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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Mon 18, 2019 9:57 pm 
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Tom:

I am doubting that what I am hearing in the adjacent radio, which is a very faint noise, is the Philco oscillator. I do not hear any whistle from the signal generator when following your instructions. All I hear is the signal generator tone in the adjacent radio. I have ordered some caps to replace the caps inside Z300 and C301. I will post results when I complete replacement.

In the meantime do you have any additional suggestions.

Thanks, Bob


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Mon 18, 2019 10:19 pm 
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I'm also a bit doubtful. I would expect it to be more like a strong, clean signal, especially if the blue and red wires are shorted (which prevents the PLL from trying to follow noise coming from the IF).

To listen for the whistle and do the zero beating, modulation in the RF generator should be turned off, since you don't want that audio tone interfering with the whistle you're trying to hear. Even with modulation on, however, you probably would have been able to hear a whistle in addition to the modulated tone.

We've got two hints that the FM1000 oscillator isn't running:

1. You were not able to get a reading on your frequency counter when coupled to the plate of the FM1000. (As a sanity check, does your frequency counter work if you connect it to the output of your signal generator?)
2. You're not picking up a clean tone from the oscillator on your nearby SW radio.

Unfortunately I no longer have a radio with an FM1000 in it, or I could try to check exactly what you should be hearing and seeing.

Having a scope would really help. Anyone you can borrow one from?

In any case, trying replacing the caps in this circuit is a reasonable next step. Also check coil continuity for both windings of Z300 if you haven't already.

Do you have a way to test the FM1000 tube, or possibly another one to swap in?

Finally, double check for any wiring errors or solder blobs that may have happened in previous recapping.

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Tue 19, 2019 2:48 am 
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Hi Tom:

I did try to listen for the whistle with the modulation on and off. Heard nothing.

The frequency counter does work when connected to the signal generator. I had it connected when doing the test.

The FM1000 tube is new and it tests good on my military I-177B tube tester.

The coils test good for continuity.

It should be a week or so before I get the caps I ordered. Will post again when I get them installed.

I should look for a working scope. I was given a Textronix 465 but it does not work. Any recommendation on something reasonably priced and easy to use?

Appreciate your help.

Bob


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Tue 19, 2019 2:54 am 
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Here’s something that I learned recently:

When a tube is actually oscillating, rectification of the signal at the control grid will generate a negative DC voltage at the control grid. I use this trick frequently to confirm oscillation in typical converter stages using tubes like the 12BA6.

However, the stray input capacitance of most multimeters and probes, digital or analog, will kill the oscillation and measure 0V if the probe is touched directly to the control grid pin.

The trick is to temporarily connect a 100K resistor to the control grid pin of the tube you want to test for whether it is oscillating or not. The resistor must be soldered to the tube socket pin with a lead length as short as feasible. 1/2” to 1” is OK. But do NOT connect the 100K resistor with a cliplead. That would have too much stray capacitance and might kill the oscillation.

Then connect the + multimeter probe to the free end of the 100K resistor. And - multimeter probe connects to circuit ground, which may or may not be the radio chassis.

The 100K resistor isolates the stray capacitance of the multimeter and the test probe so that it won’t affect the oscillation.

Most oscillator circuits will have -5V to -10V when tested with the 100K series resistor.

For the FM1000 tube, the place to connect the 100K series resistor for making this test is the junction of R306 (22 ohms) and pin 2 of the FM1000 tube. This is labeled as point “F” on the schematic.

-EB

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Tue 19, 2019 5:34 am 
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That's a good idea. Nice way to get the voltage reading without loading down the oscillator and potentially causing it to stop even if it was running before.

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Tue 19, 2019 4:30 pm 
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Tom Albrecht wrote:
That's a good idea. Nice way to get the voltage reading without loading down the oscillator and potentially causing it to stop even if it was running before.
Using a series resistor to connect to the test point is valuable for most DC voltage measurements in RF and IF stages. Reduction of unwanted capacitive loading can make a huge difference.

I believe this to be the reason why tube-era VTVMs always had a series resistor located near the tip of their DC voltage probe. Traditionally this DC probe resistor was 1 megohm and the VTVM was calibrated to account for the voltage drop across the probe resistor.

