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 Zenith 10S669 low B+ voltages

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marcus0331



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PostSubject: Zenith 10S669 low B+ voltages   Wed Oct 13, 2010 10:38 am


Hello,
Recapping a thread that started in the old forum/server, I have replaced all of the caps and most of the resistors, as well as almost all of the rubber covered wires in this radio. I also had to replace the main transformer with the same item from a 10S668 I had been keeping for spares. The radio is working on all bands with good volume when connected to an external antenna. The problem is that after B+ peaks when first switched on, it drops in a hurry by 50 volts or so and there is about 140 volts of AC appearing between the rectifier and the field coil. It disappears when I remove the coil. I have two 5Y4 tubes and the AC persists with either of them. Could both of them be bad even though they seem to check out ok with my EICO666 tube tester. Of course there is also significant hum present. Thank you all.

Marc Schlemmer
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pixellany



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PostSubject: Re: Zenith 10S669 low B+ voltages   Wed Oct 13, 2010 12:29 pm

could you post a link to the schematic?
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marcc

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PostSubject: Re: Zenith 10S669 low B+ voltages   Wed Oct 13, 2010 12:35 pm

Hum suggests a possible short or bad electrolytic. Check for reversed electrolytics. There will normally be a voltage drop across the rectifiier. 5Y4 is a filament tube and heats quickly producing the surge as the heater tubes lag behind. There will also be a voltage drop across the field coil.

I would need to see the circuit. make sure rectifier is wired correctly 5Y4 filaments pins 7 & 8 on a 5V winding of their own. Anodes Pins 3 & 5. Other socket pins may be used as tags which will feed AC in if there is a wrong connexion or a solder dag?

You need to replace all papers & old electrolytics before making any assumptions.

Eg 80 (same tube different pinout & base) 350VAc in 330VDC out.

Marc
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richfair

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PostSubject: Re: Zenith 10S669 low B+ voltages   Wed Oct 13, 2010 1:44 pm

Are you completely certain the transformer is wired right? Something like swapping one end of the HV winding with the centertap would possibly do what you describe.
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PostSubject: Re: Zenith 10S669 low B+ voltages   Wed Oct 13, 2010 1:49 pm

Even though I haven't seen a schematic, 140 volts on an AC set seems way low. Have you measured the secondary volts of the power trans when the voltage drop occurs?

The field coil could also be shorted, which would produce more hum and less voltage.
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marcc

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PostSubject: Re: Zenith 10S669 low B+ voltages   Wed Oct 13, 2010 3:47 pm

Confucius say .. "picture is worth thousand words". That's why a circuit would be useful for all of us to see what we have?

Take the rectifier out. Make sure that you are interpreting the bases correctly. Octal pin one left of spigot, then clockwise: Looking at wiring side.

Noting my last pin comment check socket voltages (tube out). Use the 1000VAC range in case you broad side the HV. The CT may not go directly to ground; It may go via resitors or the field.... we just don't know.

Marc

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OldWireBender



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PostSubject: Re: Zenith 10S669 low B+ voltages   Wed Oct 13, 2010 11:52 pm

The good news, is I found the schematic. The bad news is, you need the djvu viewer to see it.

Zenith 10S669 Schematic

And, yes, there's R22, a 40 ohm resistor between the power transformer CT and chassis ground. AND the negative of the two filter caps, C19 and C20, DO NOT connect to the chassis.

John
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marcc

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PostSubject: Re: Zenith 10S669 low B+ voltages   Thu Oct 14, 2010 5:37 am

Ok handy format the one on Nostalgia Air is a user friendly pdf. Assuming I got the correct one?

The 200 Ohm resistor is the cathode resistor for the 6V6's. its 40 Ohm above ground

Below the pdf circuit is the pinouts of all of the tubes with the voltages on them. I will assume with reference to chassis, as some one (not uncommon) failed to mention.

