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 Post subject: philco 48-482 restoration- Distorted sound
PostPosted: Sep Fri 27, 2019 12:58 am 
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Location: Davis, CA
Circuit diagram for this radio is here: http://www.nostalgiaair.org/Resources/637/M0013637.htm

There are two paper caps in this radio that have working voltages of 1000 volts (these are the voltages printed on the old caps). They are C105, a .003mf cap attached to the filament of the 5Y3 tube, and C205, a .006mf cap attached to the plate of the 6V6 tube. Are 1000volt caps necesssary?

The only 1000volt caps I have are ceramic discs in values of .0047 and .01. Are ceramic discs suitable for these applications and will substituting .0047 for the .003cap and .01 for the .06 cap be ok?

Thanks, Bob


Last edited by rck46 on Oct Thu 03, 2019 4:46 pm, edited 1 time in total.

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 Post subject: Re: philco 48-482 restoration
PostPosted: Sep Fri 27, 2019 7:51 am 
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Use in both places 0.0047uF 630v foil capacitors.


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 Post subject: Re: philco 48-482 restoration
PostPosted: Sep Fri 27, 2019 1:17 pm 
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Many people will say that you should always replace parts with ones that have the same rating--or better. The one on the plate of the output tube is there primarily to suppress "kick-back" from the output transformer if the tube cuts off due to a transient. The kickback voltage** can be higher than the typical film rating of 630 volts.

The 0.003 at the rectifier is there to provide filtering at higher frequencies--since the electrolytics have a relatively high impedance--due to self-inductance and other factors. I don't know why it needs such a high rating---maybe the turn-on transient??


**An inductor develops a voltage which is the inductance times the rate of change of current.

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 Post subject: Re: philco 48-482 restoration
PostPosted: Sep Fri 27, 2019 3:53 pm 
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Yes, and many people have no clue what they are talking about.


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 Post subject: Re: philco 48-482 restoration
PostPosted: Sep Fri 27, 2019 4:07 pm 
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upsss wrote:
Yes, and many people have no clue what they are talking about.

Excuse me??
If you disagree with something, please discuss the substance.

Specifically with reference to the transient capacitor on an OPT, there's a lot of information here on why it needs to have a higher than normal voltage rating. This said, it is certainly true that not all radios follow this.

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: philco 48-482 restoration
PostPosted: Sep Fri 27, 2019 4:14 pm 
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Let's see if this statement is more palatable:
"Unless you have done your own analysis of the circuit in question, the safest approach is to always use the parts specified by the manufacturer."

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"Measure voltage, but THINK current." --anon.


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 Post subject: Re: philco 48-482 restoration
PostPosted: Sep Fri 27, 2019 4:26 pm 
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pixellany wrote:
"Unless you have done your own analysis of the circuit in question, the safest approach is to always use the parts specified by the manufacturer."


+1, and that depends on having lab grade equipment to make such an analysis as well as knowing what you are checking for. Those specs, particularly the one on the plate of the output tube, were made for a good reason because that circuit can create spikes under the right conditions as previously observed. While a modern 630 volt cap may survive in that position, it's also quite possible that it may fail someday.

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 Post subject: Re: philco 48-482 restoration
PostPosted: Sep Fri 27, 2019 4:44 pm 
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The capacitor across the filament to chassis is probably to provide some degree of decoupling for the FM IF and FM frequencies. Electrolytic caps may not be very effective for RF bypassing.

The cap from the audio output (plate to ground) is often called a "tone cap." It rolls off the higher audio frequencies and also provides some degree of transient protection for the audio transformer primary winding.

Personally, I always change the way those caps so they are connected. As shown,, that cap is dealing both the AC voltage and standing DC power supply from plate to ground. I move the cap so it is directly across the primary winding. Moving the cap to the primary windings means it just has to deal with developed AC voltages and whatever minimal DC voltage exists across the resistance drop for that winding. Best to use at least a 1kV rating. Use two 630 caps in series if need be.

