[morzh]

Hi guys!

I am now up to plugging the thing in and trying to revive it.
Out of all the tubes I have, one 71A ( one of the two in the the output stage) did not have filament on, so I buzzed it - it was open, so I bought two new ones (the seller claims they are matched pair tested on Hikock).

Anyway, all tubes light up, no sound coming out.
(The hi-voltage rectifier works, which means my 80 diode tube is fine).
The brute force approach failed so far.

The speaker was restored in The Speaker Shop (I trust them, then what choice do I have?) and the output transformer I bought from Radiodaze is Hammond 125C, and the turns I use make the turn ratio roughly about 1:100 (I believe this is what original had), which makes the impedance (out of 0.7 Ohms) 7kOhm.
I hear light scratching when I touch the output socket pins with the meter, so the speaker is somewhat alive, judging by this.

So, short of buying my own Hikock (I realize there is more to tubes than just filament being intact), and testing all the tubes, right now I need the voltage chart for Philco 20, to start figuring out what else is wrong.

Anyone has it?

Any other advice?

Will appreciate any pointers.

Mike.

[Carl Travis]

Mike,
The voltages are listed in the Rider's service info:
http://www.nostalgiaair.org/PagesByModel...029582.pdf

[morzh]

cwtravis

Thanks, this should help for now.

Actually, voltages will vary, I understand, as the outlet voltage is 120V where I am, and there are no voltage stabilizers in use in old radios.

I.e., "D" voltage I have is almost 500V vs 425V in the chart.

[morzh]

1. All 5 wire-wound resistors, 50K, 100K, 250K, 500K have run away to the values that are anywhere 30-100% up.
Wow. Granted, I have no experience with these that are 80 years old but the resistors I did not expect to behave like so.

Need to buy those.


2. Also, the tapped resistor last section (1-2) of 1.4K that ties the 71A filament's Power Xfmr winding center-tap to all other Xfmr centertaps was just opened.
I even think I know why - the output stage probably smoked, as the output audio Xfmr primary, one half, was fried, and so was one of the tubes. There was something catastrophic, so the current probably burned that 1.5Kohm section.

Once I put an external 1.4K resistor there, the speaker came alive, and I clearly hear a reasonably loud scratch when I rotate the volume (the antenna attenuator) var. resistor.

I wonder what else's in stock for me, but I will start with resistors.

I want to buy "authentic" parts, so what do you tfhink of "Philcorepairbench.com"?

[Ron Ramirez]

"Philcorepairbench.com" (Chuck's site) is great for schematics/service info - you can't get any better service info anywhere, and the price is well worth it.

Chuck does not sell parts (neither do I). Both of our sites have Resources pages which list parts vendors, though:

http://ww.philcoradio.com/resource.htm
http://www.philcorepairbench.com/links.htm

I don't think you'll find anyone who sells the original old "dogbone" resistors. Some guys make their own by molding new parts that have new resistors inside. I don't have the details on that as I have never done this, but a Google search might turn up something.

[morzh]

Hey Ron

Thanks, this is what I thought, so I'm glad I am on the same page (I did pretty thorough research on resources 2.5 years ago but then got sidetracked, and then forgot what I placed and where - I even had that voltage chart, as I just found out ) with you guys with my opinion about the websites you've listed.

Dogbone, huh...is this what they're called? Are there specs on them (or maybe you know) power-wise?

Philco 20 uses 50K, 100K, 250K and 500K resistors, and I hadn't seen the power rating though from UI specs on the schematics I can make a guess as to what I need.
It's a pity - the ones Mouser sells won't look as good

BTW on e-bay people sell Dubillier dogbones, for Grid-leak detectors and such, but not the values I need unfortunately. It'd be ideal.


Last question:

How was the antenna connected inside the cabinet? I have to pretty frayed wires; not sure if they formed a closed contour (from antenna contact to GND contact), or if it was an open one?
I don't think the GND was really required to be connected to the house Earth GND.

[Arran]

1. All 5 wire-wound resistors, 50K, 100K, 250K, 500K have run away to the values that are anywhere 30-100% up.
Wow. Granted, I have no experience with these that are 80 years old but the resistors I did not expect to behave like so.

Need to buy those.


2. Also, the tapped resistor last section (1-2) of 1.4K that ties the 71A filament's Power Xfmr winding center-tap to all other Xfmr centertaps was just opened.
I even think I know why - the output stage probably smoked, as the output audio Xfmr primary, one half, was fried, and so was one of the tubes. There was something catastrophic, so the current probably burned that 1.5Kohm section.

Once I put an external 1.4K resistor there, the speaker came alive, and I clearly hear a reasonably loud scratch when I rotate the volume (the antenna attenuator) var. resistor.

I wonder what else's in stock for me, but I will start with resistors.

I want to buy "authentic" parts, so what do you tfhink of "Philcorepairbench.com"?

