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 Post subject: Grid cap on 75 Tube Is Touch Sensitive on my Philco 38-15
PostPosted: Mar Tue 31, 2020 12:34 am 
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Recently recapped a Philco 38-15 and changed most resistors as well. The radio plays beautifully and receives many stations with no hum or static. That is until it intermittently starts making a whooshing static noise over the audio, even at min volume setting. Audio is still fairly clear at higher volume, but the whoosh is heard on top of it. I went over every solder joint, changed every tube and tapped on every component, wire, and joint in the set trying to induce the noise with no luck. Now here’s the screwy part of it all: when the set gets into the “whoosh” mode, all I have to do is to barely even touch the grid cap on the 1st audio 2nd detector tube (75) with a screwdriver blade, and it magically disappears, even at minimum volume. That is until it decides to come back intermittently on its own 5 min, 15 min, 1/2 hr later. I’ve tried 3 different 75 tubes. No rhyme or reason why or when static comes back; it’s not heat nor mechanical shock related or poor / cold joints. Something about that circuit that’s hair triggered enough to eliminate the static. Sometimes the static will go away on its own. Any suggestions as to what it might be?


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 Post subject: Re: Grid cap on 75 Tube Is Touch Sensitive on my Philco 38-1
PostPosted: Mar Tue 31, 2020 12:53 am 
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All of the 75 tubes that you've tried have the same issue: sheared off solder joint inside the top (grid) cap. Remove cap, clean leadout wire with sandpaper; melt old solder in cap, knock it out, reinstall using fresh solder.


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 Post subject: Re: Grid cap on 75 Tube Is Touch Sensitive on my Philco 38-1
PostPosted: Mar Tue 31, 2020 2:22 am 
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Could the radio be going into oscillation? Try putting the grid wire inside the 75 tube shield. This stops the problem on some Philco radios.

http://www.nostalgiaair.org/pagesbymode ... 013239.pdf

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 Post subject: Re: Grid cap on 75 Tube Is Touch Sensitive on my Philco 38-1
PostPosted: Mar Tue 31, 2020 2:36 am 
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You might be onto something, Norm, as all it takes to subdue the whoosh noise is the slightest and lightest contact with the grid cap with a metal object. I've forcefully pressed on the grid cap of all 3 of the 75 tubes with fingers with no squelching of the whoosh, and I have no reason to think there's an intermittent contact under the grid caps. The "shield" for the 75 tube is an odd bracket that semi surrounds the tube. I'm thinking that this might be a lead dress issue and will try routing the grid wire to different positions. Perhaps a capacitor placed in the grid circuit somewhere might kill the oscillation? Will experiment.


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 Post subject: Re: Grid cap on 75 Tube Is Touch Sensitive on my Philco 38-1
PostPosted: Mar Tue 31, 2020 5:33 pm 
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i have seen the 75 and 6D6 be so sensitive to oscillation and squeals. i too have seen the grid cap wire alongside the tube inside the shield.

try that or try another tube as a last resort.

they those tubes can be very finicky, even from set to set.

here is a thread i did that may be of help.

https://www.antiqueradios.com/forums/vi ... 6&t=302323

steve

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 Post subject: Re: Grid cap on 75 Tube Is Touch Sensitive on my Philco 38-1
PostPosted: Mar Tue 31, 2020 7:14 pm 
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Thanks for suggestions, Steve. I'm not done yet. I tried many different things now with no success in removing the noise. No motorboating, no squeal; the offending whoosh noise sounds exactly like the silver mica disease degradation noise I’ve heard a thousand times before on newer radios with those ¾ inch square Ifs, but this is a 1938 radio with a big If transformer without silver mica caps inside. Most baffling thing is that all you need to do is barely graze any component on that 75’s grid circuit with a metal object and bingo, the noise disappears….for 5 or 10 minutes anyway.
Tried an extra shield on the 75 tube. No help.
Tried various value capacitors on the grid circuit to ground. No help
Seems like it would be such a simple solution, not one that requires hours of fussing around with trial and error. Maybe I need to replace the grid wire with a thin shielded cable to snake into the chassis and solder the center conductor to the proper places? Any other suggestions appreciated.


