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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sat 17, 2020 8:49 pm 
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Joined: Aug Tue 25, 2020 10:22 pm
Posts: 76
Focusing on the S meter now.

I measured the signal going through the S meter and it doesn't seem to by varying in voltage as I go to a strong station and then to dead air.

I measured in AC and DC volts nothing.

I used my continuity setting and traced each wire through the radio.

The positive terminal wire goes to a connector on the S meter adjustment pot on the back of the radio see the pic.

I traced the negative to a terminal on the underside that connects through resistors to the two 6sq7 tubes see pic.

If anyone has some ideas i am all ears.

Mike.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sat 17, 2020 9:00 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1965
As I recall, the S meter simply measures the plate current of the IF stage(s) which responds to the AVC voltage. Does the S meter move at all? If so, does it change when you back off the RF gain control? Does the zero adjust on the rear panel have an effect?


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sat 17, 2020 9:25 pm 
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Joined: Aug Tue 25, 2020 10:22 pm
Posts: 76
The rear panel adjusment does move the meter. But otherwise nothing else seems to affect it at the moment.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sat 17, 2020 9:47 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1965
Is the meter at zero or full scale? As I recall it's a reverse meter, with zero at the right. So the zero adjustment is really a full scale adjustment to account for the various tube emissions and line voltage variations. When the signal appears, the AVC reduces the plate current and the meter moves to the right.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sat 17, 2020 10:37 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 3659
Location: Seattle WA US
Very similar looking S-meter adjustment pots in SX-28 receivers have earned a reputation for weird behavior caused by flaking of metal plating inside the pot causing random shorts to ground. If all normal reasonable explanations fail, try a pot change.

-Chuck


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 18, 2020 2:21 am 
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Joined: Aug Tue 25, 2020 10:22 pm
Posts: 76
Thanks everyone. K7MCG I am going to follow your suggestion and order a replacement for that pot.

cheers,

MK


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 18, 2020 4:09 am 
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Joined: Oct Fri 09, 2009 9:25 pm
Posts: 396
Location: Cleveland, Ohio
In an earlier post, you mentioned that you applied a direct 'voltage' to the 'S' meter. Did you first confirm that you did not damage/short the winding or the mechanical movement? Typically these are 1ma FS movements and it obviously takes very little to destroy them.

If that pot is defective, it is easy to determine if that really is the case, vs. simply assuming that is the case. If I recall, the resistance is 500 ohms. Check the two outside terminals for that resistance. The wiper to winding continuity and integrity can then be checked to see if there are any dropout/open areas, by watching the ohm meter (analog or digital with a bar graph) as the control/wiper is rotated. This component is not your problem if continuity is verified. Also, the pot is only intended to add enough resistance to zero the meter, not affect it's range or sensitivity.

You might want to look at the operating procedures and schematic, to see how the 's' meter circuit functions and how the deflection is derived. If I recall, (I don't remember for sure) it comes off the AGC buss.

Is the sensitivity/RF gain fully advanced? What mode are you in? If I recall, the meter functions only in AM with AGC on. The rf gain pot should have a switch that when engaged, the meter will read a carrier correctly. Backing off the rf gain will result in reduced meter circuit readings.

Todd
ka8gef

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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 18, 2020 5:25 pm 
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Joined: Aug Tue 25, 2020 10:22 pm
Posts: 76
Ya the meter is still good I took it apart nothing is damaged. all the tiny wires are intact. I was very careful when I tested it. I applied voltage starting from zero and stopped as soon as the meter made a motion.

I pulled the pot and put in a 500 ohm resistor.

The radio works well but the S meter still does not work as it should. It does respond to when you turn the tiny adjustment screw though and will shift left or right.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 18, 2020 5:51 pm 
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Posts: 311
Location: Rockford, IL
My SX-25 meter works 100% and responds as it should - I can take pix of areas in mine if you need to. Sounds like your meter works - I gave my S-meter pot a spritz of DeOxIt and then calibrated the meter as per the manual. So it works great and I can actually use it as it was intended (nice bonus).


