Zenith H-500 Transoceanic Quandry...

[Kico]

Just finished completely re-capping my Zenith TO H-500 along with replacing a lot of out of spec resistors. I replaced all the 1Megs at the tubes bad or not just to be sure and powered it up on my variac. At about 90V it came alive and sounded great--no hum and lots of BC stations and a few SW.

It continued to play for 2 hours or so and then just suddenly quit! At first glance it appears the filament string is dead. I checked the obvious, the new diode (replaced the SR) and ohm tested the filaments on the the tubes--all good, I'm using a 1R5 for testing so as not to put at risk the 1L6.

I'm not a diagnostic expert by any means and really need some help to determine what could have happened and what steps to take next.

Thanks guys--

[timmy]

well im not an expert either but how about the obvious, is any of the tubes lit at all, any sound? i would carefully start at the ac in, if this helps at all,i would also retrace everything that you have done from the beginning, including the out of spec resistors as well.

[bob91343]

Trouble is, you can't see those tubes lit. Not enough glow. Measure the voltage across the filament pins; if one tube is burned out, all the voltage will appear across that filament and nothing on the others.

If you replaced the rectifier with silicon, that combined with the overvoltage on the line that is now the norm, will stress the filaments excessively. You probably should reduce the filament voltage by increasing the dropping resistor.

[Reece]

The tubes are in series: if one goes out they all go out, and right, you can't see them lit, they are so dim. It's been a while since I worked on one of these but I believe I put something between a 50 and 100 ohm 5 or 10 watt resistor in series with the new silicon diode to drop the voltage. The silicon rectifier drops less voltage than the old selenium does and needs the series resistor. Whatever value resistor it was, it put the voltage across the whole filament string right on the money per spec. I seem to recall it's supposed to be 8.6 volts or so? Sorry I don't have the exact value but there is some variance in thought. I'd suggest reading up across the net on power supply modifications specific to these sets.

What you can do once you figure out if you need a new tube is plug in the tube and put your voltmeter across the filament string and bring the set up on the variac. See if you've arrived at max filament voltage before you're at standard line voltage. With no series resistor, you will be, so you can try something about 75 ohms to start.

[bob91343]

It's faster to calculate the value. Assuming 50 mA filament current, figure the voltage excess. Use a resistor with high enough power rating. When in doubt, err on the side of higher resistance to get the most tube life.

[Findm-Keepm]

I assume that's what a filament resistor is for?




Cheers,