I started tinkering with electronics about two years ago to learn (in a practical way) all of the things is should learned and have forgotten in college. Rather than continue breadboarding solid-state projects (I did build an cool pH controller for my aquarium) I decided that an antique radio would be proper furniture in my house since it was built in the late 30's. The first two radios I worked on (Zenith 5S319 and a Philco 38-9) were in fairly good shape. All that was needed electronically was recapping the paper condensers and the electrolytics. The project I'm working now is leap. I couldn't pass up the 40-201 I found in an antique mall for $80. The cabinet and case were in excellent condition so I decided it was time to jump in and find out how deep the water is.
The first thing I did (other than a good dusting and vacuuming) was recapping. The radio had prior service so I replaced the electrolytics with what I found in the radio and did all of the other replacements according to the schematic. I powered it up gradually using a light bulb in series and found that the radio is functioning but not working well. For a short time I had weak, scratchy AM but all I have left now is noise. The volume control varies the loudness of the noise and the tone control seem to work. I inverted the radio and measured the voltage at the electrolytics and they are 30V above the drawing (330V and 225V spec, reading 360V and 259V, respectively) . Here's what I see on the plate on the tubes:
RF spec 182V, reading 213V
Det-Osc spec 210V, reading 254V
IF spec 225V, reading 262V
1st Audio spec 100V?, reading 165V
Phase inverter spec, 140V, reading 151V
2 push-pull 2nd audios spec, 225V, reading 191V and 157V
2 outputs spec 225V and 215V, reading 249V and 242V
I'm concerned about the high voltage and I'm wondering if I should go back to the spec electrolytic of 18 uf and 25 uf rather than the 20 uf and 40 uf replacements that were in the radio for previous repair or servicing. I've been reading Elements of Radio Servicing by Marcus and Levy to help but there's a lot there to digest. I've got a signal generator and oscilloscope but I have a hunch that may be premature. I'm a little cautious about doing anything until I get the voltage issue resolved.
If any one has any thoughts regarding the high voltage I'm seeing or where to start troubleshooting, your advice would be greatly appreciated.
Hello and welcome.
There are two factors contributing to the higher voltages you are seeing.
One is the fact that today's line voltage is higher than it was in early 1940 when your 40-201 was built. Back then, 110 to 115 volts was the norm; today, it runs between 122 and 128 (125 on average).
The other factor is the types of meters used back then compared to now. Back in the day, voltmeters with a sensitivity of 1000 ohms per volt were used. These tended to load down the circuit being tested, and as a result gave readings which were actually a bit lower than what the actual voltage was.
Now, with today's modern high-impedance digital multimeters, very little load is placed on the circuit under test when voltage measurements are taken, resulting in a far more accurate reading.
The higher line voltage of today makes the voltages inside the set higher than they were in 1940. And you are measuring them accurately with your modern multimeter - unless you are using 1930s vintage test equipment.
Do not worry about the electrolytics you have used - as long as you have installed them with the proper polarity (positive to B+, negative to B-), and they have a working voltage higher than the voltage being applied to them, they will be fine.
A quick look at the voltages you have posted indicates a problem in the output stage, since your plate voltages are low on your output tubes. Check your audio output transformer - these are notorious for going bad in 1939-1942 Philcos. Check resistors in the audio output circuit to make sure they have not drifted more than 20% off posted values. Test the tubes.
And finally, you did replace ALL of the paper and electrolytic capacitors, right?
Good luck - the 40-201 is an excellent radio when properly restored, with plenty of volume and tone. Very sensitive too with its built-in loop.
Thank you for a little history and technology. It means a whole lot more that don't worry
Yes, I did replace every paper capacitor and it took some desoldering of a wire or two to get to the capacitors. I also took the liberty of replacing the old wires with cracked plastic insulation. Curious, my radio has a mix of cloth and plastic insulation. Would I be correct to assume that was from prior service or were companies using what they could by 1940? Some of the wires with the plastic insulation are sure tucked away nice, neat and tight around the sockets and I can't imagine a service repair is the source of that.
