My repair efforts on my 1936 Philco Cathedral Radio (I think Model 60B) have run into a snag and I’m seeking advice from anybody on the Philco Phorum on a possible cause and fix. After re-capping most of the paper capacitors I found in the chassis and cleaning and lubricating the variable capacitor the set runs but the sensitivity is very low. However, when I put my finger on the grid cap of the 6A7 pentode the set come to life with a big jump in sensitivity! I changed out the 6A7 tube for two other 6A7 tubes I have on hand and it makes no difference, weak reception until I put my finger on the grid cap of the 6A7 tube. I also would appreciate it if someone would point me to the correct schematics for the 1936 Philco Model 60B and its component specs.
Please take a look at the video at this link:
https://youtu.be/Dj54lcvULMA
I would like to get some opinions on where the problem is (or at least may be). If you may have any suggestions as to what might be the cause and what I may need to do to fix it, please let me know!
P.S. Unfortunately I don’t have an extensive background in electronics but I’m trying to learn. I also don’t have an extensive set of testing tools /scopes for radio repair but I do have the basic VOM, soldering irons, and hand tools. Of course, I’m open for suggestions on what other testing tools or equipment I may need to buy (without breaking the bank)!
Welcome to the Phorum. So, by touching the 6A7 cap and getting good reception you have proven that the 6A7 is doing it's job. Since you fed a radio signal into the 6A7 in place of the antenna, that's where to look next. Check the continuity in the antenna transformer (part #4) and make sure the band switch contacts are clean.
Thank you RodB. Can you point me to a copy of the schematics so I can locate the "part#4". I assume it's inside the can shielding where the lead going to the 6A7 grid cap comes from.
In the library section of the main site you will find the service information for Philcos from the beginning right up through 1953. Here is a link to the model 60 for 1936. The B is the cabinet style.
https://philcoradio.com/library/download...l.%201.pdf Hope this helps.
Thank you mikethedruid! This will certainly help!
Hi Bill and welcome,
You don't a whole bunch of equipment to do the service work on these old guys. Aside from the normal stuff a signal is very useful to do a proper alignment and testing to see what rf/IF stage is working.
Quick service note on the 60. When installing the tube shield on the 75 tube be sure to keep the grid wire inside the shield. If not it will squeal !!!!!
Here's a little more info on the '60 and you don't have to flip the schematic around.
https://philcoradio.com/library/download...20164A.pdf
GLOM
de
N3GTE
Bill, if you haven't already, make sure you replace the coupling cap between the 75 tube plate and the 42 output tube grid. It is part of the bakelite encased cap marked #33 on the schematic and chassis layout drawing.
Any leakage through this cap will throw off the 42 grid bias, making it less negative and causing the plate current to increase. This can cause a runaway condition which can take out the tube, output transformer and power supply.
As to the sensitivity problem, probably an open winding in the antenna coil. It is the one in the can where the 6A7 grid cap wire comes from. This is a fairly common problem and usually results from a nearby lightning hit discharging through the antenna coil primary winding.
Good luck,
Mike, WA2YGA
Dear Terry,
Thank you very much for the link to the Philco 60 service page. This is so very much clearer that the one in Part 39 Vol 1.
AND :
Dear Mike,
Thank you so much for the heads up on the the coupling cap between the 75 tube plate and the 42 output tube grid. There must be a way to go across this #33 bakelite encased cap without digging it out?
Kind Regards, Bill
You could drill out the connections at the eyelets but NOT THE EYELETS on the blocks where the capacitor's wires come thru and are soldered to the terminal. If you are going to that it's not much more work to clean out the inside and replace the caps therein.
I use a heat gun w/the block unbolted to heat the sides an bottom. Use a needle nose pliers in the screw hole and a small jewelers screwdrivers in the center eyelet. I can usually push the whole mess all together.
Digging and chipping at it can result in cracking or breaking the block and it's messy. Lots of little specs of tar to cleanup.
Most of the blocks will have one cap or two caps of the same value. In the case of #33 there two of differing values. If the get them switched in the wrong order the audio will be level low and sound muffled. Nothing bad will happen but it may take a little time to troubleshoot if you didn't know what to look for. Don't ask me how I know...
Thanks Terry. The use of a heat gun to remove the whole block sounds like the way to go! Thanks again.
Bill
Too much heat gun can be overkill. Just enough to loosen the tar on the sides of the bakelite block so it slides out is enough. You do need to also snip (or drill) the wire coming through the eyelet as Terry mentioned. Here's a link to a how to in our library.
https://philcoradio.com/library/index.ph...soldering/
Bob, Wow, Nice illustrated step by step instructions! I really appreciate it! Thanks for the Link!
Bill
morzh put the tutorial together. Folks forget that our library has a wealth of tips, service info, history and more!
Dear RonB, Terry, Mike and Bob,
I can't thank you guys enough for all the good help and advice. Before removing the shields on the antenna coil (part#4) I took a hard, close look at the band switch as was originally pointed out by RonB. Oh boy, every contact was rusty and some had green deposits on them, I presume from some of the copper in the brass alloy contacts has been leaching out and oxidizing. I uses a very fine sand paper that I sliced into fine strips. After sanding followed by a little Deoxit 5 on a q-Tip the AM broadcast band came to life. The short wave band required further cleaning with both fine sandpaper and Deoxit and working back and forth several times, but finally that band came to life as well. I will post a video of the cleaning and the results as soon as I can put it together from the four video clips I took while I worked on it.
Again, I can't thank you guys enough.
73's Bill
P.S. I have a 1942 Philco Counsel with (the FM band in a different place) push buttons that I need to work on next.
Switch contacts, usually one of my first items attended to.