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  #16  
Old 05-30-2010, 05:18 PM
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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...

Last edited by Rodzilla; 05-30-2010 at 05:46 PM.
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  #17  
Old 05-31-2010, 03:13 PM
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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."
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  #18  
Old 05-31-2010, 04:11 PM
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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.

Last edited by Rodzilla; 05-31-2010 at 04:26 PM.
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  #19  
Old 06-13-2010, 05:08 PM
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*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
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  #20  
Old 06-16-2010, 03:44 PM
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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.
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  #21  
Old 06-16-2010, 04:15 PM
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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"

Last edited by Rodzilla; 06-16-2010 at 08:30 PM.
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  #22  
Old 06-19-2010, 09:02 PM
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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
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  #23  
Old 06-20-2010, 09:54 AM
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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.
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  #24  
Old 06-20-2010, 12:30 PM
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Quote:
Originally Posted by Reece View Post
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.
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  #25  
Old 06-21-2010, 05:42 AM
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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.
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  #26  
Old 06-29-2010, 06:36 PM
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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...
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  #27  
Old 07-14-2010, 09:02 PM
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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?
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  #28  
Old 07-15-2010, 03:40 AM
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I would say that ceramic disc is wrong, both in type, and, value. That's a replacement.
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  #29  
Old 07-15-2010, 03:44 AM
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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.
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  #30  
Old 07-16-2010, 04:36 PM
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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...
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