The PHILCO Phorum

I'm restoring this Bakelite Transitone (which is actually a model 49-504-125), and I'm starting this thread to address electronic issues.  But first I need to replace a riveted Loctal socket.  I hate to change the chassis appearance, but all I can think of is to use small nuts and bolts.  Any better ideas?

Electronic questions to follow.

If interested, this link is to a photo of the electronic restoration so far:

https://goo.gl/photos/MCQngsbLVBTVjAUo7 (click on photo for high-res image)

Thanks.

Well, if you can find a good loctal socket, great, they are getting rare. You can use a small screw and nut to secure the transplant, then ever so carefully grind down the head and tail with a dremel or dental technician's handpiece tool, and remove all dust and filings and top off with a tiny drop of clear nail polish, and nobody could ever tell what you have done. Of course this might be the least of your problems, but this is a start.

That's exactly what I've done in the past. I'm glad to see that I'm not the only one who thinks of doing these absurd things. I'll make the screw head look like a rivet head.
Thanks much.

Now that I have the Loctal socket replaced, I can look into getting this radio to work.



When I received the set it had a toggle switch in the back labeled “radio/phono,” and the switch wiring had been removed or cut.  After restoring all the wiring and components I checked the continuity using a Sams resistance table.  That showed a problem with B+ return to B-.   Each B+ point should have given either 90Kohm or 560Kohm.  Instead, I got multiple Megohm readings which varied, always increasing while I tested, from 2 to 30 Meg.


I went ahead with the variac and gradually increased the voltage to 120V and the pilot light and tubes all lit up.  When I powered up again, I apparently killed the 50A5--first in the string of heaters.

Weeks have passed, and I’ve gone over the circuits dozens of times and found no problems.  That is, it looks just like the schematic to me.  But when I look at the schematics, I can’t see how B+ is supposed to return to B- or the chassis without jumping over a capacitor.

I replaced the 50A5 and went on with voltage testing at 60VAC input.   The voltages all look pretty good.  If the values are doubled, they approximate what it says in Sams and Rider.  During voltage testing at max volume/no signal the radio starts playing when I touch certain B+ points (Is the meter providing a path to B-?).
So, I think the whole problem is that I’m missing some connection from B+ to B-.  This is a link to a very legible scan of the Rider schematic:

https://goo.gl/photos/z4KG89Ze1LSp6PaY9

It’s been marked up to show modifications I’ve made (red) and voltage readings (red).  Also, there’s one cap that wasn’t in the schematics (green).

Please excuse the long explanation above.  I had to express how much effort I put into this before asking for help.  The question is:  

How does B+ return to B-?

Thanks.  I appreciate any advice.

Hi Wink,
I'm Not quite sure what you are asking but the B+ returns to B- though the the tubes conducting. B+ is applied the plate of the tube as the cathode heats and emits electrons. The plate having a high dc potential (B+) the positive electrons are drawn to the plate. The grids just control the amount of electrons that get from the cathode to the plate. The more electrons that hit the plate the more conduction (current flow from B+to B-) The cathode is at B- more or less. (Usually though a low value resistor)

I think what you maybe asking is if I measure from chassis to B+ I'm not seeing the proper resistances. Because B- is not connect to the chassis is floating. B- is actually where the - side of the filter cap are connected. Use that for your - meter connection when measuring dc voltages.

Have a stupid question for you did connect that cap with the coil around it back up?? Doesn't have to have the coil.

Terry

Terry,
I did replace the "special" capacitor with an ordinary cap (no coil).

I'm sorry that my misuse of the jargon confuses the issue of how much I understand in the electronic realm.
I do understand the flow of current through the tubes, and your "I think what you maybe asking..." is correct.
I am measuring resistance to the common negative (negative side of the filter caps) and I'm not getting the expected resistance from from the plates and few other points. I also tried measuring to the chassis, but I get the same results from the plate. And I can't see on the schematic any pathway from a plate to common negative (or to the chassis). When I say "I can't see it," I mean that if it's there, then I don't understand it.

