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  #1  
Old 05-18-2010, 09:57 AM
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Rodzilla Rodzilla is offline
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modernizing?

so soon on it's way to me i have a new project radio...

a 1948-49 Bendix 75P6U AM/FM...i'm not sure how well these perform but i like the look ...the radio is claimed to be working,but original and untouched,working or not i plan to do a bit of a rebuild...it's my first tube radio rebuild project.

i got to looking at the schematics over at www.nostalgiaair.org .caps will be dealt with,the selenium rectifier will get replaced with silicon,and it looks like there is already a dropping resistor after it that may need to be upped a bit in value to compensate for the higher voltages...but i notice that the heater string is upstream and seperate...adding up the heater voltages gets me to 117v...exactly what the set is rated at,but my household voltage in 2010 is 125V...being that a few extra volts on the heater string could be enough to potentially run the heaters at or above thier max ratings i'm wondering...should i also go to the trouble to add a dropping resistor to the front of the heater string to bring this back into line as well? am i misunderstanding some basic concept on how this works or am i just being overcautious?

Last edited by Rodzilla; 05-26-2010 at 07:45 AM.
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Old 05-22-2010, 05:13 PM
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For some reason I can't open the schematic at NA today. Do you have the tube lineup of this set?
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Old 05-23-2010, 09:58 AM
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tube lineup consists of 12BA6(x3),12AT7,19T8,50L6
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Old 05-23-2010, 11:37 AM
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It never hurts to drop the voltage to protect tubes. 50 ohms would drop 7.5 volts, and at the 0.15 amp draw of your tube string that would require the resistor to dissipate 1.125 watts: use a five watt wirewound to have it run cooler. You could even go 56 ohms. I also like to put a CL-90 inlet surge supressor in series with heater strings to slowly ramp them up. They'll start out around 60 volts and over twenty seconds or so the CL-90 will heat, its resistance drops, and the voltage comes up to just below line voltage. The CL-90 will drop another couple of volts so you'd be down to where you want to be. I get mine from Mouser, and need to order some more, BTW.
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Old 05-23-2010, 02:40 PM
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The schematic for your radio can be found at radiomuseum.org; search on "75p6u" for the info. This radio was made probably in the early to mid 1950s by Bendix, a division of Bendix Aviation, in Baltimore. It is a 6-tube superhet covering 550-1600kHz/88-108 MHz, and was marketed in at least two versions: as a standalone table model and as a radio/phono combination set.

This radio should perform excellently once it is properly restored, as it seems to be a well-built set, as were most of the better sets of the '50s; with the RF amplifier tube ahead of the antenna (I don't know if it uses the same RF amp tube on AM as well as on FM), it should be a great DX set, and it probably sounds wonderful as well. Radios of the early-mid 1950s had to be built for DX reception, as in those days not every city or town had a local radio station; hook up a decent antenna to one of these sets and you may well find your radio pulling in more stations than you can imagine. I have a Zenith C845 that regularly gets FM stations 50 miles from my home near Cleveland; at night, the AM radio dial on this set lights up as well when the DX starts rolling in--and all this is with just the antennas built into the radio. Your Bendix will probably be just as hot, as any radio with an RF stage ahead of the antenna should be, if the tube is in good shape.

I'd test all six tubes and replace any doubtful ones, although if the cathode emission is still half decent, and the radio works well and sounds good (no hum or other distortion), you can just leave them alone until they either burn out or you notice symptoms such as reduced audio output or falling RF sensitivity (i.e. the radio will only receive nearby or local stations). By "doubtful" I mean tubes that test in the "weak" range of a tube tester. If any tube in your radio registers in the "replace" region of your tester's meter, that tube is so weak it is practically unusable, and needs to be replaced if the radio is to perform well.

I am not a fan of "shotgun" replacement of every capacitor in any vintage radio unless there is a problem, but I do feel that the filter capacitors should be replaced as a matter of routine, as the filters in any radio made 50+ years ago are almost certain to be defective. I have a Zenith H511 from 1951 (59 years ago) that still plays well, very little hum, and still has its original 3-section filter; however, this is an exception to the rule--again, nine times out of ten the filters in any vintage or antique radio you, I or anyone else here at VK or elsewhere may run across will either already be defective or are failing, and of course must be replaced if the radio is to operate hum-free. However, in transformer-powered radios it is very important to replace borderline or outright defective filter caps, even if there is little or no hum. Failure to do so will likely result in damage to the power transformer or other sections of the radio when, not if, the cap eventually fails. This could and likely will complicate servicing since replacement power trannies for vintage, and almost certainly antique, radios can be difficult or even impossible to find, especially if the original has special taps for the rectifier tube filament, pilot lamp(s), etc.