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 18, 2020 6:08 pm 
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Location: Cleveland, Ohio
What signal level are you using? Are you looking at a strong local broadcast band signal, that typically will be well over 20 to 30db over S-nine....is that type of signal reading near zero as well? Is the rf gain/sensitiviity advanced as noted? I don't recall if there is an AVC mode (should be), if so is that 'on'.....

Todd
ka8gef

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'Itoshiki oshieo idaki'
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Last edited by KA8GEF on Oct Sun 18, 2020 6:15 pm, edited 1 time in total.

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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 18, 2020 6:14 pm 
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Joined: Aug Tue 25, 2020 10:22 pm
Posts: 76
When I put in the 500 ohm resistor. There is now more voltage going to the audio tubes so a resistor smoked there and I heard a pop so I believe it also affected my can cap so I am having a look.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 18, 2020 6:45 pm 
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Posts: 396
Location: Cleveland, Ohio
This receiver has AVC/MVC switching on the front panel....make sure the BFO is off, AVC on.

...while in there you might check R6, R7, R10, R11....and remove that 500R resistor. Can you zero the pointer with the pot? How much pointer movement do you see when turning the pot from min to maximum?

Todd
ka8gef

_________________
'Itoshiki oshieo idaki'
-Let us never lose the lessons we have learned.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 18, 2020 8:01 pm 
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Joined: Aug Tue 25, 2020 10:22 pm
Posts: 76
When I put in the 500 Ohm resistor things went strange. It seemed to work well. Really loud volume on band 3 everything good then the resistor from rect 80 to the first audio tube started to burn.

Then my 50v 22uf cap in the filter section went pop.

So now I put back in the variable resister and read the resistance. It goes from about 10 ohms to 90 ohms!

I have an early version of the SX25 so maybe that was that model spec.

The voltage at that filter cap was increasing at a level that would exceed the 50v rating with that 500 ohm resistor in place.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 18, 2020 8:41 pm 
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Joined: Aug Tue 25, 2020 10:22 pm
Posts: 76
Ok one of my audio tubes went caput. Put back in the old 6F6s and all is good again.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 18, 2020 8:45 pm 
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Joined: Aug Tue 25, 2020 10:22 pm
Posts: 76
When i turn off the s meter the meter goes to zero if that helps.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 18, 2020 8:46 pm 
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Joined: Aug Tue 25, 2020 10:22 pm
Posts: 76
Oh and the radio is not calibrated so there is that!


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 18, 2020 8:55 pm 
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Posts: 311
Location: Rockford, IL
This is a strange thing about the SX-24, 25, and 28: when you max the RF gain and switch of manual gain, the meter light comes ON. When on manual mode, my use of the RF gain can dial in something around the S9 setting with no problem. Strong local channels require me to use 1/4 to 1/5 the RF gain to prevent overloading. But when using the manual RF gain, the meter light goes OFF and then you can modulate the RF gain directly.

When I select the auto gain (switch clicks OFF) then my S meter goes all the way left and moves with respect where the station is at - but it does respond however seems to use a different scale from what I can tell (experience using a SX-28A, SX-24, and this set they all seem to work the same). I would have thought the automatic gain control would use the same scale, but I am not seeing that with the other three sets that are similar. From what I can tell my other two sets work this way too with the S-meter behavior and how there appears to be two scales in response depending on manual or auto RF gain.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 18, 2020 8:56 pm 
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Location: Rockford, IL
Oh yes: calibration is a game changer if it appears it is not working correctly lol.

mk2020@ wrote:
Oh and the radio is not calibrated so there is that!


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 18, 2020 9:06 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1965
I am sorry but your explanations are confusing. I don't know what you mean by some of the comments. I don't know what you call zero on the meter.

Maybe if you take more time in composing your messages I can understand you.


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 Post subject: Re: Hallicrafters SX25 restoration
PostPosted: Oct Sun 18, 2020 10:23 pm 
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Joined: Aug Tue 25, 2020 10:22 pm
Posts: 76
Bob zero on the meter I believe is the where the meter is not reading anything on the scale.


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