Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Aug Sun 25, 2019 1:08 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 21 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: Power Resistor
PostPosted: May Thu 30, 2019 5:36 am 
Member

Joined: Apr Wed 10, 2019 3:48 am
Posts: 71
The ac power in my house is pushing 125v, and the radio I am working on (Westinghouse WR480 - transformer based) is from the 40s when house power was probably around 110v. I have a 2 watt 390ohm resistor tied to the output tube that is getting overly hot, and I noticed that the voltages throughout the chassis are higher than spec. This follows a full recap and replacement of many resistors that were out of value.

I noticed that some posts suggest adding a power resistor (25ohm 50watt or 30ohm 50watt) to get the higher house voltage down to a more reasonable level. Apparently, the heat sinked resistor gets mounted to the chassis to dissipate heat.

Is this commonly done?

Should it be placed in series at the 125v ac neutral power side coming in?

Would the resistors shown in the links below be acceptable?
(They appear to meet these parameters, but seem to be frequently used for ring and nest doorbells on the secondary 24v ac side of transformer.

https://www.amazon.com/TiToeKi-Resistor ... way&sr=8-4

https://www.amazon.com/uxcell-Aluminum- ... way&sr=8-1


Top
 Profile  
 
 Post subject: Re: Power Resistor
PostPosted: May Thu 30, 2019 5:46 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 32453
Location: SoCal, 91387
I restored a Zenith 8S-463, and used a power resistor on the AC return line (opposite switch connection from the one with one leg of the AC connected, wired in series with the line, bolted to the inside of the chassis.

It reduced about 121 volts down to about 110 or so, and rendered the power transformer touchable even after being on for over an hour. Here's a link;

https://www.mouser.com/ProductDetail/Vi ... esLacR8%3D

_________________
\\\\\\\\\\\\\\\\\\\\\\\\He Who Dies With The Most Radios Wins//////////////////


Top
 Profile  
 
 Post subject: Re: Power Resistor
PostPosted: May Thu 30, 2019 8:15 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3235
Location: Nr London, England, SS1 3PT
I use something similar, mine are gold painted and not the horrid green :)

Normally, I'm fitting a new control and get it with a double pole switch so the resistor/s go after this. I say resistors as sometimes its easier to fit two small ones to get the same wattage.

You can put a dab of semiconductor heat sink paste under them if you have it.

Now for something school boy :D Its easy to mark through the resistor onto the outside of the chassis but when you put it inside it wont fit. You have to flip the holes first. Easy done: put the resistor on a piece of cardboard, mark the holes on it and punch through. Now flip the cardboard and mark the chassis through that and all will be well.

Gary


Top
 Profile  
 
 Post subject: Re: Power Resistor
PostPosted: May Thu 30, 2019 11:43 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 1413
Location: Arlington, VA USA
fifties wrote:
I restored a Zenith 8S-463, and used a power resistor on the AC return line (opposite switch connection from the one with one leg of the AC connected, wired in series with the line, bolted to the inside of the chassis.

It reduced about 121 volts down to about 110 or so, and rendered the power transformer touchable even after being on for over an hour. Here's a link;

https://www.mouser.com/ProductDetail/Vi ... esLacR8%3D


Why would it matter on which side of the line for the resister mount?


Top
 Profile  
 
 Post subject: Re: Power Resistor
PostPosted: May Thu 30, 2019 12:16 pm 
Moderator
User avatar

Joined: Feb Sun 01, 2009 2:56 pm
Posts: 10417
Location: Victoria, Australia
I have actually used those feed through panel mounted resistors as a replacement for a field coil. Often I will mount them on the metal side panel on the inside. Provided its live wires cannot be touched (covered) I have also mounted them topside (but never with utility /mains power on them); And as with Radio Fixer, heatsink compound under them. Convention would see the resistor & switch on the hot wire ( Line / Active) albeit a mains transformer is not usually fussy at to which way around the primary is. Switch in Line / Active resistor after the sw. Many Australian sets had double pole switches.

If its a transformer set I would utility ground it.

Marc


Top
 Profile  
 
 Post subject: Re: Power Resistor
PostPosted: May Thu 30, 2019 2:33 pm 
Moderator
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 22951
Location: Somers, CT
That is very effective on small tube count sets. For a larger tube count set (consoles, etc.) I'd hide a filament transformer under chassis (voltage bucking) to drop the AC line voltage at the transformer primary.

