Zenith 5R312 voltages - Printable Version
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Zenith 5R312 voltages - Buzz - 11-01-2013
Hello:
Anyone out there familiar with the Zenith 5R312.
Documentation is pretty bad on this.
http://www.nostalgiaair.org/PagesByModel/984/M0024984.pdf
Radio works,nothing getting overly warm.
I replaced all the caps and most resistors.
When I checked voltages, they looked higher than schematic.
Voltage from transformer to rectifer plates measure 568V. Pins 3 and 5 together. Dont know if that is how you should test this.
Rectifer tube pin 3 lists @ 160V AC, im getting 283V AC.
Rectifer tube lists pin 8 @ 180V DC, Im getting 240V DC
On Output tube pin 3 lists @ 170V DC , im getting 225V
On output tube pin 4 lists @ 180V DC, im getting 239V
Tried a whole new set of tubes and no significant diff.
Measuring with a digital VOM....Schematic said to use a 1000 ohm per volt meter
Anybody out there worked on this radio?
Thanks,
Buzz
RE: Zenith 5R312 voltages - morzh - 11-01-2013
Well, 1. Your inout voltage is probably today's 120V. Youy should measure at 115V. 2. Yes, you should measure with 1000 Ohm per volt meter, this will lower the values somewhat.
Make sure your replaced C12 is 8uF as required, if much more the voltage will rise.
Also of course - all tubes installed and the radio is hot; do not measure while it is still warming up. And as it says, antenna OFF, volume on full.
RE: Zenith 5R312 voltages - Buzz - 11-01-2013
ok, i give up..what is a 1000 Ohm per volt meter
i replaced with 10Uf caps
buzz
RE: Zenith 5R312 voltages - BrendaAnnD - 11-01-2013
This looks like a better copy of the schematic. Hope it helps.
http://lectroncity.com/Zenith5R312.bmp
RE: Zenith 5R312 voltages - Arran - 11-01-2013
You do have your voltmeter on the right setting? AC volts not DC volts? Between pins 3 and 5 on the 6X5 should be 320 volts AC not 568 volts. Whether it's 115 volts line or 120 line should not make that much difference, normally they used 117 volts as a working number. If they used a 1000 ohm per volt meter that means you would need a 320K resistor in series with one lead on the digital to approximate it, 180K on the cathode.
In any event if there is voltage present where there should be that's what matters. You would be better off just replacing the old caps and checking resistances and continuity rather then voltages. Also make sure that you hook up the filter caps correctly, one has it's negative connected directly to the high voltage center tape, the other to a junction between two voltage divider/bias resistors.
Regards
Arran
RE: Zenith 5R312 voltages - Mondial - 11-02-2013
Back in the 30's, meters with a mechanical pointer and analog scale were used to measure voltages instead of more modern VTVM's and DVM's.
These analog meters used a milliammeter movement with 1 mA full scale reading, in series with a resistor to measure voltage. Because it took 1000 ohms of total series resistance to make the meter read 1 volt full scale, they were called 1000 ohms per volt meters.
On higher voltage ranges, the resistance increased proportionally. For example, on the 10 volt range the series resistance was 10,000 ohms and on the 100 volt range it would be 100,000. Note that the ratio always remained 1000 ohms per volt as a constant.
The net result of all this is that an analog 1000 ohms per volt meter will place a greater load on a circuit than will a DVM which has a constant 10 megohm input resistance. So on the 100 volt scale, the analog meter will load the circuit with 100K to ground while the DVM will load it with a negligible 10 megohms. Therefore in high resistance circuits like the plate of a first audio stage, the analog meter will read a lower voltage than will a DVM. Measuring directly across the power supply caps, both meters will read the same because the difference in loading has no effect.
RE: Zenith 5R312 voltages - Buzz - 11-03-2013
Howdy:
I switched to a VTVM. Voltages did drop a bit.
I also found 2 resistors that connect to the eletrolytics to be off. Now im reading about 20/25 volts over what the schematic says on the b+. AC rectifer voltage still about 50v over whats listed. Filament voltage is 6.7 instead of 6.2. Radio plays nice, ran over 6 hours, nothing got too warm . I did notice the speaker is a replacement. The schematic says the field coil is 2125 ohms. I'm reading 1080 ohms on the replacement. Maybe that effects the voltage as well as higher line voltage.
Anyway, my AC power here is running 123V. I want to bring that down to 115V.
I saw on here that some use a voltage dropping resistor and others use a CL-80 Thermister.
I might get both and see what works best. I love to tinker on this thing.
To drop the 123 to 115 will require a 22 ohm resistor, according to a dropping resistor calaculator on line. I'm terrible at math.
Will get a Vishay/Dale wirewound chassis mount.
Power is 45 Watts according to schematic, so @ 123V it suppose to draw .36A according to ohms law. What watttage resistor would work best. Calaculator program said 2.66W minimum...
