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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Wed 11, 2012 2:02 am 
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I spent a few hours carefully tracing wires and connections. Cleaned up solder joints as well so I could see what I'm doing. I found where I had connected a ground to the wrong side of R30 so that's were our short was. I replaced R21 which had drifted a few percent up but R24 had drifted all the way up to 134k, so I replaced that one as well.

I powered it up to measure voltages and got -18 on pin 5 of the 6K6s which is good. Pin 3 of 6K6 is 264v. I don't know if this is good but the voltage across R30 is -19v.

Nothing was smoking or showing signs of problems so I attached my RP-190 to the phono input and tried it. The volume was much stronger but I still got the horrible squeal when I rotated the volume and tone pots. I was however able to find a position where it didn't squeal.

I feel like we're making progress. Thank you so much for being patient with me!


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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Wed 11, 2012 2:31 am 
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noisebox wrote:
I feel like we're making progress. Thank you so much for being patient with me!
Yeah! The load on the power transformer and audio output transformer are gone. Notice your high voltage went up about 90 volts. All the voltages you reported sound like they are in the right area. :-)

With regard to squealing, is this something that seems to be happening because of a dirty volume control?

Curtis Eickerman

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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Wed 11, 2012 2:48 am 
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What I didn't want to tell you is that I got a high voltage shock after I turned it off and was checking things out. That sir, is a very hard way to learn about electrolytic capacitors--again!!

The volume/tone squeal I don't think is from a dirty pot. That usually sounds crunchy. This is a VERY loud squealing sound. Then there's the issue of the bad loctals, the 7C6 being one of them. I've replaced two of them but I think the replacements are horrid quality and don't properly grip the pins.


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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Wed 11, 2012 3:00 am 
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noisebox wrote:
What I didn't want to tell you is that I got a high voltage shock after I turned it off and was checking things out. That sir, is a very hard way to learn about electrolytic capacitors--again!!
Been there done that. Not sure how long the radio was off, but you could add a 100K 1 W bleeder resistor across each of the 3 electrolytics and they will be pretty much self discharged within less then 10 seconds. Alternatively just make sure to ground the B+ line with something before getting in there with your fingers.

noisebox wrote:
The volume/tone squeal I don't think is from a dirty pot. That usually sounds crunchy. This is a VERY loud squealing sound. Then there's the issue of the bad loctals, the 7C6 being one of them. I've replaced two of them but I think the replacements are horrid quality and don't properly grip the pins.
That might be problem alright. This radio also uses some feedback from the speaker back into the volume control. I did not try to make sense out what they were doing with that. It might be possible to disable some of the fancy stuff temporarily to see if that is stable then start adding things back in to see where the squealing reappears.

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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Wed 11, 2012 4:55 am 
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Just a quick comment. Was the output transformer replaced or rewired? Polarity is important when it's being used for negative feedback. Reversed leads and feedback is positive causing oscillation.

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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Wed 11, 2012 5:00 am 
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Norm I think you're right. When I put together my first push pull with neg feedback it did the same thing. You recommended switching the leads as well and that stopped the oscillation. I;ll give it a shot and report back.
Ben


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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Thu 12, 2012 12:32 am 
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Yes, that solved the oscillation squealing.
I gave it a test run for a half hour playing 45s through my RP-190 and it sounds pretty decent.
I do have a loud hum problem though. It changes tone and adds higher frequencies when the volume pot is turned up--but is still present with the volume turned all the way down--with more of a 60hz hum.
Two caps, C35A and C35B have not been changed out. They reside inside of a plastic thing at the power connection. Could these be a source of trouble?
Thanks again for the help,
Ben


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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Thu 12, 2012 1:03 am 
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noisebox wrote:
Two caps, C35A and C35B have not been changed out. They reside inside of a plastic thing at the power connection. Could these be a source of trouble?
C35A&B would not be a reason for hum. They are present to keep RF noise out of the power system for the radio. This is stuff like conducted interference coming down the AC power line from florescent lights, light dimmers, switching power supplies and motors to name a few. This stuff is related to 60 Hz, but generally sounds like buzzing in the radio and is more noticible between stations.

I lost track of whether you said C36 and C37 were changed. That could be a problem if they have not been changed. There are other possibilities, but lets start there.

