1952 Zenith Bugeye AM/FM Clock Radio
 

Hello Everyone, earlier late last year I had found at Goodwill for $5 a 1952 Zenith Bugeye AM/FM Clock Radio that was in really good shape yet and still worked without hum on its original power supply capacitors.

It recently though had one of its power supply capacitors fail, but what's weird is that it only affected part of the radio's function, the alarm/radio mode is the only part that hums, the radio on mode doesn't hum and neither does the sleep mode.

Any ideas as to why it would of failed that way or is it just random?

Some Pictures of the unit in question below.

Are you sure it isn't the mechanical buzzer alarm?
Make sure the alarm set button is pushed all the way! :scratch2:

The schematic is available online (thanks Bill J.) for anyone that wants to take a look.

https://app.box.com/s/e22uy20wf2slx63b8hlm

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Nope, I already checked that, in fact that's what I thought it was at first, but when I hit the button to turn it off the humming coninued and it was coming out the speaker (kind of sounded like the way Sony did their alarm for their Digimatic alarm clocks for years where they would produce a 60 Hz hum out of the speaker for the alarm buzzer, rather than a mechanical buzzer).

The funny thing is that when I test the old capacitor's leads they test normal, no shorts or anything, charge up to over 15 Megaohms and then slowly discharge like they're supposed to.

I might put it back together and see if it was maybe just a fluke.

EDIT: just put it back together and tried it out again, and it seems that now the power supply cap is completely shot as now its humming in all 3 different power settings not just the alarm/radio mode. So it seems I will definitely have to replace the power supply caps in this radio, as well as the paper caps in there.

Thanks, I just downloaded it and I noticed it is slightly different than mine but not by much, but it'll work.

The Chassis on my radio is 7J03 and the service manual you linked to me was for a 7Z03 which I'm not sure what all the differences are exactly but there must of been some other wise they wouldn't of revised the chassis design from the 7J03 to the 7Z03, but from the looks of it one of the differences was possibly the addition of an extra knob of some sort to the side of the radio cabinet, which mine doesn't have.

Well I have a good head start in recapping this radio, I have all but 3 of the coupling capacitors replaced (2 of them I won't bother with becuse they're just going across the AC Input to ground, one on each side of the AC plug hookup on the chassis, and they don't look like they are in that bad of shape, so I won't bother replacing them for right now).

The power supply caps might be a little trickier to place in the radio because there isn't very many places for one to place the new filter caps under the chassis as far as grounding the negative leads of the filter caps to.
Also the wires and resistors and capacitors that were connected to the original can capacitors leads don't have much in the way of leads on them to go by for connecting them to the new capacitors.

And I replaced the old selenium rectifier with a 1N4007 rectifier diode under the chassis, but left the old Selenium rectifier in place for appearances sake, same for the old can capacitor.

OK so I finally finished recapping the radio and went to plug it in and something made a sizzling noise in the radio and it started smoking so I quickly turned it off and unplugged it and took it apart to see what happened and nothing looked burned inside the radio, all of the filter caps were fine (they none of them were reversed) the 1N4007 Diode still looked good and it wasn't reversed and like I said nothing else looked burnt inside the radio, component wise.

The clock and the neon pilot light still work but the tubes aren't glowing so I don't know if maybe a tube failed or what, but either way its kind of got me stumped because nothing inside the radio looks burnt and none of the stuff that I thought of that could of caused that issue was the cause.

Any ideas as to what could of happened?

See photo below for the finished recap job.

You'll have to do some circuit tracing. I never reassemble a set until I'm satisfied it's working properly.
I have almost the same radio that I have to get back to.

I'd remove or replace those line to chassis caps if you plan to have that radio plugged in unattended for any length of time...They are time bombs at this point in their lives.

I'd try variacing it the chassis out of the cabinet and see if it smokes again and where...If it does not smoke look at all the wiring on the chassis and clock and see if anything could be moved by flexing or bumping into something during installation in a way that could cause a short. Magic smoke can come from anywhere so finding the general vicinity of the source is key to diagnosis.

The only problem is that with the way this radio is built I have to put it back together in order to test it because it uses a funky 4-pin safety interlock plug on the back of the radio for the power cord, and a normal two pin safety interlock plug won't work on this radio, so that's why I had to put it back together to test it which was how it got smoked, because something in the radio wasn't hooked up right or something and it blew out some magical smoke, that was pretty rank smelling, it smelled like burnt rubber almost.
So it's kind of hard to test the radio chassis outside the case.

