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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Thu 31, 2019 1:33 am 
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rck46 wrote:
I don't have a 3000pf silver mica. I do have a 4700pf, 1000v ceramic disc cap. Would this be a suitable replacement? I looked at the parts list and C400E is 3pf.

Bob
For C400E, it may be easier to find 3.3pF because that is more commonly used than 3pF. But 3pF is also available from some vendors. This C400E isn’t part of a tuned circuit so it is probably OK to use a slightly different value, such as 3.3pF rather than 3pF.

If you don’t have an exact replacement for the 3000pF then it’s OK to keep using the old capacitor. 4700pF would be too far distant from the correct value. This capacitor is part of a tuned circuit, so it must be the same pF as the original to avoid disturbing the alignment of the radio. Also it is best to stick with silver mica capacitors for tuned circuits in RF and IF sections. Silver mica capacitors have better temperature stability than other types.

-EB


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Sun 10, 2019 1:55 am 
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Location: Davis, CA
I had to order some caps to replace C400B (3000mmf) and C400E(9mmf) so haven't worked on this radio since my last post. For some reason I thought C400E was 3mmf so that is what I installed and didn't realize my error until after the !st IF was back in the radio. I did an AM alignment. With this setup I am still getting the distortion and the control grid voltages with the radio tuned to a strong station are:

1st IF: -1.4v
2nd IF: -2.4v
3rd IF: 0v

The 1st and 2nd IF voltages are not constant and vary +or- .2 volts while testing.

Do these voltages look reasonable and would changing C400E back to 9mmf make a difference?

EB: I didn't get a chance to check the voltages back from the AM detector towards the 1st IF as you suggested but will do so tomorrow.

Bob


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Sun 10, 2019 4:34 pm 
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When tuned to a strong local AM station the AVC voltages should all be in the range of -5V to -10V.

Typically the AVC voltage when the radio is tuned to a “quiet” place on the dial (no station, no signal) will be around -1V. Then, as the radio is slowly tuned towards a strong local station, the AVC voltage will go more and more negative, reaching the largest negative voltage when tuned precisely to the center frequency of the radio station. Continuing to tune “past” the station will cause the AVC voltage to move back towards the “no signal” value of roughly -1V.

Note: Accurate voltage measurement in the AVC system is challenging because of all the series resistors that are 1 megohm and larger. I’m lucky that I have an HP412A VTVM which has a 200 megohm input resistance.

One way to check AVC circuits with a modern DVM (these often have 10 megohm input resistance) is to connect the voltmeter probes across the large value series resistors in the AVC circuit. For this Philco radio, the resistors to measure “across” are:
R417, 1 megohm
R412, 330k ohm
R407, 1 megohm
R401, 1 megohm

There should NOT be a large voltage drop across any of these resistors.

For example, if there is a fault in the radio, you might observe a rather large voltage drop ACROSS R417, R407, or R401. For example, a voltage drop >5V across any individual resistor in the above list is a sign of trouble. This won’t happen when everything is working properly. The voltage drop across any individual resistor in the above list should be <2V

Finally, one more question for you: Do you have a signal tracer with an RF probe? This is a very helpful tool for diagnosing this type of problem.

-EB


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Mon 11, 2019 12:37 am 
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EB: I checked the voltages across the four resistors as you suggested. They are listed below:

R417: 7.0
R412: 2.4
R407: 7.6
R401: 0

I then started checking the wiring around R417 and noticed that I had miswired R418, the 270k resistor attached to one end of R417. It was not grounded. After correcting this I am now getting clear reception on AM but still seem to be having some distortion on FM. The FM reception is clear when the station is not quite tuned in.

I checked voltages across the four resistors again. They are now:

R417: 6.0
R412: 2.0
R407: 5.9
R401: 0

I checked the rest of the wiring in the area of these resistors and did not find any other problems.

I do have a signal tracer with a RF probe.

Bob


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Mon 11, 2019 7:50 pm 
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rck46 wrote:
EB: I had miswired R418, the 270k resistor attached to one end of R417. It was not grounded. After correcting this I am now getting clear reception on AM but still seem to be having some distortion on FM.
The FM reception is clear when the station is not quite tuned in.
I do have a signal tracer with a RF probe.
Bob
Good to hear that the AM issue is corrected.

The FM section in these Philco radios is unusual and unique.
Do a search on ARF for “FM1000” and read those posts. Some people love Philco radios that use the FM1000 tube. Others dislike them.
I haven’t worked on a radio that contains the FM1000 yet.

-EB


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 Post subject: Re: Philco FM1000 FM Demodulator
PostPosted: Nov Fri 15, 2019 12:34 am 
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Ref: Tom Albrecht's post on Philco FM1000 Demodulator: viewtopic.php?f=6&t=284598

I am getting FM reception but on some stations it is distorted.

