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 Post subject: Hallicrafters S-76 — S-meter Potentiometer Replacement
PostPosted: Mar Sat 03, 2018 4:33 am 
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Joined: Feb Fri 22, 2013 5:04 am
Posts: 170
Good evening,

I am restoring a very nice Hallicrafters S-76. It is working quite well with the exception of the wonderfully huge S-meter.

While closely following the Hallicrafters S-meter adjustment instructions, the meter cannot be adjusted to “zero” with the adjustment pot on the back of the radio. The meter pointer rests at about two-thirds of the scale with the left side of the meter being zero. The meter is not dead and does respond to a strong signal that will sometimes almost peg the pointer all the way to the right.

I am virtually certain the problem lies within the potentiometer itself. With some difficulty I was able to remove the back cover of the pot and found a 3/8” wide “bubble” in the wire wound internals of the pot. It looks as though the potentiometer got VERY hot as the fine wire is very discolored around the bubble. The wire is as fine as a human hair but it is not open. However, the wiper in the pot stops when it hits the bubble and will not cycle all the way to the stop inside the potentiometer. Also, there are gaps between the wire winding due to the stretching caused by the bubble.

The potentiometer is 500 ohms and I need a new one. What would be a good replacement? My assumption is that your typical $2.00 potentiometer from Radio Daze would not be satisfactory.

Thank you for any help you might provide.

Jamie


Attachments:
B41E6C15-9297-45F4-B197-12FD1049FBA2.jpeg
B41E6C15-9297-45F4-B197-12FD1049FBA2.jpeg [ 96.3 KiB | Viewed 667 times ]
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 Post subject: Re: Hallicrafters S-76 — S-meter Potentiometer Replacement
PostPosted: Mar Sat 03, 2018 5:06 am 
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Joined: Feb Wed 07, 2018 6:52 pm
Posts: 279
Location: Stone Mountain, GA
Any marking on the pot specifying wattage?

Once you know the power rewuirement, finding a pot (rheostat) rated at that power is easy.

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 Post subject: Re: Hallicrafters S-76 — S-meter Potentiometer Replacement
PostPosted: Mar Sat 03, 2018 5:27 am 
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Joined: Feb Fri 22, 2013 5:04 am
Posts: 170
The markings are:

25C022
137104

I only know the ohm value because it’s specified on the schematic and parts list. No wattage is given.

Jamie


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 Post subject: Re: Hallicrafters S-76 — S-meter Potentiometer Replacement
PostPosted: Mar Sat 03, 2018 5:49 am 
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Joined: Feb Wed 07, 2018 6:52 pm
Posts: 279
Location: Stone Mountain, GA
Looking at the schematic, it is right across the meter. So it must be low watts.

Probably anything will work. Looks like a 2W wirewound one. search for that.

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 Post subject: Re: Hallicrafters S-76 — S-meter Potentiometer Replacement
PostPosted: Mar Sat 03, 2018 8:11 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 11547
Location: North of Mpls, Minnesota
500 ohm 4 watt, Clarostat RTV-25

Dave


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 Post subject: Re: Hallicrafters S-76 — S-meter Potentiometer Replacement
PostPosted: Mar Sun 04, 2018 3:40 am 
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Joined: Feb Fri 22, 2013 5:04 am
Posts: 170
Thanks to each of you for the replies.

There are a number of 500 ohm 2 watt potentiometers on eBay but 4 or 5 watt pots seem hard to come by. I haven’t been able to locate a Clarostat RTV-25, either. I will continue to search but will likely settle on a 2 watt and see how it goes. I would much prefer to find that 4 or 5 watt as easyrider8 suggested.

Also, I am trying to determine what caused the gross overheating in the first place. No luck there yet.

Jamie


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 Post subject: Re: Hallicrafters S-76 — S-meter Potentiometer Replacement
PostPosted: Mar Sun 04, 2018 4:15 am 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 5421
Jamie,

A shorted C18, a short to ground in the primary of T2, or some defects with tube V4 could all cause excessive current flow through that wirewound variable which is fed directly by the B+ bus. Normal power dissipation is pretty low BUT if the resistor goes open then all of the current flows through the meter which will likely destroy it and that is why a very conservatively rated pot is used in this circuit.