You should have 325VAC on the plates of the 5Y4 & 356VDC on the filaments. Naturally with the tube removed the 325VAC will be slightly higher & no DC. We need to establish if that one is wired correctly first.

B+ should be around 282VDC after the choke. pin 4 6V6 Pin 3 6K7. I note the specs say 450V electrolytics. I would not use anything below 500V. I would be interested to know what the actual start up surge Voltage is?

Marc



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marcus0331



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PostSubject: Re: Zenith 10S669 low B+ voltages   Fri Oct 15, 2010 6:07 am

Hi, Thank you for taking the time to consider my problem. I work long hours as a medical professional and can't spend as much time with my tar baby as I would like but I am off this weekend and will revisit this, taking in your generous advice.

Marc Schlemmer
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marcus0331



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PostSubject: Re: Zenith 10S669 low B+ voltages   Sat Oct 16, 2010 9:31 am

Hello, I was very careful to mark the wires leading from the main transformer and put them in their proper places during the replacement process. I have double checked and it still looks right. The rectifier socket is not oem, replaced by a previous owner with an aftermarket item. The filter caps negative leads do connect to the CT, as prescribed, not to chassis ground.
I am getting 345 VAC on each leg without the rectifier tube in. When the tube is in and the set is first switched on I observe, on pin 8, a peak DC voltage of 477 which quickly settles down to 272. At the same time, on pin 8, AC voltage immediately appears at 15VAC drops to zero (0) then increases to 140VAC. By then the radio is working, picking up a strong local BC station. Another measure was taken on the downstream side of the field coil. Here, when first switched on, DC voltage surges to 478 then settles down to 227. At the same time, here, there is a flash of AC at 58 volts which quickly goes to zero. To recap, when the radio settles down, B+ voltages are lower than normal and there is AC voltage in an area unexpected...and, of course, there is hum. I am just a humble newbie here, this is my little diversion from work, and I greatly value your learned advice on this.

Marc Schlemmer

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richfair

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PostSubject: Re: Zenith 10S669 low B+ voltages   Sat Oct 16, 2010 3:46 pm

The schematic is on notalgia air site as 10S699, a typographical mistake.

The AC you measure at pin 8 is not alternating current but is DC that pulsates at twice the power line frequency. It is what comes out of the 5Y4 tube (what is smoothed out by C20, C19, the speaker field coil, and other parts). Your meter will read an AC component and a DC component at that point, perfectly normal. What is more important is the B+ voltage that is not meeting expectations. Either it is being sucked away before it gets to the radio circuits or the audio output stage (most likely culprit) is drawing too much current.

Here is what I would do next,
With power off, I'd verify a few important resistances:
1. Measure ohms from chassis to transformer center tap. It should be 40 ohms.
2. Measure ohms from pin 8 of each 6V6 to chassis. Each should be 240 ohms.
3. Meausure ohms of speaker field coil, which should be around 670 hms when the coil is at operating temperature. I'd expect something like 630 ohms when it is cold.
4. Measure ohms between pins 5 (grid) of the two 6V6s. It should be around 700kOhm (total of R20+R21+R20, if I read the schematic right)
5. Measure ohms between pins 3 (plate) of the 6V6s. It should be 560 ohms, the resistance of the audio output transformer.
6. With a piece of wire, short circuit filter cap C20 for 10 or 20 seconds (remember, power is off. This is to remove residual voltage on both C20 and C19.) Now, if you have an ohmmeter that has different scale settings, set it on a middle range. If it is an auto-ranging meter, that's okay too. Measure the ohms across C20. It will not have a specific value, but you should see the meter immediately jump to almost zero ohms and then "glide" up to a very high resistance reading.

If any of the above steps do not yield the right reading, stop.

If those measurements seem correct, next I would do something unusual. I would pull out both 6V6's. With all other tubes in place power on the radio.