I am still having difficulties with step two of Tom's preceedure. I have removed the grounding of pin 2 of the FM1000 tube, have put a jumper between the blue and red leads of the last transformer (Z300), and am using no modulation on the signal generator. I set my signal generator at 9.1Mhz.

I tried to use the zero beat method, keeping the loop around the mixer tube, but hear no tone in the radio. I set the radio at 9.1Mhz to do this and adjusted the tuning up and down the dial but hear no tone any place on the dial. I do get some FM reception.

I then removed the loop from around the mixer tube and tried the frequency counter method. I get no reading on my frequency counter no matter how it is coupled to the FM1000 tube. I am using a Heathkit IM2420 frequency counter.

I have a feeling I am not doing something correctly.

Bob


Last edited by rck46 on Nov Wed 20, 2019 10:49 pm, edited 1 time in total.

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 Post subject: Re: Philco FM1000 FM Demodulator
PostPosted: Nov Fri 15, 2019 1:30 am 
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Hi Bob,

Do you have a scope? The first thing that needs to be checked with whether the oscillator is running at all. With AC coupling, using a 10X probe, check for the oscillator signal on the plate (pin 4) of the FM1000 tube. Should be aroudn 9 MHz. If there is no oscillator signal, that will need to be fixed first before proceeding.

If you don't have a scope, try connecting your frequency counter to pin 4 through a .001 uF capacitor. Will it read frequency now? If not, either the oscillator is not running, or the frequency counter may have some problem?

For the zero beat, you don't need to worry about the FM dial frequency setting (it plays no role). You are trying to zero beat the 9.1 MHz signal you're injecting on the loop around the mixer and the FM1000 oscillator signal. The idea is to adjust the trimmer so that you get a zero beat.

If nothing seems to be working right, verifying that you have an oscillator signal from the FM1000 is the thing to do.

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 Post subject: Re: Philco FM1000 FM Demodulator
PostPosted: Nov Fri 15, 2019 1:50 am 
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Hi Tom:

I do get FM reception across the dial - The sound just seems to be distorted somewhat on some stations, especially at higher volumes. Wouldn't the fact that I am getting FM reception indicate the oscillator is working?

I do not have a scope.

I did try connecting the frequency counter to pin 4 of the FM1000 through a 10 pf cap but did not get a reading on the counter. Maybe the counter is not sensitive enough to give a reading. Specs for my Heathkit IM2420 frequency counter are on pages 71 and 72 of this link: http://tubularelectronics.com/Heath_Man ... 20_man.pdf Do the type of counter leads effect the ability to get a reading?

Bob


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 Post subject: Re: Philco FM1000 FM Demodulator
PostPosted: Nov Fri 15, 2019 3:15 am 
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Actually, if the oscillator in the FM1000 is not working, but the rest of the tube is functioning normally, you will get marginal FM reception. With the oscillator not functioning, it works like a bad AM detector. And if you have an AM detector with a selective IF stage (like all superheterodyne radios have), you can receive FM by tuning slightly off center via an effect known as "slope detection." So that might be what you have.

Do you have loud static/hiss between stations, or pretty quiet? If it's pretty quiet, most likely you're getting AM detection.

Somehow we need to know if the FM1000 oscillator is running. Do you have a short wave receiver? Place it next to your Philco, and see if you can pick up a signal around 9.1 MHz (it might be off a fair amount) when the Philco is turned on. If the signal goes away when the Philco is not on, that will prove you are hearing the FM1000 oscillator.

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 Post subject: Re: Philco FM1000 FM Demodulator
PostPosted: Nov Sat 16, 2019 1:16 am 
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Tom:

I think you may have diagnosed my problem. I do not hear any noise between stations - very quiet. Also, do not hear a signal in an adjacent radio around 9.1Mhz.

Interestingly, I can hear a hissing noise in the 48-482 from the adjacent radio when both radios are tuned to 9.1Mhz. When I change the station or turn off the adjacent radio the noise goes away.

The FM1000 tube is NOS.

Any further suggestions will be appreciated.

Bob


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 Post subject: Re: Philco FM1000 FM Demodulator
PostPosted: Nov Sat 16, 2019 2:06 am 
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Hi Bob,

Not sure what you mean by "when both radios are tuned to 9.1 MHz," since 9.1 MHz isn't a frequency you can tune to on either AM or FM. You may be hearing the front-end oscillator (not the same as the FM1000 oscillator) from one radio on the other.

Try unshorting the blue and red wires (if you haven't already done that) when checking for oscillator signal.

To troubleshoot the FM1000 oscillator, I would suggest the following steps:

1. Clean tube socket pins (spray tube pins with DeOxit, then insert into socket and wiggle back and forth)
2. Verify voltages on tube pins:
- cathode (pin 3): 0 volts
- grid #1 (pin 2):0 volts or a few volts negative
- grids #2 and #4 (pin 5): 40 volts
- grid #3 (pin 6): within a few volts of zero
- anode (pin 4): 200 volts
3. Check continuity of both coils in Z300
4. Replace mica cap C301 and all mica caps inside Z300 (assuming all other caps in entire FM1000 circuit already replaced).

