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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Sun 20, 2019 1:52 am 
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Location: ZIP 23831 South of Richmond, VA 25 miles down the pike.
Bob, the two resistors mentioned read good and resistance ok. The schematic little hard to read. But been looking it over off and on. What does the voltage read off the 5Y3.

Bill J.


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Sun 20, 2019 12:54 pm 
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Location: Utica, NY 13502 (USA)
I wonder if there is a problem with the AVC? Are weak stations equally as distorted as strong stations?

Dave


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Sun 20, 2019 2:02 pm 
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Location: Near Brandon, Iowa
I agree with those who think that there is a problem with the AM demodulator/AVC circuit: shorted cap, open resistor, or miswire. An issue here can cause the radio to emit absolutely horrible sounds when the set is perfectly tuned to a strong station; tuning slightly off-station (on either side) can cause the sound to improve in quality but lose volume.


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Sun 20, 2019 8:49 pm 
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Location: Davis, CA
Bill J: The voltage at the 5Y3 (+ side of C102C) is 243v.

Dave and lorenz200w: I do notice that there is a slight improvement when tuned off station and slightly less distortion when on a weak station. What items do you recommend I check? I have changed all of the paper caps and most of the resistors in the radio but left all the old micas in place.

Bob


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Sun 20, 2019 10:55 pm 
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lorenz200w wrote:
I agree with those who think that there is a problem with the AM demodulator/AVC circuit: shorted cap, open resistor, or miswire....

rck46 wrote:
...My radio has a 7B6 tube in place of the 6SQ7G tube shown on the circuit diagam.

It's essentially the same tube, but 7B6 is specified for 7V filament voltage but there's only 6V at the socket intended for 6SQ7.

http://www.r-type.org/pdfs/7b6.pdf

There's probably another 6SQ7 in one of your sets, swap out the tube to see if it makes any difference. Not guranteed to fix it but worth a try.

Check R407, R412, R417 and everything near them.


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Mon 21, 2019 12:39 am 
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Location: Near Brandon, Iowa
rck46 wrote:
Bill J: The voltage at the 5Y3 (+ side of C102C) is 243v.

Dave and lorenz200w: I do notice that there is a slight improvement when tuned off station and slightly less distortion when on a weak station. What items do you recommend I check? I have changed all of the paper caps and most of the resistors in the radio but left all the old micas in place.

Bob

Check the passive components between the secondary of the 2nd IF transformer (AM section), the high side of the volume control, and B-. If you had to replace any deteriorated rubber-insulation hookup wire going to the 2nd IF can, that would be a clue as to where to start looking. If you didn't replace any hookup wire, look for flaked-off sections of insulation that could be causing a short to another line or the chassis. Otherwise, just wring out the physical wiring against the schematic.


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Mon 21, 2019 1:44 am 
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Location: Utica, NY 13502 (USA)
Another possibility is grid emission in one or more of the IF tubes. I've often seen this problem in communications receivers. After the radio has been on for 10 or 15 minutes, measure the control grid voltages with a 1 meg resistor in series with the probe with no stations tuned in. None should show a positive reading.

Dave


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Tue 29, 2019 1:38 am 
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Location: Davis, CA
Dave:

I haven't had much time to look at this radio lately but did get a chance to check the control grid voltages of the IF tubes as you suggested. They are:

1st IF: +.3v
2nd IF: -.1v
3rd IF: 0v

Do these results suggest anything?

Bob


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Tue 29, 2019 3:44 am 
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rck46 wrote:
Dave:

I haven't had much time to look at this radio lately but did get a chance to check the control grid voltages of the IF tubes as you suggested. They are:

1st IF: +.3v
2nd IF: -.1v
3rd IF: 0v

Do these results suggest anything?

Bob
Grid voltage should never be positive. So take a very close look at that first IF stage. Try a different tube first. If that doesn’t help, then measure the voltage at the grid terminal on the tube socket without having a tube in the socket.

Are the IF transformers slug-tuned? If so, silver-mica disease SMD could be the problem.

-EB

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Tue 29, 2019 3:59 am 
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When the radio is tuned to a strong local AM station the grid voltage on the first IF amplifier tube should be negative, and have a value between -5V and -10V.

Check the AVC line. There are 1 megohm resistors R401 and R407. Also 330K ohm R412. Is there a large DC voltage change as you travel back from the AM detector towards the 1st IF tube grid? If so, there is DC leakage either inside a tube or inside an IF can.
Do this DC voltage measurement while radio is tuned to a strong local station.