7. Measure B+ after the other tubes have warmed. Notate what it is, but it should be well above 280 volts.
8. If B+ is NOT well above 280, I would measure DC across the speaker field (still with radio operating without 6V6s). It is helpful to know how much voltage is being lost within the speaker field.
9. Shut off the radio as soon as these measurements are taken.

Having these readings will determine where to look next.


Last edited by richfair on Sat Oct 16, 2010 3:48 pm; edited 1 time in total (Reason for editing : added note to nostagiaair location of schematic)
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marcc

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PostSubject: Re: Zenith 10S669 low B+ voltages   Sat Oct 16, 2010 10:16 pm

It would be an idea, if not already done to print the pdf version of the circuit.

Aside from the other recommendations. Did you test all of the tubes for "shorting"?

I hope you are not running the set with the 5Y4's sideways. If they are not orientated correctly sideways a sagging filament can cause issues.

I will wait & see, as to what the other results are before further comment.

Marc
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marcus0331



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PostSubject: Re: Zenith 10S669 low B+ voltages   Sun Oct 17, 2010 9:55 am

Hello,
Here are the results, 1. 40ohms 2. 240ohms 3. 580ohms (cold) 4. 702ohms 5. 513ohms (lower than predicted) 6. this occurred as predicted

Then, a new problem.... 7. 312volts with 17volts across the field , this with both 6V6s out...there was a mild pop and B+ went to zero....I tried to be quick about this test but obviously I stressed something beyond limits....what did I likely do, here? I hope I didn't kill my transformer. I probably should have stopped at 6, right?

Marc
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marcus0331



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PostSubject: Re: Zenith 10S669 low B+ voltages   Sun Oct 17, 2010 10:26 am

Hello,
Some good new, the main transformer is still making the big ac voltages, so maybe I merely trashed the 5Y4.

Marc
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marcc

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PostSubject: Re: Zenith 10S669 low B+ voltages   Sun Oct 17, 2010 5:04 pm

Hark back to my comment re filaments & check the 5Y4. Yes it may have self destructed, however One must distingush between "Snap Crackle & Pop".

Pop sounds more like C19 or C20 (more likely) letting go due to over voltage. A modern 450V electrolytic will only stand about 500V. I would be surprised if one of those had not already let go if that is what is in there?

That set runs particularly high B+ and I am surprised that it has not got screen dividers on the 6V6's to reduce their voltage.

Transformers tend to crackle as do field windings. Shorted power transformers with shorted loads tend to growl.

My other obsevation is that all hell seems to break loose at the point that the other tubes are coming into conduction and one of them could be shorted? As I have seen it happen several times, there is also the possibility that the field coil insulation has failed.

An electrician with an insulation tester can help here. You remove the two B+ wires from circuit & check for conductivity between the body of the speaker and the coil (one wire of tester to coil: One to metal frame. You can test the speaker transformer primary in a similar manner. It also must be "floating" & you can use CT to ground.

It's getting away from original but: My solution to problem radio's with massive surges of that nature, is to use the Sovtek 5Y3. This is a more rugged tube built like 6X4 etc. It has a massive high thermal inertia filament. I doubt it would sag. It comes into conduction verv slowly thereby cutting the surge to a similar level to a heater type....Heater type rectifiers do not have massive surges.

Marc
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richfair

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PostSubject: Re: Zenith 10S669 low B+ voltages   Sun Oct 17, 2010 7:50 pm

I'll bet the experience bumped up your heart rate a bit, no? If the replacement power transformer was working before, as it seemed to be, this little incident did not hurt it. No worries there. It takes time for a power transformer to generate enough internal heat to do real damage.

I posted an earlier reply here, which I deleted immediately. Sunday evenings are busy around my homje and I wanted to make sure I was fully focused on this. You are in good hands with marcc as well.

My suspicion all along is that either there is a wiring mistake around one or the filter capacitors C19, C20 (applogies, I know you have double checked, but still....) OR that C20 was not functioning properly or at all. I am going on that assumption.