Nearly all modern digital multimeters have a 10 megohm input resistance. Adding a 100k series resistor at the end of the probe introduces only a 1% error. This is inconsequential for measurements in most vintage radio circuits.

My informal test results show that a 100k ohm series resistor is sufficient to prevent circuit loading. I generally don’t hear any audible change in the sound coming out of the radio when probing with the 100k series resistor. In contrast, probing RF/IF circuits directly with a DVM frequently causes the audio from the radio to drop out almost totally.

-EB

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Wed 20, 2019 1:04 am 
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Tom and EB:

I attached a 100k resistor to pin 2 of the FM 1000 and measured voltage between that point and chassis ground with my DVM. Voltage when a station was tuned in was in the range of -3.0 to -3.2. In between stations the reading was -3.8 to -4.0. Does this indicate the oscillator is not working?

Bob


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Wed 20, 2019 1:30 am 
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rck46 wrote:
Tom and EB:

I attached a 100k resistor to pin 2 of the FM 1000 and measured voltage between that point and chassis ground with my DVM. Voltage when a station was tuned in was in the range of -3.0 to -3.2. In between stations the reading was -3.8 to -4.0. Does this indicate the oscillator is not working?

Bob
This indicates the oscillator is working. If was not, the measurement would be much closer to 0V.

However, the voltage measurement doesn’t tell us anything about the frequency of oscillation.

-EB

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Wed 20, 2019 6:35 am 
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I agree this should indicate the oscillator is running. I wonder if it is quite far off frequency, and that's why you didn't find it on your SW receiver? You might hunt over a little wider range. Still a bit odd that you don't get any reading on your frequency counter if you couple to the plate through a capacitor.

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Wed 20, 2019 7:54 pm 
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Tom Albrecht wrote:
I agree this should indicate the oscillator is running. I wonder if it is quite far off frequency, and that's why you didn't find it on your SW receiver? You might hunt over a little wider range. Still a bit odd that you don't get any reading on your frequency counter if you couple to the plate through a capacitor.
This is one of those projects where an oscilloscope is incredibly useful. Any oscilloscope. It doesn’t need to be fancy or expensive.

-EB

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Thu 21, 2019 2:05 am 
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In response to Tom's comment I went back and coupled the frequency counter to the plate of the FM1000 through a capacitor to check my previous work. I confirmed I get no reading on the frequency counter when a jumper is placed between the blue and red leads of Z300. I discovered when I remove this jumper I do get a reading which was around 8.7 Mhz. I adjusted the trimmer cap so the frequency counter read 9.1 Mhz. I then tried adjusting the slug coil after grounding pin 6 of the FM1000. The frequency counter reading did not change with large adjustments to the slug coil. Still getting distortion on FM.

Bob


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Thu 21, 2019 7:04 am 
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Although it's somewhat counterintuitive, the FM1000 oscillator should be able to run freely with the red and blue leads shorted (this shorts out the "left" coil on the schematic diagram of the two coils in Z300. Since yours doesn't seem to run with those leads shorted, it suggests something is weak or compromised. I'd probably hold off much further work until you get your replacement mica caps. That's what brought mine to life, even though it was not clear what was really wrong with the mica caps that were in there.

Shorting the red and blue leads obviously loads down the oscillator coil a bit, so if gain is marginal, it can stop oscillating. That prevents going through the two steps of the coil alignment separately, and that might well result in a system that cannot lock properly.

Unfortunately, connecting the frequency counter to pin 4 would have relatively little effect on the oscillator with the red and blue leads shorted, but with them unshorted, it will load down the left coil and change its resonance. So whatever alignment you try to do in that configuration is likely to shift badly when the frequency counter is disconnected.

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Sun 24, 2019 9:18 pm 
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I have now completed the replacement of all the caps inside Z300 as well as R300, which measured 9.3k. Also replaced C301, R 301, R306, R302, C305, R304, and R305. I previously had replaced the paper caps C302, C303, C304, C307, and the electrolytic C306. Continuity of L300 was measured at 12 ohms. Continuity of the coils in Z300 measured around one ohm each.

I am getting the same results - no change.

As I was reviewing the above and checking against the circuit diagram I noticed that the TC300 coil may be a 10 ohm coil rather than 1 ohm as I thought previously. Circuit diagram hard to read. Do you suppose this is the source of my problem?

