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Philco 91-121 sensitivity - Printable Version

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Philco 91-121 sensitivity - grandntl - 10-05-2017

Hi everyone, I have a Philco 91-121 that has the electrolytic and bakelite block capacitors replaced along with many new resistors; I bought it that way. ( The blocks were bypassed so I can see the capacitor values).  My signal generator is not currently working so I  found that the oscillator was running at 268 khz above  a received station using a digital radio and I peaked the IF transformers for maximum narrowing of the working shadow meter on several stations . All seemed well except that when I removed the one piece tube shield all stations came in significantly stronger with no squeals etc. With the shield in place and I touch the grid cap of the 36 detector/oscillator the reception also improves. What is causing this and thanks in advance for responding.
Bob

RE: Philco 91-121 sensitivity - Ron Ramirez - 10-05-2017

Hi Bob,

Most likely a bad RF coil. Check the RF amp plate to see if it has voltage. If not...you've found the culprit (the coil connected to the plate of the RF amp tube). Icon_smile

If the RF amp tube does have plate voltage, then the antenna coil is likely faulty (open winding).

The IF of the 91 is 260 kc so you're in the ballpark.

RE: Philco 91-121 sensitivity - Radioroslyn - 10-05-2017

+1
Hi Bob,
You can test the ant coil primary (this is the section that is bad most of the time) by measuring the resistance from the ant to the gnd post If it's good you should see about 25ohms. Much higher is a problem. Some of these eary sets have a small coil that is mounted inside the top of coil form. Have seen these broken wire that was a simple repair.

GL

RE: Philco 91-121 sensitivity - grandntl - 10-06-2017

Thanks, Ron and Terry, for your help; I checked the DC voltage on the plate of the 44 RF tube, it's 272v on a strong signal and 260v between stations with motorboating when the probe is in place. I used the chassis and electrolytic capacitor negative as ground points in separate tests. I double checked to make sure I had the right tube pin and checked the plate voltage on the 36 detector/oscillator(272v) and 44 IF (250) . Looks like the RF and detector/oscillator plate voltages are high.
Terry, I checked the resistance between ant and gnd posts... 22 ohms. It looks like the coils are okay. What should happen when the tube shield is removed? I'm sure the radio should not play better after doing it, I would expect squeals etc.
Bob

RE: Philco 91-121 sensitivity - Radioroslyn - 10-06-2017

Hi Bob,
>With the shield in place and I touch the grid cap of the 36 detector/oscillator the reception also improves.
Forget to ask is this with an antenna connected?? Does the reception get much better when you touch the cap of the rf vamp tube??
> I used the chassis and electrolytic capacitor negative as ground points in separate tests. 
Should use chassis gnd for voltage measurements. When you use the - side of the filter in adds any negative bias voltage to the total reading so it can be confusing. Also your modern meter has a very high input impedance so it doesn't load the circuit like the 1930's meter that has 1 or 2K input.
On your AA5 sets the chassis is floating, the - side of the power supply isn't tied directly to the chassis so in this case you would use the - side of the filter to test voltages.
> What should happen when the tube shield is removed?
Best case is that it won't have any effect worst case it will squeal like mad and tuning the tuning cap may cause some changes better/worst.

My other thought is at perhaps #5 and #15 are out of alignment. These would be the other two trimmers on the tuning capacitor, the third one tunes the osc section. Set your dial at 1480kc and an antenna connected you should hear WDAS. If not adjust the osc trimmer till 'DAS is at 1480. Next adjust 5 and 15 for best reception (loudest). See if that helps.