Carbon pots get noisy very quickly with DC flow through them so you want to use a wirewound replacement just like the original or very quickly the S meter will become unstable as the resistance of the calibrating pot becomes unstable. Be sure and wire the replacement exactly like the original because that setup means that if the connection between the variable contact and the resistor fails the entire resistance will be in circuit instead of just providing an open circuit when wired as a simple two contact device; wired this way if the rotating contact fails then the meter is toast.

Mouser has a good possibility: https://www.mouser.com/ProductDetail/CT ... QxPgguk%3d

If you run into a really oddball high wattage wirewound pot where you cannot find a good replacement if it is a set and forget type you can always use a wirewound adjustable resistor but that isn't necessary for this fairly common size/wattage pot.

I would keep the meter disconnected from the circuit until you figure out whether there is excessive current draw. A pair of back to back diodes across the meter terminals can offer some protection but you will need to determine the coil resistance and meter sensitivity (full scale current) to choose a proper pair of diodes.

Rodger WQ9E


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 Post subject: Re: Hallicrafters S-76 — S-meter Potentiometer Replacement
PostPosted: Mar Mon 05, 2018 6:46 am 
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Joined: Feb Fri 22, 2013 5:04 am
Posts: 170
Thank you, Rodger. I appreciate your detailed insight as usual.

This radio was a gift from a gentleman in Virginia who had the radio restored in 2010. When I received it, the S-meter was not working and the BFO was not functional. I began “re-restoring” it. I quickly found five out of tolerance resistors, some cold solder joints and four connections not soldered at all. One of the wires connected to the S-meter pot simply popped loose when I began to remove the pot from its mounting hole. The connection was heavily oxidized.

The work I did fixed the BFO and it now works perfectly.

My hope is that one of the problems mentioned above lead to the overheated potentiometer.

As you suggested, I ordered the new potentiometer from Mouser. I should receive it mid-week. I will post a follow-up to this after the new pot is installed.

Again, I appreciate the help each of you contributed.

Regards,

Jamie


Last edited by Jamiecj5 on Mar Mon 05, 2018 10:50 pm, edited 1 time in total.

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 Post subject: Re: Hallicrafters S-76 — S-meter Potentiometer Replacement
PostPosted: Mar Mon 05, 2018 8:32 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 5421
Jamie,

You are welcome and good luck with the S76!

Rodger WQ9E


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 Post subject: Re: Hallicrafters S-76 — S-meter Potentiometer Replacement
PostPosted: Mar Thu 08, 2018 6:00 am 
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Joined: Feb Fri 22, 2013 5:04 am
Posts: 170
I would like to post a follow-up to my earlier posts, if I may.

Tonight I found what I think might be the reason the S-meter potentiometer overheated and failed. I have almost finished double-checking what the previous restorer did back in 2010 and found the errors shown in the images below. The one that particularly got my attention was the incorrect resistor, R16, attached to pin 6 of V4, the 1650 kHz IF Amp tube. (This tube is directly involved in the functioning of the S-meter.) It should be an 8,200 ohm resistor but the previous restorer had used an 82,000 ohm resistor instead. Over time, could this much too high value resistor have lead to the overheating and eventual failure of the S-meter adjustment pot?

Other discrepancies are noted in the images, as well.

Regards,

Jamie


Attachments:
E7959685-7ABA-47CE-BC98-351B66427F14.jpeg
E7959685-7ABA-47CE-BC98-351B66427F14.jpeg [ 173.29 KiB | Viewed 469 times ]
F9FA38FC-B0B9-43B1-A4D5-1AB0DEC321D1.jpeg
F9FA38FC-B0B9-43B1-A4D5-1AB0DEC321D1.jpeg [ 145.19 KiB | Viewed 469 times ]
B34DD830-9590-47C4-B1D3-C6FB6D368AD6.jpeg
B34DD830-9590-47C4-B1D3-C6FB6D368AD6.jpeg [ 149.69 KiB | Viewed 469 times ]
08CF3744-F9D9-4933-AB8B-B39D55E5E1F0.jpeg
08CF3744-F9D9-4933-AB8B-B39D55E5E1F0.jpeg [ 166.51 KiB | Viewed 469 times ]
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 Post subject: Re: Hallicrafters S-76 — S-meter Potentiometer Replacement
PostPosted: Mar Thu 08, 2018 6:28 am 
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Joined: May Fri 01, 2015 2:33 am
Posts: 1127
Location: Roslyn,Pa 19001
Probably not. By have a having a high value resistor there would low the screen grid voltage and cause the tube to draw less plate current rather than more which is what moves the meter movement. It would make it difficult to zero the meter and cause poor sensitivity.

