Stewart Warner R1711

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Stewart Warner R1711

Post by Ed in W. PA on Fri Mar 18, 2016 12:11 pm

I have two questions regarding this radio. This is a 5 tube radio manufactured in 1936, and uses tube line=up of 6A7 converter, 6D6 IF amp, 75 2nd det.amp & AVC, 43 audio output, and 25Z5 rect.  My first question is about the line cord. The schematic shows 3 conductors, onegoing to the on-off sw, one going to the plates of the 25Z5, and the third is shown as a 130 ohm resistorgoing to the filament string. This conductor consists of a fine piece of resistance wire spiral wound  around the cotton insulated conductor that goes to the plate of the 25Z5. This cord is 80 yrs old andmust be replaced. Is there any good reason they designed it this way? I intend to replace the cord and install whatever value resistor necessary to obtain the proper filament voltages. The second question deals with a component connected to the grid of the 75 tube through a 500K resistor. The schematic calls it a "grid bias cell { 1 volt }". Has anyone encounterd anything like this before?  Any help greatly appreciated. Thanks, Ed.

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Re: Stewart Warner R1711

Post by Wildcat445 on Fri Mar 18, 2016 1:45 pm

The line cord is a resistance cord or "curtain burner" in old radiospeak.  The 130 ohms of resistance is necessary to lower the voltage to the series-connected tube heater string.  A suggestion would be to use a 130 ohm, 20 watt resistor to replace the one in the line cord, then use a regular cord.  Those bias cells are little batteries whose purpose is to bias the grids.  

If you will refer to Riders, Volume 9, pages 5 and 6 under Stewart-Warner, you will find that the schematic does not show the bias cell.  The 75 grid is biased by a 500k and a 1 meg quarter-watt resistor, connected to B- and the grid.  That would be the way to by-pass the bias cell.  I could not remember how it was done.   I hope this helps.  Good luck.

Just for reference, your radio is listed in Riders under model R-171. R-1711 is listed on the schematic. R-1711 is not listed in the Riders index.

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Stewart Warner model R1711

Post by Ed in W. PA on Fri Mar 18, 2016 3:44 pm

The service literature that I am using is Riders volume 9 pages 5 and 6, thatis where I saw this cell it is labeled item 11. To bypass this can I connect it to B- { the chassis in this case}, or do I need to connect it to a positive potential? Other similar circuits I have looked at { not with a 75 tube } seem to lead me to believe that it needs to be slightly positive or 0 volts. Thanks, Ed.

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Re: Stewart Warner R1711

Post by Bill Cahill on Fri Mar 18, 2016 4:08 pm

No, you must put a resistor between that grid, and, B-. Connecting is directly will not work.1 meg is the usual resistance.

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Re: Stewart Warner R1711

Post by Wildcat445 on Fri Mar 18, 2016 4:16 pm

I printed the schematic for reference.  I missed item #11.  (I looked on the wrong tube.  In my defense, the schematic is sideways in Riders.)  B- appears to be drawn connecting to one side of the power switch.  The bias cell is connected to one side to chassis.  I would do as Bill suggested and run a 1 meg resistor to B- to bias that tube.  This is a rather oddly-drawn schematic.  All the radio manufacturers were trying to cheat RCA out of their royalties by drawing their power supplies in "innovative" ways sometimes.  I never did understand the reasoning behind using bias cells.

I would want the grid to be 0 to slightly negative.  The bias voltage is typically taken from B-.

Referring to the "fine print" at the bottom right hand corner of the schematic, under "note B" is states that "the control grid bias of the triode section of the 75 tube is -1 volt as supplied by the bias cell (number 11), however this voltage can be measured only with a vacuum tube voltmeter."  The positive end of the bias cell is connected to chassis ground, the negative end goes thru the 500K resistor to the grid.  You should be just fine if you used a 1meg resistor connected to B- as stated above.

More reading of the "fine print" reveals that "R-171" refers to the chassis number, while "R-1711" refers to the radio model number. Nice. Very Happy

The fine print also states that all voltage reading are taken with a meter with 1,000 ohms per volt sensitivity. If you use a VTVM or a more modern meter with, say, 20K sensitivity, you may experience higher voltage readings than those listed.


Last edited by Wildcat445 on Fri Mar 18, 2016 5:14 pm; edited 1 time in total

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Stewart Warner model R1711

Post by Ed in W. PA on Fri Mar 18, 2016 5:11 pm

Thanks guys, I will give that a try. Thanks again, Ed.

