Recently finished repairing a Philco 84b. Had to replace all the capacitors, resistors, and wiring as it was all in pretty bad shape. Surprisingly, all the coils testing good and no signs of the green disease typical of these units. The first time I powered it on, it gave an instantaneous humming. Tracked that down to a poor ground connection on the #6 pin on the 77 tube for the 2nd IF and fixed it with a short jumper wire soldered to the chassis.
Now, I'm having a heck of a time with a very weak volume control. I can tune in a few stations, but get a really weak volume. When I hover my hand over or touch the cap on the 1st Det/Osc tube, the volume jumps right up. Same thing happens when I touch a metal probe to the #1 and #2 tabs on the bottom of the antenna coil (one coming in from the volume control, the other going out to the tuning condenser). Pulled the antenna coil a second time to double check for continuity and measure the resistance. Measured 3.0 and 6.4 ohms respectively. Went back through and double checked solder connections and looked for any inadvertent groundings taking place and didn't find any issues.
At this point, I'm completely baffled. Any help would be greatly appreciated.
Attached are the schematics and a link to short video showing the issue.
https://www.youtube.com/watch?v=aR-IPtOLr88
Maybe C3 leaking / shorted? It appears to be a "gimmick" (2 insulated wires twisted together., but not shorted.) Make sure that only the insulated wires are twisted but that the conductors don't touch. I am not sure why it is there, but it is in the 37-84, a set similar to yours except that it uses Octal tubes. It is not in the 80 and 81 which use a 36 Tetrode as an Autodyne Osc / Mixer, nor in the 89 version that uses a 77 Osc Mixer like the 84. If the 2 conductors of this twisted pair touch, it essentially shorts the antenna to the cathode of the tube.
That was one of my lines of thinking, but I don't know much about gimmick capacitors. When I rewired the chassis I tried to get the gimmick to look the same. I've attached before and after pics. As you can can see, only one end of the two wires terminates to a dead end. I guess I can experiment with the length of the gimmick and the number of twists and see if that makes a difference. I also used 20 awg wire. Could that affect how the gimmick works?
Check #20. When replaced the caps inside if you got them switch around it will couple very little audio and it will be muffled. Don't ask me how I know...
GL
Checked #20 again. The .001 cap goes from pin 1 to 3, and the .015 cap goes from pins 2 to 3 per the schematic for the Bakelite block model listed on this site. I'm going to ohm out the oscillator coil next where the gimmick connects on a hunch that the coil may have opened.
Have you done an IF alignment? And does it have it's fiber sensitivity adjustment nut? The IF alignment can be done without the ant or the osc circuits working properly. Just have to get some 460kc signal onto the grid of the 77 mixer tube.
Actually, I haven't started the alignment yet. Usually, I have to troubleshoot any issues after working under the chassis, but from what I've been reading the past day, as this is a "regenerative" radio, the alignment appears to be connected to the strength of the volume on this set. I don't have a signal generator, but I do have a variable AM transmitter. I think it starts at around 525 kHz. May not be the best option, but I'll see if it helps improve the volume output.
Thanks for the suggestion.
Update: Completed the alignment and amazingly the volume control works now. Thanks to all of you who helped resolve this issue. Had to actually align the set twice to get the maximum volume output, but the volume output now is substantially better. Nothing better to help the learning process than having to solve a problem. Thanks again!
Congrats, PercyJD!!
Sorry, I didn't think alignment because when you touched the grid, you got good volume. This is a deceptively simple set, and when I ran into my first 80 (the predecessor to your 84), it messed up my head because there was no IF Stage. Do you know which adjustment did the most good? Was it C5, C13, C15 or C17? I would think C5, but in an Autodyne, anything is possible.
I have an RCA R28 ca 1933. At the time, this was RCA's cheapest set and was introduced in the throes of the Great Depression, at a price of $28.50. It, too, has no IF Tube, just the transformer, but has a 58 RF Amp before the 2A7 Pentagrid Convertor, so this is a 5 tube set. When the Philco 80 (similar circuit to your 84 but with tetrodes instead of pentodes for the 1st and 2nd Detectors) was introduced at $18.99, it was meant to be a "loss leader" to get people into the store where the sales rep would attempt to "upsell" the Customer to a better and more expensive model). However, the set was a hit. RCA had to drop the price of the R28 to $25.99. Philco countered by dropping the cost of the 80 to $14.99. RCA dropped their price too $19.99 and likely lost money on every set sold. Philco kept this model in one form or another for an additional 4 years with the 37-84 being the last iteration and sold a lot of them to the consternation of RCA and several others.
BTW, nice job on the wiring. With cloth covered wiring most leave it in place unless it is deteriorated. The real nightmare for restorers came a few years when rubber wire came out. The rubber insulation almost always deteriorates, cracks and falls off, requiring rewiring.
MrFixr55,
In response to your questions, adjusting the rear tuning capacitors, C7 and C11, initially did the most to improve the volume control. In fact, I ended up having to tighten C11 almost as far down as it would go. C5 also seemed to greatly improve the volume levels as well, but mostly in the upper bands. I actually went through the alignment procedures twice trying to maximize the signal and volume output.
When working on this set, I removed C5, C11, C13, and C17 in order to clean the mica wafers as best as I could. That's probably why adjusting C7 and C11 resulted in the best improvement as I completely loosened the nuts to get under the mica. Didn't have to adjust C13 or C17 much at all.
The volume output still increases when I hover over the 77 tube or touch the cap, but not nearly as much of a difference as their was before the alignment. At least now I don't have to turn the volume control almost to maximum to get anything audible through the speaker except for some of the more distant stations.