I hope this helps. These older radios will, as a rule (if they are in good shape), run rings around today's flimsily-built plastic one-chip portables; once you get your Bendix in top shape, as I mentioned, it should pull in stations like crazy and sound excellent. As I always say regarding Zenith and other premium brands of radios, they don't make them like that anymore. From your location in Nova Scotia, Canada, you should be able to DX up and down the east coast and into part of the midwestern and southwestern United States after dark with this set if, as I suspect, the RF amplifier tube is used on both AM and FM, as it is with many of Zenith's better AM/FM radios of the 1950s; I've heard AM stations on my vintage radios as far west of Ohio as Houston and Dallas, Texas, using only the radios' built-in AM antennas.

Nighttime DX on frequencies normally occupied during the day by small 500- and 250-watt (or less) local-service stations that presently do not operate under the relatively new FCC rules giving daytime AM stations the option of operating at night with low power, but which sign off at sunset local time--and there are still a few of those left, mostly in small, one-horse towns that can't afford to run the local station 24/7--can be had as well, although you will have to listen to a hodgepodge of weak signals competing with one another; however, every once in a while you may hear a station hundreds of miles away coming in with a listenable signal above all the others. You may be pleasantly surprised, once you get your Bendix radio in peak shape, to hear -- well after sundown -- a 250-watt peanut whistle AM station 1000 miles away or more, booming in as well as if it were local--or at least strong enough to be listened to.
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Old 05-23-2010, 08:07 PM
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thanks for the info guys,this radio is most often described as 1948/1949 model...and tho that could be wrong,it certainly looks the part with art deco-ish 40's styling...

the filter caps if original are no-brainer replacement candidates being 55-60 years old..even if they are ok now i wouldn't expect them to hold up long under regular use...likewise the set uses several paper caps,and a selenium rectifier...these will be replaced for the same reasons...i realize that they might all happily function for years more, but all fall into the "when,not if" category for me...and it should be easy enough to replace them,eliminating the worry..

the tubes,if the description is accurate may be ok,it's described as working quite well as is...i don't own a tube tester but do know someone who does and who can likely source any replacements from what he has on hand if any are needed.

i expect the radio to arrive about mid week,i'll be able to post some pics and might have some more questions then...
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Old 05-25-2010, 06:23 PM
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Rodzilla Rodzilla is offline
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the Bendix has landed...a little more cosmetically challenged than i had hoped,it's actually quite filthy but it should clean up to look much better than this



the chassis is worse and will take some work to clean up it looks only a little grubby from the front...



it's much worse inside due to the carrying handle on the top of the cabinet also serving as a rather large vent...if you look at the "deck" of the chassis...it should be the same color as the silver base plate!



the cleanup aside...it has a few issues,the dial light is burnt out,but it's wired across the ac line not in series with anything...so until i replace it,its just a cosmetic issue really....when i first peered down though the vent in the top i spied some "foreign matter" in the speaker frame...these turned out to be two very old wads of toilet tissue wedged between the speaker cone and the frame...i expected to find a torn cone.but the cone is ok...since it had been tested tho it was against my better judgment...i plugged it in...it does work,no magic smoke and no hum whatsoever...however the reason for the toilet paper immediately became obvious...tho it appears intact,the speaker has a nasty metallic rattle of some type at anything above a whisper...if i can't figure it out,i guess a modern 4 ohm 4x6 auto speaker should fit in the available space with a bracket of some type i'll need to fab up to hold it...anyway here's to the start..of something
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  #8  
Old 05-26-2010, 05:40 AM
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Reece Reece is offline
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After you brush the dust out of the speaker, front and back, try this: remove the "padding" somebody put in there. Dip your finger in water and thoroughly dampen the front of the cone, all over, stay a half inch away from the center felt, and wet the outer corrugations also. Leave the speaker sitting face upwards overnight to dry. Many many times this will correct the problem. The cone absorbs moisture from the air and warps, making the voice coil drag, and the cone dries out in that position. When you dampen it, it relaxes and the spider, which positions the voice coil in the gap, is able to pull the cone back into alignment.

If that doesn't work, it can be aligned another way with shims.
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Old 05-26-2010, 06:07 AM
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David Roper David Roper is offline
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I just noticed the link above to Nostalgia Air is misspelled, that would definitely account for it not opening.

Here's the schematic
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Old 05-26-2010, 07:46 AM
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Rodzilla Rodzilla is offline
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Quote:
Originally Posted by David Roper View Post
I just noticed the link above to Nostalgia Air is misspelled, that would definitely account for it not opening.