In the meantime, a couple of hours ago I was doing the voltage checks again. I touched one pin, heard a half-second "Bz-z-t," and the radio went dark and silent. I have to figure that one out, then go back to the original question.

Thanks for the reply. It's likely that I've overlooked something too obvious for you to consider. If you figure out what I'm asking, then you'll probably be able to explain it to me. I really appreciate your help.

I suspect you're not quite understanding the flow of current through a tube... you can't just measure resistance from B+ to chassis because what you will probably end up measuring is a phantom... the charge in the capacitors will give you some kind of reading... which will change slowly since when you're doing a resistance measurement you are applying a small voltage to the circuit under test, which will build up the charge in the caps.

There are only two component types in the radio (this is best-guess since I can't find your schematic - I'm basing on the 49-500) that will pass DC from B+ to B- and one of those will only do that if its failed - the capacitors.  Assuming they are good (because you've recapped the set) then the only path for electrons to travel is inside the tubes.  

You can't just measure that with a resistance meter because (just as they are drawn in the schematic) each element in a tube is separated by an air-gap inside the glass envelope - there is no direct connection.  The electrons will not flow without first being heated and attracted.  Heating is done by applying a voltage to the filament, which excites the electrons on the cathode and creates a cloud of happy free electrons whizzing about.  The second stage is applying a large positive voltage to the plate (and/or screen grid but ignore this for the moment) which draws the negatively charged electrons from the cathode to itself in a constant stream.  Thus, electron flow occurs and the circuit completes from B+ to B-.  No heater and/or no plate voltage = no current flow = no path from B+ to B-

As far as I am aware, there is no way to measure what I think you're trying to measure the way you're trying to measure it.  That is, of course, if I understand what you were trying to do correctly?

Also, you may have also inadvertently shorted one or more of the filaments out of the circuit while taking voltage measurements - causing the other filaments to have to carry significantly more voltage - so that last problem you mentioned could be another dead heater (or two).  Hopefully not, but thats my guess from your description.

Hope that helps

Steve

If you are trying to check the value of specific resistors, you can disconnect one end of the resistor from the circuit and then measure across that resistor. Doing the measurement this way ensures there are not any alternate parallel pathways to give lower values and no interference from capacitors as mentioned by Steve.

My first concern is to avoid frustrating my mentors to the point that they give up on me.  Please bear with me.
My response is slow because just last night I figured out that emails from the Phorum were going into the spam box.

I do have a basic understanding of the vacuum tube, how it can function as an amplifier or a rectifier.  I also understand how a capacitor acts as a filter following rectification and why it can work as a bypass for AC but not DC.   And I understand how a coil lets DC pass easily but resists AC.  

I know that the resistance reading across a capacitor or between anode and cathode is expected to be infinite.

Link to schematic:
https://goo.gl/photos/z4KG89Ze1LSp6PaY9

So, looking at the schematic and my resistance readings chart; with radio unplugged; connecting one lead from my ohmmeter to the negative side of capacitor C101-C:

From 50A8 pin 7 (cathode) the chart says to expect 130 ohms.  I read 130 ohms.  I can trace the electrical path on the schematic from pin 7 to C101 (negative), and there is a 130 ohm resistor in that path.  That makes sense.

From 50A8 pin 2 (anode) the chart says to expect 90 Kohms.  I read 12 Megohms and the resistance slowly increases as long as I have the meter connected.  If I trace the electrical path from pin 7 on the schematic, each direction ends at C203; the positive side of C101 A, B, or C; or the 35Z5 cathode.  I would expect infinite resistance (setting aside that the meter is charging a capacitor).  But I can’t see how I could expect to read 90 Kohms.

I’ve restored about a dozen prewar radios and I’ve never run into this issue.  What am I missing?

(Regarding the Bz-z-zt:  The first thing I did was check filament continuity in the 3 most suspect tubes, but they were OK.  Next I’ll check all tubes in a tester and look for a break in the continuity all the way back to the wall socket.)