How would I variac the radio out of the cabinet? I can't run the radio outside the cabinet because of how the interlock plug is made on this radio.

Also I had figured out that the diode I wired into the radio to replace the selenium rectifier shorted because I wired it in backwards unintentionally because I was wiring it up how it was on the selenium rectifier which had the cathode marked with a (+) sign which made me think it was the anode side because of how it was marked.

But the only thing is that when I replaced and properly wired in properly, there was no audio but the tubes were lit.

What would be going on?

Having the diode wired backwards could have damaged your electrolytic caps , but first things first there is usually a resistor between the rectifier and and the filter caps , that's the first check you should make and hopefully if that resistor smoked it saved your filter caps in the process .

Also , if at all possible when replacing a chassis mounted multi cap with single caps , it's best to mount a terminal strip and use that to install your new caps .

I agree. If you powered it up with the diode backwards you likely ruined the lytics. If you don't have a leakage tester that can test them at full working voltage it is probably best to change them again.

Do you own any clip leads? You can bridge the interlock with those. IIRC when I changed the power cord on mine you can unscrew the female phenolic part of the interlock from the back and grab those terminals with your clip leads... Be careful not to mix up the connections or you could release more smoke... even the least serviceable designs can be worked around with a bit of enginuety.

I didn't have a resistor in line with the diode on my radio... I just wired the diode straight up to the radio because I didn't realize you needed a resistor online with the diode, so yeah the capacitors are probably toast, but would that cause there to be no sound to come out of the speaker or the tubes not to light?

Your sending contrary messages , in one post you say "No audio but the tubes were lit" and in this post you say "The tubes not to light?" , , , So , which is it , ARE the tubes lit or are they not ?

Now , to get down to the troubleshooting , we need to talk about troubleshooting itself and what it means to diagnose and repair vacuum tube circuit electrical failures . The "Shotgun" method of replacing parts on a vague "It could be this part" notion is not the right way to go . A LOT of perfectly good parts end up replaced that way at great cost till the bad part is finally changed by the luck of the draw . Yes indeed changing out the failure prone electrolytics was a good action , but now that you've had the episode of reverse polarity and have no sound the best course would be to start over completely .

Were this my situation to rectify I'd start by replacing the filter capacitors again because as Electronic M and I have said , they very well could be damaged . Consider mounting a terminal strip to the chassis and attaching the new caps to it as in my picture . I'd put the Selenium rectifier back in the circuit (polarized correctly or you'll REALLY learn what it means to "let the magic smoke out"* !!!!!) and then with a DC voltmeter on a scale of 300 volts or higher , and the schematic diagram , see exactly where B+ voltage actually is and isn't . This of course presumes that the tubes ARE lit and you have no sound . If the tubes aren't lit , start with an AC voltmeter on at least 200 volts scale and using your schematic follow the tube's heater AC path till the open is found .

A multimeter , a schematic , and about 5 minutes are all it should take you to diagnose this , once you replace the filter caps and put the rest of the circuit back to original status . Once you've got it running if you want to eliminate the Selenium rectifier , fine , but DO install the proper dropping resistor for the excess voltage of the silicon diode , circuits have "as designed" operating voltages for a reason , and especially in electronics "somewhat higher" is just about never equated with "better" .

* If you don't already know , an overloaded or shorted Selenium rectifier lets out a noxious smoke that smells like a bucket of rotten eggs X 10 , so make damn sure you polarize the Selenium rectifier correctly , and also using your ohm meter mode of your multimeter make sure that the new caps you install are wired correctly and aren't presenting any kind of short circuit to the rectifier BEFORE you power it on again .....

If you smoke the selenium have a fan and an openable window ready...Selenium gas is VERY toxic as any chemist worth their salt will tell you.

Oh , and , Thanks to the schematic graciously provided by Jim :thmbsp: , I can see that there is a resistor between the rectifier and the first filter cap . That resistor , R32 , which is a 22 ohm 1 watt , is very likely what made the smoke . I'd still change the caps , and then check R32 as you may well find it open (infinite ohms) . That is also the resistor you'd want to increase the resistance of if you do try again to install a silicon diode .

IIRC from when I worked on mine that resistor looks like a thin mica cap. It was color-coded in dots...It may or may not be the same on your set depending on production run and any possible previous repair work.