I'll check the output transformer, resistors and the tubes in that order since I can do the first two rather easily. If that doesn't work I'll have to hounddog someone with and tube tester that can accept the older tubes.
I'm a little slow a first but I'll keep the post going until it's working right.
By the way, the jewel is missing. No big surprise.
Also, just as a added note to what Ron suggested already, make sure all the tube-pins are clean & shiny especially the loctal types, ( if your set has a few). Loctal types are notorious for causing bad reception,no reception, or intermittent reception after all else has been properly done. Try gently "rocking" the tubes in their sockets, while your set is in operation. If it comes on loud & clear, bingo, you have found bad ( corroded)tube pin connections. If so, you can carefully scrape all the tarnish off all the tube-pins one at a time, carefully using a exacto pointed knife tip. Just make sure you dont accidentally cut yourself. You can also spray a small amt of WD40 in the sockets, while set is "unplugged" !!!, and help clean out the sockets pins-connections, by installing loctal base tubes, then carefully removing them,repeating several times, to acheive good tube pin-to -sockets contacts. Best of luck restoring your vintage Philco 40-201!!!
I've done some electronic troubleshooting but until I get a moment to gather my thoughts here are some additional notes.
Pushbutton maintenance: The pushbutton switches were not functioning well mechanically. Those I could depress would locked in place. To get a better look at the assembly of eight buttons I removed the switch carriage and coils carefully from the tuner frame, taking great care to avoid any tension on the wiring. A very soft natural hair paint brush was to clean any of the dust on the coils and switches that I could not blow away with my breath. I did not use compressed air. Too may fine wires. Once that was done it was time to loosen up the switches. One drop of Marvel Mystery Oil on the contact surface and points that move did the job after cycling through the buttons perhaps a dozen times. The excess was removed either with a paper towel or cotton swab. The contact points looked varnished over so broke off the tip of a swab and twisted a pinch of 0000 steel wool on the bare end. A drop of Marvel Mystery Oil was added for lubrication. All exposed contact surfaces were cleaned well. Any that I couldn't reach were cleaned using strips of 400-grit SiC wet-dry sanding paper. Reassembly was painfully slow but in the end the pushbuttons work as they should. Smooth motion to depress and lock as the previously depressed button unlatches. The only loss encountered was due to age. I beleive there are small rubber pads that would cushion that shock as buttons retract. Age and heat has allowed those to degrade.
If necessary, you can test your set without using the pushbuttons assembly,period. Those are only used for station-presets when the PB/ switch(s) are depressed. I have found many bad contacts points in the old P/B assys, nomatter how clean they are! Some are completely un-reliable, no matter how much you clean them,period!
To bypass this potential-prob, is by following your schematic closely to where the points of "manual-tuning" engages in the old Pushbutton assy, then using a jumper-wire with a alligator-clip on each end, to bypass the switch assy altogether. Make sure your getting good contact for "manual-tuning" only, ( according to your schematic),then once you get your set working properly, you can go back and check your P/B switches later, and those small coils (small screws) will need to be tuned one at a time using a RF gen, or by ear, across the band, one at a time in khz steps corresponding to each coil(s), for your local stations broadcast khz (s).
Once you completely by-pass your pre-set buttons, and make sure your tuning your receiver by hand, turning the variable condenser, you will know you have the set ready to receive stations, once other probs are corrected first.
I would still suspect any corroded tube-pins on any of your local-type base tubes for bad recept!, since you already carefully recapped your set. Also, make sure your antenna is working properly, no shorts, & if need be for test, use a pc of wire approx 5'-10' long, to bypass your orig antenna, to the chassis' main antenna terminal, to receive your local stations while your testing your set. Never trust those vintage pushbutton assemblys! Those can completely "kill" any good radio under restoration! Wait till you get your set working fully in "manual-tuning" first, then you can go back & check your PB stations-presets assembly later!
Just tryin to help! Best of luck with your vintage Philco!!
Voltage and resistance checks:
Field coil: Spec = 680 ohms, actual = 814 ohms
Voltage divider: Spec = 140 and 27 ohms, actual = 137 and 29 ohms.
