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 Post subject: Philco PT-95 has high-pitched oscillation in the output
PostPosted: Jun Wed 13, 2018 9:31 pm 
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Joined: Jun Wed 13, 2018 9:18 pm
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Howdy. I recently got a nice Philco radio. I replaced all the capacitors and the radio works great, with one problem. Mixed in with the audio appearing at the speaker, there is a high pitched low-amplitude tone that sounds kind of like a CRT television, but much more piercing sounding.

The capacitors I used were mostly random old-stock of the correct values, measured good and tested showing no DC leakage. However, because my oscilloscope is broken and they are not marked, I could not check for the polarity of the outside foil, which people say is important in audio electronics. Could this high-pitched oscillation be caused by a backward capacitor or two?

I'm thinking that if I'm unable to find an in-circuit solution without modifying the radio, I'll just put a low-Henry inductor in series with the speaker wire as a high frequency choke.

What do you all think?


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 Post subject: Re: Philco PT-95 has high-pitched oscillation in the output
PostPosted: Jun Wed 13, 2018 11:35 pm 
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Location: Mountains of Mourne. Ireland.
Is the antenna... viewtopic.php?p=1070222#p1070222

Schematic: http://americanradiohistory.com/hd2/IDX ... e-0101.pdf

Greg.


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 Post subject: Re: Philco PT-95 has high-pitched oscillation in the output
PostPosted: Jun Wed 13, 2018 11:42 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
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Location: Near Brandon, Iowa
Check for correct installation/value of C22 (0.02 uF) located between the plate of the output tube and the floating ground bus. This needs to be a 1000V rated part.

Orientation of the "outside foil" lead on any cap in this radio is probably not important.


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 Post subject: Re: Philco PT-95 has high-pitched oscillation in the output
PostPosted: Jun Wed 13, 2018 11:48 pm 
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The 250pF (17) is also important. If you are getting RF into the output tube you will get an oscillation.

Jay


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 Post subject: Re: Philco PT-95 has high-pitched oscillation in the output
PostPosted: Jun Wed 13, 2018 11:55 pm 
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Location: North of Mpls, Minnesota
SixFiveZeroTwo wrote:
The capacitors I used were mostly random old-stock of the correct values, measured good and tested showing no DC leakage.
Were the capacitors tested at the rated voltage? New capacitors are inexpensive.

SixFiveZeroTwo wrote:
I'm thinking that if I'm unable to find an in-circuit solution without modifying the radio, I'll just put a low-Henry inductor in series with the speaker wire as a high frequency choke.
Putting a band-aid on it is not the way to go. Try detuning the if transformers a little to see if that makes a difference. The other thing to watch for is lead dress, are all the replacement components in the same position as the old parts were?

Dave


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 Post subject: Re: Philco PT-95 has high-pitched oscillation in the output
PostPosted: Jun Thu 14, 2018 4:49 am 
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Not just lead dress, but routing as well. And as stated, new caps are cheap; I wouldn't waste time with "NOS", which could possibly be the problem once they are energized, regardless of how they tested.

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 Post subject: Re: Philco PT-95 has high-pitched oscillation in the output
PostPosted: Jun Thu 14, 2018 7:57 am 
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Thanks everyone for all the quick replies!

Two things: 1. I thought the oscillation might have been the piezoelectric effect from one of the capacitors, so I stuck a tube in my ear and listened to points all over the radio inside and out, and the oscillation is coming from the speaker. It's not a mechanical vibration of any parts under the chassis.

B) One thing I omitted is the topic of this thread: http://www.antiqueradios.com/forums/viewtopic.php?f=6&t=214247. I omitted the coil because everyone said it's fine if I do.



egg wrote:


Is not the antenna... I swapped the leads and the oscillation is still present. The lug is also connected properly to the back of the chassis.

lorenz200w wrote:
Check for correct installation/value of C22 (0.02 uF) located between the plate of the output tube and the floating ground bus. This needs to be a 1000V rated part.


This capacitor, C22, was not present when I acquired the radio. Where it appears in the rough diagram in the datasheet does not seem to match up with my radio because there is not a reasonably close floating ground near its location in that diagram. I attached it to a floating ground point on 50L6 initially, and I moved it to the floating ground on 35Z3, where it most closely would be according to the diagram, and I also relocated one of the C26 power supply capacitors to a ground point away from 50L6, but neither of these changes eliminated the oscillation.

