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?

For some reason I can't open the schematic at NA today. Do you have the tube lineup of this set?

tube lineup consists of 12BA6(x3),12AT7,19T8,50L6

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. :yes:

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.

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...

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

http://www.videokarma.org/attachment...1&d=1274829037

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

http://www.videokarma.org/attachment...1&d=1274829037

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!

http://www.videokarma.org/attachment...1&d=1274829037

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 :)

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.

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

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?

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.

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.

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?

I have used amber tinted spray lacquer from on-line woodworkers supply shops to simulate that "brass" plating on metal parts.

ok...so..actually measuring the thing...looks like at 125v into the selenium it puts out about 108-109..[looks like it's still working about right anyway]..accounting for the increased input of 125V over the original design specs of 117v AC...i'm thinking the design voltage should be about 100V out of the rectifier...sound about right?

adding up the plate current for the tubes gets me to 94 Ma...so call it .1A...sound right?am i still on track?

so if i need to drop 25v...running this through various calculations it becomes a 263ohm resistor carrying about 2.5 watts...sound right?...there is a wire resistor off the selenium of 33 Ohms connecting it to the Can cap..handy that

300 Ohms is a value i saw tossed around a bit in other online forums and that's pretty dang close if i remove the wire resistor and just use a 300 Ohm resistor from the diode to the cap...i do much better with hands on testing,pretty sure this will work...but i'll give it a shot,measure and see...

Are you going to change out the selenium? The voltage would go up if you do. If you stick to the selenium, and want 100 volts, you only need to drop 9 volts, not 125. Maybe sub the diode for the selenium and see what it puts out.

Good idea posted about the tinted lacquer simulating "gold" on metal. Another source could be, if you can find it at a crafts store, tinted lacquer used for making "stained glass."

yes i plan to take the selenium out...the 100v figure is my best guess after measuring the output voltage of the selenium rectifier with the radio playing,it puts out 108v-109v..

Accounting for the increase in household AC voltage over the last 60 years...100v seems a reasonable assumption.most of what I've read about selenium rectifiers quote about 85% efficiency give or take a few %...108-109V is about 87% of today's 125V...the same 87% of 117 is about 101v...i think a 100V target going to the filter caps is probably a "safe" option,or at least probably within a small tolerance of the original design...I'm aware I'm ignoring the very small voltage drop of a silicon diode,but i don't see a volt or so as terribly significant in this,the circuit should easily accommodate that little bit of variance...sorry if i didn't explain that very well.

*update*

after not having much time to work on the bendix lately,i've finally gotten the P/S and all the electrolytic and paper caps replaced. the set plays decently enough,tho still with a small buzz that goes up and down with the volume...checking voltages here and there...it would appear i need to dig into testing resistors,even after trying a few different resistor values after the rectifier...voltages are still low,and not in sync with one another,ie not uniformly low as one would expect if the PS was just not delivering the right voltage...so i guess it's multimeter gymnastics for a bit to find the culprits

Re: buzz: picture seems to show a couple of tubes with tube shields. Does the 12AT7 have a shield? If so, it needs to have a good clean ground to chassis. If not, try wrapping some foil around it and clip leading it to chassis as a test. Sometimes first audios pick up hum. Dress leads and cap from volume control to first audio grid close to chassis to reduce hum.

hmmm...thanks for the suggestions Reece,i will investigate!

i did suspect that maybe the relocation of the PS caps might be an issue,leads to the original can cap that were laid more or less flat to the inside top of the chassis had to be rerouted somewhat as necessary...i'm not real happy with where the caps ended up,but the chassis is quite cramped in that area..the idea has occurred to me that i might just be picking up internal line noise...

*edit*..perhaps i should say as well,tho this noise I'm getting is surely power line related,its a definite buzz,not really a power supply hum...with the volume at zero the set is dead quiet,with a station tuned in, it's audible below the audio..but just a light "bzzzzz" you hear mostly in quiet passages or below voice...if it were just slightly less audible i might even be able to overlook and live with it...off station it increases with volume to a point,and then seems to vanish at a point just before the inherent hiss and noise of the circuity becomes audible..if one could play it in that max volume range,the buzz would probably be gone entirely....?

and yes both the 12at7 and the 19t8[listed as 1st audio] have shields,the parts list does list a "tube base shield" but i don't know if these are what they are referring too?BUT i figured somebody must have put them there for a reason and i have left them on,but they look almost homemade to me.i'm not sure if they're original or some former owner's replacement or "tweak" :)

while waiting for a parts order to come in next week,today i said "screw it,lets do this right" so i bit the bullet,disconnected a few things,got some really tiny pliers to straighten the twist tabs and removed the FP can cap from the chassis ..then got the dremel out and decapitated it :)

my plan is to have the base reinstalled and ready to accept some new caps when they come in...i think i just need the base,the original can has a cardboard cover which is in excellent condition and once the base is reinstalled and wired back in properly under the chassis,it should suffice to cover the evidence nicely as well as provide easy access should it ever be needed later ...once modified a tiny bit,i should be able to treat the base as a terminal strip,install the caps from the topside and just slap the cover on over them..