Bob


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Sun 24, 2019 10:29 pm 
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I have the original service manual for this radio, so here's a much nicer scan of the FM1000 demodulator circuit:

Attachment:
Philco 48-482 FM1000 Circuit.jpg
Philco 48-482 FM1000 Circuit.jpg [ 117.96 KiB | Viewed 1837 times ]


The three lines going outside the frame are:

1. Lower right: B+
2. Upper right: audio out
3. Line going to "D": FM IF in

Coil resistances from schematic: Left side of Z300: 1 ohm; right side of Z300: less than 1 ohm.

Here are the FM Alignment notes from the service manual:

Attachment:
Philco 48-482 FM Alignment Notes.jpg
Philco 48-482 FM Alignment Notes.jpg [ 285.71 KiB | Viewed 1837 times ]


Attachment:
Philco 48-482 FM Alignment Chart.jpg
Philco 48-482 FM Alignment Chart.jpg [ 262.71 KiB | Viewed 1837 times ]


Attachment:
Philco 48-482 FM Alignment Locations.jpg
Philco 48-482 FM Alignment Locations.jpg [ 147.98 KiB | Viewed 1837 times ]


And a section on FM troubleshooting for this part of the circuit:

Attachment:
Philco 48-482 FM Troubleshooting 1.jpg
Philco 48-482 FM Troubleshooting 1.jpg [ 436.46 KiB | Viewed 1836 times ]


Attachment:
Philco 48-482 FM Troubleshooting 2.jpg
Philco 48-482 FM Troubleshooting 2.jpg [ 229.92 KiB | Viewed 1836 times ]

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Sun 24, 2019 11:13 pm 
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OK, with all that posted, let's get back to what you are seeing. Basically, it appears your problem is that the FM1000 oscillator won't run with the red and blue wires of Z300 shorted, which is preventing you from doing one of the critical alignment steps (Step 8 on Philco's chart above).

Notice that in the "FM Troubleshooting" section (last two pictures posted above) they use precisely the same method as electricboyo mentioned for checking whether the oscillator is running -- coupling to the oscillator grid through a large value resistor. Philco suggests 50K, but electriboyo's 100K will work just as well (or even a little better). Note that they say you should get 2.5 V negative if the oscillator is running; however this is with a 20,000 ohms-per-volt meter, which should mean that when set to the single digit volts scale, that meter would have had an input resistance of 20K. With 50K in series, it would only be measuring 2/7 of the available voltage, so I think you should get something like 9 volts negative when it is running. If you use a modern meter, you'll see almost the full voltage, even with 100 K in series.

I think you measured a lower voltage, which possibly suggests that your oscillation is weak.

Anyway, the thing to check right now is whether this measurement shows the oscillator to be running with the blue and red leads shorted, or only when they are unshorted. What do you see?

Studying the circuit, I now see that connecting a frequency counter to the plate will not work when the red and blue leads are shorted. Doing so effectively kills any oscillator signal on the plate (even though the oscillator should still be running). That's why it's better to use the DC voltage on the grid as an indicator of the oscillator running.

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Mon 25, 2019 12:44 am 
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Tom:

Thanks for posting that info. Much clearer than what I have.

I checked voltages again with my dvm. Measurements made after the radio was on for about 20 minutes. They are:

Pin 2: -1.0 +or -.3 (measurement made through 100k resistor)
Pin 3: 0
Pin 4: 185
Pin 5: 52
Pin 6: 1.4

Pin 2 voltage is now lower than when previously measured.

I also Checked the FM IF’s again. With pin 2 grounded, a wire loop around the mixer tube and connected to a 9.1 MHz modulated signal I hear loud undistorted tones at 95, 100, 107, and 108 on the FM dial. I hear some faint tones at a few other spots on the dial. I do not hear any tone with the tuning gang completely closed( the point where The Philco alignment instructions say to set the dial). I tried readjusting the trimmers with the dial set at the 95 point on the dial and did not see much improvement. Fairly large changes in some of the trimmer settings makes little or no change in the output. I was using an analog voltmeter across the voice coil to judge loudness.

Bob


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Mon 25, 2019 1:38 am 
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Looks like the FM IF stages prior to the FM1000 are not aligned properly, so I think it's important to get those fixed first.