Here's the schematic

oops...you're quite right it was mispelled!!i've edited the original post as well
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Old 05-28-2010, 06:51 PM
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Rodzilla Rodzilla is offline
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so i have the chassis "clean" or at least clean enough to work on,i have had a peak inside and scared myself[if you ask me,the greatest invention of the 20th century was not the automobile or the computer...but the printed circuit board!...yikes ]

i plan to make a parts run in the morning...but am stuck trying to figure what i need to get with regards to the rectifier...i just keep reading stuff that doesn't really apply to what i need to do or contradicts other stuff i just read..so before i bust out my DMM and just measure the output voltage of the selenium feeding the B+...can someone tell me what i need to do to figure out what size resistor i need to install to use a silicon replacement??...is there an accepted rule of thumb that works for this?

i have just read on another board that i may not even HAVE to use a resistor at all,but just slap the diode in it's place since the filament supply is not taken from the rectifier and won't be affected..and the overall difference in B+ voltage will not be large enough to bother correcting[+/-20% tolerance]does THAT make sense?...it's been a while since i learned any of this basic electrical theory and i just need a simple answer...what do i gotta do here?
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Old 05-29-2010, 12:13 AM
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Jeffhs Jeffhs is offline
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Quote:
Originally Posted by Rodzilla View Post
so before i bust out my DMM and just measure the output voltage of the selenium feeding the B+...can someone tell me what i need to do to figure out what size resistor i need to install to use a silicon replacement??...is there an accepted rule of thumb that works for this?

i have just read on another board that i may not even HAVE to use a resistor at all,but just slap the diode in it's place since the filament supply is not taken from the rectifier and won't be affected..and the overall difference in B+ voltage will not be large enough to bother correcting[+/-20% tolerance]does THAT make sense?
I've never heard of that before. If you just put the silicon diode in without the dropping resistor, it could work well, or you could see too much voltage at the output of the rectifier--which could and likely will damage other parts of the radio. If you have a schematic, look at the value it gives for the voltage at the rectifier output. This is the value that must be at the output of the rectifier as measured by a DMM or DVM. The reason a dropping resistor must be used with a silicon diode rectifier is that the output from a silicon diode is somewhat higher than that from a selenium rectifier.

As to calculating the value of the dropping resistor, use Ohm's law: R=E/I, where R is the value of the resistor, E is the voltage at the output of the silicon rectifier, and I is the circuit current. This formula is all you need to know to find the right value for the dropping resistor. I'd just ignore all the other irrelevant and/or contradictory information you seem to have read; it seems to me that it will probably just lead you up a blind alley or, worse, down the wrong road.
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Last edited by Jeffhs; 05-29-2010 at 12:17 AM.
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Old 05-29-2010, 06:52 AM
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You may be OK without the dropping resistor, as long as the voltages you measure at the plates of the tubes are less than their stated maximums, as determined from a tube manual, or by looking up at

http://www.nj7p.org/Tube.php

Add up the plate currents of all the tubes and that's the "I" to use. If your plate voltages are high, figure about how many volts you need to drop and use that for "E." This will give you the resistance. I x E will give you the wattage of the resistor, but go a size or two higher in watts to help it run cool.
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Old 05-29-2010, 12:51 PM
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Rodzilla Rodzilla is offline
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and i have parts...

i picked a small range of a few resistors to hopefully have what I'll need on hand,the schematic doesn't show any voltages so when i get it open again,I'll just measure whats there and work the numbers to match at that point,the selenium doesn't look in bad shape,and the rest of the radio still works as well,so hopefully whatever voltage i find won't be too far off spec[accounting for the increased AC voltage going in]...alltho being pressed for time to get to a supplier that closes very early on saturdays...i got the resistors,caps...and yet forgot to buy the diode!!...but i may be able to scrounge one from my junk pile...seems to me the IN400x is not an uncommon part in SS amplifier and receiver power supplies...i have a few carcasses laying around to check

*EDIT*...parts are all accounted for...found a few 2A06 diodes in an old PC power supply that should work,going by the data sheets it looks to be roughly equivalent to an IN4006 but with 2amp current ratings instead of 1amp...

the issue with the speaker has turned out to be silly one....it's not that the cone has sagged so much,it's that the glue holding the outer edge of the cone to the frame has mostly failed allowing about half the cone to noisily vibrate against the frame,if i just hold the thing tight to the frame the noise goes away...so i also picked up some glue that should work and some clothespins to use as make shift clamps if i need them today lol

oh and has anyone tried the home electroplating kits from caswell? [!]

http://www.caswellplating.com/kits/index.html

this may be for a bit later once i get the rest of the thing sorted out,but i may try one of these just to see how it works...after getting it all apart to clean,i'm pretty sure the bronze bright work on this radio is plated brass,and lookie,they have a bronze kit..as well as silver,gold,chrome etc..prices are pretty reasonable,especially if the result is as well...anyone have any experience with these?

Last edited by Rodzilla; 05-29-2010 at 05:31 PM.
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Old 05-30-2010, 06:40 AM
akent36 akent36 is offline
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I have used amber tinted spray lacquer from on-line woodworkers supply shops to simulate that "brass" plating on metal parts.
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