Zenith help please!

[dieseljeep]

Originally Posted by old_coot88

I'm lookin' at the Riders for Zenith 7S-558, ch.# 7A02, and it clearly shows the speaker field in the traditional position coming off the rectifier cathode.
http://www.nostalgiaair.org/Resource...=\M0025336.pdf
I'm wonderin' if we're looking at different schematics for the same radio.

I quoted that observation wrong.
I also looked at the entire Zenith schematic source in Riders volume 12 that I have on my notebook computer. The all-important page 12-16 is not shown in the N/A entry.

[radio nut]

Well primary is powering the rectifier only, secondary shows as open. Without a Sam's diagram I am not sure what transformer to buy. I ended up years ago on a site that had granny's that might work but not sure how much high voltage the b+ line should even be......

[old_coot88]

Originally Posted by radio nut

Well primary is powering the rectifier only,..

Does that mean only the 6X5's heater is lit, and the other tubes' heaters are not lit? Or what?

...secondary shows as open.

Presumably you mean the HV secondary which connects to the plates (pins 3 and 5) of the 6X5. Have you measured between pins 3 and 5? It should be a few hundred ohms. The winding is center-tapped, so it should measure half that value from either plate to the CT**.

**Half the winding will show a slightly higher resistance than the other half. That's because the winding is layered, and the outer layers have larger radius (more wire per turn).

[radio nut]

This set has a 5y4 instead of 6x5. Not sure why. The secondary shows as open infinite resistance. That is taking into account the 5y4 , which has a different pin out as far as heater is concerned .

[dieseljeep]

Originally Posted by radio nut

Well primary is powering the rectifier only, secondary shows as open. Without a Sam's diagram I am not sure what transformer to buy. I ended up years ago on a site that had granny's that might work but not sure how much high voltage the b+ line should even be......

The set's too old for Sams coverage!
The H/V winding should show 370 ohms from the center tap to each plate, pins 3 &5 of the 6X5 socket.
The replacement transformer should have around 310 vac from C/T to each plate @ 40ma. The 6.3 volt heater winding should good for 4 amps.

[old_coot88]

Originally Posted by radio nut

This set has a 5y4 instead of 6x5. Not sure why. The secondary shows as open infinite resistance.

Measuring from the center tap, do both halves show infinite resistance? If so, a severe B+ short downstream of the rectifier might have popped 'em.

[radio nut]

Only one side is open, I have checked and I am not "seeing" a short to ground on the blown side

[dieseljeep]

Originally Posted by radio nut

Only one side is open, I have checked and I am not "seeing" a short to ground on the blown side

If the set is using a 5Y4 it can't have the original transformer! Is there a part number stamped on the transformer?
I ran into a similar model Zenith model, where the transformer was burned out. I rewired the 6X5 socket to take a 5Y4. There's not enough contacts on the 6X5 socket to use a 5Y3.

[old_coot88]

If the radio has been running with the 5Y4, the xfmr has to have a dedicated 5V filament winding for it. He said something about the xfmr "powering the rectifier only". I interpreted that to mean only the rectifier was lighting up and the other tubes were not lit. Dunno if that was the right interpretation or not.

[radio nut]

Rechecked and 1 tube was not powering up but the "7" series of tubes are harder to see if filaments lit..

[radio nut]

Will check if tranny has a number on it

[dieseljeep]

Originally Posted by radio nut

Rechecked and 1 tube was not powering up but the "7" series of tubes are harder to see if filaments lit..

Keep the rectifier tube out and run the radio for 15 or so minutes and see if they get warm. The transformers probably bad anyway if the HV winding is partly open.

[Electronic M]

If it is your set and you don't mind a possibly short-lived jury-rig repair you could disconnect the HV winding completely tape up the open leg and connect the remaining two to a SS full wave diode bridge rectifier, or diode/capacitor voltage doubler circuit and hook the plus/minus outputs of that to the field coil input and neg bias resistors respectively...If the primary is already half open there ain't much to lose trying to coax enough B+ out of the good half...

[radio nut]

One thing I have never understood with this set. The voltage on the line where the electrolytic caps grounded was about 335 or so minus. Isn't that an awful high number?