The IF alignment should not depend on the setting of the tuning dial, so I would definitely set it to the low end of the dial (definitely not one of the places where the tone comes through louder right now).

Ground pin 2 of the FM1000 to turn it into an AM detector. Set your signal generator to exactly 9.1 MHz using your frequency counter (with modulation off), then remove the frequency counter, turn on the modulation, and couple the RF generator to a loop of wire around the 7F8 mixer tube. If you can get at least some signal through (you may have to run the output level on the your signal generator up to its max to hear something), then tune the four FM IF transformers for maximum audio tone. There are 8 trimmers to tune:

C400C and C400D on the 1st IF
C401B and C401C on the 2nd IF
C402C and C402D on the 3rd IF
C403B and C403C on the 4th IF

If these are very far out of alignment, you won't be able to get a signal through at the start. In that case, do it like this, using the weakest signal level from the signal generator that's loud enough to hear, and readjusting at each step to keep the signal weak:

Connect sig gen to pin 6 of 3rd IF tube (7H7) and adjust the trimmers on the 4th IF.
Connect sig gen to pin 6 of 2nd IF tube (7B7) and adjust the trimmers on the 3rd IF.
Connect sig gen to pin 6 of the 1st IF tube (7H7) and adjust trimmers on the 2nd IF.
Connect sig gen to loop of wire around mixer and adjust trimmers on the 1st IF (now your signal is going through the entire IF chain).

When you do this, pay attention to whether each trimmer has a clear peak. If any trimmer fails to show a clear peak as you go through these adjustments, there may be a problem with the IF transformer you're adjusting.

Let us know how this goes, and then we can suggest the next step.

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Mon 25, 2019 1:44 am 
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Quote:
Notice that in the "FM Troubleshooting" section (last two pictures posted above) they use precisely the same method as electricboyo mentioned for checking whether the oscillator is running -- coupling to the oscillator grid through a large value resistor. Philco suggests 50K, but electriboyo's 100K will work just as well (or even a little better). Note that they say you should get 2.5 V negative if the oscillator is running; however this is with a 20,000 ohms-per-volt meter, which should mean that when set to the single digit volts scale, that meter would have had an input resistance of 20K. With 50K in series, it would only be measuring 2/7 of the available voltage, so I think you should get something like 9 volts negative when it is running. If you use a modern meter, you'll see almost the full voltage, even with 100 K in series.
I don't think the negative voltage will be -9V. I calculate that it will be between -3V and -3.25V.

Why?

Answer:
    Service manual says "make this test using 10V range of 20k ohms per volt multimeter." Therefore, the input resistance of the multimeter will be 10 x 20k = 200k ohms when the meter is on the 10V range. The series resistor for the service manual is 50k ohms.
    Using the voltage divider formula for 50k/200k resistors: When the multimeter reads -2.5V, the actual negative grid voltage (FM1000 pin 2) will be -3.125V.
    Because modern digital multimeters typically have an input resistance of 10 megohms on every DC volts range, the voltage loss through the 100k series resistor that I recommend is so small that it can be ignored when using a modern digital multimeter.
So the measurement with a modern digital multimeter should be about -3.1V.

I believe that the left side of coil Z300 is only "loosely coupled" to its right side, which is the oscillator section. Therefore, the negative voltage at pin 2 of the FM1000 should only change a small amount between the two conditions of Z300 red/blue wire shorted vs. red/blue wire open circuit.

Note: If I am reading the schematic and alignment instructions correctly, Z300 blue wire goes to FM1000 pin 4 (plate) [test point E] and Z300 red wire goes to test point B which is also the junction of R302, C304, and C305. Again, whether these two test points are shorted or not shouldn't cause a large change in the negative voltage at pin 2 of the FM1000.

The oscillator section appears to be "Colpitts:"
    2 series capacitors (C300C, C300D) provide the positive feedback that makes this circuit into an oscillator.
    A Colpitts oscillator has one inductor (right side of Z300), which is tuned by variable capacitor C300B.
    All of these components, including the coil itself, must be very close to the schematic values in order to oscillate at the proper frequency of 9.1MHz.