Thanks
Buzz
RE: Zenith 5R312 voltages - morzh - 11-03-2013
Well,
(123/115)*6.3 = 6.73V, this is the answer for why the filament is 6.7V.
Your field coil is indeed 1/2 the resistance specified and this will affect the voltages some.
RE: Zenith 5R312 voltages - Arran - 11-04-2013
If it were mine I would go the bucking transformer route, connect a 6.3 volt or higher voltage transformer in series with the primary. The working line voltage was normally 117 volts on a 60 cycle set, unless it was a 25 cycle set then it was 115 volts at 25 cycles.
Regards
Arran
RE: Zenith 5R312 voltages - Buzz - 11-04-2013
Arran:
Can you expand more on this 25 cycle thing..My schematic says line voltage 115V. What was the standard in the late 30's? 60 or 25? and whats the difference and why? Parts list says they sold power transformers in 50-60 cycle and 25 cycle. Would this cause the B+ to be higher?
Also expand on this bucking transformer if you can. I dont have much room in chassis.
thanks Buzz
RE: Zenith 5R312 voltages - BrendaAnnD - 11-04-2013
Generally, only sets made in Canada had 25 Hz power transformers. If yours is 25 Hz, it would be HUGE in comparison to a 60 Hz unit, as they require a lot more iron.
Design specifications varied from manufacturer to manufacturer. Zenith tended to use the least expensive transformers they could get away with. They were borderline at 115V, and pushing it at 117, which was the de-facto standard in the 30's. Of course, they've never been able to hold to any given voltage because of varying line losses, etc. (but the US is WAY better than here.. our line voltage varies from 210-230 volts depending upon how much is being used at any one time.)
RE: Zenith 5R312 voltages - morzh - 11-04-2013
Frequency does not affect transformer action - you still get the output equal input times the turn ratio.
However your ripple voltage after rectifying is inversely proportional to the frequency meaning that if you have Vripple=I/(2*f*C), you ripple will double when you go 25Hz vs. 50 and more than double if vs 60Hz.
Which simply means you have to use a bigger cap or your ripple will be too...ripply. Notwithstanding the fact that it will lower your rectified voltage.
As for the bucking transformer, it is simply a transformer connected sort of like the autotransformer where the secondary winding (good for the full load current) is connected in the opposite direction with the primary, thus dropping the voltage without power dissipation. This is the most accurate and clean waty of reducing the line voltage. For practical purpose you simply buy a, say, 120/12V transformer, connect the windings and end up with approximately 127-12=115V AC.
You might ask what's wrong with the regular step-down transformer 127/110V. The thing is the step-down transformer makes all winding carry the full load current (primary is almost the same as secondary) making it a big heavy expensive thing.
The bucking transformer makes only the small voltage secondary carry the full load current making it a much less power-hungry (10% of the step-down) and thus a whole lot smaller, lighter and less expensive thing.
RE: Zenith 5R312 voltages - Buzz - 11-04-2013
thanks guys for your replies..
think i'll get a 120/12v transformer and play with it.
like i said..i love to tinker..
Buzz
RE: Zenith 5R312 voltages - Arran - 11-04-2013
There were 25 cycle power grids in both Canada and the U.S, though they were more common in Canada. From what I was reading there were also some 40 cycle power grids as well. There is a summary on Wikipedia about what power line frequencies were used and where. I've never heard a or read a comprehensive explanation as to why 25 cycle was used, one theory is that it was for industrial uses like rolling mills for steel. My theory is that a lot of early power companies in Canada used to buy equipment from Britain, where they used 230 volts at 50 cycles per second, so I think they may have modified the equipment to run at half voltage and half speed just to use it here.
In any event if you look at the specs of a 25 cycle transformer not only is there more iron in the core, to prevent the core from being saturated, there is also a higher DC resistance to the primary, so they are really not bothered by a higher line voltage. Most radio manufacturers in Canada offered sets with 60 cycle transformers or 25/60 cycle transformers, Rogers-Majestic/Standard Radio used 25 cps transformers in all their sets regardless of where they were sold.
Regards
Arran
Regards
Arran
RE: Zenith 5R312 voltages - morzh - 11-04-2013
http://sound.westhost.com/articles/buck-xfmr.htm
Figure 3 shows the bucking arrangement. The dot denotes the beginning of the winding: as you can see they are in opposite to each other. Disregard the Earth wire, it is not necessary unless you have the Earth wire. Also disregard the voltages shown - just know that on the left is your primary and on the right - secondary. I say 10-12V ransformer should work.
Choose one with the output current rating exceeding your radio's Mains current draw.
For instance if your raido is 50W total and so your primary draws roughly 0.5A choose the transformer with the secondary rated for at least 1A (the primary is not important - it automatically follows the power rating over the input voltage).