Wire wound resistors do not drift, they either go open or some of the turns may short out which would reduce the resistance not increase it, you may be thinking of body-end-dot code carbon resistors and yes they can drift 30-100% upward in value. As far as I know nobody sells authentic resistors for these sets, to have authentic looking resistors you would have to take a modern resistor and cast in plastic resin to look like an old B.E.D "Dog" bone style resistor.
Wattages can be tricky to figure out but most carbon resistors aren't any more then maybe 2 Watts in size, the vast majority of the small ones will be 1/2 Watts, the medium sized 1 Watt, and the larger size 2 Watt. Sometimes you can figure it out the wattage by where it is used, grid bias resistors can be as low as 1/4 watt, cathode bias resistors usually 1 watt for example, plate resistors can be as low as 1/2 Watt for a single tube, voltage dividers are usually wire wounds but in sets with a fixed bias supply they can use a series of large carbon resistors from 1 to 2 Watts. If you had the factory data it would likely have a parts list to tell you exactly what they used.
May I suggest using your voltmeter here? There should be B+ going into the center tap on the primary of the output transformer, then each leg should be going to the plate pin of each of the #71A tubes. There should be an audio interstage transformer in the amplifier circuit as well, the primary has one end connected to the B+ circuit and the other connected to the plate of the first audio tube, either a type 27 or a 56 tube. The secondary of the audio interstage transformer should have the center tap grounded or connected to B- somehow, the two legs of that transformer should be connected to the grids of the #71A tubes. The resistor between the center tap of power transformer winding © and B- is either for hum control or for tube bias, it would be unrelated to whether the output transformer primary was open because the output transformer is in the plate circuit.
So the plates of the #71As should be getting 200+ volts B+, the plate of the 27 first audio tube should be getting something maybe 20 volts less, so if those are getting B+ everything ahead of them is working. There are no voltage divider resistors ahead of these points, just a filter choke, and the output transformer primary in the case of the 71As, and a choke, the speaker field coil, and the primary of the interstage transformer in the case of the 27 tube.
The volume control is in the antenna circuit as you noted, turning it may result in scratching noises but that does not necessarily mean that everything after it is working. See if you can find a voltage chart and then check all of the voltages on each pin of each tube, if anything is missing or way off then check the resistors, capacitors, and coils connected to it or in line with it.
Regards
Arran

P.S Here is a voltage chart and schematic located on another web site, it may be enough to get you by:

http://www.nostalgiaair.org/PagesByModel...029582.pdf

[morzh]

Arran

Thanks,

yes those are carbon resistors, and I ordered everything from Mouser yesterday. All 5 of them.
Plus, what bugs me (I am sure it does not matter) - the mica cap bypassing the detector's (24) anode, which is supposed to be 250pF, when disconnected and measured, was 400pF.

I thought mica caps were long-term stable. I am sure this does not matter much in this case, but ordered another mica cap for the 250pF just in case.

I want to have all resistors right, so to compare the DC voltage values with the chart first, otherwise it becomes a guessing game. My non-tube troubleshooting experience tells me "get all values right, then troubleshoot". Usually makes all that much easier.

The last thing I will check is those potted caps and the two caps in metal boxes (so far I measured two of the potted caps I could easily unsolder, and their values were right where they are supposed to be). Hopefully the rest is like this also.

[Arran]

Arran

Thanks,

yes those are carbon resistors, and I ordered everything from Mouser yesterday. All 5 of them.
Plus, what bugs me (I am sure it does not matter) - the mica cap bypassing the detector's (24) anode, which is supposed to be 250pF, when disconnected and measured, was 400pF.

I thought mica caps were long-term stable. I am sure this does not matter much in this case, but ordered another mica cap for the 250pF just in case.

I want to have all resistors right, so to compare the DC voltage values with the chart first, otherwise it becomes a guessing game. My non-tube troubleshooting experience tells me "get all values right, then troubleshoot". Usually makes all that much easier.

The last thing I will check is those potted caps and the two caps in metal boxes (so far I measured two of the potted caps I could easily unsolder, and their values were right where they are supposed to be). Hopefully the rest is like this also.

Those potted caps would be the first thing I would change, even if they test good now it's like playing a game of Russian Roulette, I would say that at least 75% of the problems in old radios are caused by leaky, shorted, and open, paper caps, the black Bakelite block condensers especially. The ones in the steel can sometimes work and sometimes they don't, some old timers will tell you that they don't need replacing and will last forever but these caps are 80 years old now and are not to be trusted. As for the micas those don't increase in value, sometimes they leak, sometimes they go open or short, if there is a 400 mmf mica in there and it looks factory it could be a factory revision, there are early and late model 20s so they may have decided to increses the value from a 250 to a 400 or reduce the value from a 400 to a 250 for some reason.
Regards
Arran

[morzh]

In the Philcorepairbench there's a way described to fix those potted caps, where he simply warms it up and the whole thing can be pushed out, and then new caps are put inside and potted.

[Arran]

In the Philcorepairbench there's a way described to fix those potted caps, where he simply warms it up and the whole thing can be pushed out, and then new caps are put inside and potted.