_________________
Just because it can be done doesn't mean it should be done.


Top
 Profile  
 
 Post subject: Re: Power Resistor
PostPosted: May Thu 30, 2019 7:33 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 32453
Location: SoCal, 91387
ErikD wrote:
fifties wrote:
I restored a Zenith 8S-463, and used a power resistor on the AC return line (opposite switch connection from the one with one leg of the AC connected, wired in series with the line, bolted to the inside of the chassis.

It reduced about 121 volts down to about 110 or so, and rendered the power transformer touchable even after being on for over an hour. Here's a link;

https://www.mouser.com/ProductDetail/Vi ... esLacR8%3D


Why would it matter on which side of the line for the resister mount?

Now that you asked the question, I guess it really doesn't matter. That's just the way I did it. Seemed that all circuitry being on the return side was just a more correct arrangement.

_________________
\\\\\\\\\\\\\\\\\\\\\\\\He Who Dies With The Most Radios Wins//////////////////


Top
 Profile  
 
 Post subject: Re: Power Resistor
PostPosted: May Fri 31, 2019 12:32 am 
Moderator
User avatar

Joined: Feb Sun 01, 2009 2:56 pm
Posts: 10417
Location: Victoria, Australia
Sideways but of interest: In DC the positive is the hot wire and the one on the automotive battery that gives the most trouble. On positive earth vehicles (have two) the lead from battery positive to chassis is an open braided lead; And the same with body to engine (engine mounted on rubber). The theory behind that is that as the positive is hot that dissipates the heat & it cold switches. I did note in Mitsubishi vehicles (neg earth), often the headlight is live and the relay operates on the negative side, grounding the filament.

Food for thought (Active / Line is hot).

Marc


Top
 Profile  
 
 Post subject: Re: Power Resistor
PostPosted: May Fri 31, 2019 1:56 am 
Member
User avatar

Joined: Dec Wed 25, 2013 7:57 am
Posts: 3718
Location: USA
This specific set is rated for line voltage up to 125V. Knowing how engineers like to add a safety factor beyond normal operating conditions, components won't immediately start to smoke even if your household voltage hits 126V. As long as the radio is properly restored and right components in place, may I suggest not spending too much time worrying about it :)
http://www.nostalgiaair.org/Resources/414/M0024414.htm

Are you talking about the 390ohm resistor on 6F6's cathode? By definition, power resistors are supposed to very warm; you can put a volt meter across it to see how much heat it's actually dissipating.

Note the notations on the schematic.


Attachments:
Screen Shot 2019-05-30 at 5.53.26 PM.jpg
Screen Shot 2019-05-30 at 5.53.26 PM.jpg [ 74.04 KiB | Viewed 538 times ]
Screen Shot 2019-05-30 at 6.04.03 PM.jpg
Screen Shot 2019-05-30 at 6.04.03 PM.jpg [ 65.62 KiB | Viewed 537 times ]
Screen Shot 2019-05-30 at 6.09.25 PM.jpg
Screen Shot 2019-05-30 at 6.09.25 PM.jpg [ 28.51 KiB | Viewed 536 times ]
Top
 Profile  
 
 Post subject: Re: Power Resistor
PostPosted: May Fri 31, 2019 5:25 pm 
Member

Joined: Jun Fri 19, 2009 6:34 pm
Posts: 8611
Location: Long Island
If it is just one particular resistor that is getting excessively hot, it probably isn't a line voltage problem. A gassy or leaky output tube, grid bias resistor that has gone up in value, or a leaky grid coupling capacitor will cause the cathode resistor to run hotter than normal. The most reliable test of high line voltage is to measure the tube heater voltages. They should be at their nominal ratings, +/- 10%. For the 6.3-volt tubes in your set, the range should be 5.67 volts to 6.93 volts. Adding dropping resistors to the AC line when the voltages are not excessive at the heaters can lead to poor performance and might actually shorten tube life.