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 Post subject: Re: Philco FM1000 FM Demodulator
PostPosted: Nov Sat 16, 2019 9:58 pm 
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Tom:

I removed the jumper and gave the tube pins and sockets the dioxit treatment then tested for oscillator signal.

With the adjacent sw radio dial set at about 8.5 on the dial I pick up the FM stations from the Philco on the sw radio. Is this the indication that the oscillator is working?

FM1000 pin voltages are as follows:
Pin 3: 0
Pin 4: 191
Pin 5: 47
Pin 6: 1.5
I assume these are ok.

Still have the distortion.

I will remove Z300 and check coil continuity and change caps.

Bob


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 Post subject: Re: Philco FM1000 FM Demodulator
PostPosted: Nov Sat 16, 2019 10:21 pm 
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What's the pin 2 voltage?

Picking up the FM stations from the Philco at 8.5 MHz on the SW radio could mean one of two things:

1. FM1000 oscillator is working, and is tracking modulation (working properly as PLL detector)
2. You're just picking up the IF signal from the Philco, and the FM1000 oscillator is not necessarily working

Better test:

No antenna connected to Philco, and tuned to a place where you do not expect to receive any FM station. Now can you hear the FM1000 oscillator on the SW receiver? It should sound like a strong "station" with no audio (maybe a little hum). Should appear when Philco is on, and go away when Philco is off.

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 Post subject: Re: Philco FM1000 FM Demodulator
PostPosted: Nov Sun 17, 2019 1:12 am 
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Hi Tom:

Pin 2 voltage is -.4

I disconnected the FM antenna. Hear a light hissing or buzzing noise on the sw radio at 8.5 on the dial that goes away when the Philco is turned off. Volume on the sw radio has to be turned up all the way to hear this. The noise is not a clear tone like you would hear from a signal generator.

Bob


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 Post subject: Re: Philco FM1000 FM Demodulator
PostPosted: Nov Sun 17, 2019 1:43 am 
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I previously was thinking your oscillator is running, but I have some doubts. Try adjusting the trimmer on Z300 a little. Does it affect the frequency of the signal you're picking up? (i.e., does adjusting the trimmer on Z300 move the signal a little on the SW receiver dial?)

If it does not have any significant effect, the FM1000 oscillator is dead, and you're picking up a very weak IF signal (basically just interstation noise) from the Philco. Check coils and replace capacitors in the FM1000 circuit as I mentioned above.

If it does have a clear effect of shifting the signal, I'll give instructions on what to do next when I hear back from you.

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Sun 17, 2019 6:13 pm 
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Tom: Is The trimmer you are referring to c300b?


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Sun 17, 2019 8:23 pm 
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Bob, I wasn't any help, tried looking at SAMS, still nothing. But the link below is for SAMS data. Maybe something more noticeable. Just ignore my markings.

Bill J.

https://app.box.com/s/qjtac179u3yt4m4a2kkfwqvd1vuhpy1b


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Sun 17, 2019 8:32 pm 
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Yes, C300B is the one.

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Sun 17, 2019 8:50 pm 
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Tom:

I adjusted trimmer C300b slightly and it does have a clear effect on moving the signal a little on the sw receiver dial.


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Nov Sun 17, 2019 10:06 pm 
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I guess the oscillator is alive, and perhaps the reason it is noisy is simply because it is trying to lock onto no signal.

We can use your signal generator (assuming it is a nice precise digitally controlled one) and SW receiver to "walk" the FM1000 oscillator to the correct frequency of 9.1 MHz. Assuming you've already peaked the FM IF transformers to 9.1 MHz (not including the final FM1000 transformer Z300), this will end up making it all work properly.

Tune your SW receiver so that you can hear the FM1000 oscillator. Now place your signal generator probe near the SW receiver antenna terminal (but don't connect it). Then tune the signal generator frequency until you hear it whistle (this is the signal generator beating with the FM1000 oscillator). Center the tuning of the signal generator so it "zero beats" (center with low or zero pitch) and that means the signal generator matches the FM1000 oscillator. Read off the signal generator and you've got the frequency. Now keep adjusting the trimmer C300B until you've successfully moved it to exactly 9.1 MHz by stepping your way there and checking repeatedly by zero beating the signal generator with it.

Do this first with the red and blue leads of Z300 shorted, and adjust C300B to get the oscillator to 9.1 MHz. Then unshort the red and blue leads, and making sure you're not at a dial frequency that tunes any station (we want NO station) then adjust the slug in the coil of Z300 to bring it back to 9.1 MHz once again (it will have moved when you unshort the red and blue leads). Once this is done, the FM1000 demodulator should work properly.

Tuning should now be easier, audio should be mostly distortion free, and audio level should be higher for FM reception.

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