I'm not sure if my buzz problem is originating from the PS but even if it's got nothing to do with it, this will be a better solution for the caps in any case :)

Check that the tube shields make good contact with the chassis. There will be a spring "finger" that presses against them. If that's rusty and the shield too they need to be brightened up.

already done Reece...no change unfortunately...things are a bit dirty/dusty and lightly oxidized in spots,but fortunately there's no actual "rust" to deal with.

i did take some fine sandpaper to the "fingers" which you refer too tho and also tested for continuity/resistance between the shields and chassis...it all seems good,my meter beeps for continuity and measures less than 1 ohm from any part of the chassis to the top of the shields, which i don't think is bad considering they're actually just made of a heavy aluminum foil over a cardboard sleeve ...i don't think there's much more i can do there.

Check all chassis grounds. Sometimes there's a ground made through a rivet or screw that has oxidized some and doesn't make good electrical contact. Shine up around it and solder it down to chassis. Not looking at the schematic but the low side of the volume control is usually circuit grounded; in any case needs to have a good tight connection.

Sometimes if you turn the (non polarized) line plug over the hum goes away.

so...caps are done,back in the can on the topside of the chassis where they belong...tho with no change to the buzz...i guess it was mostly unnecessary but it does free up some fairly tight real estate under the chassis,and my sense of aesthetics likes this cleaner "hidden" setup much better. :)

i also got to borrow a friends tube tester over the weekend and ran all the tubes through...surprisingly most test as new,of the three 12ba6's the lowest came in at 98%,the RCA 50L6 bounced off 100% and then pinned the needle down hard...she's a good'un! the 12at7 and 19t8 were less thrilling testing at around 75% each,but the tester's owner tells me those readings are quite good for those types on his tester,he didn't think he had any that tested much better in his surplus inventory...so the tubes are good to at least quite serviceable..

i started to change out a few moderately drifted resistors around the output tube socket,and took the opportunity of doing the caps and having a few other things disconnected to reroute and clean up some wiring...which seems to have reduced the buzz by about half..it's not quite gone yet but it's significant progress!!! i think i need to do something more with the diode/resistor rectifier combo as these are still mostly just in a temporary/testing config at the moment,but i suspect that's the actual noise source,the cleanup has not so much moved any lines farther from each other or closer to the chassis...but has tucked them up farther away from the rectifier...I'm not sure that that's the key to this, but I'm happy just to have made any progress in the right direction!

and the adventure continues... :)

still dealing with the buzz...and a very aggravating schematic...I've already found a few errors and outright circuit differences,and also found some evidence of work done on this radio in the past as well...i don't know which to trust,the schematic which at times disagrees with it's own parts list as to what is what or the physical radio that may or may not have been "fixed"at some point.... :(

case in point c32 on the schematic...it links the center pin of the volume control to the19t8 1st audio amp[buzz gets louder if i touch this cap,but that may be normal]the schematic says it's a ".03",the parts list says it's .05uf paper cap and that's close enough to overlook i guess[i replaced a couple of those same caps in other area's...i didn't miss it,they're kinda big!]...the problem is that with the real life radio..this is a ceramic disc cap and if i'm decoding the color code right...it's a .003...now i would think a cap on the order of a factor of 10 times smaller there might cause some low freq cutoff.some sort of audio weirdness maybe...but frequency response has no issues,other than the slight buzz it sounds just fine...but it seems an audio coupling cap to me,which would lead me to think the schematic may be right...but would anyone care to give a better informed opinion than mine...which would be more the expected range of cap in this circuit?

