Suggestions on a Zenith R723 Radio

[Keefla]

Hello all. Im new to posting but not to the site. I ran across it about a year ago and have been an avid reader of the Color & B&W Tube TV forums. I have to admit i dont know much about tube electronics, or electronics in general, but am fascinated by how they can do so much without semiconductors. My little electronics experiance comes from the Automotive end of things. Me and a friend run a small auto shop and i usually get stuck solving wiring and sensor problems so im quite proficiant with a VOM and a schematic. I know basic resistance and capacitance theories, transistor theory and random other bits and pieces. Basically enough to get me by when dealing with computer controlled engines. I know my way around most engine wiring harnesses, but this electronics stuff is quite intriguing to me...i gotta say i give you all my upmost respect for fixing these things and keeping the history alive. Thats why im trying it for myself.

Ok enough intorduction. Time for the radio. I happened across a Zenith R723 radio for 5 bux at a yard sale. Chassis # 7H04z2. I downloaded the Rider schematic PDF file for it and went through the radio looking for any discrepencies (along with many downloaded web sites covering vacuum tube pinouts and vacuum tube theory...fascinating stuff!). So far so good as far as changes to the wiring go. But thats as far as i got. What little i know i picked up from this site and reading about vintage TV restoration stories online. I know not to plug it in till i re-cap it (paper and plastic molded caps). I also ordered the electrolytics and the line bypass caps (well, line to ground on this radio). You guys think i should be OK with that lineup. I dont have a variac (im working on getting one) so thats why im replacing the bypass and electrolytics. By hooking the ohmmeter across power cable what should i look for (as far as resistance) to detect a possable problem before i power it up the first time?

Suggestions? Comments? Im sure i'll have a few questions as i go. But this is the first real resto project im getting into. It was cheap, the case is not in the best of condition, dail cord is broken and i know it doesnt work cause the guy i bought it from said the last time he went to use it (over 10 years ago) it didnt do anything. Im thinking maybe a burnt out tube, the tube heaters are in series so if it doesnt fire up (maybe fire is the wrong term) ill check the voltage drops across the heaters. I figure its a good first project. Ive re-capped an old RCA Victor RFD11V, but that already worked somewhat, the motivating factor was the filter caps were shot and it hummed badly (AGC was much better after the paper caps were replaced too).

Ok enough for now. I hope I can get this set running and put it to use somewhere in my house or shop. Thanks all for listening, and thanks for humoring me in my learning stages of electronic troubleshooting.

Keith

EDIT: Attatched is the Rider PDF file im using.

[Tom Bavis]

If you have continuity through the power cord, tube filaments are good. If not, try wiggling tubes in sockets - could be contact problem. REPLACE that selenium rectifier (square finned thing)! A 10 cent 1N4007 will do - band end is +. This will increase voltages, not really a problem though you might have to increase the output tube (35C5) cathode resistor to keep it from running too hot. Or just plug in a 35EH5 instead... it will draw less current.

I think I have the Sams Photofact folder in my "to be scanned" pile... email for a copy. TBavis(at)rochester(dot)rr(dot)com

[Chad Hauris]

One thing I have done is to use a 100 ohm 10 watt resistor between the silicon diode and the power line...this helps pad down the voltage to the right level and helps provide surge protection to the diode. Make sure that the old selenium is electrically disconnected from the circuit.

Also inserting a fuse in the power line is a good idea for extra protection...1/2 amp will probably do.

I don't believe in the Variac tricks for reforming electrolytics...just replace those old caps. It's not too difficult and you will have many more years of good performance. To check for shorts I do use a series light bulb (60 watts) on the power line to the set. Watch out, the chassis is connected to power line.

[Keefla]

Thanks for the advice everyone. I have a question about the rectifier....how reliable are these selenium rectifiers. You say change them, but other sites say in radios they are not that unreliable. What are the pro's and con's? Would it damage the radio if it shorted? Or is it just a reliablity issue? Not being cheap i was just wondering for my own knowledge. Ill replace it is its a safety/damage issue. Also good point of the fuse. I didnt see one in the radio now that you mentioned it! Ill definatly get one and wire it up.