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Stewart Warner R-1711

Post by Ed in W. PA on Thu Apr 07, 2016 12:48 am

I don't know who worked on this radio, But if there is a special place "down there" where stupid, lazy, and just plain inept repairmen go this person or persons should go there. It would appear that the electrolytics failed several times in this set, the first time someone just jumped new ones in without disconnecting the original. The second time(?) they jumped the new ones across the ones from the previous "repair". The last two weren't even soldered but just lightly wrapped around the leads from the two that had been stuck in before. As for the rest of the set, it is just as bad. Almost no solder joints were made where the wires or leads had any evidence of good mechanical contact on tube sockets or terminal strips before soldering. It almost looks to me like this was manufactured this way, or some unskilled person had almost every component disconnected at some time. I have seen things butchered in the past, but this has to be the most complete job I have ever seen, a new low. All this just adds to the fun of fixing this stuff. Will keep you posted, Ed.

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Re: Stewart Warner R1711

Post by Wildcat445 on Thu Apr 07, 2016 1:08 am

I have a "fast and dirty" book on radio repair from back in the day.  It actually recommends paralleling the old caps with new.  I have seen that a lot.  We do a lot better job fixing these old sets than the pros did when the sets were newer.  We restore them to run for another 50 years.  They would just fix what was broke at the time, get it out the door, and get paid.  Cars are the same way.

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Stewart warner R-1711

Post by Ed in W. PA on Thu Apr 07, 2016 1:43 am

WC , I know after working so many years in the TV repair business, I have heard the statements that their neighbor, friend or brother in law just looked at it for them but I found missing or wrong parts, missing screws, and lifted or completely missing copper traces on the circuit boards. The customer would swear that they "just look at it" but they either severely damaged it or destroyed it. This was just a part of the business one had to endure. Back in the days of rental tapes, if a rental tape got stuck in a VCR the customer would frequently "remove" the tape to avoid paying late charges, then bring the utterly destroyed VCR to the shop for repair stating that "all we did was get the tape out". Either way I was the bad guy. Ed.

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Re: Stewart Warner R1711

Post by Wildcat445 on Thu Apr 07, 2016 2:14 am

I have often thought that if I actually had to make my living working on old electronics, I might not have so much fun with them. Your thoughts are well-taken. Bringing a hack job back from the brink is especially rewarding sometimes. Anybody can fix the easy stuff....... Very Happy

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Stewart Warner R1711

Post by Ed in W. PA on Thu Apr 07, 2016 2:24 am

Amen!

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Stewart Warner R1711

Post by Ed in W. PA on Fri Apr 08, 2016 10:26 am

WC, if you still have that schematic handy take a look at item #23. This is a .005mfd @ 600v cap shown on the diagram as connected across the primary of the output transformer. What I found was this cap was connected from the plate of the 43 tube to ground. This looked original as the cap was the same brand as most of the rest of the paper caps and its connection to ground looked untouched. I'm thinking the schematic is right. What do you think? Thanks, Ed.

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Re: Stewart Warner R1711

Post by Bill Cahill on Fri Apr 08, 2016 11:53 am

Don't mean to barge in, but, I'd say you are on the pre amp instead, If I'm wrong, both could be right. Both ways of wiring it up would be to improve the tone quality of the radio.
Is the cathode of the 43 connected to ground? If so, they have found the best way to get tone out of radio. The only problem I see might happen is if that capacitor shorts, it would cause the output to have a melt down, and, probably the power supply as well.

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Stewart Warner R1711

Post by Ed in W. PA on Fri Apr 08, 2016 12:13 pm

Bill, the 43 tube is the output tube in this set and the cathode is grounded. I always thought that the cap across the output transformer helped with impedance matching in the plate circuit. Your comment about the cap shorting is all the more reason to wire it the way the schematic shows. Thanks, Ed.

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Re: Stewart Warner R1711

Post by Wildcat445 on Fri Apr 08, 2016 5:50 pm

Ed, sorry for the tardy reply. I misplaced the schematic I had printed and had to look up Riders again. My vote would be to wire it like it was originally, if you think it IS original. There may have been a running change in production, not noted on that schematic. It might be a good stunt to check in the back of that Riders manual to see if there has been a production change on that chassis. I also agree with Bill's comment. A grounded cathode requires a good cap to protect transformers. Good luck

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Stewart Warner R1711

Post by Ed in W. PA on Fri Apr 08, 2016 10:15 pm

I am now into the part of the set where the bias cell is and took a few measurements. The bias cell reads infinity and the old dogbone resistor in series with it reads 508K, only 8K high. It would seem likely that the bias cell went bad early in it's life because there is a dogbone resistor jumped right from the grid to ground reading 68K. I plan to use a 1meg resistor like Bill suggested then measure the grid voltage and go from there. Regards, Ed.