No the tubes aren't glowing, what I thought was tubes glowing was actually the neon pilot lamp glowing.

As for the capacitors I ordered some new ones, and as for the selenium rectifier smell goes, I'm well aware what they smell like, that's why I wanted to eliminate it from the circuit so that it didn't have a chance to fail on me, and I did order a 47 ohm 2 watt resistor to install after the 1N4007 diode.

And I did use a terminal strip to install the electrolytic caps.

I would start by testing the tubes... the heaters are in series, so if one heater is open, none will light up.

jr

I did test the tubes that's the thing and they all tested good yet on my tube tester.

But they still don't glow.

If the tubes are good, then I would use an ohm meter to trace around the heater string from one ac input to the other end of the string, following the path shown in the schematic. Could be a bad power switch, a broken wire, bad tube socket, dirty heater pins on a tube or whatever that is causing the open circuit.

This test can be done with the radio out of the cabinet and, of course unplugged.

jr

Is the heater string marked on the schematic, or do I have to find it and Mark it myself?

I haven't seen one of those resistors in ages. The flat Micamold resistors. They're both 470 ohm, 2 watt resistors.
I had to take mine apart today. It's a 7X03 chassis with OEM part code dates from late 1955.
The under chassis view from the OP shows the band selector switch is mounted on an angle with some form of a universal joint, which I thought was rather novel. Mine is mounted straight.
I unsoldered all the leads going to the back to make it a little easier to work on. Of course I made a drawing where each one went. :scratch2:

The heater string is shown toward the bottom of the schematic. :thmbsp:

Ok, will check it out. :thmbsp:

OK So I figured out that you were right there were some micamold resistors in the radio, two of them in fact, one that was coded Red-Red-Black, which is 22 ohms which is the R32 you were referring to on the schematic, and just like you suspected, it was blown wide open (it measured infinite) but the capacitor was fine yet, which I suspect that maybe the capacitors were actually spared any damage from the reversed diode by that resistor and that R32 was actually where the magic smoke came from.

So If I understand you guys correctly I'll need to replace R32 the 22 Ohm 1 watt resistor with a 47 Ohm 2 watt resistor if I intend to use the 1N4007 Diode in place of the Selenium Rectifier? Also would R32 being blown open be the cause for a dead radio? If so It seems the repair may be fairly easy to get this radio up and running again, a lot easier than we thought at first.

EDIT: there are actually 4 Micamold Resistors in the radio, and 3 of the 4 Micamold resistors are measuring as open, the two 470 Ohm 2 Watt Resistors (R30 and R31) attached to the one 22 MFD 160 Volt electrolytic capacitors are both measuring as open when measured in Ohms, but when measuring in kOhms it measures as .475 kOhms, and then the 22 Ohm resistor (R32) measures open no matter how you measure it.

Then all but one of my filter caps measure normally, meaning that they are not destroyed, but the third filter cap, (the 22 MFD 160 Volt cap that R30 and R31 are attached to) measures 29 PF and that's all that shows up on the capacitor tester, I don't even get an ESR value from my ESR meter/Capacitor tester, so I don't know if that means that the capacitor is shot or if its because it has resistors in line with the capacitor and its affecting the reading or what the deal is.
I know that when I still had the original can capacitor hooked up yet and when I went to test the capacitor lead that those two 470 ohm 2 Watt resistors were attached to (the 20 MFD 150 Volt lead that was marked with the triangle), it measured the same exact way, and the other two leads measured as they were supposed (even with resistors attached to their leads).

What's really bizarre is that R30 and R31 when measured as resistors they don't measure anything (they measure as open) but when measured as capacitors they measure 29 pF (MMFD) which is strange because why would resistors be measuring as capacitors?
Also for some reason the filter cap that those 2 resistors are attached to is also measuring as 29 pF on my capacitor/ESR tester but it doesn't give an ESR Value for that filter cap, which its like the capacitor tester is testing that capacitor as a mica or ceramic disc capacitor rather than an electrolytic capacitor like it should be, which is weird because all of the other filter caps test like they should, and they have resistors attached to them.
Even more strange is that this same phenomenon I'm describing that's happening with R30 and R31 and C1B that they are attached to also happened with the original can capacitor when it was attached still before i changed out the old can cap for the new filter caps.