Just waiting for the caps to come in.

Keith

[blue_lateral]

Welcome Keefla!

Selenium rectifiers had a reputation for being unreliable even when they were new. When it goes, it will stink up the place like rotten eggs. I dont think they usually short, but they have some voltage drop when they are new (thus all the cooling fins) and it gets worse with age. When it gets bad enough, there will be more heat generated than the fins can dissipate. You'll know.

John

[Keefla]

so basically i *could* leave it in place till it goes? Id like to keep it as origonal as possable, but i dont wanna damage anything.

[Sandy G]

I dunno, I think leaving that ol' selenium gizmotron alone-Everything i've ever read or heard, you're sorta sittin on a time bomb. It might take out an IF coil or something when it goes-and those are made out of Unobtainium. And I think it's a cheap permanent fix-THEN leave the old rectifier in place, just dis-connected.-Sandy G.

[Keefla]

eh....well if its got the potential of damaging something as you suggested then its gonna go. Ill leave it in place just disconnected. My luck it will go out the first time i power the radio up and it'll toast something.

Caps should be in tomorrow or friday. Hopefully tomorrow, i got the day off Can i get that new diode at radio shack? Do you guys/gals all feel this excitment waiting for parts to your projects to come in? Its 10 dollars worth of capacitors but its like christmas in July!

Keith

[Keefla]

ok all. me again. I re-capped the set with the exception of the electrolytics (so far). and powered it up to see. I have new caps for the electrolytics, which it seems i will need to use casue the can gets moderatly warm and there is a slight hum. And about the hum....thats all i get. that and static when i change bands. when i put the soldering gun on the wiper of the volume pot i get a buzzing, so it would seem the output stage is ok. but nothing on AM or FM at all. Any ideas on where to begin. all the paper caps are gone. there seems to be a molded plastic cap with 3 colored dots on it. I should have a replacement for it. is this usually a reliable cap? I doubt it would kill the AM and FM thow(??)
Any ideas on where to start? im new to this so be gentle.

thanks again

[Keefla]

UPDATE: i got it to make noise! AM and FM. its recieving a signal on both however its intermittent and very distorted on FM, and very weak on AM. Im getting a can of de-oxit and im gonna spray the selector switch with that. one thing i noticed thow is that there was NO output at all unless 2 resistors on the audio output tube were touching leads (they arent supposed to be according to the schematic.) I dont remember what they went to off the top of my head, ill post that tomorrow, but im thinking the resistors are high in value and cutting off the output. I have to say they look kinda textured looking, not like the other resistors. Ill give it another go tomorrow after work. Also after running it a bit the humming has gone down and the cans are a bit cooler, and so is the rectifier (still selenium) is a lot cooler too, im monitoring its temp very closely and im gonna pick up another diode tomorrow when i get the de-oxit. Any other suggestions are welcome.

Thanks guys!

[vintagecollect]

Restoring a set with all new caps, getting rid of selenium rectifiers is good idea for reliablity. Selenium rectifiers are often found faulty when tested on older equipment. I have only seen decent heavy duty versions of selenium rectifiers on some 50s military radios. The work put into radio is well worth years of enjoyment set will bring.

[Keefla]

Originally Posted by vintagecollect

The work put into radio is well worth years of enjoyment set will bring.

i agree. Ive been working late at my garage thow lately and ive only had sporatic time to work on it. Seems this is the 2 weeks of overheating GM cars. 3 cavaliers, a grand AM, and a saturn. then randomly a Mazda pickup that the guy tore the engine down on and gave up....sent us the truck with the engine pieces in a box on the passenger seat He's gonna love the bill we give him for sorting through that mess.
Back to the radio.....ive tested 2 resistors that looked suspect in the output stage, both had drifted to almost open circuits, and one was a 470 ohm! So im gonna get new resistors and just replace any bad looking ones and go from there. Ill start in the output stage and test as i go. Also i got the new rectifier diode and resistor to replace the selenium. Ill keep you posted as i go.

