Philco Model 91 - Printable Version
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RE: Philco Model 91 - Radioroslyn - 07-15-2022
I need a little clarification. Are we talking about #2 on the diagram? It should have 5 connections. The coil on the top is the primary and has the 10,000 ohm resistor in parallel with it. One end is grounded the other the ant. The larger part of the coil is the secondary which consists of the 2 end connections and a tap. 5 total for the coil, I have no idea about pin numbers as they are not noted on the diagram.
Top of the winding goes to the stator of the tuning capacitor. Tap connects to grid cap of the 39/44. Bottom of the winding goes to bakelite block #6 .05 and #3 2 Megohm resistor.
The .05mf cap provides a rf ground for the coil (not a dc ground). Without this the secondary has no "ground" return. The 2 Meg resistor supplies the avc voltage thru coil to the control grid of the rf amplifier tube.
If your saying that the primary is bad that is simple enough to rewind. Remove old wire and rewind with 25' of 40G magnet wire.
RE: Philco Model 91 - texan george - 07-16-2022
Terry,
That is what I was referring to I rewound the coil at the top and hooked one side to the antenna/10k resistor and the other goes to item 6 and 3.
According to the schematic on service bulletin 129-B, there is another tap on the secondary that went to the on-off-freq. switch. That tap had the wire cut off. My radio did not have a three-position switch, just on-off.
Also found open primary on the det. coil item 12 which I will rewind later today. I have checked the osc coil and it seems to be good, and I am checking the 1st and 2nd IF. I think I need to get better at checking the whole radio before starting, instead of just jumping in.
I appreciate your help
George
RE: Philco Model 91 - texan george - 07-17-2022
Rewound the Det coil and still no reception. Checking more things, I found that the section of the tuning cap that goes to the Det coil had 7.2 ohms resistance between the stator and rotor. No evidence of fins touching, and the resistance is constant thru ought the travel of the rotor. That doesn't seem right to me but have no explanation for it. Any ideas?
Thanks,
George
RE: Philco Model 91 - Ron Ramirez - 07-17-2022
Yes - you are actually measuring the secondary of the converter (RF) coil, part (12). This is connected between the 36 grid and ground, as is the section of the tuning condenser which you referenced. The resistance you read sounds about right to me.
RE: Philco Model 91 - MrFixr55 - 07-17-2022
Hi George,
Are you checking the tuning condenser "in circuit" (connected to the osc. coil)? If so, the trimmer condenser marked 23 on the schematic (Between the osc. coil and ground) is shorted or very leaky. Check C18 also. If out of circuit, then the trimmer (labeled 27 on the schematic) that is part of the tuning condenser may be shorted or very leaky. The plates of the tuning condenser develop corrosion that are almost like "stalactites". These will also short the condenser, however, not when the condenser is set for no plate mesh (Highest frequency on the band you are using). In addition, the resistance reading will usually change drastically when the condenser is rotated between full mesh and no mesh.
In addition to repair and restoration, consider adding a "radio - phono switch". That push pull amp with 42 Tubes will really kick @$$ when fed by an MP3 player or computer playing or streaming big band music or old time radio programs!
Good luck!!
John, MrFixr55
RE: Philco Model 91 - MrFixr55 - 07-17-2022
Oops, sorry, I stepped on Ron Ramirez. Sorry, Ron.
George, are you measuring the section of the tuning condenser that goes to the grid of the 1st detector or are you measuring the oscillator section of the tuning condenser? If measuring the grid circuit, yes, you are measuring the secondary of the RF output transformer. If measuring the oscillator section of the tuning condenser, it should read open circuit even if "in circuit", and my immediate previous post applies.
Another Phorum member had a similar issue (dead oscillator) that was caused by leakage in the oscillator circuit of the tuning condenser. Out of circuit, the condenser measured in the 10s of K Ohms between rotor and stator.
RE: Philco Model 91 - Ron Ramirez - 07-17-2022
MrFixr - No worries.
It sounds like George is describing the converter or "RF" coil stage, just prior to the 36 autodyne converter-oscillator tube.
RE: Philco Model 91 - texan george - 07-18-2022
Thanks to Ron and MFixr for your responses. Ron was right, that's what I was measuring. As for today I checked some voltages and here is what I got. It didn't seem to matter is I went to chassis or the filament. RF tube- plate 224, Screen 71 control 2, cathode 1.6. Det. tube- plate 232 screen 70 control grid 9 cathode 9.28. IF tube- plate 223, screen 68 control 2, cathode 1.7.
