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 Philco 38-620 Dead?

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radionovice



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PostSubject: Philco 38-620 Dead?   Thu Oct 14, 2010 11:03 pm

I've finally gotten back to this Philco I started a while back. It's been recapped and all weak/bad tubes replaced. It had been worked over before I got it, so I started at a disadvantage. Anyway, the only audio I get is an occasional 'pop' from the speaker. The screen and plate voltages are off on every tube; the screens are all high. The original speaker had a bad xfmr, so I replaced it with one from a 37-610. Also, the 6F6 gets scalding hot in just a couple of minutes. I know the metal ones get hot, but... Thanks in advance for your help.

Jerry
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Johnnysan



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PostSubject: Re: Philco 38-620 Dead?   Thu Oct 14, 2010 11:35 pm

I would want to use a glass 6F6 just to see the plate (and if it got red).
Is your field coil OK (should measure 1140 ohms)?

Next step is to set the volume control at mid position and touch the center wiper (you should get hum). You should have a negative voltage at the grid of the 6F6; if not, the power supply and filter caps probably have a problem (like the previous owner may have put the caps in wrong).
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radionovice



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PostSubject: Re: Philco 38-620 Dead?   Fri Oct 15, 2010 12:03 am

Okay, I was a little mistaken when I first wrote this. The symptoms I gave were old ones, and I really didn't check as completely as I should have before posting;engaging mouth before the brain again. So, to address what you said Johnnysan: I did the recapping and ensured they were right with the schematic, no problem there. I find that I do get a little sound through the speaker when I touch either a meter probe or solder iron tip to as far back as the plate of the 1st AF (6Q7). It only makes scratching sound instead of hum when touched with soldering iron. Voltages are good on 6Q7, and only marginally high on 6F6. I also find that the same sound happens when I move the RF section, indicating, not surprisingly, that the rubber washers are worn through. As old as this radio is, I'm also thinking that many of the resistors have drastically changed value. I have a -11 volts on the grid of the 6F6.
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Johnnysan



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PostSubject: Re: Philco 38-620 Dead?   Fri Oct 15, 2010 12:21 am

What is your grid voltage? This is important because you could burn up your output tube; it should be -16 to -20.

Next you should clean all the switches. If you can, inject audio at the center tap of the volume control; you should have full audio at the speaker. When you have audio, any problems will be in the IF or RF stages. Do not try to align the radio--it isn't fixed yet.

Go ahead and check the resistors now; they may have changed value.
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smeezekitty

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PostSubject: Re: Philco 38-620 Dead?   Fri Oct 15, 2010 12:33 am

Johnnysan wrote:
What is your grid voltage? This is important because you could burn up your output tube; it should be -16 to -20.
Quote :

I have a -11 volts on the grid of the 6F6.
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Johnnysan



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PostSubject: Re: Philco 38-620 Dead?   Fri Oct 15, 2010 12:56 am

I have to assume that this radio uses a candohm; has it been replaced? If so, is the resistance 202, 8 and 35 ohms? This will affect the negative voltage used for bias at the grid of the 6F6.

I'm not sure what the bias voltage should be, but the 6F6 grid should be run at -16.5 to -20 under normal operation. -11 seems low.
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radionovice



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PostSubject: Re: Philco 38-620 Dead?   Fri Oct 15, 2010 2:06 pm

The candohm appears to be good, it reads 40, 10, 220. All within 20%, though maybe high. It appears to be the original. I'm running my variac at 120, perhaps I should reduce it to 115?
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Steve Chambers

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PostSubject: Re: Philco 38-620 Dead?   Sun Oct 17, 2010 2:45 pm

Is it possible you made a mistake in wiring when you replaced the output transformer?

Steve Chambers
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radionovice



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PostSubject: Re: Philco 38-620 Dead?   Mon Oct 18, 2010 4:00 pm

I didn't replace the transformer, Steve. You may be thinking of another of my threads about my Farnsworth. What I did find late last night though, is the 70k ohm grid resistor on the 6F6 is reading 90k, which means too much of a voltage drop. That would account for such a high grid voltage, which could keep the tube cut off. If I have a new one, I'll replace it tonight and see what happens.
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Steve Chambers

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PostSubject: Re: Philco 38-620 Dead?   Mon Oct 18, 2010 4:54 pm

"The original speaker had a bad xfmr, so I replaced it with one from a 37-610."

This is the sentence that led me to think you had replaced the output transformer. I must be misreading or misunderstanding. Keep us posted.

Steve Chambers
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radionovice



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PostSubject: Re: Philco 38-620 Dead?   Mon Oct 18, 2010 5:17 pm

My bad, Steve, I misread you. I replaced the entire output transformer/speaker assembly. It is always a possibility that I did miswire when I replaced them. And thanks for the reminder to check it again.
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radionovice



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PostSubject: Re: Philco 38-620 Dead?   Tue Oct 19, 2010 6:29 am

The field coil is good, as is the output transformer. The transformer is wired correctly. The candohm appears to be good. The 70k resistor in question is not the grid resistor for the 6F6, but rather is in the plate circuit of the 1st AF. The plate of the 1st AF reads 9 volts instead of 150.
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