Zenith help please!

[radio nut]

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?

[dieseljeep]

Originally Posted by radio nut

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.

[radio nut]

290 is

[radio nut]

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

[Electronic M]

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.

[radio nut]

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.

[dieseljeep]

Originally Posted by radio nut

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.

[radio nut]

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

[radio nut]

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

[radio nut]

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......

[dieseljeep]

Originally Posted by radio nut

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.

[radio nut]

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.

[zeno]

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
LFOD !

[dieseljeep]

Originally Posted by zeno

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
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!

[old_coot88]

Originally Posted by dieseljeep

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.

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.