I would say that ceramic disc is wrong, both in type, and, value. That's a replacement.
Bill Cahill

Normal value for cap to volume control center tap would be .01-.03, depending on radio. But, in that set, who knows? .05 Could be correct....
The disc is wrong. It could also introduce buzz in audio due to the type, and, might be picking up other things, or, even emitting it's own noise. I'd reccommend against using another disc capacitor.
Bill Cahill

thanks Bill...I'm more than a little frustrated with this schematic.it's got a few obvious errors,and the parts list that goes with it seems to take some liberties...it's also for a 75P6,where my radio is actually a 75P6U..the few times I've seen that issue addressed on the net nobody seems to know what the difference is,or if there even are any...its obviously the same basic design,but there are certainly a few differences mostly in the power supply i think...having to follow wires and then guess occasionally all adds to my difficulty lol

anyway...

i didn't have a .03 cap to try,i did have a .047 tho[schematic or parts list??? :( hmmmm]...this works..mostly...still with a bit of the buzz tho greatly reduced to really a pretty tolerable level and might be able to be reduced further with some care in installation...i think i do need to try a .03 tho,the larger cap seems like it might be tilting the tone of the radio lower slightly and at high volumes where the buzz used to be replaced by amplifier noise,i now get some mild 60hz hum with it that was never present before...

while waiting on more parts and a new speaker to arrive[the old one works ok,but has some issues with voice coil alignment as well as a few small holes and previous patches that the cone's extreme fragility makes me reluctant to even attempt to fix]

in the meantime I've started on the cosmetic side..the "bronze" trim is indeed brass,and as was suggested earlier,it's not actually bronze plated but has some kind of tinted lacquer applied...some brasso and a little elbow grease quite nicely took the badly deteriorated finish off the large trim piece below the dial,that cross piece is now gleaming clean brass...i guess the question now is,being that i rather like the bronze look,what can i find to duplicate it?tinted lacquer is not something i see at wally world or home depot... :)

What makes you think it's tinted lacquer? It's not. It simply has decayed with age. It happens. Takes longer than no lacquer at all, but, it still happens.
Bronze plating was done with special acids, and, other chemicals. Difficult to duplicate.
Bill Cahill

i suspect it was spayed on as it came off very easily...once i had determined that i wasn't going to try to save it as it was,i could actually scratch off the "bronze" with my fingernail,it was not metal plating...I'm pretty sure it was lacquer or maybe some other type of coating.but it was definitely not real bronze...the brasso reduced it too sludge and had it down to the bare brass in a matter of a few minutes...

as i mentioned near the beginning of this thread...the real deal can be done.I'm not sure of the quality of the results,but like most things these days...if you can dream it up,it's on the web...as i said I'm not sure of results one can get from these systems or of my due diligence in the polishing and cleanliness that's absolutely required for a decent result in ANY plating process...but it SEEMS easy enough.unless there turns out to be an easier way to replicate what was there...i may give it a whirl

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

Are you sure the trim is solid brass? Could be brass plated steel. Test with a magnet. No matter, if the finish is shiny brass "looking," look in a crafts store for "stained glass" spray lacquer. Some have used an orange tint of this stuff to make brass (or even shiny steel) look more like an antique brass.

yup it's brass,tho "solid" is probably misleading,it's formed brass channel for the main cross piece,the logo is actually solid cast brass..its non magnetic and the real tipoff was the hint of green corrosion on the backside :)..even the knob brights cleaned up similarly..tho I'm not sure of their actual composition...and you've read my mind on the crafts store,was thinking that just last night,there's a large one i've not been in,that i need to check out

i think i have a plan on the brass,but i'm gonna wait till last on that...but by way of an update on this project

i had ordered a new 19t8 and 12at7 for this radio since they were the only tubes that didn't test near new. i was hoping that possibly the new 19t8 MIGHT do something with the slight buzz,since i'm about out of ideas and even theories at this point to solve it.... as well before i started messing with oscillator coil spacing that has existed for some 60 years,in order to fix the slight offset in the dial like the service info instructs,.i figured i'd get a new 12at7 and see if that affected anything there..both tubes work out to about a 20 dollar experiment,and i figured I'd at least know that all the tubes were then tip top even if it affected no change at all

other than these issues the set has been performing well enough,i find the heat from the 50l6 a bit worrisome but being that it seems to be operating on proper voltages,if not actually even a bit on the conservative side,i guess thats just the way it is...it gets warm enough on top that plastic would surely soften and warp,but i guess the bakelite can take it....??and i guess i know where to set a cup of coffee i need to keep warm

anyway i've had the thing playing off and on for a while now,and for a good while last night with good volume and sound quality,no issues whatsoever beyond those i've mentioned,so i shut it down for a while and when i came back later...dead,the dial lights up,but no sound...damn,a quick look inside revealed no lit heaters on the tubes...double damn,now of course in a perfect world the filament that went would have belonged to the 19t8 or 12at7 that i had on order,the ones that actually arrived just today...but nope,after checking across the heater pins of the four other tubes...I've lost a perfectly good testing 12ba6 to an open filament...dang setback #112 on this project,i got new tubes to put in,some stuff to try and i can't fire it up until i locate a working 12ba6 lol