Other critically important components are C301 and L300:
    If C301 is missing, then the signal to FM1000 grid will be greatly attenuated going through R301 (15k) and so the FM1000 might not oscillate.
    If L300 is open there will be no DC current flow through the FM1000, therefore it won't oscillate.
    If L300 is shorted, then there will be no positive feedback signal to C300C/C300D, and once again, the circuit won't oscillate.
    The values of C301 and L300 aren't all that critical, but both components must be present and functional.
    Note: The function of RF choke L300 is to provide a DC path to ground for the cathode of the FM1000 tube, but without attenuating the RF signal at this point. The choke (should) have a very large impedance at 9.1MHz, but a low resistance ( <10 ohms) for DC.

Some other thoughts:
    A good place to monitor the oscillator output is the cathode, pin 3, of the FM1000 tube.
      The alignment instructions don't mention this.
      The amplitude of the RF at the cathode, pin 3, should be roughly in the same range as the RF amplitude at the oscillator control grid, pin 2.
      But it would be difficult to directly measure the RF amplitude at the control grid, pin 2, because this point has a very high impedance.
      That's why the series resistor (50k or 100k) is used to indirectly observe the oscillation amplitude by measuring the negative DC voltage.

    On the other hand, the cathode of a tube always has a relatively low output impedance.
      I think that a 10:1 scope probe could be connected here without upsetting the functionality of the oscillator.
      I suggest that both the oscillator frequency and the oscillation amplitude can be measured at the cathode.

    BTW, I normally use a standard 10:1 scope probe to feed signal into my frequency counter. The purpose of the 10:1 probe is to reduce loading on the circuit being tested. Most counters are sensitive enough to respond to a signal which is attenuated by a factor of 10 by a scope probe.

    Pin 5 of the FM1000 supplies B+ voltage to its oscillator section.
      Pin 4, FM1000 plate, isn't relevant to operation of the FM1000 oscillator section.
      I think the oscillator should run whether or not pin 4 (plate) is connected to anything.

    But pin 5 is absolutely required because this grid actually serves as the "plate" for the FM1000 oscillator section.
      Note that the schematic suggests the voltage on pin 5 should be about 40V.
      I suggest checking to confirm this is reasonably close to 40V (+/- 10V is probably OK for its range).

    The raw FM input signal is applied to FM1000 pin 6. This can be considered to be the control grid of the FM1000 "demodulator" aka "mixer" or "multiplier" section. Compare this to where the antenna signal goes into the converter tube (12BE6) of an AA5 AM radio. The FM1000 tube will have some gain in this section, so that the signal on its plate, pin 4, should be considerably larger than the amplitude of the input signal on pin 6. Also, the signal at the FM1000 plate is the "mixed" signal and therefore contains both a 9.1MHz component AND the demodulated audio signal.

    Next I've been thinking about what the left side of Z300 does:
      I think its purpose is to couple just enough of the incoming 9.1MHz FM signal into the FM1000 oscillator section to keep it "locked" to the average frequency.
      As I was thinking above, the coupling between the left and right side of Z300 is "loose."
      The actual "direction" of RF signal travel through Z300 is from left to right.


Please understand that all of my comments are based on reading this thread and the schematic/alignment instructions. Sadly I don't (yet) own a radio that uses the FM1000 tube so I cannot test this myself. I intend to look for FM radios that use either the FM1000 or the 12BN6 because I am very curious about how these "FM quadrature detector" circuits actually work. I know quadrature detectors were commonly used to demodulate the audio in TV sets. But most vintage FM radios contained either a "Foster-Seeley discriminator" or a "ratio detector." The "quadrature detector" is a whole different animal compared to those.

-EB

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Mon 25, 2019 1:49 am 
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You're right about the voltage that should be measured. After I wrote about expecting 9 volts, I also realized that you'd be on the 10 V setting of the old style multimeter, so indeed the input resistance should be 200K. So I agree that around -3V would be correct if it is oscillating correctly.

Let's get the IF chain working properly first, and then come back to the FM1000 oscillator. Thanks for posting all the helpful info about the oscillator design above.

Note that he does not have a scope. I agree that having one would really be helpful for troubleshooting the FM1000. But I think the voltage measurement on the grid can be used, and either zero beating or listening for signal on the SW receiver can do the job.

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Mon 25, 2019 6:50 am 
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Tom:

Is there a proper order to adjust the IF trimmers. I noticed that you listed them in a different order than Philco did.

Bob


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