B+ voltage on zenith k731

Hello to all members, this is my first post and believe me i read all the threads in this forum and still not finding answers to my questions...http://techpreservation.dyndns.org/s...ics/Zenith.htm dont work or cant access this website so i have no schematics to refer to.

I've got questions that some of you guys hopefully might be able to help..first is the plate voltages on zenith k731: Is it the same for all the tubes and what are the exact voltages? I'm concerned about this after reading that over B+ will shorten its life, you see i have done the recap but did not find exact values so i've used a 50, 70, 100 mfd. in place of the original which is 40, 60, 80 mfd. From reading i learned that increasing capacitance would lead to increase in B+ , is this true? . By the way i also did the silicon diode with 100 ohm resistor mod.
My B+ as measured is 124V. is this on the spot?. I've not yet measured the plates voltages for i dont know which pins to touch on each tubes??..

The radio is working and i'm magnetically hooked to its sound, in fact ive forgotten my expensive stereo. So you see i really need to give this one the best that i can offer together with your help..so, should i say HELP please..thank you everybody.

[Tom Bavis]

Larger caps won't increase voltage much, if at all. With tube rectifiers, there is a limit due to peak current, but solid state rectifiers have a much higher peak capability. Larger caps may decrease hum - a good thing.

All the tubes can operate on higher voltage than 124V... what matters is dissipation (voltage times current). The two tubes that wear out fastest are the FM converter and the audio output - they're the only ones that run close to their ratings, and will show problems sooner if they become weak. But they are $5 parts - I guess I wouldn't worry about it too much. If you want the tubes to last a REALLY long time, take out the 35C5 and put in a 50C5. Performance may suffer a little, but ALL the tubes will run cooler.

Thanks for the info Tom, 50c5 seems a good idea and i'm really glad to know that, but unfortunately i'm also a performance guy, the reason why, after recapping and replacing almost all resistors the next thing i wanted is to make sure all voltages are according to specs. I don't know if you call this obsession but for me, knowing that everything is spot on extends my pleasure of this hobby

Hoping anybody out there knows these voltages or a schematic (zenith k731), please,.. i will really appreciate if you can share it and please dont tell me to buy the schem. for i only need these exact voltages (the B+ and plates) and i'll be a happy man, thanks guys.

PS.http://techpreservation.dyndns.org/s...ics/Zenith.htm is this working?

[Chad Hauris]

The voltages will also vary from the schematic due to variations in local line voltage.

Appreciate your thoughts Chad, so i went and look at the back of my k731 which stated 117v -ac/dc, then measured my line V which came out 118.1v and said wow, that close.
But after thinking for awhile, this line variation wont even matter (except for filaments) if we can adjust the voltages going out to the circuits by using the optimum resistance, paricularly the Resistor thats in series with the selicon diode... My next chore would be to monitor the average of my line voltage and calibrate my B+ from that average so more or less i'll be on target. If there is a better way please let me know?.
Am like blindfolded right now as i cant do anything if i dont get that B+ specs. I'll probably wait for that website to get fixed. Meanwhile, somebody wants to HELP???.....my anticipated thanks.

[Nolan Woodbury]

I don't have the K731 schematic, but for what it is worth I think you're fretting over minor details. You're recapped the radio with replacements of close, or increased value (good) the rectifier, and the reisitors too? You should be good to go. The tubes will certainly handle any minor voltage variance and even if you fry one, that's an easy fix. Sit back and crank it up! You've already done more to it than it probably needed.

[David Roper]

Schematic Bank is back up, but there's no K731 schematic there either. Forgive me for being blunt, but you've now been told by three of the forums most expert members that you're questing for more precision than is necessary, or perhaps even attainable. You can ignore them and imagine the work you did on the radio counts for naught, or you can heed them and enjoy it. Which makes more sense to you?

Thanks for the opinions. The way i see it , none of you bothered to check your radios voltages and nobody has the equivalent schematics. I'm sorry if i ask too much but all along i thought there's no harm in trying. I tried to inquire more info about my newfound hobby which i thought, in my opinion, could improve it's performance but i notice this is not so important for some people.

As for me, i'll continue to do my research, (again, no harm in trying) on my own. I regard this as a hobby, that is my reason why i keep keeping on..(don't you think so

[RetroHacker]

Keep in mind that since you've recapped the set with modern capacitors and replaced resistors with modern ones, that your radio is probably more in-spec than it was when it was new. Those old components were like +/- 20% or worse. Replacing the filter caps with higher ratings is good - helps reduce hum. But once you do find a schematic with voltage ratings on it, trying to get your set to match is going to be hard - you can't always trust old numbers! That was done with a VTVM and with original +/- 20% components. So if you find the voltages in an old manual, don't try to get them exactly - as long as you're close it doesn't matter. As for you B+, it sounds about right for a table radio like that - and all the tubes are meant to operate at that voltage. I don't have a K731, if I did I'd measure it for you. It looks like that radio is covered in Sam's 653-12. Try to see if your local library has it, or maybe someone here has it and can send you scan. Having the schematic is really useful.

Have fun - and try not to drive yourself too crazy getting things exact - with these kinds of electronics, exact is always going to be +/- 20% <grin>.

-Ian

[QUOTE=RetroHacker] I don't have a K731, if I did I'd measure it for you. It looks like that radio is covered in Sam's 653-12. Try to see if your local library has it, or maybe someone here has it and can send you scan. Having the schematic is really useful.

Very good pointer and i fully agree. I promise if i find the specs first i'll post it here for anybody who wants to do more for their hobby.

Please dont get me wrong, i'm really having fun doing this. I think i got used to my other hobbies; building/ flying model helicopters, which requires so much challenge and precision (and i like it). Now that i've got this old radio from a neighbor which was humming so badly, my instinc kicked in,..not expecting that i will be real crazy hearing the sound after doing the mod plus the real satisfaction of doing it.. Well i always thought old things are always inferior especially with electronics, but boy, was i wrong. Then, i asked myself what if i do more and who knows?
So excuse me for my obsession guys, but i can't stop the "too much fun" now. .. Looking for that B+ is getting more exciting and challenging for me ( and i like it)!!

Ps. Planning to start a thread pertaining to models that sounded better than k731.

[Chad Hauris]

For the most part tubes are very forgiving of voltage variations...as long as the voltages are within the guidelines for the tubes as you will find in a tube manual you are fine. The power supplies in radios like this is about as basic as you can get...half wave rectifier, unregulated, so there is no compensation for either load current variation or line voltage variation, so you can't hold it to too close of a tolerance.

[David Roper]

OTOH, a puppy finds great pleasure in chasing his tail.

Thanks Chad, we have the same observation regarding tolerance specs of this old radio. I wonder what if we add more parts; regulators..etc.. to this old circuit and make it better?..You see i have four high-end Amps, two of them almost the same model except for the price. The reason?..they claim it's basically the same circuit but the magic, they say is the 2% tolerances of the caps and resistors being used inside.....and believe me it sounds better, the reason why i bought it...
I did a research about these low tolerance parts and they actually exist and very expensive;..$2.80 for one (1/2W) resistor, whew!....So what drives these people to produce such exactness?? ...I figured very few could understand the quest for improvement... and probably should not talk about it here.

Originally Posted by David Roper

OTOH, a puppy finds great pleasure in chasing his tail.

Funny but true if i dont get out of this thread for i dont see puppies going out looking for better things than their tail..

[Sandy G]

In any case, Welcome To AK, Beda4...Guess you've figgered out we've got quite a few tech-heads here.. What other old sets you got ? These damthings are like Tater Chips..Nobody can have just one...<grin>