Voltage measurements given on old schematics and in old service literatire are a slippery slope. First of all, many of the parts in old radios had 20% tolerances, which means the voltages could have varied as much as 20%. On many antique radios, the readings were taken with 1,000 ohms-per-volt meters. If you use a modern DMM with a one or ten megohm input, it will not load the circuits as much and some voltage readings will be higher. Many techs put resistors in parallel with their DMM probes to mimic the loading of an old analog VOM. This is done by simply taking the likely range a VOM would be on and multiplying by 1,000 or whatever the ohm/volt sensitivity of the meter is that you are trying to emulate. For example, a 1,000 ohm/volt VOM on its 250 volt range would have 250,000 ohms between the probes.

Replacing all the capacitors in a radio with modern ones can lead to increased DC voltages too; paper and electrolytic capacitors manufactured years ago had intrinsic leakage the day they left the factory. Modern ones have far less leakage at least when new, so they don't load circuits as much. This is usually not a problem and there's no easy way to fix it, but it is mentioned because it may make some voltage readings higher than what the old schematics and charts show.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


Top
 Profile  
 
 Post subject: Re: Power Resistor
PostPosted: May Fri 31, 2019 9:15 pm 
Member

Joined: Jan Tue 16, 2007 7:02 am
Posts: 3035
Location: Lexington, KY USA
I have a 2 watt 390ohm resistor tied to the output tube that is getting overly hot,

Yes. Measure the voltage across this resistor. Is it close to the 16V seen on the schematic fragment, above?

No need to worry about the meter input resistance for measuring this one.

Measure the resistor's resistance, as well. Should be able to do this with the resistor in the radio circuit.

It would be good practice to get the radio working properly first, with nothing overheating. Then add an AC line voltage dropper of some sort, if you want to increase the life of the set.

Ted


Top
 Profile  
 
 Post subject: Re: Power Resistor
PostPosted: Jun Sat 01, 2019 12:26 am 
Member
User avatar

Joined: Jun Wed 14, 2006 11:49 pm
Posts: 7609
Location: Leo, IN or Zellwood, FL
I would put the resistor in the high side of the AC line. This schematic shows a Phonograph so put the line drop resistor in the primary lead of the power transformer ONLY. See where is says RED on the schematic. Thats where the resistor should be placed. This is only a 5 tube radio so you will need more ohms to lower the line to 110 or 115 than the 8 tube that Fifties mentioned. Probably in the area of 35 to 40 ohms. I worry more about the temperature of the power transformer than I do the resistors in the set. Lower the Juice Bruce your 80 year old power transformer will thank you. Yes it says on that schematic that the line was 117vac when they took the readings. If you want to squeeze the last dB of gain out of the radio then OK just keep on running it on full line voltage. I would rather the radio last another 80 years with minimal maint issues than trying squeeze it for all that is worth.

John k9uwa

_________________
http://www.JohnJeanAntiqueRadio.com


Top
 Profile  
 
 Post subject: Re: Power Resistor
PostPosted: Jun Sat 01, 2019 1:17 am 
Moderator
User avatar

Joined: Feb Sun 01, 2009 2:56 pm
Posts: 10417
Location: Victoria, Australia
Accuracy is not attainable with an electrolytic cap across the resistor. As the current & voltage is noted the wattage at the 6F6 cathode @ 40ma is around 0.7W, running on specified volts.

Marc


Top
 Profile  
 
 Post subject: Re: Power Resistor
PostPosted: Jun Sat 01, 2019 2:46 am 
Member
User avatar

Joined: Jun Wed 08, 2011 2:33 am
Posts: 9005
Location: Ohio 45177
I myself would just put in a resistor with a higher power rating, unless there is actually a malfunction in the audio output stage anyway. I have seen what seemed to me to be underpower resistors in cathode circuits that had drifted high and looked a bit heated. So if I can get the value in say, a 5 watt wirewound, just one of the dirt cheap imported sand filled square jobs if not another type, I would pop that in. The wirewound may run warm or hot too, but they are rarely overstressed by a bit of heat and hold their value.

_________________
Reddy Kilowatt says; You smell smoke? Sorry about that!


Top
 Profile  
 
 Post subject: Re: Power Resistor
PostPosted: Jun Sat 01, 2019 7:07 am 
Member

Joined: Jan Tue 16, 2007 7:02 am
Posts: 3035
Location: Lexington, KY USA
Yeah. If the resistor gets hot, put in a bigger one. If the fuse blows, put in a bigger one. If the screw won't tighten down, use a bigger hammer. You can always sell the radio on e-bay, once you are done.

