Zenith help please!
 

I am working on a zenith 7s558 chassis 7a02. Recapped the set and it ran. Sprayed electronics cleaner and ran it more, then it quit. Found what looked like a mica cap that was bulging and removed it. Here it was a wire wound resistor.
Replaced it with a 3 watt resistor and I get nothing but a little hum with the volume all the way up. My digital voltmeter is showing 60 volts positive b+ on the positive side of the electrolytic caps and 302 negative on the ground side. The electrolytic caps ground in the circuit and not to the chassis.

Well, what do I check now?

The schematic shows a 250 ohm WW resistor 1.5 watts, 3 watts OK! Is it a 250 or so ohms.

290 is

270 is as close as I could find, did some checking and the rectifier tube had a dead side. Will replace it after work and try again

Another thing to watch out for is shorts from bad rubber wiring. One of my sets had only a few rubber wires, and one was for B+ to the eye tube...Insulation flaked off that wire at the point it passed thru the chassis and it was shorting B+ to chassis.

I put another rectifier tube in and the b plus is zero, beginning to think the transformer is bad. The negative side of the electrolytic caps is now 489 volts.

The N/A schematic is lacking page 12-16. That's where all the voltage data is!
Remove rectifier tube and measure the HV winding resistance, about 400 ohms. Of course, with the power disconnected.
That Zenith has a strange approach to the grid bias circuit. You have to go through the entire power supply to make sure there isn't an error in the wiring, easy enough to do!
Also, make sure the speaker field coil checks good, 1250 ohms. :scratch2:

Speaker field checked around 900 ohms, will recheck. Not showing b+to ground short , ohm meter reads infinite. Also now tubes aren't glowing....

Will pull tube after work and measure resistance across transformer. Did that already with tube in no power to unit

Speaker field 1045 ohms. Transformer reads open......
So, I guess I have a dead radio.

The customer did mention that the set made a pop type noise the last time it ran. Wonder if this was the windings breaking down......

What part of the transformer reads open, the primary?
I looked at the Zenith in my collection, but couldn't make out the chassis number. Is it the model that uses the potted power transformer?
It looks like the transformers used in the old car radios at the time. It could be easily replaced by any other transformer of similar ratings. :scratch2:

The secondary that leads to the speaker field.
Unfortunately I dont know the proper voltages for the high voltage output to know what transformer to use or who to call to get it!
The transformer looks like any standard old radio tranny though.
I did call Moyers and they are going through their book to see if they have a replacement.

If the tubes are not lighting look to the primary being open.
AS A RULE you have a 2 green secondary for filaments 6.3 VAC
2 Yellow for the rectifier filaments only. 5 VAC
2 red & a red / yellow center tap to ground.
2 black primary.

2 reds go to rectifies plates.
Rectifier cathodes go to 1st filter cap. Then the B+ goes through the choke
thats in the speaker. Then another cap. This is the B+ for the outputs.
Runs 300- 400VDC
Other B+ sources are dropped by resistors & filtered.

73 Zeno:smoke:
LFOD !

If you follow the schematic on N/A, you'll notice a rather strange approach to the fixed bias circuit. The speaker field circuit, plus the voltage divider circuit is in the negative source.
There is a strong possibility that one of the electrolytics was installed reverse polarity.
I have a radio like it in my collection that I bought from a fellow collector and it didn't seem to work very well. I checked to connections to the 'lytics in the p
ower supply, only to find that one went to chassis ground instead to the B- line. It happens!
I haven't played that radio in twenty years! :sigh:

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. :sigh:

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......

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

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).

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 .

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. :scratch2:

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.

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. :sigh:

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.

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

Will check if tranny has a number on it

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. :sigh:

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...

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?