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 Post subject: Re: Grid cap on 75 Tube Is Touch Sensitive on my Philco 38-1
PostPosted: Mar Tue 31, 2020 7:32 pm 
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make sure you reflow the grid cap and make sure you see the little lead sticking thru the hole from the glass into the grid cap before you solder as lorenz said.

this is a long shot, but, ground or B- a .05 capacitor with a alligator clip lead.

start touching the other end of the capacitor on the tube pins and any point in that offending circuit.

then again, if it goes away when you get near it, you might have a lot of "hook up" and walk away time on your hands to see if it comes back.

you could also do this: bounce everything that you did against the schematic. then, bounce the schematic against your work. do this three times.

is it possible that you omitted a component, did a miswire, or change the lead dress on anything ?

swap the 75 ? other tubes ? do the others have the necessary shields and wires in the shield ?

could a replaced mica be going belly up ?

i had that happen but mine was constant:

viewtopic.php?f=6&t=322945&hilit=+rework

i'm pulling at strings here to eliminate the obvious.

maybe you omitted a bypass capacitor by accident ?

that is about all that i have for suggestions as these radios are not my fortee. i am totally a very late 30s to mid 40s AA6/5 guy with the cap tuned IFTs. i will have absolutely have nothing to do with with radios that have the stinking little POS cheapo garbage IFTs that will get SMD 8) .

hope this helps and hopefully some others can chime in who are more seasoned in these radios as i am not.

:)

steve

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 Post subject: Re: Grid cap on 75 Tube Is Touch Sensitive on my Philco 38-1
PostPosted: Apr Wed 01, 2020 7:53 pm 
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Back at the old Philco again. Studying components inside vs schematic is showing me at least 3 factory wired components in this radio that do NOT match Riders schematic. I even looked at other Philco look-alike radio schematics from same time period, but find no match. Is anyone aware of a different schematic version of Philco's 38-15? I've replaced just about every component now, and each and every time that set is turned on that same distortion is present. Detuning IF's and other components will not kill it. It just comes and goes when it damned well pleases.


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 Post subject: Re: Grid cap on 75 Tube Is Touch Sensitive on my Philco 38-1
PostPosted: Apr Wed 01, 2020 9:14 pm 
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that is par for the course with stinking philco and their documentation.

i have no time for stinking philco for numerous reasons.

steve

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 Post subject: Re: Grid cap on 75 Tube Is Touch Sensitive on my Philco 38-1
PostPosted: Apr Fri 03, 2020 9:32 pm 
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Wasn't aware that Philco had documentation issues on their schematics. After beating this intermittently dead horse endlessly for several days on and off trying desperately to match the factory installed parts inside vs schematic, I will avoid getting another late 30's Philco like the plague. This one's ready for the glue factory.


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 Post subject: Re: Grid cap on 75 Tube Is Touch Sensitive on my Philco 38-1
PostPosted: Apr Fri 03, 2020 9:51 pm 
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i had all the documentation for a philco 610. i had the underside chassis part map, the part list chart, and the schematic. in the packet, i even had pic of the exact radio.

it wasnt even physically or technically close.

of the three docs, the parts map, the parts list chart, and the schematic, NONE OF THEM MATCHED the radio.. two of the docs did not match each other.

the kicker ?

this packet was sent to me FROM PHILCO in philadelphia back in the 80s. they were not sams, riders, or biettman.

the radio went in the garbage along with the worthless documentation.

i despise philco and have no time for their rhetoric or proprietary rederick.

steve

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 Post subject: Re: Grid cap on 75 Tube Is Touch Sensitive on my Philco 38-1
PostPosted: Apr Fri 03, 2020 11:53 pm 
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The symptoms suggest RF oscillation in the audio stages. With a good scope that could be confirmed looking at the plate circuit of the 75. But without that there is a "test" you could do to confirm it. Place a 4.7k to 10k resistor in the wire leading to the grid cap of the 75, this will suppress RF oscillations.

