as there is, well, nothing to conduct.
And even if you think the N is Earth, in fact it is not quite it, as it has the same noise as the L.

What I have is an X-cap. The X-cap (we went through it before, as to where it is used, I am repeating it to make the picture more detailed) is used across the L/N and not from L/N to the chassis/GND.
As for the purpose of it, it is used to filter the Differential noise. That is the one mostly made by conducting currents. Because it is differential, the cap will happily conduct the noise from L to N, and so this part of noise will be routed away from your device.
Because often times the frequencies of the noise go quite low, the value of this cap is picked to be quite a bit larger than the Y-caps;' it is often between 0.1uF to a few uF. It also works well with the differential inductance, that is those place in-line with L (or L and N but uncoupled from each other); this forms one-pole LC filter. Sometimes the leakage inductance of the CM choke (which is differential in nature) is relied upon for this part.

So, in my radio (AC/DC) due to the absence of a separate isolated chassis, there is no reason to have a Y-cap.
In a transformer radio, there is.
Better yet, there is a reason to have both, same as they do in today's off-line SMPS (Switching Mode Power Supplies): two Y-caps to the chassis, whether it is connected to the PE or not, and one X-cap across the AC Line. I would go with 10nF for Y-caps and 0.1-0.3uF for X-cap.
If the X-cap is placed before the Power Switch, use a 2M bleeder resistor to drain the charge when the radio is unplugged.

Coming back to two Y-caps making one X-cap: yes, it will, but it will be a bad X-cap, as it is small in value (2x10nF make 5nF X-cap, which is not much).

And the last note: in today's devices the function of the input filter is opposite to the one in the old radios: in the latter it protected the radio from the interference via the power line; in the former, it protects the power line from the device.
But the filter remains largely the same.

[morzh]

OK, well, so after staring at the sch again, I realized the problem had to be the #12/12A padder.
I put a scope on the oscilaltor: it worked fine on the SW band but switched to 11.8MHz at the BC upper part, while quickly failing to oscillate soon after starting moving toward the lower end of the scale.

So, I was able to restore the oscillation by adjusting #12 padder (the nut), which was somehow well out of range, and then I got reception on the BC, and then I was able to align it.
Now I have nice reception on both BC and SW.

The dial look sad: it was in bad shape in the first place; I removed the bulge in the middle, so the pointer could move freely. As I said before, if ever found, the dial will be replaced.

Now I have to go to cleaning the cabinet.

https://www.youtube.com/watch?v=cuy0afTDv-A


The video of the radio reception while on the bench.


PS. And, yes, the speaker works. I cannot explain what I saw and the behavior of the Field coil resistance, but the speaker behaves quite normal, sound-wise. I'll take that for now.

[radiorich]

Hello morzh,
Nice explanation on the x and y capacitors and glad you are narrowing down the issue with this set !

Sincerely Richard