[davemc]

The last two radios I restored (Zenith 216 and Sparton 67) exhibit the same problem. The B+ is WAY to high. In both cases, the original filter caps were dual 8mfd units. I replaced them with 10mfd/450 volt caps (I actually measured them, and they were very close to 10mfd).

In both cases, the B+ reached excessive values with only 100 volts AC input! In the case of the Sparton, the schematic says the B+ should be 275 volts (+/- 15%) with 119 volts AC in. I measure 312 volts DC (B+ to the TUBES, not raw B+) with only 103 volts AC input (from a Variac). There's no way I would every apply normal line volts (123 volts) or even AC from by bucking transformer (110 volts).

In the case of the Zenith, I changed the input filter cap to 4mfd. This helped some, but there was some low level hum and I still had to power it from the bucking transformer, else the B+ would be 360 volts!

I don't think the impedance of the meter (10meg vs. 1000 ohms/volt) would matter in this case.

Has anyone else encountered this problem?

Thanks!

[rickd]

Are you measuring the voltages with the tubes installed? Without the tube loads, the voltages will be higher.

[davemc]

Tubes in place, radio playing, both radios. Maybe I should not worry so much, but the maximum screen voltage on a type 42 (6F6) output tube is 250 volts. With 20 volts on the cathode, it could go as high as 270 volts. But 350 volts or even more?

[codefox1]

I assume you didn't replace the rectifier tube with a solid state device. If you did, that's the answer. How are the filament voltages, are they normal? Any extra hardware on board, (remember bucking transformers can go up or down.) Is there more than one primary winding on the power transformer? Well that's a start.One more wild guess, are these radios export models originally sent to Japan or Canada?

[AI2V]

Don't rule out a voltmeter problem. With two radios reading high, it might be worth checking with another voltmeter.

[exray]

Another tip.
Shorted turns in the power transformer primary can cause this. But thats usually associated with excessive heat or current draw. (depends much on 'how' the short occurs.)

I'd recommend checking the total AC current draw of the set compared with the nameplate. Some transformers are beefy enough to where the heat is not so excessive to raise a red flag.

[davemc]

The original rectifier tube (80) is in place and tests good. My meters Fluke 75 and HP973 are very accurate and agree. The Variac dial also says 100 volts. The power transformer is original - no extra hardware has been added. Measured wattage (a REAL wattmeter) is normal (60 watts operating - but I do not have the specs) and no heat in the power transformer. Also, AC voltages on each side of the high voltage are within 1 volt. The power transformer has only one primary and no taps. The filament voltage measures 6.2 volts AC with 110 volts AC input - it measures low with 100 volts in (but the radio works OK).

One other thing: the Zenith (Zenette) 216-2 is a 25hz model! It works well on 60hz and the good folks on ARF said operation at 60hz would not be a problem. But I wonder if the result is that the B+ is too high operating on 60hz. On that set, I changed the input filter to 4mfd, but the B+ is still way to high (again, normal B+ with only 100 volts in).

On the Sparton, there is no label or model information - the set was identified from the schematic, part numbers, and chassis photos in Riders. So that set could theoretically ALSO be a 25hz model! The transformer on the Zenith is very large - not so on the Sparton.

I have had no problems like this with any other sets in my collection, at least operating at 110 volts (I run all my sets through a bucking transformer, unless the set has a 125 volt AC input tap).

[AI2V]

It looks like you've covered everything! The only thing left to do is: ignore it, or put about 400 ohms/5 watts between the 80 rectifier cathode and the first filter cap. The latter would be preferred for several reasons.

[Arran]

Are there any open resistors, resistors that have drifted too high in value particularly around the audio output section? Is there a cathode resistor on the power output tube? Did you check the AC output of the variac? I assume that you did? A 25 cycle transformer shouldn't cause any problems with high B+ voltage, they never did on any of my 25 Cycle sets. How do these sets read when plugged directly into the wall outlet?
Best Regards
Arran

[davemc]

All the resistors are within 20% and most within 10%. I have gone through the radio and replaced all out of tolerance resistors. The bias on the output tube is correct. I did measure the output of the variac (103 volts AC with the dial set at 100). All tubes have also been tested and are good. The radio works very well.

One thing I did not do, and will do, is measure the high voltage INPUT to the rectifier and compare THAT with the voltage table in Riders. I'll post the results.

No way would I plug the radio directly into 123 volts. I did try it through my bucking transformer (110 volts), but the B+ is still WAY to high (325 volts vs. 275 listed in the voltage table in Riders). I suppose the voltages in Riders could simply be WRONG. But even if so, the B+ greatly exceeds the tube manual maximum screen voltage even at 110 volts in, let alone 119 volts (the basis of the table).

[davemc]

I measured the AC outputs of the power transformer with 110.8 volts input:
Input 110.8 AC (spec is 119)
B+ is 325 VDC (spec is 275)
High voltage 370 AC (spec is 440 each plate of 80)
Filament 5.85 AC (spec is 6.1)
Rectifier Filament 4.71 AC (spec is 5)

And the filters are 10mfd (spec is 8mfd)

So what is left? THE SPEAKER FIELD!!!! It measures 908 ohms (spec is 1600). So apparently it has some shorted turns - although the radio works fine (the part number on the speaker is correct, so it has not been changed). So I will do as suggested and add a series resistor chosen to produce 275 volts of B+ with 119 volts in.