Bz-z-z-t. I shorted the pilot light. That killed the bulb and also blew the fuse in the variac. I'll drop the question about resistance/continuity for now and probably discuss it with my brother, K3KU.

I put everything back together, powered it up to ~115V, and it works well. Good local reception with no antenna.

Question on hum: For the electrolytics I've replaced 30-25-20 with 33-22-22. Can I increase the capacitances to reduce the hum?

Thanks to all. I know it's nothing to write home about, but I'll post something when it's done.

Glad it was something simple and inexpensive

My gut feeling is that the info you're working from has a typo - because the readings you're getting are what I would expect... there is no path I can see to measure resistance from the negative lead of that cap to the plate of the output tube other than through the electrolytic itself.

Regarding hum - 40uF is the max rated smoothing cap for the 35Z5 - so you could try increasing C101A to 40 if you can find that value... alternatively have you moved any of the wiring or component placements?

Cheers

Steve

Just for the fun of it if you would like to experiment a bit to remove hum disconnect R-100 and replace it with a small iron core choke. This can be one from a power supply, the HV secondary from a power transformer (disconnected of course) or it could be the primary of the tube type audio output transformer. Two things will happen one is that a choke is much better in filtering than a resistor and the other it has less of a voltage drop. So you'll get less hum and a little increase the voltage.

Terry
ps Grew up near where your bro lives. I'm from Sandy Spring about 15min from the other SS.

Steve:  That’s exactly what I was asking.  It’s not likely a typo.  Possibly there’s some instruction missing or I’m failing to do something that they assume I know to do.




As far as wiring changes, they’re shown in red on the Rider schematic at this link:

https://goo.gl/photos/z4KG89Ze1LSp6PaY9

And in blue on the Sams schematic:

https://goo.gl/photos/xesfhPaBPayFQWfB8

The only other change was that the 7A8 cathode pin had no B- connection, so I connected it to the 14A7 cathode pin (the nearest B-).

Hum:  Is the 40uF rating for the first cap only or for each cap?

Terry:  Before I play with the capacitors, I’ll look through my junk for an appropriate choke.  I haven’t been saving stuff for very long, but I never throw away anything.

My brother does contests a lot, and he always mentions any contact with hams in my area.  You’re the first actual connection between us.  Can you tell me this:  Some years ago, when he assumed the K3KU identity (formerly K3OAE) I asked him why she chose that call sign.  He said to look at the code.  I did, but I couldn’t see any pattern in it.  Can you tell me what’s unique about K3KU?

Well K3KU is pretty short in terms of dots and dashs (14) but K3AOE is shorter(13). He probably wanted a call that reflected the earning of the Extra class license. The latter call could be used by any operator who pass the Technician, General,(Advanced) or Extra class examine. The one by two call (K3KU) would only be issued to a Extra class op.
The Advance class license isn't issued any more. (I hold one of those) Same with the Novice license. They had a two by three call . Back in my Novice days I was KA3TSY (circa 1988 ).
K3KU is easily recognized as the letter K is used a lot in CW communication. A lot of callsigns start with K and K is use as the invitation to transmit on CW. It also is rhythmic.

Terry N3GTE
mine is only(10) 1988 isn't supposed to have a face. Stuppid computer.
He MUST be your BIG brother. Saw his age.

Well if it isn't a typo then someone is going to have to explain it - the only other thing I can think of is maybe the original electrolytics had a FAR lower internal resistance than modern ones and you could measure them this way - but that seems unlikely to me... Maybe check some other similar models service info and see what you can glean from them?

I've never used SAMS stuff before... but if that schematic is indicative of how they drew them I think I'll avoid them... compared to the Riders schematic thats just awful to try and read. I never understood why some schematics would have two points going to earth and rather than connecting them, both would have an earth symbol and remain separate... drives me nuts... but thats just me

The 40uF rating is just for the first cap - each cap after that is part of a series of progressive RC filters that isolate them somewhat from the first (reservoir) cap.

Cheers

Steve