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Last edited by Eickerman on Apr Thu 12, 2012 1:08 am, edited 1 time in total.

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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Thu 12, 2012 1:08 am 
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Yes, C36 was changed--a double electrolytic, which is now in a can.


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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Thu 12, 2012 1:09 am 
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noisebox wrote:
Yes, C36 was changed--a double electrolytic, which is now in a can.
And C37?

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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Thu 12, 2012 1:39 am 
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C37 also done, though mounted under chassis. For C36 I used a 10mf and 33mf. C37 is 10mf.


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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Thu 12, 2012 1:45 am 
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Cap 37 was 20mf in the schematic . Your 10mf may not be enough to remove all hum?

http://www.nostalgiaair.org/PagesByMode ... 013588.pdf

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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Thu 12, 2012 2:40 am 
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I'm sorry that was a typo. C37 is a 20 I put in. C36 is a 10 and a 33.


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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Thu 12, 2012 3:16 am 
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Over the next few days I have to work 10 hour shifts so I won't be online much. Hopefully in that time I can check the rest of the resistors in the receiver section and maybe deal with the bad loctal socket. Is there a way I can check those electrolytics to see if that's where some of the noise is coming from?
Ben


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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Thu 12, 2012 3:20 am 
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Ben

You can connect a 10 mf @ 450 volt across an electrolytic cap to see if hum is reduced.

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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Thu 12, 2012 3:33 am 
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A couple of other things worth checking. Since you were replacing components you probably should make sure the areas highlighted in orange below are tucked pretty tight against the chassis and as far as practical away from either the AC power line wiring or filament wiring in the radio. In addition if you want to try something else you could drop the value of R17 as low as 1 Meghohm without a detectable effect on low end frequency response and that grid circuit will become much less sensitive to picking up stray 60 Hz fields.
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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Thu 12, 2012 5:27 am 
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As you can see there's a lot of spaghetti going on here! All I had to do was move the electrolytic seen on the left and the humming changed. I might just mount that one in the can on top. Now I know why they did that.
The lead from the 7C6 is shielded with the lead running to pin 3 of the 7C6 and the shield soldered to R30. That orange lead is coming from pin 8 of the 5Y3 and goes to C36 so there's probably a lot of noise there as well. I'm sure this mess is where most of the hum is coming from. I just need to get in there and sort things out.

The 7C6--I looked it up to see what it does and it's pretty confusing to me--a Duo Diode Hi-Mu Triode. I get the part where the signal from the phono comes in to the grid, pin 3, and affects the anode voltage/signal, but all that other stuff is strange. I know it somehow passes the RF signal on its merry way to the anode as well, but how? 1 and 8 are the heater, 7 is the cathode, so what are 5 and 6?

http://frank.pocnet.net/sheets/084/7/7C6.pdf


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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Thu 12, 2012 6:02 am 
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5 and 6 are the diodes.

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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Thu 12, 2012 8:49 pm 
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Quote:
you could drop the value of R17 as low as 1 Meghohm


Sorry Curtis, that's a bad idea. The circuit is relying on the high resistance to develop grid bias for the 7C6. It's called "contact potential" biasing. On their way towards the plate, a few electrons strike the grid. This tiny current, through a huge resistor, develops the half a volt or so the tube needs to keep the grid from going positive on peaks and distorting.

I agree with everything else you said. I wish I'd had you as an Elmer when I was a kid.

Dave Wise


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 Post subject: Re: Strange Philco problem --need help
PostPosted: Apr Thu 12, 2012 9:00 pm 
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Vintage Dave wrote:
Sorry Curtis, that's a bad idea. The circuit is relying on the high resistance to develop grid bias for the 7C6. It's called "contact potential" biasing. On their way towards the plate, a few electrons strike the grid. This tiny current, through a huge resistor, develops the half a volt or so the tube needs to keep the grid from going positive on peaks and distorting.
Oops, sorry. You are right. I missed the fact that they directly grounded the cathode of that tube. With a little cathode resistance and suitable bypass cap "then" you could turn down that resistor and help with hum immunity, but that's going a little far. Looks like it would take about a 820 ohm to 1 K resistor and 8 to 10 mfd 5 volt cap.

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