For the alignment, I used a small AM transmitter to broadcast a 1 kHz test signal at 600 kHz on the dial which I calibrated using my Kenwood stereo that displays the frequency on a digital readout. Not perfect, but the set seems to pick up stations close to their locations on the dial. I probably pick up 4 stations with only a 3ft antenna.
I want to thank you and others on this site. This forum has been a great well of knowledge. This is only my third radio, so I've got lots to still learn. A Philco 41-608 was my first set, and it didn't give me any problems after replacing the caps, resistors, and some of the wiring. Still have the Beam of Light tonearm to tackle, but the radio portion works great. The 84b looked like it would be a lot simpler to fix, but that turned out be more work...but more rewarding too.
And thanks for the compliment on the rewiring. So much of the wiring was already deteriorating that it was simpler to just tackle it all at once instead of having to go back and fix something else down the road. Besides, it's been very helpful in learning to read the schematics and understanding the operation of the radios.
Jim
Hello percyjd ,
Nice Job glad you got your issue fixed yes good old alighment I just got done restoring a 1960s RCA am transistor radio and well my next step is alighment .
Sincerely Richard
Jim, you're welcome.
The talent that is available on this site, especially from Ron Ramirez, RadioRoslyn, Paul Philco, Morzh, Aaron, Nathan, RFenstra, Phlogiston and many others (and sorry for not naming them) is unbelievable. Looking through some of the old posts that have pictures of some of their restorations, they are "phantastic".
Yours is a very interesting set. When I first started working a Model 80 (similar to yours), I thought it was a very simple TRF like the Emerson 25, RCA M-17 or other very cheap sets that popped up during the depression. After looking at the schematic, I was surprised to learn that it is a superheterodyne.
The key to the surprising performance of this set is the "regenerative detector". This is an old school grid- leak detector with a feedback circuit which causes a lot of gain. This is the basis for some of the original Armstrong 1914 patent 1 tube radios like the Westinghouse Aeriola Sr, one of the first commercially available "consumer" radios. Adjusting C17 adjusts the detector "gain before oscillation". Consideringthe poor fidelity and high distortion of grid-leak and regenerative detectors, this set sounds surprisingly well!
I find a lot or radios with good reception will get louder when one touches the loop antenna or, in the case of these older ones with grid caps, touching the grid cap. I guess our body mass makes a good tank circuit for the broadcast band. Based on my size, the lower end!
These sets really require a "long line" antenna of at least 25 ft horizontal length, along with a good ground. The antenna should point in the direction or exact 180 degree opposite direction to the desired stations.
Keep up this hobby of preserving the past. It is cheaper than restoring cars (Unless you get into some of the Atwater Kent Breadboards, EH Scotts, Zenith Stratospheres, Radiobars, RCA CT100 Merrill (first RCA Color TV), McIntosh HiFi, etc.)
Hello jim,
Well MrFixr55 said it and that is a nice set that you picked up and for me I really like Collection Philco's Zenith and RCA made some nice radios too and so did Crosley .
Sincerely Richard
Folks, that is not MY Collection, unfortunately. I only wish I could own a CT100, a pre-war TV, Scott or Zenith Stratosphere. Among the more interesting sets I own are:
1922 Westinghouse Aeriola Sr 1922 Westinghouse RC
1923 FADA 160 (Factory) 1924 RCA Radiola III
1923 (4?) Atwater Kent 10C 1924 Atwater Kent 20
1926 Atwater Kent 30 1927 Federal 10
1927 RCA Radiola 16 1927 RCA Radiola 17
1928 RCA Radiola 60 1930 Philco Model 20
1931 GE S22A (RCA R7A) 1933 RCA R28
1933 RCA R-17 1934 Crosley 5C2
1936 Zenith 6S52 Chassis only 1936 RCA 6BT
1937 RCA 5T7 1940 RCA 14BT2
1941 Philco 41-255 1946 Philco 46-350
1948 Zenith 8G005 Trans-Oc. 1948 RCA 1R81 AM-FM
And a host of others, along with TVs from 1946 to present, Fisher, Scott, Heathkit and KLH HiFi.
Personally, I'm most intersted in the 1930s and early 40s sets with wood cabinets. Not too fond of anything in bakelite, although I do rather like the Westinghouse H-188 "Pagoda". My sets so far include:
- Philco 41-608 Beam of Light, probably the one I'm most proud of so far. Found it on the curb and had to refinish the cabinet and replace the zebra wood photo finish. Still have the record player to get up and running as there is not much literature on how to actually replace the photocell in the tonearm. Hopefully I'll get brave enough to tackle it in the near future.
- Philco 84b, which is probably my second favorite now that it's operating again. I'll post a pic once the cabinet is refinished.
- Westinghouse WR-258 which needs a new transformer, but I love the look of the wooden case.
- Crosley 66TA American Overseas, which was actually my first tube radio; picked it up at an antique store - was already repaired.
I'd like to find a Wards Airline model with the "green eye" and probably a Silvertone with an Art Deco design. An Atwater Kent would also be nice at some point.
Not interested in acquiring any that have already been refurbished as my interest lies in the challenge and satisfaction of repairing a set. And no more big floor consoles as my wife is already not happy with the four radios and three phonographs (Columbia, Silvertone, Edison) around the house, almost of of which I've repaired/refurbished. Says our house is looking like a museum or antique store, which of course I'm totally fine with.
Thanks again for all the information and insight.
Jim