You don't have to repot the Bakelite block condensers, that was just to keep moisture out of the old condensers, the new replacements are made out of plastic or are already sealed up so they don't need to be potted. When it comes to the big condenser can it may or may not need to be repotted, I'm thinking it won't on a Philco but with some RCA ones the metal box will fall apart without the tar inside. The Philco ones also have sets of terminal strips on the bottom so you can attach the replacement caps to the inside of those strips.
Regards
Arran

[morzh]

Well, today brought some good news - the "Lowboy" just came to life.

I received the Mouser package with resistors, and changed all 5 of them, plus restoring the 1.4Kohm open section of the multi-tap voltage divider bu just soldering 3W 1.4K resistors in parallel.

Now, I started touching anodes (plates) and once I touched the plate of the 24 detector tube, I heard a loud 60Hz buzz from the speaker, so I knew my Audio amp works, and my speaker works.

Then I took off the screen (the one protecting the tubes in HF section) and was able to tune to some buzzes and whistles. Then I heard a faint signal.
Then I swapped couple of tubes places and the reception became very loud, and when i connected the GND wire to the actual Earth GND, it became even louder. I was able to achieve a nice low distortion sound by reducing the volume. At high volume it would go into excitation.

I think one of the three 24 tubes is subpar, but I lack the tester, and a Hickok in a working state is over 300 bucks, so I will start from buying one extra 24 and trying to see if it makes things better.

So far only one station is received clearly and is loud.

The last thing is (have to find out why - thoughts welcome) - when replacing the protection screen over the 24 tubes, I lose the reception almost completely - I can barely hear that station that comes in very well without the screen.

And another question (about the voltage chart you guys gave me the link for):

I am not sure, on the voltage chart, what those figures on top of the tubes mean. The plate voltages etc are in the table, which lists the tubes and their finctions vs. Plate/grid etc voltages. But in the sch on to of the tubes there are volt/curr. figures like "225V 3ma" / "130V 3ma"/"30V"/"115V 3mA".
Not sure. Are these the voltages measured at 3mA through each tube?

And in what mode then the table voltages are measured (the plate voltages are fairly close to what I have now; Grids - different).

PS> The reception is achieved without rebuilding the potted/canned caps. Do you think I should start looking at those?

[Radioroslyn]

>Now, I started touching anodes (plates)
You are lucky you didn't get knocked on your Bum!!
Please don't do that as most of the anodes has around 200-300vdc on it. It smarts ALOT!

I suspect that the reason that you are losing reception then you installed the protection screen is that you didn't have an antenna connect to the set. It's actual propose is to stop the 3 tubes from interacting with each other. It also blocks out any radio signals too. Not to protect tube from breakage.Another thing to check is the primary windings on all the rf coils. Can be done without removing them but the ant coil you'll have to test with the set unplugged and the volume full up lest you will be measuring the resistance of the volume control. All should show a very low resistance like a few ohms if not it's time to rewind!

The voltage and current listing on the diagram is just to let you know how much the anode current of each tube should be drawing. If you have close to what the stated voltage is then you'll fine.

Probably should rebuild the caps if you don't want to burn up the power transformer or your house. Safety First!
Terry

[morzh]

>Now, I started touching anodes (plates)
You are lucky you didn't get knocked on your Bum!!
Please don't do that as most of the anodes has around 200-300vdc on it. It smarts ALOT!

Terry, though I indeed lack a huge tube experience, I am an EE, and have designed few off-line power supplies in my life, plus some measuring voltage probes for 25kV and current sensors for 25kA (it's a lot of current). I know what voltage tube plates command, and I was touching them with my meter's probe to see the voltage, and that's how I discovered the effect.

I suspect that the reason that you are losing reception then you installed the protection screen is that you didn't have an antenna connect to the set.

Actually I did. Plus I connected the GND to the Earth GND.

It's actual propose is to stop the 3 tubes from interacting with each other. It also blocks out any radio signals too. Not to protect tube from breakage.

Yes, I kinda figured that out too. I am familiar with a screen concept. This is why I called it "screen" and not "that metal box that keeps tubes from breaking"

Another thing to check is the primary windings on all the rf coils.

That was done in the early stage of the debug. The coils seem to be intact. They show low-res. continuity on my meter.

Probably should rebuild the caps if you don't want to burn up the power transformer or your house. Safety First!

Which ones? Those in the big can? They indeed can fail creating potential hazard; the potted ones in small backelite boxes #8 and #22 (0.05uF) also face direct plate voltage "D" so, yes, they also can be a problem.

Actually, it was my intent today to measure the capacitance of those by de-soldering the surrounding wires. I wonder myself how they have survived these 80 years


Thanks Terry, I'll let you know how it goes.

[Radioroslyn]

Well all for best results. The big one definitely! It's a mess to take apart and clean the tar. After that it's a piece of cake. Keep the value of the input cap the same the others aren't as critical.
There are a few bakelite block caps and a metal can with a few .25 or .5 in it if memory serves me correct. When you replace all it makes troubleshooting easier and you don't have to keep pulling it apart time after time when another one fails.
Terry
PS
>Actually, it was my intent today to measure the capacitance of those by de-soldering the surrounding wires. I wonder myself how they have survived these 80 years

It's the leakage that is problematic.