However, to look back and assess what might have happened, based on your earlier post, we know that the peak voltage the capacitors will encounter is about 10% above their ratings and when the other tubes warm up B+ will fall quickly below that. Brief over voltage should have been tolerated by your new capacitors! Besides, even without the 6V6's in place, B+ should have dropped quickly below 450. Whatever went "pop" was bound to fail soon. Check the 5Y4 and see if it has failed. I have never experienced a rectifier tube failure due to bad operating position but it is very possible, as marcc has cautioned. If B+ suddenly went to zero a failed tube is most likely, a test will confirm.

Step 5 where you measured 513 ohms across the output transformer, let's not worry about that now, it is probabably nothing. It is not a factor in this failure.

Please recheck step 4, you said you measured 702 ohms. You should have measured approximately 700,000 ohms, 700kohms. Please confirm.

Also clarify,
Quote :
Then, a new problem.... 7. 312volts with 17volts across the field , this with both 6V6s out...
Across what circuit points did you measure across? Less that 10 volts? Really? What you've said seems unlikely.
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PostSubject: Re: Zenith 10S669 low B+ voltages   Sun Oct 17, 2010 8:01 pm

Hello,
I think you are correct, I probably did lose the input filter capacitor. I tested a spare 5Y4 that now glows cherry red. I will verify and I hope that this is all that is broken here. I still have normal ac voltages. In error, I meant to say 703KOhms not 703Ohms, thanks for pointing that out. As for the field, query #7 .... 312volts, then 17 volts measured across the pins leading out to the field coil....wrong?

Regards
Marc
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richfair

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PostSubject: Re: Zenith 10S669 low B+ voltages   Sun Oct 17, 2010 10:03 pm

Quote :
As for the field, query #7 .... 312volts, then 17 volts measured across the pins leading out to the field coil....wrong?
I mis-read what you wrote but now I understand. I saw 7.312 volts, not "step 7, 312 volts". Me bad.

You measured only 312 B+ volts, even without the 6V6's installed? Wow, that is well within the 450v rating of the filter caps. You measured that at pin 5 of the 5Y4 without the 6v6's installed? That is good information.

A 17 volt drop across the speaker field (513 ohms) is maybe more than I would have guessed but is not alarming. It says there is 33ma being drawn by the rest of the circuit with only 312 volts of B+. However, this is at odds with the fact that the voltage peaks at 477 then drops to 312. Those two "facts" do not jive.

I'm still guessing that C20 was faulty. I would disconnect both C20 and C19, replace them with new parts, replace the 5Y4 and start over. (marcc's suggestion to sub a different rectifier tube is smart, but without it for now your spare 5Y4 will be good for testing)

This is where a dim bulb tester would really help as an indicator and as a protector. A variac would be very helpful also. Either (or both) would allow a lower voltage into the radio, thus lower current drawn by a fault condition, and better safety while testing. Otherwise, we're just asking for more trouble.
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PostSubject: Re: Zenith 10S669 low B+ voltages   Mon Oct 18, 2010 5:45 am

Do note that the DC resistance of the speaker primary & field is on the pdf diagram.

I assume the 5Y4 was in the set when it suffered embarassment due to overload. If you look at the tube data for the rectifier, there is comment on positioning. A sagging filament will hit the plate and fuse.

Normal voltage drop across the field should be 74V with 356V at the rectifier. a 250V rail will in some sets peak to close to 450V & this, with 356VDC at normal load is going to go way above that. If you find C19 & 20 have failed, put 600V ones in. Most times one works on twice PIV for power supplies. If C19 or C20 went pop there will be a hole in the side or the rubber will be blown out on the + end. if the choke is at 670 ohms hot with 74V thats 110 mA from a tube rated 125mA max.