I also hooked up my signal generator to the control grid clip of the det-osc tube at 260 kHz and the dial at 55 and got the tone thru the speaker. I then hooked up the sig gen to the antenna connection at 1400 kHz and the dial at 1400 and got the tone.
Not sure what that tells me other than some of the voltages don't match the chart. Still working on it.
Thanks,
George
RE: Philco Model 91 - MrFixr55 - 07-18-2022
Hi George,
The plot thickens. The good news is that your "Autodyne" 1st Detector appears to be working unless the output of your signal generator is sufficient to blast through. If you set the generator to 1400KHz and tune above and below 1400 KHZ and the signal fades, that is a good sign. If you have another radio, place it near the 91, set its tuner to 1000 KHZ. If that radio starts squealing as you tune the Philco between 700 and 800 KHz, then the Philco's oscillator is definitely working.
A nice divide and conquer procedure is to connect a long wire antenna directly to the 1st Detector grid and if you can tune stations, then you merely need to troubleshoot the RF Section. Disconnect the long wire from the 1st Detector Grid cap and connect to the RF grid Cap. If the stations are received even louder than when the Long Wire was connected to the 1st Det grid cap, then troubleshot the antenna circuit.
Voltage-wise, the 1st Detector voltages compare fairly well to those listed in the Schematic. a 10 - 15 or even 20 % difference in plate and screen voltages is not USUALLY fatal. The RF Section is a bit of a puzzler. Both the RF and IF Amp Tubes are '44s and share the same cathode resistor and bypass condenser. I don't know why the cathode voltage would be as high as the schematic states that it should be for the RF tube when it is much lower on the IF Amp tube and both cathodes connect to the same resistor and bypass capacitor. Schematic calls for 25V for RF Amp cathode, but 5V for the IF Tube cathode. Also don't understand why the screen voltage for the RF amp is listed as 50V when the screens for the RF, 1st Detector and IF Tube are fed by the same resistor and bypass capacitor. The only difference resides in the fact that the readings are listed as to cathode and the RF cathode reading is low, but circle back to the beginning of this paragraph and scratch your head like I am.
Best Regards and good luck.
RE: Philco Model 91 - texan george - 07-19-2022
I now have a very feint, very distorted reception on the upper end of the dial. I didn't understand the schematic where the wire from the cathode on the 36 tube went thru a coil to the junction of the cap/resistor items 20 and 21. On the radio wire from the cathode went to a terminal on the osc. coil, and the wire from the cap/resistor went to another terminal on the osc coil. That small coil was open, so I rewound that part. So maybe on the road to something.
George
RE: Philco Model 91 - texan george - 07-25-2022
After many hours going over oscillator problems on the phorum, and many rewinding tries of the oscillator coil I have decent reception across the dial. Also tried different values of resistors for item 21. Was 10k but worked my way down to 5.1 k ohms. Next items are to restring the dial. The shadow meter starts out with thin line upon power up then gets thicker but doesn't change much when tuning. I will look at the info on the phorum for that. Just happy with reception, I was getting frustrated and walked away for a few days but can't stay away long. The answer is there, just have to find it. Thanks to all
George
RE: Philco Model 91 - texan george - 08-10-2022
Finally got back on the radio, dial restringing was fun
. Started on the cabinet. Everything was looking fairly good until I applied the second coat of clear gloss. As it dried, I got white streaks in several places. Today I used 50/50 lacquer thinner and acetone to wipe off the bad spots. What would have caused the streaks? The stripper I used was citristrip and stripper wash. Did I not get it clean enough?George
RE: Philco Model 91 - RodB - 08-10-2022
Most often this is caused by humidity when applying the lacquer. The lacquer picks up moisture and it affects the finish with white marks. I don't use lacquer any time the humidity is above 50 percent.
RE: Philco Model 91 - Radioroslyn - 08-10-2022
If it is moisture Mohawk sells a product called No Blush. It's designed to remove the trapped moisture that is in the lacquer. As Rod said keep an eye on the temp and humidity. General rule is don't use lacquer below 60 deg or with the hum above 60 percent.
RE: Philco Model 91 - texan george - 08-11-2022
Thanks Terry and Rod. Come to think of it I was doing it in my garage, and it was raining. Good old Gulf Coast weather. Waiting on some more Mohawk products and for the weather to dry up. Good to know about the humidity which is high here.
George