Go to a heating/AC contractor and ask them for a scrap of ductboard, all you need is a piece about 3 x 4 inches. They will have a dumpster full of it. Ductboard is a compressed fiberglass material with a foil facing. It's an inch thick. Use a long knife and slice through so you have a piece about 1/8 of an inch thick with the foil face on it. Glue that inside the top of your radio with the foil facing down towards the hot tube or tubes. Cooler top.

ok i give up...the thing has a buzz,will always have a buzz,and probably always did...i don't know what else to try short of replacing every single component and rewiring the whole chassis...and I'd lay odds it would STILL buzz!

can anyone add ideas to this list...

caps are done...all except the couple hundred ceramic discs[seems that many looking anyway :) ]

tubes are good or new

tube shields are in place and well grounded to chassis

AC plug orientation makes no difference

cap lead dress or positioning makes no difference

grounds are tight solid and good..there are only a few chassis grounds in his thing...components that were originally routed to them are still...

the speaker and associated bracket has been replaced...the ground through there is also 100% solid

the volume control ground is solid...and has not been disturbed at any rate

i finally got around to finishing the install of the new speaker and bracket today,i also redid one of the caps to ground after looking at some "before pics" last week,i noticed i had it all of a inch or so down the rail from where it was originally[yes I'm down to THAT level of desperation!i'll try shit that i know will make no difference!]....i powered it up and the buzz seemed quite nearly gone,the new speaker sounded good!i put the case back on and the buzz is now not only back but louder than ever.....wtf?!??

i have no doubt that this is just a stupid problem,an iffy connection somewhere or at worst one or a few of the multitude of ceramic discs may have failed...or something...but i just give up,the only way i think i'm gonna get any satisfaction out of this thing is to find a very large hammer and smash it to bits...

i christian thee...Bendix...dysfunctional shelf queen...

Let's track down this buzz.
1. Do other radios working off the same wall receptacle buzz? If so, could be something in the house: light dimmer, phone charger, shaver charger, etc. etc. Could even be at the neighbor's.
2. The AC switch is on the volume control, which is hum sensitive. How are the wires to the control routed? They should be tightly twisted to reduce hum and routed away from audio grids.
3. Did you replace the cap from volume control to the grid of the first audio with a new cylindrical cap? Routing of this cap may be critical. Some radios shield this cap. Try this: wrap some foil around the cap careful not to short to the end wires, fasten with tape, and connect foil via test clip to chassis ground: hum may be reduced. If so, devise a permanent connection for same. Sometimes where space permits, placing this cap tight against the chassis has a similar effect, the chassis acting as a shield.

If you could post an underchassis view, maybe two, up close of both sides of chassis, we might spot something.

I hate to ressurect old threads, but almsot 5 years later I have found one of these radios, intact, complete, pretty and of course not working.

I have the Rider schematic for the 75P6 and like the OP noticed, it is very different, especially in the area of the power supply.

the OPs pics line up with what I see, there is a 2nd transformer between the filter cap and the output tube that is used as a resistor between the rectifier cap section A and B which then goes to the actual output transformer.

Also missing are the 470ohm resistors (many sets use a 1Kohm resistor to drop the voltage from the first cap section to the voltage used by the rest of the plates, I find a 390ohm 2 watter carbon comp in there. Plus a lot of other differences to darn near make the schematic unusable.

In the intervening 5 years does anyone have a line on the 75P6U model? Beitman does not, Im getting no hits for Sams and Rider just has the non-U version.

Also of curiosity is a 5mfd 50v wax cap with a + end marking where the + is on the B- and the other end is, well elsewhere....it is not in the SCM at all.

90% of the 1/4w resistors in there look like pure-dee-crap so I will swap them out, but I am using a 'replace with what you find' motif rather than 'RTFM' since the SCM is so very wrong.

Id be interested in hearing what you used for a dropping resistor across the Se rectifier. In other sets where I have done this that use the std 22ohm inrush limiter, I settle on a 120-150 ohm resistor given the higher line voltages. (and remove the 22 ohm to boot)

As part of my work I am using a polarized cord and rewiring to the switch to force the rectifier to be hot, that way chassis ground is at or near earth and not -125. Biggest issue I had to get over what I utterly assume is a schematic error where the diode symbol is drawn backwards, try as I might, there was no way I could work it out in my head where that circuit could ever work - if it did it would reverse the polarity across the filter caps.