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 Post subject: Weak Firestone AA5
PostPosted: Sep Sun 15, 2013 2:55 am 
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Hi everyone, I'm working on a Firestone S-7403-2, and it plays very very weakly. The audio is barely audible. All the tubes check good, all the resistors are within tolerance as far as I know. The caps have been replaced. I tweaked the IFs and they seem fairly close, I don't have a working signal generator yet. Not sure what else to check.
http://www.nostalgiaair.org/PagesByModel/284/M0006284.pdf


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 Post subject: Re: Weak Firestone AA5
PostPosted: Sep Sun 15, 2013 5:15 am 
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Location: Livermore, CA
Have you checked voltages? Pin #3 and #4 on 35L6 should be positive, around 100 volts.

Does touching center terminal on the volume control cause loud noise?

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 Post subject: Re: Weak Firestone AA5
PostPosted: Sep Sun 15, 2013 7:08 am 
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Norm Leal wrote:
Have you checked voltages? Pin #3 and #4 on 35L6 should be positive, around 100 volts.

Does touching center terminal on the volume control cause loud noise?


And as a reminder, if you go sticking your fingers in there, for goodness sake hold the other one behind you and not touching anything else. A soldering iron stuck to the volume control is a little safer.

Brett


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 Post subject: Re: Weak Firestone AA5
PostPosted: Sep Sun 15, 2013 12:46 pm 
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Quote:
I tweaked the IFs and they seem fairly close, I don't have a working signal generator yet.
Without a signal generator, how would you know the IFs were "close"? But--if you tweaked them all up to the same frequency while tuned to a station, then that is not likely to be your issue.

At least here, the noise at the low end of the dial is significant---if you tune to 600 or so and crank up the volume, do you get noise? If not, then the issue is at the audio output end. In addition to checking all voltages, see if there is any evidence of overheating.

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 Post subject: Re: Weak Firestone AA5
PostPosted: Sep Sun 15, 2013 5:48 pm 
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I tried touching the center tap of the volume control with the soldering iron, nothing happened. I haven't checked the voltages, I will after I get home from work today. The schematic also doesn't list the voltages. I used my AM transmitter and tweaked the IFs for the loudest signal, didn't change much tough. I don't see any evidence of overheating.

Thanks for your help so far.

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 Post subject: Re: Weak Firestone AA5
PostPosted: Sep Sun 15, 2013 6:35 pm 
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Jaycebot wrote:
I tried touching the center tap of the volume control with the soldering iron, nothing happened. I haven't checked the voltages, I will after I get home from work today. The schematic also doesn't list the voltages. I used my AM transmitter and tweaked the IFs for the loudest signal, didn't change much tough. I don't see any evidence of overheating.

Thanks for your help so far.



Iron was on, right? If so, that means there's something wrong with the audio section (which may in turn be a function of the power supply). I would get that working, check the supply voltages, then move on.

The general rule (if you have no other indicators like blown/gassy tubes or parts obviously burned up or failed) is
1 power supply
2 audio
3 IF
4 RF

Brett


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 Post subject: Re: Weak Firestone AA5
PostPosted: Sep Sun 15, 2013 8:31 pm 
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When you get home measure voltage on pin #3 and #4 of 35L6. We should be able to tell from those readings if voltage is low. Expect somewhere around 100 volts. Pin #8 should have around 5 volts if the circuit is operating properly.

While you are at it measure pin #6 on 12SQ7. If correct that pin will be more like 50 volts. If you have proper voltages on 35L6 and 12SQ7 audio circuits should operate.

Using a soldering iron, plugged in or not, should give hum out of the speaker. The soldering iron will act like an antenna and pick up 60 cycle. Your finger would have the same effect but as Brett mentioned a person could get shocked it they also touch other places in the radio.

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 Post subject: Re: Weak Firestone AA5
PostPosted: Sep Sun 15, 2013 9:29 pm 
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Don't forget, the soldering iron on the volume control terminal trick only works if the volume control is turned-up, off zero volume.

And don't use a grounded soldering iron on an non-isolated AC/DC radio! You should get a good buzz from the speaker with the iron unplugged from the wall.

Ted


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 Post subject: Re: Weak Firestone AA5
PostPosted: Sep Sun 15, 2013 9:36 pm 
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Verify with your meter that the loop antenna has continuity, and is connected correctly.