Once again I'm not sure if the differences are significant or not on the power supply end of the circuit. I suspect this part of the circuit is fine.
Output transformer: Black, white and center tap black-white reads 252, 265 and 250 volts. I applied a 10V, 400 Hz sine wave (with a 0.1 uf cap between the circuit and the function generator) on the grid of the number 42 output tubes. Audible with nice tone. Looks like the speaker works.
I checked all of the resistors from the first audio to the speaker and decided to replace them since some were reading high and share a common junction with two or more resistors. I ended up changing all of them. I'm still getting low volt on the second audio number 37 tubes: 168 and 163 volts. Voltage at the cathode (?) is also low: spec 95 reading 69. Tubes? When I applied <10V, 400 Hz sine wave on the grid, big time volume. I'm not sure if that tells me much.
Voltage off the phase inverter looks normal. Spec 140, actual of 186. There is (what I believe) a capacitor that makes a low pass filter on the grid of this tube. It reads 360 pf and the spec is 250 pf. I guess all that will do is draw off some highs. Not sure if it's important. This number 37 tube glows brighter than the audios.
Interesting note on the first audio tube. There are two inputs to the tube that are bridged by a 2.2 Mohm resistor that I replaced because it simply looked old. I've got a a decent multimeter and when I try to measure the voltage at each node. One of them clicks and the other actually makes reception better - good enough to manually tune in a station. The radio also plays better when the ground is disconnected. This is odd to me.
One the whole the signal is absent. Depending on the tone setting I get either load white noise or lower white noise (and I don't believe that's motorboating). I can only hear the radio signal through the noise faintly. I've got a 75 foot long wire connected as an antenna. It's connected to the one of four posts that gives the best reception, which isn't very good at all.
I'm at a loss now where to begin although it sure looks like teh second audio stage is the problem. Perhaps new tubes.
g8wayg8r Wrote:I've got a 75 foot long wire connected as an antenna. It's connected to the one of four posts that gives the best reception, which isn't very good at all.
I think we've found your problem.
It sounds like your set is missing its loop antenna. It MUST have this in order to function properly.
The 40-201 does not need an external longwire or ground.
The 1940 loop is an oval, cylindrical affair covered in brown paper. A loop from any of the larger 1940 Philco consoles will work in your radio.
Oh my. I have to do that installed in the cabinet. Maybe its time to take it out and hook it up later tonight
Talk about other silly oversights, my fluorescent lights about 3' over my bench are large contributor to the distortion. It still sounds a little gritty but I'll see how it goes when I hook up the broadcast and SW loops.
I'm still curious how measuring the voltage at the junction of the IF assembly and the 1st audio tube eliminates some of the background noise and gives a slight boost in volume.
Ron, thanks so much for a knowing eye over my shoulder.
I've got the both antenna connected per the schematic. I marked the wire color on the lugs before I removed the radio from the cabinet. As reconnected the numbers impressed on the connections match the numbers and wire colors on the schematic. so I feel good about the connection. The sound is better and the volume is fine. The sound, however, is still scratchy or buzzy. Although the speaker is working I'm not sure if it's working well.
Anyone know how or what makes a good test speaker? I figure eight ohms is good but I'm lost where to find a replacement other than taking one out of my 38-9.
I've been working on the radio in my basement so I brought it upstairs to see if reception was a problem. No change so it's back downstairs.
I've got a lot of noise that I can control the loudness with the volume control. There's also a good deal of buzz noise that's the same regardless of the tuning frequency. On talk radios stations, the voices are garbled. The louder they speak, the more garbled it sounds, just like turning up the volume. Guess it time to start reading about motorboating again.
I looking at the schematic and there's a 3.5 ohm RF coil in series with a 100 pf mica capacitor between the RF tube plate and the Det-Osc tube grid. The ohms are hard to check because it's in parallel with a 10k resistor (guess that shouldn't affect it that much). The wax coating is cracked and I can see the copper wires through the cracks. The mica capacitor looks dirty and both are very close to an solder lug so I don't want to get into that much with a hot iron. If I were to replace the capacitor I would simply clip it off and clean a point as far as possible from the coil to reconnect. I may even scrap enough old wax from all of those old paper caps to seal the coil.