Also, the datasheet says C22 is a .02uF 400V capacitor. I replaced it with a .022 600V capacitor. Where do you get the 1000V figure from?

JnTX wrote:
The 250pF (17) is also important. If you are getting RF into the output tube you will get an oscillation.

Jay


C17 is present and connected properly. It's a mica capacitor, so I did not replace it.

easyrider8 wrote:
Were the capacitors tested at the rated voltage? New capacitors are inexpensive.

Putting a band-aid on it is not the way to go. Try detuning the if transformers a little to see if that makes a difference. The other thing to watch for is lead dress, are all the replacement components in the same position as the old parts were?

Dave


I went through the junk room at my university's electronics lab and tested lots and lots and lots of capacitors with the Carlson LV Capacitor Tester for DC leakage. The capacitors I selected were tested for multiple failures and are as good as any new capacitor for this application. That said, there were some I had to order like the electrolytic ones and the .2uF 400V one.

All the capacitors I replaced are as close as possible to the locations of the ones that were replaced, with the exception of C22, which was not present when I acquired the radio. I tried it connected to the 50L6 pin and a floating ground on that same tube, and with a different floating ground on another tube. Both configurations produced the oscillation.

fifties wrote:
Not just lead dress, but routing as well. And as stated, new caps are cheap; I wouldn't waste time with "NOS", which could possibly be the problem once they are energized, regardless of how they tested.


The capacitors are placed as close as possible to the locations of the ones that were removed. Also, I went through hundreds of different batches of capacitors at my university's electronics junk room and selected capacitors which passed multiple failure tests. I'm aware of the risks/unreliabilities of using "NOS" capacitors because it's age and not usage which causes some failures. I selected capacitors very carefully and intentionally because of this problem.


Thanks again everyone for your recommendations, but I'm still troubleshooting away.


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 Post subject: Re: Philco PT-95 has high-pitched oscillation in the output
PostPosted: Jun Thu 14, 2018 8:07 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: North of Mpls, Minnesota
Testing capacitors with the "Carlson LV Capacitor Tester" is not a valid test. I would suggest starting over with new capacitors.

Dave


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 Post subject: Re: Philco PT-95 has high-pitched oscillation in the output
PostPosted: Jun Thu 14, 2018 2:53 pm 
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Location: Cleona, PA
Does the oscillation disappear when you turn down the volume all the way or does that have no effect?

Edit: bridge the 250pf cap with another (100-200pf, exact value not important) and note effect on oscillation.

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 Post subject: Re: Philco PT-95 has high-pitched oscillation in the output
PostPosted: Jun Thu 14, 2018 3:24 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 4199
Location: Near Brandon, Iowa
SixFiveZeroTwo wrote:
lorenz200w wrote:
Check for correct installation/value of C22 (0.02 uF) located between the plate of the output tube and the floating ground bus. This needs to be a 1000V rated part.


This capacitor, C22, was not present when I acquired the radio. Where it appears in the rough diagram in the datasheet does not seem to match up with my radio because there is not a reasonably close floating ground near its location in that diagram. I attached it to a floating ground point on 50L6 initially, and I moved it to the floating ground on 35Z3, where it most closely would be according to the diagram, and I also relocated one of the C26 power supply capacitors to a ground point away from 50L6, but neither of these changes eliminated the oscillation.

Also, the datasheet says C22 is a .02uF 400V capacitor. I replaced it with a .022 600V capacitor. Where do you get the 1000V figure from?

There is no floating ground connection on either the 35Z3 or 50L6 tubes; at least, not one which is used by either of these two tubes. In this case, the node to which the negative ends of the electrolytic B+ filter caps are both tied defines this set's "floating ground", AKA the "B- bus". Check the schematic.

C22 is important because it dumps out supersonic (and, as in your case, near-supersonic) audio before it makes it to the output transformer. It also dumps out some amount of normal-AM-audio-range high-frequency sound, so it also functions as a fixed, high-cut tone control. A previous owner probably removed the cap because he didn't like the missing high-frequency content.

A high-voltage cap is desirable here because a fast collapse of the magnetic field in the output transformer can cause a high-voltage spike (think "automobile ignition coil") which can punch through and short a lower-voltage-rated cap. A short in this cap can ruin the output transformer due to overcurrent.


