What looked like a routine restoration on a Philco 84b has gone wrong; not because of a lack of information, but an abundance, with lots of contradictions. Had it working briefly with new filter caps and rebuilt 28 and 9 capacitor blocks. Between the 24 and 29 blocks something happened. That’s when I dug myself deeper between schematic discrepancies and dumb mistakes. The radio is silent and drawing.5 amps at 90 volts. I’ve checked out a half dozen wiring photos and no two are the same. Any past experience or plan of action to move forward will be appreciated. Thanks! Rob Q.
Hi Rob and Welcome,
Let me pull up the diagram so we're on the same page.
https://philcoradio.com/library/download...l.%201.pdf
Ok so 28 is the bias filter for the - voltage going to the 42 and 9 the the screen bypass for the 77 mixer. When you say it was working was it receiving stations? And all was well? At this point it seem as though you have a short loading down the HV. #28 & #9 are kinda unlikely as if the bias voltage was gone I don't think it's would cause the 42 to pull down the HV that much. #9 has a fair amount of series resistance connected to it don't think that's it either. Thinking #29 could be it if the 2nd cap is wired backwards. Either or both could have too low of a voltage rating (450v).
One way to troubleshoot it would be to set your variac to 90v or so. Connect a DC voltmeter + from one of the filament pins on the 80 tube and the - to the chassis. Turn the set on for only abt 4 or 5 seconds. What you should see from the time you first snap it on is the dc voltage go up and go to abt 250 - 325v or so. It may start to drop down a bit this is ok.This is good. If it only goes up to 25 - 100v in the 1st 4 or 5 seconds that's bad. It's an indication of a short in the hv line pretty close to the rectifier tube.
A set left on in that condition the both plates in the 80 will start to glow red. Then sparks will start to fly around inside the tube. The plates melt and short against the filament. This in turn shorts the hv winding in the power transformer. After that hot tar and smoke. Need to sort it out before some bad happens.
GL
Thanks GL for your quick reply! At 90 v. getting 343 v. between 80 filament and ground. 4 MFD going to field coil #27, pin 2 of cap block #24, and 16k resistor #12.
8 MFD going to field coil #27 and pin 1 of 80 tube. Attached is a photo of the chassis. We appear to be referring to the same schematic so that should help. Thoughts? - Rob Q.
Hi Rob,
First off, in addition to looking at the Rectifier plates, watch the '42 plate and screen grid for glow. If the plate glows red or if the Screen grid glows, shut down immediately. If the output transformer is open or the connection is broken, the screen grid will draw enough current to glow bright yellow, burn up the screen grid and short the tube. Check all 3 wires between the radio and the speaker (42 plate, and both field connections), especially if you separated the radio from the speaker. The wire from the '42 plate goes to the transformer, the connection that is common to the transformer and the field goes to the 4uF cap connection of 29 and the other field connection goes to the junction of the 8uF cap connection of 29 and the '80 filament
The draw of 0.5A is likely good and not excessive. 343V between chassis ground and Filament of the '80 is an indication of low current draw. Check between the negative terminal of Cap 29 (the power supply filters and each "+ Terminal+ of the cap. The 8uF cap will likely read the 343V that you saw or higher, because there is a 325 Ohm resistor between the negative return of the B Supply and the Chassis, mainly for cathode bias of the '42 Audio Output tube. If there is current draw, the voltage between the negative terminal of 29 and the 4 UF terminal will be lower, usually in the order of 250V. If the 2 voltage readings are the same, the output transformer may be open.
If the voltage is zero, either the field is open or there is a short totally loading down the B+ and a 450 some odd volt drop across the field coil. This would cause a strong magnetic field to be developed at the field pole of the speaker.
If Cap 24 is shorted, that would kill the audio output and cause all B+ to drop through the field coil and output transformer. A hum would likely be heard, however.
In Summary, connect the negative of your DVM to the junction of the B- of the power transformer, the common negative of C29 and R30. Note that this not the same as chassis ground. With the positive lead of the meter, check the following points for the following voltages (all voltages are +/- 20%, especially with the Line voltage being 90V):
8uF terminal of C29: 350V
4uF terminal of C29: 250V
Plate of '42: 235V
Chassis ground 15V
Let us know the results.
Best Regards, and a Blessed Thanksgiving,
John, MrFixr
Thanks John- I’ll run through your voltage check as soon as I address one more thing i.e. the routing of pins 3 and 4 of the 42 tube socket. What do they connect to?