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 Post subject: Re: Weak Firestone AA5
PostPosted: Sep Mon 16, 2013 1:35 am 
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I measured the voltage on pin 3 of the 35L6, I get 107 volts, on pin 4 I get 87 volts. Pin 8 reads 7 volts. Pin 6 on the 12SQ7 reads 48 volts. I do not get a buzz when I touch the soldering iron to the center terminal on the volume control. The antenna also checks good and is wired correctly.

Thanks for your help so far

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 Post subject: Re: Weak Firestone AA5
PostPosted: Sep Mon 16, 2013 4:07 am 
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Those voltages are reasonable and should give sound. Is the cap across output transformer, C9, shorted?

Is cap C8 on the right pins? Pin #6 of 12SQ7 to pin #5 of 35L6. Does cap C6 connect between center of volume control to pin #2 on 12SQ7?

These are things that could stop sound to your speaker. Speaker being bad could also but you do hear some sound.

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 Post subject: Re: Weak Firestone AA5
PostPosted: Sep Mon 16, 2013 4:23 pm 
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Hi Norm,
I'll check those after school today. Thank you for your help so far. Earlier, I tried another speaker and it had no effect on the amount of volume.

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 Post subject: Re: Weak Firestone AA5
PostPosted: Sep Tue 17, 2013 3:15 am 
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Cap 8 is correct. Cap 6 is also correct. However, cap 9 was not connect across the primary of the output transformer, one end went to ground. That's the way it was originally, however that is incorrect according to the schematic. I soldered it to where it is supposed to go, and it had no effect on the amount of volume. Getting closer though.

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 Post subject: Re: Weak Firestone AA5
PostPosted: Sep Tue 17, 2013 11:36 pm 
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With the radio on and warmed up try shorting pin6 (12SQ7 plate) to chassis using a screwdriver . If all is well thru to the speaker, you'll hear a loud pop. If not, chances are the speaker is bad as all voltages are good.
Willard


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 Post subject: Re: Weak Firestone AA5
PostPosted: Sep Wed 18, 2013 1:04 am 
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test resistance of output transformer with 1 side disconected on both sides , I had one that had internal shorts making radio play low volume

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 Post subject: Re: Weak Firestone AA5
PostPosted: Sep Sun 22, 2013 5:38 am 
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Hi Willard and Daniel,
I'll give those a try. I've been very busy with school lately, hopefully I can take a look at it again tomorrow sometime.

Thanks!

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 Post subject: Re: Weak Firestone AA5
PostPosted: Sep Sun 22, 2013 3:55 pm 
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Usually Lurking wrote:
Don't forget, the soldering iron on the volume control terminal trick only works if the volume control is turned-up, off zero volume.

And don't use a grounded soldering iron on an non-isolated AC/DC radio! You should get a good buzz from the speaker with the iron unplugged from the wall.

Ted
For my learning, couldn't one accomplish the same task with an insulated screwdriver and avoid the lightning rod issue altogether? Don't mean to hijack the thread.
-Mike


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 Post subject: Re: Weak Firestone AA5
PostPosted: Sep Sun 22, 2013 4:30 pm 
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All the voltages indicate no open in the output transformer primary and the 7volts on the cathode indicates output tube conduction with bias. The check that
I recommended is just a quick and dirty way to check the audio output after you find you have no hum when touching the volume control center lug with the VC turned all the way up. The check I suggested quickly proves whether or not the output stage is the culprit. If you hear a loud pop when shorting the C8 side of the 12SQ7 plate to chassis there is at least a 98% chance the output stage thru to the speaker is operating.
Another quick and dirty check on AA5 sets with PM speakers with no audio is to listen carefully for hum even, low hum. If you hear it this almost always means the speaker and/or audio output transformer are not causing the no audio. I think you said in an early post you did hear weak audio so what I've just said may be moot here
Willard


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 Post subject: Re: Weak Firestone AA5
PostPosted: Sep Tue 24, 2013 8:14 pm 
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Mike,

The finger, or the soldering iron, usually provides more signal than an insulated screwdriver would.

You might hear only a click and then a very soft buzz, using the screwdriver.

The idea is to connect a fairly large conductive object to the audio amplifier's input.

This trick won't work on a desert island, either. It depends on nearby AC power wiring.

Ted


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 Post subject: Re: Weak Firestone AA5
PostPosted: Sep Tue 24, 2013 8:44 pm 
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When you tested the tubes, how long did you leave them on? I only say this because today, along with several other times I have had a situation where a tube, and in particular the output tubes will test "good", but if left in test mode, once it got hot the tube would then act screwy. I had this happen on another Firestone: 5Y3 tested good. But then after a minute or two the needle started jumping all over the place. A new 5Y3 fixed the problem.


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