Ted


Top
 Profile  
 
 Post subject: Re: Power Resistor
PostPosted: Jun Wed 19, 2019 10:52 pm 
Member

Joined: Apr Wed 10, 2019 3:48 am
Posts: 71
Installed a 25ohm 50 watt drop down power resistor and all my voltages fell into schematic range. The problematic 390 ohm resistor is still hot, but NOT overheating anymore and the voltage across the resistor dropped from 31v to 16v in line with the schematic. Happy with the performance now with the exception of one thing:

With volume all the way down, I hear a low hum from the speaker. The hum does not increase when the volume is turned up, however, now that I am conscious of it I can detect it slightly when listening to stations. Did full recap and resistors changes, cleaned tuner and switches. Does anyone know what this could be? I followed all specs in the schematic. Thought I read somewhere that an increase in certain capacitor values could reduce hum, but not sure which ones???

http://americanradiohistory.com/hd2/IDX ... e-0216.pdf


Top
 Profile  
 
 Post subject: Re: Power Resistor
PostPosted: Jun Wed 19, 2019 11:16 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 32453
Location: SoCal, 91387
Codepug wrote:
Installed a 25ohm 50 watt drop down power resistor and all my voltages fell into schematic range. The problematic 390 ohm resistor is still hot, but NOT overheating anymore and the voltage across the resistor dropped from 31v to 16v in line with the schematic. Happy with the performance now with the exception of one thing:

With volume all the way down, I hear a low hum from the speaker. The hum does not increase when the volume is turned up, however, now that I am conscious of it I can detect it slightly when listening to stations. Did full recap and resistors changes, cleaned tuner and switches. Does anyone know what this could be? I followed all specs in the schematic. Thought I read somewhere that an increase in certain capacitor values could reduce hum, but not sure which ones???

http://americanradiohistory.com/hd2/IDX ... e-0216.pdf

You might try slightly increasing one or both filter cap values.

_________________
\\\\\\\\\\\\\\\\\\\\\\\\He Who Dies With The Most Radios Wins//////////////////


Top
 Profile  
 
 Post subject: Re: Power Resistor
PostPosted: Jun Thu 20, 2019 12:43 am 
Moderator
User avatar

Joined: Feb Sun 01, 2009 2:56 pm
Posts: 10417
Location: Victoria, Australia
I would refrain from increasing the first filter, that will raise voltage. Also consider / try utility grounding the chassis. Also note that cathode heater leakage can cause hum.

Marc


Top
 Profile  
 
 Post subject: Re: Power Resistor
PostPosted: Jun Thu 20, 2019 1:23 am 
Member

Joined: Apr Wed 10, 2019 3:48 am
Posts: 71
Tried doubling the value of two of the electrolytics and had no improvement.
(Did not mess with 1st electrolytic that goes to the rectifier.)
Utility ground the chassis and had no improvement.
Bypassed the recently installed dropdown power resistor and had no improvement.

I guess it could be a tube? But do not have a tube tester, and assume a cathode heater leakage would require a tube replacement? Not sure how to go about testing heater leakage.


Top
 Profile  
 
 Post subject: Re: Power Resistor
PostPosted: Jun Thu 20, 2019 2:12 am 
Member
User avatar

Joined: Dec Wed 25, 2013 7:57 am
Posts: 3718
Location: USA
Quote:
...Thought I read somewhere that an increase in certain capacitor values could reduce hum, but not sure which ones???

This one.

Changing the value would affect tone, it may/may not affect the hum level. If it's very low level and doesn't interfere with normal listening, then there's probably nothing wrong.


Attachments:
Screen Shot 2019-06-19 at 6.01.06 PM.jpg
Screen Shot 2019-06-19 at 6.01.06 PM.jpg [ 74.33 KiB | Viewed 256 times ]
C12.jpg
C12.jpg [ 222.97 KiB | Viewed 253 times ]
Top
 Profile  
 
Post New Topic Post Reply  [ 21 posts ]  Moderators: Marcc, Norm Leal Go to page 1, 2  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: Dennis Wess and 2 guests



Search for:
Jump to:  




























Privacy Policy :: Powered by phpBB