Pete

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 Post subject: Re: philco 48-482 restoration
PostPosted: Sep Fri 27, 2019 7:02 pm 
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Thanks for the comments. I forgot about putting caps in series. If I use two .0068, 630 volt film caps in series I will have a .0034, 1260 volt cap. Also, if I use .01 and .015, 630 volt film caps in series I will have a .006, 1260 volt cap. Am I correct here?

Also, is it the consensus that it would be better to place the .006 cap across the primary of the output transformer rather than to ground?

Bob


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 Post subject: Re: philco 48-482 restoration
PostPosted: Sep Fri 27, 2019 7:46 pm 
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rck46 wrote:

Also, is it the consensus that it would be better to place the .006 cap across the primary of the output transformer rather than to ground?

Bob

I doubt you would find a consensus on that...Considering the transient suppression, what counts is a current path from on end of the primary to the other. For the supply side, the filters provide that path.

Your math on series C is correct.

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 Post subject: Re: philco 48-482 restoration
PostPosted: Oct Thu 03, 2019 12:36 am 
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I have finished replacement of all the paper caps and out of spec resistors. I replaced C105 and C205 with two caps in series as discussed previously.

I did an AM alignment but did not touch FM.

I am getting AM and FM reception across the dial but the speaker output sounds somewhat distorted - not bad but not right. I would describe the sound as gravelly or slightly garbled. Also, it seems that the distortion worsens as the station is tuned in. At first I thought it was a bad speaker but substituting in two different known good speakers did not change the distortion.

I checked the voltage at the plate and grid of the 6V6 output tube. Plate voltage was about 220v and the grid was about 242v. These compare favorably to the voltages shown on the circuit diagram (230v and 250v). Note that the voltages as printed on the circuit diagram are reversed.

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

I substituted for the 6V6 and 7B6 tubes to see if there was any improvement but got no change.

Any ideas on what might be wrong or what to check?

Bob


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Sat 05, 2019 1:27 am 
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I measured voltages at the points where voltages were shown on the circuit diagam. They are shown below by section numbers per the Philco circuit diagram. Is there anything here that would cause the distorted sound I am experiencing?

Thanks, Bob


Attachments:
Sect 1, resize.jpg
Sect 1, resize.jpg [ 305.31 KiB | Viewed 1324 times ]
Sect 2 & 3 resize.jpg
Sect 2 & 3 resize.jpg [ 239.31 KiB | Viewed 1324 times ]
Sect 4 resize.jpg
Sect 4 resize.jpg [ 334.18 KiB | Viewed 1324 times ]
Sect 5 resize.jpg
Sect 5 resize.jpg [ 267.96 KiB | Viewed 1324 times ]
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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Sat 05, 2019 3:19 pm 
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Location: ZIP 23831 South of Richmond, VA 25 miles down the pike.
I consider your plate voltage too low on the FM1000. Pin 5 readings are ok. So I would revisit the 47K R302 for starters.

Bill J.


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 Post subject: Re: philco 48-482 restoration
PostPosted: Oct Sun 06, 2019 4:54 am 
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pixellany wrote:
upsss wrote:
Yes, and many people have no clue what they are talking about.

Excuse me??
If you disagree with something, please discuss the substance.

Specifically with reference to the transient capacitor on an OPT, there's a lot of information here on why it needs to have a higher than normal voltage rating. This said, it is certainly true that not all radios follow this.

If he doesn't believe us, maybe he'll believe Frederick Emmons Terman, ScD. (From Radio Engineering, 2nd Edition, 1937, pp 262-263):
Attachment:
RE Book On Surges.jpg
RE Book On Surges.jpg [ 343.08 KiB | Viewed 1264 times ]

John

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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Tue 08, 2019 1:09 am 
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Joined: Mar Thu 03, 2011 8:53 pm
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Location: Davis, CA
Bill: I checked R302 (47K) and also R304 (15k) which I believe control the plate voltage of the FM1000 tube. R302 measures 51.8K and R304 measures 16.2K. Is this enough variation to cause the low voltage at the plate?