Two mistakes that I’ve found and corrected consist of an incorrect value cap in the #20 block, and I initially noted the discrepancy in the #24 cap block (uses pins 2-3 instead of 1-3) but then forgot when soldering it back in. Certainly enough to foul things up. We might almost be there. Thanks again guys. - Rob Q.
The pin out on the 42 is #1 heater #2 plate #3 screen #4 control grid #5 cathode #6 heater.
Pin 3 connects to the speaker harness (bottom of output transformer). Pin 4 connects to #20. If you wire 20 internally backwards you will get low level and muffled audio.
Btw GL is for good luck and name Terry. Oh when you get a minute snap a pic a the front of the cabinet. I'm curious what generation your set is, there's several.
Hi Terry- Made connection as per your information and the audio end of the radio is working, drawing.4 amps at 110 v. Touching the cap on the first 77 tube brings in our local AM station but nothing otherwise. I’ve rewound two of the coils twice but they may still be wrong. Thoughts? Thanks!! - Rob
P.S. - I’ll get you a picture when the cabinet returns. My friend, the owner of the radio, is working on it.
P.PS. - If you want to talk radios sometime I’m in your neck of the woods when I run trains at the Colebrookdale Railroad between Boyertown and Pottstown.
Connect your generator to the grid cap of the 1st 77 & gnd. Set it to 460kc with a modulated tone. Use an insulated tool to peak #s 11 and 15. # 11 has hv on it. As the signal gets stronger back down the output from the generator so you don't overload he detector. Next adjust the sensitivity cap #17 till the the IF starts to oscillates then back it out 1/4 of a turn or a little less for sharper selectivity.
Two other stages are suspect the local oscillator (lo) and the antenna input circuit.
To check the lo use a small radio with a loop ant near the 1st 77 tube. Tune the small set 460kc higher than the dial on the 84. If a is well you should hear a hiss or a squeal in the small set. For instance tune the 84 to 100 (1000kc) in listen in the small set while you are holding near the 77 to 1460kc. You should be able to track it all the way down to 540kc by tuning both sets. If it's not oscillating most of the time it's the feedback winding on the coil has rotted (green dots). If it has been rewound and is not working it's been wound in the wrong direction or the wrong # of turns.
The ant circuit is pretty simple. There must be a short or open in the primary of the ant coil or the volume control/wiring. Unplug the set. turn he volume full up and measure the resistance from the ant post to ground post. Should see about a few ohms. If you see a high resistance there's an issue with the coil/winding being open. If it's lower than an ohm or so I'd look for a short.
Working backwards from your list I started with the antenna coil and couldn’t get a consistent reading. I pulled out the coil and after polishing the ground and pins I got 101 ohms pin 1 to ground & 6.3 ohms pin 2 to ground. Are these numbers in the ballpark?
For anyone restoring a Philco 84 it’s possible to get 2 of the 4 capacitor blocks (7625H & 7762B) wrong when referring to Service Bulletin #289. What I’ve learned is to examine the blocks before digging in to see if they match the bulletin. Also, feel free to consult those on the Phorum who’ve successfully tackled the restoration. …….just sayin’.
- Rob Q.
101 ohms is too high and indicates a bad winding. I'm assuming it's the primary as there is no marking on the diagram as to pin numbers. That's the small winding on lower outside of the coil. Rewind with #38ga wire.
Thanks! I had to order 38 AWG wire so I’ll put things on hold till then. Should I also rewind the oscillator tickler while I’m at it? I’m assuming that the answer will be “yes”.
Is this radio so sensitive because there’s not much to it? Every radio seems to have its own personality. - Rob
Does the osc run and track across the whole band 460kc above the dial setting. Does the resistance seem good. And with a close inspection are there any green dots.
When I first pulled the oscillator there was enough green that I unwound the tickler coil and rewound it with 34 AWG wire. In fact I did it twice and never got the results I wanted.
When I did the oscillator on a Philco 60b I used the same gauge wire (34) and got it working right away. I just followed Crist Rigotti’s posts and the restoration went smoothly. Not so with this bugger.
I’ll get back to the oscillator once I get the antenna coil right. Baby steps.
The key thing with rewinding the tickler is the phasing (direction). Must be wound in the same direction as the original. Also not a bad idea to check the 6K cathode resistor. The ant coil isn't near as fussy.