I am new to this forum and look forward to learning from the experiences all of the Philco Radio Phorum experts.
I am restoring this Radio and discovered that the ceramic base of the Compensator Condenser is cracked in half. The part number of this component is 31-6188. This condenser has two adjustable sections, items 7 & 9 on the schematic. I plan to rebuild this component and need to know the capacitance range of each adjustable section. I checked this site and could not find the capacitance values of the 31-6188 part in any of the literature.
Note: the 31-6188 is unique in that it has a built-in 250pf capacitor item 14. I plan on using a discrete capacitor to replace it.
Hello gprutchick,
First of all I wanted to Welcome you to the Phorum !!
That is a great set I have the model 38-10T radio mine my chassis is almost done few more items to finish .
If that was my set I would first put ad in out want ad group someone here most likely
will have a good used part that you could get .
My 38-10 has no shield bases and obviously no shields. I have noticed that the tube sockects for the 6A8, 6K7 and 6Q7 do not have a socket installed a the pin 1 location. I am using metal tubes at these locations. My understanding is that the metal case of these tubes is internallay connected to pin 1.
Q1. Why did Philco use sockets without pin 1? Was it because glass tubes were used? My set was oscillating and motorboating. I temporarily jumpered pin 1 of the IF tube to chassis and no more oscillations.
Q2. Would it be a good idea to replace the tube sockets and connect pin 1 to chassis in lieu of shields? I know this is alot of work but it may be more difficult to find shield bases and shields.
I worked out the values for C7 and C9 and have have rebuilt the compensator. The range for C7 is about 13pf to 50pf and the range for C9 is about 160pf to 450pf. I have done an IF alignment. The IF is very strong (470KHz). I have done the alignment on the broadcast band, not totally happly with it, and the radio does not receive any stations. I have a home made am transmitter that transmits at 1500KHz. My 38-10 receives it but faintly with alot of noise. There is alot of noise across the entire band. The antenna coil resistances seems ok. I also checked it by applying a signal generator to the antenna terminals while monitoring the G4 grid cap with a scope. The signal gets through the antenna coil and resonates at the frequency set by the dial.
I have tried several 6A8 tubes.
I checked the 6A8 oscillator and confirmed the frequency of oscillation with a frequency counter. The frequency is 470KHz higher than the dial setting. Grid voltage varies from -12 to -18V, which I think is good. The frequency of oscillation is approximately 470KHz higher than the dial setting.
What am I missing?
I may put this one asside for a while because frustration has set in.
Hi Gary and welcome to the Philco Phorum. We are a family-friendly place and are very happy to have you as a member and to help you. Do take time to "surf" the Phorum, read the rules and "What not to post" section. Note that we have sections for non-Philco radios, TVs, HiFi, test equipment for sale or want to buy, etc. Don't forget to stop in at Old Ron's Bar and Grill for a virtual burger and brew.
Now, to your radio. I have not worked on a 38-10 but I can tell you this. Metal tubes were an RCA / GE concept. Philco's (and Zenith's) tube supplier was Sylvania who did not make metal tubes but did market a "metal-glass" tube. I am not sure, but I think that there was an aquadag coating ion the inside of the glass envelope and the aquadag was connected to the cathode instead to a dedicated shield like pin 1 on the RCA / GE metal versions. If i am correct regarding this, then a shield was not needed, nor was pin 1 needed as this internal shield was grounded to the chassis by the cathode pin, pin 8.
The schematic does not show shields but the pictorial shows the IF and detector / 1st AF tubes are shielded. I wonder if someone pulled the tube shield bases off the chassis because the bases of the metal tubes would not pass through it
How to go forward depends on how "period correct" you want to make the radio. Either change the tubes or change the sockets. It may be easier to change the tubes. Alternately, jumper pins 8 and 1 on the 6A8, 6K7 and 6Q7, as pin 1 is the shield on all of these tubes, pin 8 is the cathode on all 3 of these tubes and the cathode is grounded to the chassis on all 3 of these tubes.
After you deal with the pin 1 issue (and you DO have to ground pin 1 on the above 3 tubes to use metal tubes), if performance does not improve, check the following:
How long is your antenna wire and where is it. Is it in the basement? Are you grounding the ground pin (or the chassis) to a water pipe? Grounding may be necessary, especially in a weak signal area or whenyou have a lot of interference.
Is this the only set in your collection that requires an external antenna?
The amount of electrical noise is unbelievable. Between the computer, WiFi, the Internet of Things, CFLs, LEDs, cellular phones, unshielded house wiring, meters that can be read through the power lines, etc.
Try touching a 20ft or longer wire to the grid cap of the 6A8. Is there a dramatic improvement in reception?
If so, check the antenna coil primaries for shorts. The switch A1 / A2 must work. Ensure it is cleaned.