John: I have read your attachment regarding voltage surges and noticed that "the voltage developed by these surges frequently causes the breakdown of insulation in the transformer, with resultant microphonic effects that appear as noises in the amplifier output". Could this be what is causing the distorion I am hearing?

Thanks, Bob


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Wed 09, 2019 2:03 pm 
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Welcome to the Philco Phorum Rob. :wink:

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 Post subject: Re: philco 48-482 restoration
PostPosted: Oct Wed 09, 2019 2:57 pm 
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rck46 wrote:
I have finished replacement of all the paper caps and out of spec resistors. I replaced C105 and C205 with two caps in series as discussed previously.

I did an AM alignment but did not touch FM.

I am getting AM and FM reception across the dial but the speaker output sounds somewhat distorted - not bad but not right. I would describe the sound as gravelly or slightly garbled. Also, it seems that the distortion worsens as the station is tuned in. At first I thought it was a bad speaker but substituting in two different known good speakers did not change the distortion.

I checked the voltage at the plate and grid of the 6V6 output tube. Plate voltage was about 220v and the grid was about 242v. These compare favorably to the voltages shown on the circuit diagram (230v and 250v). Note that the voltages as printed on the circuit diagram are reversed.

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

I substituted for the 6V6 and 7B6 tubes to see if there was any improvement but got no change.

Any ideas on what might be wrong or what to check?

Bob


A possible cause of the distorted sound might be the C205 cap which feeds the signal into G1 of the 6V6. A leaky cap will affect the bias of the output tube. Did you check the negative bias at G1 of the 6V6? If the voltage there is not the correct one will produce a distorted sound.


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Wed 09, 2019 5:36 pm 
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You can determine whether the distortion is either in the audio section or the RF/IF/detector section by using a test jack on the back of the radio.

Attachment:
Philco 48-482 Audio Test Point.JPG
Philco 48-482 Audio Test Point.JPG [ 57.84 KiB | Viewed 1126 times ]


Feed a known distortion free audio signal into the jack from, for example, an mp3 player or tuner. If there is distortion, then the problem is in the radio's audio circuit. You can monitor the tuner section of the radio by feeding an output signal from this jack to an external audio amp and speaker. If you hear the same distortion that you have described above, then the problem is in the radio's RF/IF/detector section.

If you hear the same type of distortion on both AM and FM, then the problem is probably in the audio section. If the distortion is only in one mode (AM or FM but not both), I would expect the problem to be in the section that has the distortion.

An IF tube with grid emission would cause distortion in an AM signal because the grid is being forced positive. With FM, the symptom would just be low gain but not necessarily distortion.

Dave


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Sat 19, 2019 10:42 pm 
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Got back to this radio today.

Voltage at G1 of the 6V6 tube is -14.3 volts.

I have no distortion on FM.

Distortion on AM sounds like the station is not quite tuned in.

I do not have the equipment to check the audio circuit and RF/IF/Detector section as suggested by Dave. I did do the following but don’t know if these tests duplicate what Dave suggested.

I imputed a 400kc audio signal from my signal generator at the point suggested by Dave. There was no distortion.

I also get no distortion when tuning the radio to RF signals from the signal generator on both the AM and short wave bands.

I replaced the used 7F8, 7H7, 7B7, and 7B6 tubes with new tubes. The remaining tubes test good.

Bob


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 Post subject: Re: philco 48-482 restoration- Distorted sound
PostPosted: Oct Sat 19, 2019 11:16 pm 
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I think audio section is ok if you're receiving FM stations clearly.

The only place I can think of where AM-only distortion can be introduced is the AM detector statge (6SQ7). If that's the case, sound distortion would happen on SW too.


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