-EB

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Tue 29, 2019 3:01 pm 
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Location: Utica, NY 13502 (USA)
With a strong AM station tuned in, measure the voltage on each side of R401 (1 meg) to the chassis. If the side that goes to the IF transformer is significantly less negative than the other side, then I would suspect a leaky cap (part of Z-404), the IF transformer has leakage to ground or B+ (unlikely), or a bad tube.

Attachment:
Philco 48-482 1st IF AVC Test Points.jpg
Philco 48-482 1st IF AVC Test Points.jpg [ 98.49 KiB | Viewed 2621 times ]


Did you replace that .01 uf cap when you did your recap?

Dave


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Tue 29, 2019 10:23 pm 
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Location: Davis, CA
Here are results to the suggestions of items to check.

1. Substituted a used 7H7 1st IF tube for the new tube in the radio. Control grid voltage was +.5v. Removed tube and voltage was +3.5v.

2. Tuned to strong local station with new tube back in radio. Control grid voltage was +.1v

3. Measured voltage on each side of R401. Voltage on IF transformer side was -2.8v. On other side of R401 voltage was-6.0v. Cap Z404 is new. The choke part of Z404 is just three turns of wire around the cap.

Based on above is it likely the problem is with the 1st IF transformer. If so, what should I be looking for.

Bob


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Wed 30, 2019 12:48 am 
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rck46 wrote:
Here are results to the suggestions of items to check. Based on above is it likely the problem is with the 1st IF transformer. If so, what should I be looking for.
I think you are correct. Seems like an open secondary winding or connection in the IF transformer exists. It will still work to a degree depending on where the break is. But the tube is likely running without benefit of the (R401) grid resistor so the grid can easily drift positive causing distortion of the top half of the modulated wave.

The different voltage readings at the two R401 test points is likely due to the loading effect of your voltmeter. I would measure resistance between the 7H7 control grid socket connection and the connection at R401 that leads to the IF transformer. It should be a relatively low value.

I'm trying to come up with an explanation for the +3.5 volts with the tube removed from its socket. What are the voltages at the two R401 test points with the 7H7 removed?

Dave


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Wed 30, 2019 3:27 pm 
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Based on your voltage measurements, I think the 1st IF transformer is the culprit.

So I recommend removing the 1st IF transformer from the radio and (very carefully) removing its metal can so that you can inspect it internally.

This is a complex and unusual IF transformer because it combines both AM and FM sections inside 1 can.

There is a fixed capacitor inside it, marked C400E, which connects from input side to output side. If this fixed capacitor has even a tiny bit of DC leakage then it will put positive voltage on the first IF control grid.

There are 4 other capacitors inside this can. 3 of those are adjustable trimmer capacitors. One is fixed. I don’t know whether either of the fixed capacitors are the exposed silver mica sheet variety that commonly develops “silver mica disease” SMD. But that can be identified by taking apart the transformer and giving it a thorough internal inspection. Do be extremely cautious to avoid breaking the hair-thin wires from the coil windings.

-EB

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Wed 30, 2019 4:49 pm 
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rck46 wrote:
Measured voltage on each side of R401. Voltage on IF transformer side was -2.8v. On other side of R401 voltage was-6.0v.
Based on above is it likely the problem is with the 1st IF transformer. If so, what should I be looking for.Bob
I agree that there is something wrong inside the first IF transformer.

As mentioned in another post, when you measure voltages in AVC circuits, the input resistance of the multimeter can throw off the measurement. Most (but not all) modern digital multimeters have a 10megohm input resistance. So if there is -10V on one end of a 1 megohm resistor, the load from the multimeter will cause the other end of the 1 meghohm resistor to measure about -9V.

I do recommend checking the specifications of your multimeter. Look for "input resistance" for its DC voltage ranges. As mentioned above, 10 megohms is relatively standard these days. However, some multimeters have higher or lower input resistances. If the input resistance is <10 megohms, this would throw off AVC measurements by a very large amount. In the other direction, I have an HP 412 VTVM which has 200 megohm input resistance. That works quite well for measuring AVC circuits. There is hardly any error when measuring voltages around multi-megohm resistors.

There is another way to check for DC leakage in these high resistance AVC circuits: Measure the voltage drop ACROSS R401. In other words, connect the leads from the multimeter directly across R401. Neither multimeter lead is grounded. When the AVC circuit and the 1st IF transformer are working properly, there will be little or no voltage drop across R401. The meter reading should be <100mV (0.1V). This same technique can be used to measure voltage drops across other resistors in the AVC circuitry.