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 Post subject: Re: Philco PT-95 has high-pitched oscillation in the output
PostPosted: Jun Thu 14, 2018 6:32 pm 
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Joined: Jun Wed 13, 2018 9:18 pm
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easyrider8 wrote:
Testing capacitors with the "Carlson LV Capacitor Tester" is not a valid test. I would suggest starting over with new capacitors.

Dave

Very helpful. Thanks for all the supporting detail.


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 Post subject: Re: Philco PT-95 has high-pitched oscillation in the output
PostPosted: Jun Thu 14, 2018 8:07 pm 
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Joined: Jun Wed 13, 2018 9:18 pm
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Quote:
There is no floating ground connection on either the 35Z3 or 50L6 tubes; at least, not one which is used by either of these two tubes. In this case, the node to which the negative ends of the electrolytic B+ filter caps are both tied defines this set's "floating ground", AKA the "B- bus". Check the schematic.


You're right about the schematic, but the unused pins on those tube sockets are used as solder posts in this radio. They are connected properly according to the circuit without connecting to an internal electrode to the tubes. I tried relocating the capacitors to different solder points, but this had no effect.

Quote:
C22 is important because it dumps out supersonic (and, as in your case, near-supersonic) audio before it makes it to the output transformer. It also dumps out some amount of normal-AM-audio-range high-frequency sound, so it also functions as a fixed, high-cut tone control. A previous owner probably removed the cap because he didn't like the missing high-frequency content.


Got it. I tried the radio with this capacitor omitted and the oscillation is gone. However, the sound is definitely a little more shrill without that capacitor. I used a .022uF capacitor rather than the .02 that the datasheet recommends. I think I'm going to take wrnewton's advice and try changing the capacitance value of this capacitor to see if that can remove the sharp high-frequency content without inducing the oscillation.

Quote:
A high-voltage cap is desirable here because a fast collapse of the magnetic field in the output transformer can cause a high-voltage spike (think "automobile ignition coil") which can punch through and short a lower-voltage-rated cap. A short in this cap can ruin the output transformer due to overcurrent.


Right, that tends to be called flyback and it's used lots in switching power supplies. It hadn't even occurred to me here though. If it is a factor worth considering, why isn't it considered by the circuit design? Were they unaware of this effect in the 40s?


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 Post subject: Re: Philco PT-95 has high-pitched oscillation in the output
PostPosted: Jun Thu 14, 2018 8:33 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
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Location: Near Brandon, Iowa
SixFiveZeroTwo wrote:
Right, that tends to be called flyback and it's used lots in switching power supplies. It hadn't even occurred to me here though. If it is a factor worth considering, why isn't it considered by the circuit design? Were they unaware of this effect in the 40s?

Radio designers have been aware of this from the early 30s when low-impedance Rice-Kellogg loudspeakers (requiring an output transformer) were introduced. But HV caps were expensive and designers of inexpensive radios were under pressure to use the cheapest part that would get the radio past its warranty period. Since your radio is a transformerless set the designer probably felt that he could get away with a 400V part. Power transformer-based sets usually have this part spec'ed at least at 600V, 800V, or more due to the higher energy in the B+ bus.


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 Post subject: Re: Philco PT-95 has high-pitched oscillation in the output
PostPosted: Jun Thu 14, 2018 9:09 pm 
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Joined: Jun Wed 13, 2018 9:18 pm
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I also have one other question about these radios. When the power switches on, the dial light starts very bright and quickly dims as the tubes heat up and increase in resistivity. Is this a common characteristic of these radios, or is there a fault in my radio?


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 Post subject: Re: Philco PT-95 has high-pitched oscillation in the output
PostPosted: Jun Thu 14, 2018 9:26 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
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Location: Near Brandon, Iowa
SixFiveZeroTwo wrote:
I also have one other question about these radios. When the power switches on, the dial light starts very bright and quickly dims as the tubes heat up and increase in resistivity. Is this a common characteristic of these radios, or is there a fault in my radio?

Flaring/dimming is normal for these sets, for the reason you gave.


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 Post subject: Re: Philco PT-95 has high-pitched oscillation in the output
PostPosted: Jun Thu 14, 2018 9:28 pm 
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Joined: Jun Wed 13, 2018 9:18 pm
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lorenz200w wrote:
SixFiveZeroTwo wrote:
lorenz200w wrote:
Check for correct installation/value of C22 (0.02 uF) located between the plate of the output tube and the floating ground bus. This needs to be a 1000V rated part.