So I'm not sure what's going on here... :scratch2::eek::dunno:

Your DMM probably on its sub k ohm range has a max resistance measurement limit of 200-400 ohms and anything above that will read as infinite unless you move the range switch to a higher range. The 470 ohm resistors are probably fine.

The lytics measuring in the pF range are almost certainly open and defective. Picofarad range capacitance is easy to get from the cutoff leads of the cap, your instruments test leads, set wiring, etc. All can present a few dozen pF to the meter easily. Heck if you have a good amount of RCA A/V cables you'll find most have well over 100pF between center and shield. Any 2 conductors not connected to each other have some finite (possibly measurable) capacitance between them.

Ok. That makes sense, so it seems I only have 1 electrolytic cap to replace and the one resistor (R32) to replace and that's it. And the radio should be good again.

How about a chemist that's on a low-sodium diet? :D

Ha! That's a good one!

He puts it in his smokes. :smoke: :D

Replacment of these items may well fix the B+ supply, but it appears that you also have another unrelated problem in the heater string that is causing the the tubes not to light up.
Unless you fix both problems, the radio will remain dead.

jr

Well I got it going, mostly it was indeed R32 that was causing the problem when I took R32 out, it had a nice big crack running down the backside of it, telling me that it had taken the brunt force of the improperly installed diode.

But now I have a new problem, and it may very well be just a bad tube causing it or an improperly seated tube, but the AM Band works perfectly on the radio, but the FM Band is very weak and barely picks up anything including a local Christian Station that puts out nearly 50,000 watts of power making it the strongest station in the area.

What do you guys think? A bad tube or something else?

V1 is the only shared tube so it would be the most likely culprit for a weak tube, but may as well test all the tubes as if they are all barely good it could be a cumulative effect.

Make sure the internal antenna is working by substituting a good external FM ant.

If the tubes are fine check all resistors for tolerance, replace any out of tolerance. Also, check voltages at the tube sockets for any telltale discrepancies and work to correct any found.

If after all that FM is still weak try adjusting the alignment.

I figured out that the 19T8 tube was the problem in there as I had tried subbing out the all of the tubes including V1, and nothing really changed except that then the AM band would go out and the FM band would start working and the effect would would swap back and forth depending on the tubes changed out.

But when I swapped out the 19T8 tube it worked consistently on both bands, making me realize that the 19T8 was the culprit all along.
So now its working fine now. thanks for the help, everyone! :thmbsp::music:

I have a similar Zenith where the FM band slowly lost sensitivity and started drifting. I found one section of the 19T8 to be just a little weak, and once replaced the FM was rock solid again.

After that, I puled the 19T8 from my H845 and checked it, and found one section far weaker than the other 2. I always thought the H845 performed well, but a replacement tube gave me a noticeable sensitivity improvement. Evidently, those Zenith's require a good, strong 19T8 to perform properly.

Yep, that's what I'm thinking, the 19T8 that was in there was an older replacement so I swapped it out for one I had nabbed out of an old Zenith I had scrapped and bang it had fixed it right up again!

OK so I've noticed something weird about the alarm clock portion of this radio, the radio portion of the alarm clock comes on 20 minues or so before the set time for the alarm to go off, and then the the buzzer starts going off 5 minutes before the set time for the alarm to go off.

But then when the time for the alarm to go off comes around the buzzer just shuts itself off and the radio remains on until you turn it off, the radio part doesn't even shut itself off after an hour like its supposed unless you turn the power knob to the off position, then when you turn the power knob onto the alarm/radio position then the radio is actually off then.

The weird thing is that the sleep feature shuts itself off like its supposed to after an hour or however long you have the sleep timer set for, without a problem, its just the alarm part that seems to have some issues with working properly.

Is what its doing in alarm mode normal operations for this alarm clock radio or is there something mechanically wrong with the clock mechanism that needs repaired?

Any helps is appreciated in this matter.

I used mine as an alarm for a while. I never set it so the buzzer operates...I'm a light sleeper and I hate waking up to a shrill tone. If it is set for just the radio to wake you up it should switch off after an hour IIRC.

That's interesting, because mine doesn't shut off on its own after an hour, you have to switch it off by hand and then wait an hour and then it shuts it off. Which is weird, as it should shut off on its own after an hour, but it doesn't, it just turns on and stays on unless you turn it off manually by switching the power switch to off and wait till an hour has gone by then it's actually powered off.