Hope this helps. Keep at it, you will succeed.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
The tube sockets for the 6A8, 6K7 and 6Q7 in my 38-10 do not have terminals for pin 1. I will start the process of changing these tube sockets so that pin 1 can be connected to ground.
I have a Protek LCR meter that I used to measure the coil resistances.
BC Selected, Resistance between A5 & A1 is 0.082 ohms (Primary of ANT Range 1 coil)
BC Selected, Resistance between A1 & A2 is 0.013 ohms (Verifies SW A1 & A2 are making contact)
BC Selected, Resistance between A6 & A7 is 2.04 ohms (Secondary of ANT Range 1 Coil)
SW Selected, Resistance between A1 & A2 is 0.14 ohms (Primary of ANT Range 2 Coil)
BC Selected, Resistance between A7 & Cap of 6A8 is 2.1 ohms (Secondary of ANT Range 2 Coil & Secondary of ANT Range 1 Coil in series)
Do try the wire on the 6A8 grid cap to see if you get more reception. Also, do try grounding the chassis to a water pipe ground. DO BE CAREFUL, if the power input filter caps are still original, they are known to short and cause the chassis to be electrically "hot". (This is why they are referred to as Death Caps.)
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
I replaced the tube socket for the 6A8 and the 6K7, and tied pin one of each tube to ground. Solved all of the instability issues!
Re-did the IF alignment at 470KHz as specified in the procedure.
I did the BC RF alignment.
Afterwards I checked the frequency of the osc against the dial and discovered that the difference frequency was 440 KHz from 580 to 1600 on the dail. Tracked good but 30KHz lower than spec!
The RF alignment was strange in that the effect of adjusting C7 and C9 were reversed! Is anyone aware of an issue with the documented locations for C7 & C9?
It is entirely possible that I accidently reversed them when I rebuilt the compensator, oops!
I re-aligned the IF to 440KHz and the radio receives stations at the correct location on the dial, however weak until I touch the grid cap of the 6A8 and WOW!!! So now the BC ANT coil is suspect. The primary of the BC ANT coil measures 0.082 ohms. I suppose there could be some shorts???
How many turns are on the primary, not too many I would guess.
I am going to chenk the coil by applying my RF generator to the antenna terminals and monitor the grid cap of the 6A8 with a scope to ensure there is good coupling and the signal peaks at the frequency set by the dial.
Grounding the set did not change much.
Not sure why the OSC tracks at 440KHz above the dial, most likely the range of capacitance on my rebuilt compensator caps are off a bit. I could adjust the values a bit. Or, a 440KHz IF is ok???
I think that you definitely on the right track! Having the metal shields of the tubes floating is just asking for interference.
Re the oscillator tracking, I think that you should try adjusting trimmers 4A and 4B as well as C7 and C9 to try to get the 470KHZ spread that you are looking for.
As far as tuning the IF to 440KHz, the only issue is if the transformers are as efficient at 440 KHz as they are at 470 KHz. If the radio performs well at an IF of 440 KHz and you can't get the RF- Osc difference to 470 KHz, then go with what's easy.
I assume that by "WOW", you meant that you received a plethora of channels when you touched the grid cap. So yes, there may be a short and likely in the primary of the coil If the secondary was shorted, then this would greatly affect the tuned circuit. I suppose that you can just cheat and run the antenna wire to the 6A8 grid cap.
An alternative would be to use a loop or ferrite loopstick, instead of the antenna coil, thereby eliminating the need for an external antenna. Loopsticks are directional so mount away from the chassis in such a manner as to maximize the signal with the radio in the direction that you desire. In addition, you would have to figure a way to put it in place of only the Broadcast Band section of the coil assembly if you want to use the Shortwave.
Congratulations on your progress!!
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
I took the IF xfmrs out to clean the trimmers. There was a black paper tube over the coil, presumably to insulate the coils from the inside of the IF can. The paper tubes just crumbled to the touch, very brittle. Is there any suggestions as to what to replace these with? Can I just use piece of construction paper or ???
Hi;
That was likely some sort of fish paper that was used over the coils, I think that they were impregnated with rubber, or some other insulative substance. I think you can still buy it from AES, maybe Mouser, but I haven't looked lately. I would not use construction paper, maybe gasket paper or plastic sheet in a pinch, others my have better suggestions.
Regards
Arran
A mylar sheet may work. I do not expect that much heat is dissipated by these coils. Total cathode current for a 6A8 is 13mA, total cathode current for a 6K7 is 6mA. Voltage drop across the transformer primaries would be negligible, given the resistance of the coils. Worst case scenario would be clear postal tape or electrical tape lining the inside of the can.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
Fishpaper.
The only thing, it is a bit rigid, so you might want to form it, and watch the fine wires so it does not try to tension them when trying to straighten up. Maybe even glue the seam after forming.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
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