Quote:
1. Substituted a used 7H7 1st IF tube for the new tube in the radio. Control grid voltage was +.5v. Removed tube and voltage was +3.5v.
This is another indication of a problem inside the 1st IF transformer: With the tube removed, the voltage at the control grid pin of the tube socket rises to +3.5V (positive).

The only way this can happen is from DC leakage inside the IF can itself. There is no other place for positive voltage to get in.

The reason why it drops to +.5V with the tube installed is because the control grid of the tube acts like a diode, preventing the voltage from going very much positive.

In fact, one of the best techniques for diagnosing DC leakage and silver mica disease is to remove the tubes and then apply B+ voltage to the radio. The control grid pin of any tube socket in the radio should never measure positive with the tubes removed.

For this radio, which has a power transformer and a 5Y3 rectifier tube, you could remove all of the tubes except the 5Y3 and then slowly bring up the AC supply voltage with a variac or dim bulb. Don't let the B+ go higher than it would be with all tubes installed. In fact, to search for DC leakage in the IF sections only requires applying enough power to the radio to get the B+ up to about 200V. That is high enough to detect any DC leakage that might exist.

-EB

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Wed 30, 2019 11:25 pm 
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Joined: Mar Thu 03, 2011 8:53 pm
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Location: Davis, CA
I removed the 1st IF from the radio. The four coils show continuity with ohm readings close to what is shown on the circuit diagram. I do not see any obvious problems. I have attached three pictures. C400E is the small mica shown in the first picture. The larger 3000pf cap (C400B) is shown in the second picture. Are there some tests I can make now with the IF removed from the radio? Are the three trimmers shown on the last picture the tyoe subject to silver mica disease?

Bob


Attachments:
IMG_0871 (2), resize.jpg
IMG_0871 (2), resize.jpg [ 252.33 KiB | Viewed 2577 times ]
IMG_0884, resize.jpg
IMG_0884, resize.jpg [ 259.21 KiB | Viewed 2577 times ]
IMG_0878 (2), resize.jpg
IMG_0878 (2), resize.jpg [ 159.76 KiB | Viewed 2577 times ]


Last edited by rck46 on Oct Thu 31, 2019 12:53 am, edited 1 time in total.
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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Wed 30, 2019 11:37 pm 
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Location: Davis, CA
EB: My multimeter is an Extech 430. I could not find any reference to "input resistance" for its DC voltage ranges in the users guide I have.

Bob


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Wed 30, 2019 11:56 pm 
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Postage stamp micas occasionally develop DC leakage.

In this case C400E is the one to be concerned about. It has the full B+ voltage across it. The other (larger) postage stamp mica doesn’t have any DC voltage across it, so it isn’t a likely cause of the problem in this radio.

Since you’ve already disassembled the IF can, my recommendation would be to replace both mica capacitors with modern silver mica capacitors of the same pF value and 500V DC rating. Modern capacitors are smaller so they should fit easily. If I’m reading the schematic correctly, the values are 1pF and 3000pF.

I haven’t personally observed any adjustable trimmer capacitors failing except from visible mechanical damage, such as extreme rust or broken ceramic pieces. Maybe other ARF members who work with a lot of pre-1950 radios could comment about this?

If you want confirmation, the old C400E can be tested for DC leakage as follows after removing it from the IF transformer: Apply 100V DC to one end while measuring voltage at other end with a multimeter that has 10 megohm input resistance. The voltage seen on the meter should be <100mV (<0.1V). This corresponds to leakage current <10nA. I’m sure a new silver mica capacitor will easily pass this test.

-EB

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Thu 31, 2019 12:14 am 
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rck46 wrote:
EB: My multimeter is an Extech 430. I could not find any reference to "input resistance" for its DC voltage ranges in the users guide I have.

Bob
According to its manual: “Input impedance >7.5 megohm on all AC and DC voltage ranges.”

For DC, impedance and resistance are equal. It wouldn’t surprise me if the Extech meter’s resistance on the DC ranges is 10 megohms. That’s pretty much the standard for modern digital multimeters.

If you have another multimeter, you could use it to measure the DC resistance of the Extech. Note that the Extech must be set to “DC volts” to do this.

-EB

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Thu 31, 2019 1:14 am 
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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


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