Terry N3GTE


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 Post subject: Re: Hallicrafters S-76 — S-meter Potentiometer Replacement
PostPosted: Mar Thu 08, 2018 1:19 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 5421
Agree with Terry that the wrong screen resistor didn't cause problems with the pot but it would definitely drop your sensitivity.

The tube would have to draw 100 mils through the pot to hit its rated power dissipation and the only way a 6BA6 could sink that level of current is through an internal short but not through any sort of tube operation since that is well beyond its cathode emission capability.

The pot could have developed an internal short to case ground causing overheating or it may have overheated due to poor contact between the wiper and the wire but it probably just died from a combination of heat and use/age.

However finding one resistor that was replaced with a clearly incorrect value makes it very reasonable to question other work done to the receiver prior to your ownership.

Rodger WQ9E


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 Post subject: Re: Hallicrafters S-76 — S-meter Potentiometer Replacement
PostPosted: Mar Fri 09, 2018 4:47 am 
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Joined: Feb Fri 22, 2013 5:04 am
Posts: 170
Success!!

The S-meter is working to perfection after installation of the new pot from Mouser recommended by Rodger. Even this wasn’t without its problems, though.

Once the pot was installed and the S-meter started working, I immediately noticed the pointer would stop 3/8” before it swung all the way to the left toward zero. It was as though it hit a hard stop.

I removed the meter and disassembled it. I found a tiny sliver of ferrous metal stuck to the magnet on the right side of the meter movement. With tweezers and a narrow strip of masking tape I was able to “grab” the sliver and remove it. I reassembled the meter and it now works perfectly.

The sensitivity of the radio is quite good and it also has very pleasing audio.

Again, I sincerely appreciate the help I received from each of you.

Regards,

Jamie


Attachments:
File comment: New potentiometer installed. It is MUCH smaller than the original.
AA177763-9C8E-42F0-A3E4-49B7C77990F7.jpeg
AA177763-9C8E-42F0-A3E4-49B7C77990F7.jpeg [ 122.43 KiB | Viewed 421 times ]
File comment: Disassembled meter
3CC6A00B-2A46-49C0-83F0-58EE765C7C31.jpeg
3CC6A00B-2A46-49C0-83F0-58EE765C7C31.jpeg [ 118.03 KiB | Viewed 421 times ]
621B328E-D1CA-44C8-824A-B73CCF6C3738.jpeg
621B328E-D1CA-44C8-824A-B73CCF6C3738.jpeg [ 94.84 KiB | Viewed 421 times ]
C697379C-CEA2-4F98-A849-ACB93EB942B0.jpeg
C697379C-CEA2-4F98-A849-ACB93EB942B0.jpeg [ 137.17 KiB | Viewed 421 times ]
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 Post subject: Re: Hallicrafters S-76 — S-meter Potentiometer Replacement
PostPosted: Mar Fri 09, 2018 3:03 pm 
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Posts: 5421
Nice job and it looks very nice Jamie! I am glad the meter repair went well.

Rodger WQ9E


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 Post subject: Re: Hallicrafters S-76 — S-meter Potentiometer Replacement
PostPosted: Mar Fri 09, 2018 4:02 pm 
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Location: Portland, TN, USA
Well done, Jamie!

That delicate meter surgery takes a good eye and a steady hand, but is always rewarding when successful - especially on a good-looker like your S-76.

Color me envious. :D

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 Post subject: Re: Hallicrafters S-76 — S-meter Potentiometer Replacement
PostPosted: Mar Sun 11, 2018 6:39 pm 
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This month's (March, 2018) Electric Radio has an article on the S-76. It offers some modifications and discusses restoration.

Norm

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