For interest check R22 (40 Ohm)

Marc
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PostSubject: Re: Zenith 10S669 low B+ voltages   Mon Oct 18, 2010 6:09 am

Marcc has posted while I wrote this, his advice is always good. I think we're going after the same points.

Obviously, cherry red is a very bad thing and will quickly destroy your remaining tube. Probably there is a near-short across the rectifier now, again I point to C19 and C20, probably C20. I think you should replace them both before proceding. Don't power up again until this problem is resolved. Take out both filter caps, measure resistance across them, you may find one of them is shorted now. If not, measure resistance across the points where they were connected. It should be very, very high ohms.

Because the B+ can soar so high, briefly, 600v filter capacitors would be a better choice than 450v units:
http://www.alliedelec.com/Search/SearchResults.aspx?N=0&Ntk=Secondary&Ntt=5071127&campaign=Google%20Base&gclid=CJXysN6n3KQCFeJN5QodBkDFKQ

From now on, if you run the radio on its side, I have read a 5Y4 can be operated horizontally as long as pins 2 and 3 are in the vertical plane. That puts the chassis at an odd angle, so it may be more convenient to use clip leads and attach your probes to interesting points before operating the radio with chassis straight up.
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PostSubject: Re: Zenith 10S669 low B+ voltages   Mon Oct 18, 2010 6:37 am

Hi,
I will re-do the filter section with higher voltage items. I had mounted the 450V caps on an ungrounded terminal strip. I agree with you gentlemen that this area is the most likely culprit. The radio is oriented upside down on a stand I made, are sagging filaments still a concern? I'm entering the 'long day' cycle of my job... it may be a few days before I can return to the radio and this forum. I really do appreciate your help on this.

Marc
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PostSubject: Re: Zenith 10S669 low B+ voltages   Mon Oct 18, 2010 6:49 am

Upside down is as good a right side up. Good solution.

By the way, 5Y3 and 5Y4 are not interchangeable due to different pinouts.

I am also heading into a couple of weeks of very long days and not enough sleep. Hopefully marcc can be more alert than I expect to be. Good luck! I am interested in your solution whenever it may happen.
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PostSubject: Re: Zenith 10S669 low B+ voltages   Mon Oct 18, 2010 7:37 am

Marc, early on you mentioned that the rectifier socket had been replaced. As it had been trouble before, inspect that socket for signs of arcing between pins/terminals. Wafer-type sockets are notorious for this. Hopefully it's a good molded socket, but even they can arc between terminals.

Reece
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PostSubject: Re: Zenith 10S669 low B+ voltages   Mon Oct 18, 2010 8:06 pm

That 5Y3 point was where I was coming from with the comment on originallity. Change the tube change the wiring. I have never had a 5Y3 fail upside down. This is also why I sugested the Sovtek. The old 5Y3 (80) design is more akin to a towel flapping around in an air conditiner duct. Sovtek has two things going for it: viz. robust & low surge.

The other thing to try with the caps if there is no cap tester (some valve testers have them)
Put an appropriate globe in series with the cap say 24V & feed it the correct DC & polarity. for the globe & source. If the globe/s (in series) continue to glow: Toss the caps. This will only work on a dead short

I too would also like some feed back on the Rectifier socket wiring (photo even).


Marc
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marcus0331



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PostSubject: Re: Zenith 10S669 low B+ voltages   Thu Oct 21, 2010 9:23 am

Hello,
Some good news! The problem turned out to be a failure of the input filter capacitor. Now the voltages are normal and there is no hum. Both of the e-caps were new and I assumed that the problem was elsewhere in the set....grasshopper is learning, assume nothing! I'm going to let it run on the bench for a few hours before I align. I still need to refinish the cabinet and repair a small hole in the speaker, grille cloth is good. It will be a 10S669 in a 10S668 cabinet. When this is done, my next project will be an RCA 13-2 I picked up on Ebay for $90, a more complex radio than the Zenith. Hopefully, I won't have to bug you gents for advice. Thank you all.

Marc
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