Of course, that does NOT explain the problems with the Zenith 216-2. I guess I'll have to write that one off as a side effect of operating a 25-30hz radio on 60hz and using 10mfd filters instead of 6mfd. By the way, the draw is only 60 watts - spec is 85 watts (at 25 hz) - all tubes are good - output tube is NOS.

[Scottb]

If you are reading the voltages with a Fluke meter the B+ can read higher. I noticed on most schematics for old radios that voltages were taken with a meter that had a 1000 ohm per volt loading on the circuit. I forgot what the loading characteristics of modern DVMs but it is very high and does not load the circuit. I would think your voltages ar OK.
Scott B

[Texasrocker]

Have you tried swapping out the Rectifier tube? Internal intermittent rectifier tube plate shorts possible? Not usually the prob tho, without the tubes giving a spectacular light-show.The 25-50 cycle power- trans will work on 60 cycles, but generally always run hot in my past experiences. Whenever I run across these sets, I always replace the 25-50 cycle ac trans with similar type 60 cycle trans I rob from other cheap parts sets as they come avail. I would suspect the power trans in your case(s)?
I remember I bought a couple nice TV sets many yrs ago for cheap at a garage sale ( without close inspection) of the mains, that never worked right. After closer inspection, I had purchased 2 TVs that had 50 cycle ac trans mfd in Mexico,for Mexico 50 cycle pwr mains, and although they worked, the "picture" was never clear & audio was distorted.Their lifespan was short also on 60 hz mains! Both ended up with smoked power trans & blown fuses. I have run across several old Canadian model Philcos with the 25-40 cycle trans that were previously totally-fried, windings internally by someone previous plugging them in using modern day 60 cycles mains due to internal excess heat inside the trans. Limiting incoming ac line voltage keeps these 25-50 cycle trans from smoking, for awhile, but its better to rid them if possible and replace them with other vintage 60 cycle types when possible.

[davemc]

I have not swapped out the rectifier, but it was tested. I don't see how a short or whatever in the rectifier could INCREASE the B+ (which is STABLE, by the way).

I wish I could get a definitive answer on the issue of operating a 25hz transformer on 60hz. Other forums as well as some individuals with which I have consulted have stated that doing so is not a problem, but doing the OPPOSITE would be a problem (running a 60hz radio on 25hz - which no longer exists). The explanation I was given was that a 25hz transformer (or 50hz for that matter) has more iron as well as a higher primary inductance than a 60hz transformer. Thus it actually runs COOLER on 60hz. That is also my experience with this radio. Eddy current losses may be higher. In my case there is no transformer heating, and the actual wattage when operating is only 60 watts (spec is 85). Now that is with 110 volts AC input through a bucking transformer. At 110 volts input, the B+ is a little high, but likely OK. For now, I will continue to operate the set on 110 volts. I suppose I could install a series resistor if I ever decide to sell the radio, or at least warn the buyer.

The probability of finding a suitable parts set for this rare Zenette is likely zero. These guys go for big bucks, at least on eBay. But I will keep an eye out just in case a parts set shows up.

Dave

[Texasrocker]

Your B+ is running higher because of the 25 cycle pwr trans being used on 60 cycles mains. It would be best to replace the pwr trans if possible. Transformers do run hotter internally when applying 60 cycles into a 25 cycle because of the size of wires windings inside a 25hz type as compared to the 60hz types. If ac trans swap is out of the question, here are a couple things ( guaranteed) so you can bring down B+ voltage in your sets that have 25 cycle trans.

(1) find your power transformers center-tap wire that usually is soldered directly to chassis. Cut & lift it off chassis, install a 25 watt wirewound power resistor ( exact best ohms needed can be found by subbing in & out a few different values with jumper wires with alligator clips in "series" with trans CT back to chassis ground) while monitoring your B+ voltages, slow upping with your variac to modern line voltage. Ohms needed could be anywhere from 3-500 ohms in my guess. Monitoring your B+ while subbing in the resistors will yield you a good match.

or try this: ( may be easier)
(2) find the socket connection (wire) off your rectifier tube socket that feeds your radios 1st [ First] B+ filter cap according to schematic. Cut & lift the wire, add a 20 watt wirewound pwr resistor in "series" ( try subbing anywhere from 250- 350 ohms) using alligator clip jumpers while monitoring your B+ while slow upping your variac to work on modern line voltage. You will find a good match by subbing in various resistors until you get your B+ voltages near schematics ratings +/- 15%

* either of the above suggestions will drop your B+ voltage. Exact matching yield by experimental subbing the 20 watt w.w. resistors via 2 jumper wires with alligator clips on each ends.


*for those that say it is ok to run a 25hz power trans on 60hz mains needs to try this to be "convinced" sometimes via "audio"!
find a vintage German or Mexico 50 hz version of a vintage tape-deck. Plug it into 60hz mains & listen to it thru any amplifier. Distortion , lousy sound, are due to "higher B+" voltages courtesy of only 10hz difference upping the B+! Try it sometime, and the nay-sayers 25hz vs 60hz can be convinced easily "by ear"! Enjoy your vintage Radios!!