RE: Philco 91-121 sensitivity - grandntl - 10-06-2017

Terry, I relied on the chassis gnd for my numbers, just tried the cap neg for comparison; my antenna was always connected and I checked the 3 tuning cap trimmers again (yep WDAS 1480). With the shield off, touching the 44 RF amp tube cap results in a slight reduction in reception, touching the 36 detector/oscillator cap results in a noticeable increase and touching the 44 IF cap eliminates the signal. Is all this as it should be? For now I will leave the tube shield off and enjoy the good sound, but something still needs fixing (or not)!
Bob

RE: Philco 91-121 sensitivity - Radioroslyn - 10-06-2017

Ok so what happens if you have the shield on and the ant off is the signal stronger if you touch the mixer cap or the rf amp cap??
Two other thing come to mind, maybe your 39/44 is flat and what kind of voltage do you have on pin 3 and pin 4.
Typically w/o the ant connected reception should be the loudest when touching the rf amp grid cap. A slight decrease when touching the mixer cap and should go dead (most of the time) when touching the if amp grid cap.

RE: Philco 91-121 sensitivity - grandntl - 10-06-2017

With the shield on and antenna disconnected the signal is a lot stronger when touching the 36 cap; the signal is as strong when touching the 44 RF cap as it is when touching the ant connection but a lot weaker than the 36 cap touch. I exchanged the 39/44 RF tube with two spares.... no change in signal strength. The cap replacement looks professional to me but I suppose I should review it ...starting where?

RE: Philco 91-121 sensitivity - Radioroslyn - 10-06-2017

Voltages??

RE: Philco 91-121 sensitivity - grandntl - 10-06-2017

Here are the voltages: 44 RF, K=27, F=3.1AC?, F=3.1AC?, P=264, SG=96 the top cap,-1.5 36 tube, SG=96,K=8, F=3.1AC, F=3.1AC, P=267 and the top cap= millivolts(varying). I used these letters instead of tube numbers because I followed a socket data diagram. I have an older Radio Shack digital multimeter. The AC volts were from pin to ground; a watt meter shows 97 but that probably isn't relevant.
Bob

RE: Philco 91-121 sensitivity - Radioroslyn - 10-06-2017

Hmmmm
I did find something that doesn't make any sense on the schematic. It shows the cathode of the rf amp and if tied together but if you look at the voltage chart there at two different voltage [Image: http://philcoradio.com/phorum/images/smilies/icon_problem.gif]. Have a look at the resistor in #9 to see if it has gone up in value. 20 or 25v is ok but it could be a little lower, this way the tube will produce more gain. Unless Philco had a bulletin and split the two apart and added a second resistor the if amp K is going to higher than is listed too.

RE: Philco 91-121 sensitivity - grandntl - 10-27-2017

I resumed my search for the sensitivity cause and found that the on/off switch has three positions with two switches. One is the on off switch, the other identical to the on off switch, makes or breaks the connection between the cathodes of the 44 RF and 44 IF tubes being connected to the part no. 9 capacitor/resistor in between. The control toggles the two switches; far left is off for both, middle only turns on the power and far right activates the cathode connecting switch. This surely looks like a factory installation and all of my problems stemmed from only turning the switch one step. This must have been a "local.. distance" option. Thanks Terry and Ron for your help.
Bob

RE: Philco 91-121 sensitivity - grandntl - 10-28-2017

This radio 91, code 121, is broadcast only. The schematic doesn't show any 2 part switch but the 91-126 ( BC Police Air) schematic shows a two part (no. 7) on off/frequency switch. Did all model 91-121 radios use this two part switch assembly with the second part used in the RF IF capacitor/resistor circuit?
Bob

RE: Philco 91-121 sensitivity - grandntl - 10-29-2017

Terry, my finding of this extra switch ties in with your comment about the 44RF and 44IF cathodes being tied together on the schematic but having different voltages listed in the tube socket data. Here is what I found; the cathode voltage on the 44RF is 32v with the switch off, 6 with it on. The 44 IF voltage is 6v with it off or on. Again this switch looks factory (same as the 91-126 band switch?) and the wiring is black cloth covered and looks old. The switch on position (far right) makes the radio"come alive".
Bob

RE: Philco 91-121 sensitivity - Radioroslyn - 10-29-2017

At +32 v on the cathodes the tube aren't going to conduct much. I've had two model 91's a 91B and a LZX but it was many yrs ago like in the late 80's. Don't remember a local /distant sw position, pretty sure both of mine were bc only models.