Initially my first thought was there might be a poor solder connection from the tube's lead out wire to the grid cap, I have seen that before with 6K7's for example. But as soon as you said you tried other tubes and it was the same, that theory was discounted.

If the series resistor cures, you can simply leave the resistor there, it won't affect the audio frequency response.

But it would be good to identify if there was anything non-standard about your radio predisposing to this condition. Even spiral configuration metal film resistors, have more inductance than the old carbon comp resistors and can result in parasitic oscillations in odd circumstances. Also, there could be a grounding issue, where it looks like you have a good chassis ground, but it is not.

Not the exact same problem as yours, I have a Philco radio, with a 6A8 in the front end, and on short wave there is a hint of instability, only sometimes, in that case it was cured with a 100R resistor placed in series with the g1 grid. Its a very marginal condition, so some designs of radios might or might not have this sort of gremlin, that could show up with some combinations of component types, lead dress and sometimes different tubes, the length, self inductance and path of grid wires can set it off. Even though the tubes you see in radios used in the audio section are being used for audio frequencies, most are capable well up into the RF spectrum, so potential trouble awaits in the wings in that area.


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 Post subject: Re: Grid cap on 75 Tube Is Touch Sensitive on my Philco 38-1
PostPosted: Apr Mon 06, 2020 10:59 pm 
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Tried the suggestion of putting in a 4.7K resistor in series with the 75's grid wire. Radio played beautifully for about 5 minutes, then the hash came back with a vengeance. I waited a few minutes, and it went away on its own, only to return a few minutes later. For the hell of it, I upped the series resistor to 10K, then 50K, then 240K, then 18 freaking MEGOhms! The radio plays beautifully each and every time with just about any series resistor in the box until the hash decides to spoil my day once more. I tried to get creative changing values of several resistors in associated circuitry to no avail. One tiny brush on the 75's grid cap with a metal object squelches the static forthwith, then the gremlin resurfaces a few minutes later. Sometimes the hash is there on a cold start, sometimes not which leads me to believe it's not a heat related issue. Tapping and wood stick probing on all components does not suggest an intermittent joint.


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 Post subject: Re: Grid cap on 75 Tube Is Touch Sensitive on my Philco 38-1
PostPosted: Apr Tue 07, 2020 1:10 am 
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Ok, in that case its not likely the 75 stage that is going into RF oscillation, just signal from it altering the conditions, it would then likely be the audio output tube.

Instead place the 4k7 in series with the grid of the audio output tube. Also check that the audio output tube is ok by substitution in case the one you have is gassy. Another trick is to place a 47R resistor in series with the output tube's anode. Also try connecting a 0.1uF (400 or 630V rated) capacitor between the screen grid connection of the audio output valve and its cathode.

What type is the audio output tube in this set ?

Then there is the possibility of the last IF stage oscillating, try connecting a 0.1uF capacitor from the screen grid of that stage to the cathode. And try adding one on the HT rail in that area as an experiment.


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 Post subject: Re: Grid cap on 75 Tube Is Touch Sensitive on my Philco 38-1
PostPosted: Apr Tue 07, 2020 1:31 am 
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radios with fat pin tubes are not my thing. however, i have worked on a handful.

i remember having quite a few 75 tubes that exhibited noise just like you describe. i had to roll many of them to find one that did not exhibit this anomaly.

i remember marking them "noisy" on the glass.