Keith

Im gonna dregde this one back up from the dead. Sorry guys its been a loooong fall. Also for some reason i had to make a new account. as my other one i couldnt log into. But its me, same old me. The radio has been sitting in my bedroom, looking pretty but not working, since my last post. Well tonite after work i had nothing to do *gasp* and as i sat down to watch TV i looked at the 'ol Zenith. I went to radio shack armed with the schematic and bought resistors to replace all of the old ones (if need be). the thought was to leave the originals if they tested in spec, but i didnt want to have to run out again and buy more if i didnt have them. Cause that would be an excuse to not git it workin tonite.

Sat down with my Fluke meter i use at the shop, wire cutters, soldering gun, and schematic, and went to town. I tested the ones that looked cooked first, starting the the audio output section. Im not that great at electronics but can read a schematic and can use a meter. I went through cutting a lead or removing a tube wichever would isolate the resistor. I found that R14 in the ryders schematic was open, where it sould be 2200ohms. Also R18 drifted from 150ohms to over 300. R3 was supposed to be 4.7Meg, tested over 6meg. And R16 was 220K ohm, up from 150K. All other resistors tested within spec (taking tolderances into account). So now the moment of truth. Plugged it in and was delighted to hear a hum when touching pliers to the volume pot. tinkering with the band selector i was able to pull in AM, lets just say, LOUD and clear (forgot the volume was up fairly high). FM didnt at first seem to work, but i realized the antenna lead had come undone from the back of the radio. fixing that brought in AS MANY if not MORE FM stations than my Sony reciever. And must i say, expremely clearly. Tuning past a rap station (shudder) the cone on that old zenith speaker got a workout. The selectivity is impressive. I dont have anything to judge it against besides modern radios and my Sony reciever that has digital tuning, but id say im most impressed.

It defintaly needs a good once over with de-oxit as the selector knob is scratchy, as is the volume controll. I let it play for about 30 minutes, the filter caps are ever so slightly warm, but there is zero hum, so i dunno if i sould leave them or change them out. Also the origonal selenium rectifier is still hooked up, i forgot to buy the new diode, and that gets QUITE warm, so im nervous about running it too long with that in place.

Im hooked guys. I fix cars, i love cars, but this is different. The cars i work on are modern and have no soul for the most part. This is the first tube device ive fixed (besides just re-capping) and seeing it play a classic rock Fm station with the lights off and just the glow of the tubes is quite romantic. Yes me and my radio had a romantic moment

All thats left is a new rectifier diode, dial cord, and haul it to the shop to replace the ailing boombox i have in my office. The sound out of this is 100 times better. Or maybe it just sound that way o me casue i made it sing for the first time in possably decades


Thanks for all the help, sorry for the long post, i just wanted to share. And im sure you wanted to hear my sucess story.

Keith

[Keefla]

The electronics seem to be 100% in order now. Replaced the Selenium recitfier with a 5404 rectifier diode. Rat Shack didnt have a 4007. This was probably overkill but its fine by me. Also added the 10watt 100ohm resister as was suggested to keep the tubes happy. The selenium recifier looks origonal with the wires still running into the chassis, however its electrically disconnected. I ran the inline fuse holder wires out through the same hole, and it sits neatly against the back of the chassis. I then hit the selector switch and volume pot with some de-oxidizer/lubricant (couldnt find any straight de-oxit). worked them back and forth a bit. let it sit for about 5 mintes and plugged it in. HURRAH no more crackling on the volume, and the AM/FM band switch is working perfectly again. Right now i got it running on my desk listening to 94.1 WHJY (rock/classic rock) and it sounds really good for just being a chassis on top of a honda accord shop manual (i need to prop it on something so the backboard with the plug would sit level.) next i gotta go online and order the dial cord, and buton it back up. Its kinda sexy just out in the open like this tho i have to say.