This capacitor, C22, was not present when I acquired the radio. Where it appears in the rough diagram in the datasheet does not seem to match up with my radio because there is not a reasonably close floating ground near its location in that diagram. I attached it to a floating ground point on 50L6 initially, and I moved it to the floating ground on 35Z3, where it most closely would be according to the diagram, and I also relocated one of the C26 power supply capacitors to a ground point away from 50L6, but neither of these changes eliminated the oscillation.

Also, the datasheet says C22 is a .02uF 400V capacitor. I replaced it with a .022 600V capacitor. Where do you get the 1000V figure from?

There is no floating ground connection on either the 35Z3 or 50L6 tubes; at least, not one which is used by either of these two tubes. In this case, the node to which the negative ends of the electrolytic B+ filter caps are both tied defines this set's "floating ground", AKA the "B- bus". Check the schematic.

C22 is important because it dumps out supersonic (and, as in your case, near-supersonic) audio before it makes it to the output transformer. It also dumps out some amount of normal-AM-audio-range high-frequency sound, so it also functions as a fixed, high-cut tone control. A previous owner probably removed the cap because he didn't like the missing high-frequency content.

A high-voltage cap is desirable here because a fast collapse of the magnetic field in the output transformer can cause a high-voltage spike (think "automobile ignition coil") which can punch through and short a lower-voltage-rated cap. A short in this cap can ruin the output transformer due to overcurrent.



Alright, I tried a couple things. I replace the .022uF 600V capacitor with an identical one which measured a slightly different capacitance within the tolerance range and the oscillation disappeared. I also tried a .033uF 1000V capacitor, this also eliminated the oscillation. So which would you recommend, a capacitance close to spec but with 600V tolerance, or a capacitance a little further from spec with 1000V tolerance?

Edit: Last question. If I were to modify the radio to add a fuse, what current rating would be the best selection for the fuse?


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 Post subject: Re: Philco PT-95 has high-pitched oscillation in the output
PostPosted: Jun Thu 14, 2018 9:47 pm 
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Location: Near Brandon, Iowa
If it were me, I'd go with the 1000V rated part, but only if the higher cap value (0.033 uf) didn't reduce the
"highs" too much. That's a matter of taste. The higher capacitance value won't hurt anything but it WILL change the tone.

As noted earlier in the discussion (I think, but maybe I'm conflating threads) you can obtain a 1200V equivalent cap by putting two 600V caps of double the desired net capacitance in series.


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 Post subject: Re: Philco PT-95 has high-pitched oscillation in the output
PostPosted: Jun Thu 14, 2018 10:09 pm 
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Joined: Nov Tue 14, 2017 5:09 am
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Location: Austin, Texas
If you want to add a fuse, I would recommend one in the 3 to 5 amp range. That's much higher than the normal operating current but you need to consider the surge current when the radio is first switched on. Repeated surges can cause a low current fuse to fail. If there is a major component failure, there will be plenty of current to blow the fuse.

Jay


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 Post subject: Re: Philco PT-95 has high-pitched oscillation in the output
PostPosted: Jun Thu 14, 2018 10:57 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 4199
Location: Near Brandon, Iowa
JnTX wrote:
If you want to add a fuse, I would recommend one in the 3 to 5 amp range. That's much higher than the normal operating current but you need to consider the surge current when the radio is first switched on. Repeated surges can cause a low current fuse to fail. If there is a major component failure, there will be plenty of current to blow the fuse.

Jay

There's a split-element (40+80 ohm) surge resistor in this set; so max cold surge current cannot exceed about one amp (the rectifier tube has a heater sleeve and won't produce voltage until it heats up, so initial turn-on-surge current is confined to the heater string cold draw). If the B+ circuit shorted after the heaters warmed up, line current could try to go as high as 3 amps but the 35Z3 tube can't support that amount of current (its normal max output is 100mA) and would self-destruct in short order. A 1A slow-blow fuse should work OK but IMO the protection it offers is marginal: in the event of a B+ fault (the most likely failure mode) the rectifier tube is probably going to burn up before the fuse can shut things down. Which is exactly what would happen if no fuse were present.


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