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Re: Stewart Warner R1711

Post by Wildcat445 on Fri Apr 08, 2016 10:42 pm

Sounds like a plan. Good luck. Very Happy

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Stewart Warner R1711

Post by Ed in W. PA on Sat Apr 09, 2016 9:41 am

Does anyone have experience cleaning a variable capacitor in a ultrasonic cleaner? The cap in my set is covered in some kind of green material. Spot cleaning with Tarn-x then alcohol cleans it up well, however I can't get into all the areas. The instructions that came with the ultrasonic cleaner say not to use it on aluminum. I am not sure what the vanes of this cap are made of, could be aluminum or could be nickel plated brass. Has anyone tried this before? Thanks, Ed.

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Re: Stewart Warner R1711

Post by Bill Cahill on Sat Apr 09, 2016 2:59 pm

Do not use an ultrasonic cleaner. It will damage the mica pieces.
I hope some of the guys can come up with a more proper way to fix this problem. It is repairable, but, not easy.

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Stewart Warner R1711

Post by Ed in W. PA on Sat Apr 09, 2016 3:54 pm

Oddly enough there are no trimmers on this cap. It is still a good idea not to use the ultrasonic. Right now I am cleaning the cap as best I can using lemon juice then alcohol, slow but shines it up nice. I had been using Tarn-x but I don't like the smell, and really don't know if it could do any damage. I should be OK with the lemon juice. I will look for a sacrificial cap to try in the ultrasonic cleaner and see how that goes. That way I can measure capacity and leakage resistance ( if any ) and compare before and after, and try to see what solutions work best. Thanks for bearing with me, Ed.

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Re: Stewart Warner R1711

Post by 75X11 on Sat Apr 09, 2016 7:10 pm

In my experience tarn-x or noxon will give a fast response, but I have noticed with some metals the tarnish returns and with a heavier deposit than the previous one. I use 91% isopropyl alcohol on tarnished silver plated contacts as a wash with brushing. The lemon juice ought to be all right with a good rinse.
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Re: Stewart Warner R1711

Post by Bill Cahill on Sat Apr 09, 2016 8:25 pm

In both cases, water is involved. in Your 91 percent alcohol, it has 9 percent water. Lemon has natural water in it. I recommend against both.

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Stewart Warner R1711

Post by Ed in W. PA on Sat Apr 09, 2016 8:53 pm

This is a point well taken. Is there a good way to do this? I can finish the process with contact cleaner and carefully lubricate the bearings and apply Deoxit to the wipers, but as of now this is the best way I know. How do others clean their varicaps? This is for cosmetic reasons only. Thanks again, Ed.

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Re: Stewart Warner R1711

Post by 75X11 on Sat Apr 09, 2016 10:26 pm

To remove surface dirt and metal oxides, you could use a non chlorinated brake cleaner spray and use a paper towel or two to absorb the runoff. I used CRC mass airflow sensor cleaner spray to clean out the bearings and contacting surfaces. It is an electrical contact cleaner with a degreaser. Take care when using it with plastics other than phenols like bakelite or printed circuit boards.
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Stewart Warner R1711

Post by Ed in W. PA on Wed Apr 13, 2016 8:55 am

Fired it up yesterday, and it plays well considering it hasn't been aligned yet. One problem I noticed right off was I had a steady hiss coming from the speaker even with the volume control all the way down. To remedy this I tried a 10meg resistor to ground off the grid of the 75 tube instead of the 1meg resistor I had put there in place of the bias cell. I haven't measured the grid voltage yet ( should be -1 volt ) but will fine tune when I align it. Also I have to move the 130 ohm 25 w resistor I installed in series with the filament string to take the place of the resistance in the "curtain burner" line cord because I mounted it right under the filter choke and its heat is overheating the choke. All in all I am pleased at the performance of this set. Hooked to the longwire it picked up really well across the whole band. Regards, Ed.

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Re: Stewart Warner R1711

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