steve

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 Post subject: Re: Grid cap on 75 Tube Is Touch Sensitive on my Philco 38-1
PostPosted: Apr Tue 07, 2020 9:10 pm 
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I tried each and every one of your resistor and capacitor recommendations, Acorn, and this p.o.s. hash spewing radio has steadfastly thumbed its nose at every one of my repair efforts. I changed all tubes again, to no avail, and even installed a 42 tube in the output socket where the 41 normally resides, but the coronahash reemerges every time....sometimes on turn-on, sometimes after radio plays for 5 or 10 minutes. When the hash emerges, you can turn the volume all the way down, and the hash still comes blasting through the speaker. Touch the 75's grid cap.....BINGO! it's gone again. Touching any of the other grid caps will not have this effect. The schematic for the Philco 38-15 is available on NostalgiaAir, but as I said in a previous entry, there are factory installed components in my set that don't even appear on the Riders schematic.


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 Post subject: Re: Grid cap on 75 Tube Is Touch Sensitive on my Philco 38-1
PostPosted: Apr Tue 07, 2020 9:34 pm 
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i am going to go with experience and say that you have a few 75 tubes that are just a PITA.

unfortunately, i don't think they are cheap to keep buying and rolling until a good one is found.

i've been there with those PITA tubes (and PITA philco) and have not gone back in nearly 35/40 years.

steve

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 Post subject: Re: Grid cap on 75 Tube Is Touch Sensitive on my Philco 38-1
PostPosted: Apr Tue 07, 2020 10:55 pm 
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Wait a minute....YES, I think I found the problem. I had cleaned the pot and band switch some time ago, but just for the heck of it I exercised and twisted the AM / short wave switch back and forth a bit. The problem is gone! I tried everything to induce that hash, and this pathetic Phoolco is now pumping out static and hash free AM for an hour now without interruption. Thanks all, for the tips.....much appreciated. Time to button it up Who knows, years from now some other hapless schlub might have to deal with a 1938 Philco, and this posting will save him days of frustration.


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 Post subject: Re: Grid cap on 75 Tube Is Touch Sensitive on my Philco 38-1
PostPosted: Apr Wed 08, 2020 1:23 am 
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capzapper wrote:
Wait a minute....YES, I think I found the problem. I had cleaned the pot and band switch some time ago, but just for the heck of it I exercised and twisted the AM / short wave switch back and forth a bit. The problem is gone! I tried everything to induce that hash, and this pathetic Phoolco is now pumping out static and hash free AM for an hour now without interruption. Thanks all, for the tips.....much appreciated. Time to button it up Who knows, years from now some other hapless schlub might have to deal with a 1938 Philco, and this posting will save him days of frustration.


If there was a poor connection in the AM-Shortwave switch it doesn't really explain why it appeared that the audio section was giving trouble. So it might be that the problem has gone away coincidentally and it will return when you least expect it as intermittent faults have a habit of doing.

If the problem comes back and if there was a suspicion that say the wiper of the vol control was going open circuit at times, you can add a 1M resistor experimentally across the wiper to the ground (or earthy side if not directly grounded), so if the wiper does go O/C there is still a current path. Normally though, if a G1 grid is left O/C, it charges negatively and the tube cuts off, but sometimes odd things can like oscillations can happen. Though it depends on the circuit how the vol control is connected, if there is a usual coupling cap to g1, and a grid leak resistor there an O/C control would have no effect other than killing the volume.Other circuits make a direct connection.


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 Post subject: Re: Grid cap on 75 Tube Is Touch Sensitive on my Philco 38-1
PostPosted: Apr Wed 08, 2020 11:04 pm 
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I was very suspicious of the volume control from the beginning and tested and rattled it at length trying to find fault, but it didn't reside there. Radio has been playing on and off all day now and most of yesterday without a single incident. When it was on the fritz, it wouldn't play any longer than 5 or 10 minutes without horrific hash, and it could easily be induced by fussing around with the 75's grid circuit. Not now. I know what you mean by coincidence though, as I've often encountered radio problems that will go away at the very same time you've messed with an unrelated circuit. Cold joints are absolutely insidious this way, and tapping on one corner of the radio can often establish / break contact on a far corner of the chassis.


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