What substitutes for mica capacitors now? Good grief, what substitutes for a 3.5 ohm rf coil (an inch long and about 1/4 diameter). There's 180V going into the coil so it looks I put my solid-state capacitor stock away for another project.
....mica caps are still avail from several good sources online when needed. I use <!-- m --><a class="postlink" href="http://www.justradios.com" onclick="window.open(this.href);return false;">http://www.justradios.com</a><!-- m --> for all my replacement caps needs, personally. Fair pricing, great quality, & friendly service too boot!! However, Im not sure that has anything to do with your reception prob at all. Sometimes, it can be a simple-solution, instead of moving too fast, and possibly reading into the prob to0- much,sometimes,.. on restoring vintage Philcos radios. The 75' antenna was "way too much"capacitance indeed!! Best of luck in nailing-down your prob in your vintage Philco! Keep searching, you WILL find it!!... usually something overlooked after full recap. Best of luck!!!
Thanks for the encouragement. The thing that really caught my attention is the fact that everything flows through that coil and mica cap right out of the RF tube. It looks like a low pass filter but I'm sure how the configuration with the coil in there affects that. Time to do more reading and not be hasty with the solder iron. I've got a signal generator and a scope that I need to put to good use. For now I'm going to assume that all of my coils are OK. I don't know what to do about checking the caps that are tied up with them. Until I find some really good evidence to crack into the coils. I'm staying clear of them.
Tubes: I couldn't find anyone local and I had one glowing purple just after my recap so I got replacement that were tested. Only my 42s on the output are not new/tested so I'm not too worried about the tubes being the problem. Loctals are an annoyance but at least they will stay seated in an earthquake.
My father was am electonics technician before becoming a full-time electrician since that job wasn't as much of a political football. He worked on unmanned spacecraft at Honeywell during the 60's space boom. A lot of that was solid-state but radios and TVs were still tube operated. If memory serves me correct, he assembled a kit TV. I showed him the schematic of the 40-201 and he knew it was built in 1940. The first thing he focused on was the second audio push-pull stage. Thought they can be a bit tricky (what ever that means) and the went on the let me know that the plate can bite pretty hard and that working on televisions could be an even more enlightening experience. At 86, the conversation started to meander on to other relevant topics. He agreed that it could be possible that the mica caps were bad but thought I should start using my oscilloscope and signal generator to track the noise. I think that a good idea but I'm not sure if I can tell nosie from a good signal. We'll see.
The website All About Circuits has a nice review about resonance in series and parallel. The plate off the RF tube goes through a seires resonator with a resistor in parallel with that coil. That opens up the high frequency end but crops off the lows.
I'm getting negative voltage at my 1st audio grid. That seems like it would really limit the plate voltage. The negative voltage to the grid comes through a 10 Mohm resistor thats connected to the voltage divider that taps the center-tap, high voltage lead of the power supply. This is a fixed-bias power supply per Marcus and Levy. Now I know were the slight negative voltage is coming from the RF coil. I'll have to do some checking to see what the correct voltage should be.
Note to self: Since the original line capacitors were replaced I need to check my connections of the input and output electrolytics are indeed correct. The input capacitor does not ground to the chassis. I'm going to assume that the standard ground triangle symbol is a chassis ground.
I walking in the dark now so I could be missing something very obvious or just see one more bit of the puzzle.
OK. Here's the story. I've missed something really obvious. I replace components one at at time unless there's a gang. I had a resistor out of place. There's a voltage divider coming out of the second IF coil assembly that the volume control taps into through a capacitor. I didn't connect the 330k resistor to ground
. When that's done, all I get is the normal squeal and much clearer reception. I think I'm almost home or at least close enough to focus on the cabinet hardware.
I've checked my connections several times. Now I see the error. I think I'll take this as an opportunity to see what's normal with my scope and run through the diagnostics with my signal generator. It time to keep learning and be less concerned about restoration
.