Zenith 7h822-z tubes not lit

[Tennangler]

Someone gave me this radio to try to get working. The bulb on my DBT did not light when plugged in. The Pilot light is on but none of the tubes light.
Can someone tell me the first thing I should check?
Thanks
James

[init4fun]

Originally Posted by Tennangler

Someone gave me this radio to try to get working. The bulb on my DBT did not light when plugged in. The Pilot light is on but none of the tubes light.
Can someone tell me the first thing I should check?
Thanks
James

This is a 7 tube series string set, with a neon light for a pilot light. Your first check will be a continuity check of all seven tube's heater pins, and it's VERY highly likely that you've got at least one tube with an open heater.

It's also highly likely that at least the electrolytic B+ filter cap(s) are gonna need replacing, along with any wax paper caps


Here is a link to the schematic, click on the little eyeball icon next to the icon of an old time floppy disk to view the pages. I see no pilot light in this schematic, but then this lists as 7H822 only, and not 7H822-z, perhaps the "z" designation was the model that includes the pilot light. I do have several variants of this type of Zenith and the pilot light is always a neon type. With no tube rectifier, that traditionally had a heater tap for driving a standard #47 incandescent (Such as the 35W4 rectifier tube did) it was easier for Zenith to use the neon pilot light (it and a resistor provided direct 120 V operation) VS attempting to derive the 6 V needed for a type 47 bulb in some other way.


http://www.nostalgiaair.org/resources/309/M0025309.htm

[old_coot88]

The audio out tube is the most likely to have an open heater, as it runs the hottest.

[Tennangler]

Init4fun,
Thanks for the fast reply. Are the heaters the pins that go straight to ground on the schematic? Do I measure to chassis ground?
By pilot light I mean the bulb that lights when I turn it on.

[init4fun]

Originally Posted by Tennangler

Init4fun,
Thanks for the fast reply. Are the heaters the pins that go straight to ground on the schematic? Do I measure to chassis ground?
By pilot light I mean the bulb that lights when I turn it on.

Here is a link to a website that will provide you with the pinouts of every tube in your radio. Here I have specifically linked the 35B5, but you can use this site for all the others as well. And no, the heaters do not go straight to ground, they are in a series string, as Old Coot said they are like modern Christmas lights, one burns out and they all go out (old time Christmas lights were in parallel)

35B5;

https://frank.pocnet.net/sheets/049/3/35B5.pdf


12BA6;

https://frank.pocnet.net/sheets/127/1/12BA6.pdf

19T8;

https://frank.pocnet.net/sheets/093/1/19T8.pdf

and so on......

[Tennangler]

Just to make sure I'm checking pins 3 and 4, is 1 the gap and pins numbered clockwise looking at the bottom of the tube?

[old_coot88]

No, the gap is '0', and the numbers run clockwise. Same with 9 pin tubes, octals and locals.

On 9 pinners, the heater is most always pins 4 and 5.

[Tennangler]

I check all pins on all tubes and all had a pin on each with continuity to ground except one. The 35B5 power amp did not have continuity on any of the pins. I tightened the pin sockets and sprayed with deoxit but still no continuity.
Can one bad tube cause none of them to light?

[old_coot88]

Just to double check, pull the 35B5 and check the heater continuity (pins 3 and 4). And yes, since the heaters are all in series, an open anywhere in the string will cause them all to go out (just like the old time Xmas tree lights).

[Electronic M]

There were series and parallel Christmas lights in the old days and both still exist as LED lights today. The big ones tend to be parallel and the small ones tend to be series.

It's a good idea to have some understanding of series and parallel circuit principals before working on vintage electronics otherwise you're stuck at parts changer status and mostly helpless (only recourse is spending lots of money on a parts cannon that may or may not get the actual problem) to track down the root cause when it doesn't work post recap.

[old_coot88]

OOPs Double post

[old_coot88]

A vital factoid to hardwire unto one's knowledge base is this -

Series = constant current

Parallel = constant voltage

To elaborate, in a series string, constant-value current runs thru every element of the string. Voltage across individual elements may vary depending on each element's resistance.

In a parallel setup, constant voltage is across every element, though current thru individual elements may vary according to each element's resistance.

[Tennangler]

I looked up the pinout after I wrote. Should have done that first. I checked continuity across pins 3 and 4 and it was open. A friend with a tube tester tested it as bad. He told me a 35c3 he has will be a replacement. I was reluctant to use it but tried it anyway. The radio came on but has a hum. The hum does not vary with the volume control.
First, is it ok to use the 35c5? Second, if it is does the hum indicate a grounding issue?

[old_coot88]

Originally Posted by Tennangler

First, is it ok to use the 35c5? Second, if it is does the hum indicate a grounding issue?

35c5 is a different pinout and wouldn't work without rewiring the socket.

https://www.radiomuseum.org/tubes/tube_35c5.html

35f4 would be a drop-in but most likely unobtainable

https://www.radiomuseum.org/tubes/tube_35f4.html

Hum is bad filter capacitor(s).

[Tennangler]

I will try to find the correct tube.
Which one is the filter capacitor?