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Zenith 7H920 AM/FM set 7 tube - Printable Version

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Zenith 7H920 AM/FM set 7 tube - jcassity - 11-30-2018

happened upon a free bee and it was pretty dirty till i hit first with air inside and out.

**** I need a typical page from a manual that shows the bottom view with designations on what component is what.. if anyone can help with that.


i took it apart to inspect and found out it looked pretty clean underneath... and the caps dont look like your usual blistered bunch of misfits.

I noticed some round disc type mica caps and wondered if those were original or not.  i will use the parts list in the manual to verify this.

i totally dislike the tone control location.. its just plain stupid- it prevents easy access to the speaker.

All the tubes were in place so that seems to be a plus if the fire up!

It appears someone has installed a filter cap over top the original filter cap terminals... this makes no sense!!!
if you have a can that has a 4 section filter cap and it goes bad... why would you replace it with a 2 section dongle aftermarket,,**AND** parallel it (or should i say "series" it with a bad filter cap?


based on the bottom looking so good and even the resistors looked really great, i decided to fire it up and it works and AM comes in easily.
The FM has me stumped,, i added a rabbit ear antenna to the termials and still nothing,,  its all flooded out with other noises that souund like a symptom of a bad filter cap... but not exactly if that makes sense.

when i am on AM or FM,  there seems to be an audible rumble noise in the speaker.  
when i am on AM and on a station,, i can feel the cone and feel the broadcast but riding atop the signal is another frequency that is from what i can only describe as a muffle noise/ rumble noise.  this noise reminds me of when you have a speaker cone thats coming off or something.. but its very close to the noise of a bad filter cap.  in my case the rumble noise seems to follow the volume knob as i rotate it,, but not drastically.
oddly after an hour or so this issue seems to go slightly away.

so here are some pics.

apparently this radio was sold in the same town i live in,, and has a service sticker of the former repair shop where i live as well.

a few pics..........

RE: Zenith 7H020 AM/FM set 7 tube - jcassity - 11-30-2018

more pics

RE: Zenith 7H020 AM/FM set 7 tube - jcassity - 11-30-2018

and another

RE: Zenith 7H920 AM/FM set 7 tube - David - 11-30-2018

http://www.nostalgiaair.org/pagesbymodel/312/M0025312.pdf    Looks like your set

RE: Zenith 7H920 AM/FM set 7 tube - David - 11-30-2018

   
   
Zenith made a lot of set during this time that were alike except for cabinet and layout of chassis.

RE: Zenith 7H920 AM/FM set 7 tube - Paul Philco322 - 11-30-2018

Like the way the cram the selenium Rectifier up against the speaker too, but they are good players. Good luck!

Paul

RE: Zenith 7H920 AM/FM set 7 tube - jcassity - 12-01-2018

thanks David,,,
your link you posted, thats the one i downloaded yesterday.

my chassis is 7F01 , model 7h920

those prints are very legible.

RE: Zenith 7H920 AM/FM set 7 tube - jcassity - 12-01-2018

i am down to three caps to replace yet but.......

just replacing the single bumble bee cap helped bring FM back in.

now to the earlier issue in post 1.

i powered up just now and the sound when on a station sounds sort of "gargled" if that makes sense.

RE: Zenith 7H920 AM/FM set 7 tube - David - 12-01-2018

Did you replace the selenium rect. with a silicon diode you will also have to install a voltage dropping resistor. The selenium rect drops voltage about seven volts the silicon diode less than a volt. Please replace all paper caps before turning the set on again.

RE: Zenith 7H920 AM/FM set 7 tube - jcassity - 12-02-2018

silicone is six tenths of a volt drop. is there a typical standard resistor ohmic value?

i will get the remaining caps replaced.

one of the remaining wax caps has three legs.. the hot, gnd then another outter layer for shield , its tied to my volume control.
i will have to fabricate a modern cap to include a shield. i have a decent plan for this.

my main thing of asking about the "gargle" sound was if anyone happened to have enough experience to know what this is typically caused by.

RE: Zenith 7H920 AM/FM set 7 tube - jcassity - 12-02-2018

also,,
who is it if anyone know that i can go to for a legible schematic?

RE: Zenith 7H920 AM/FM set 7 tube - David - 12-02-2018

   
I do not know of another source for a better copy of the sch than the one from Nag. Air. The problem with Beitman is they did not do every model radio but there copies are clear at least my set. I have the complete set of Beitman and try and post info that I hope is useful. The resistor on the plus side of the sel rect is around 22 ohm on most of the sets from Z  that are AM/FM table models of this time period. The ones I have worked on will replace with a 50 ohm  at least 5 watts better to go with 10 watts. Thinking about what you said about the sound, try taking a second speaker let the set warm up and using jumpers connect to the output for a few minutes if the sound coming out of the 2nd speaker is clear you may have dirt and dust around the voice coil. Try replacing the cap without the shield it will probably work. Going to add another from Beitman has the 4 sec filter cap hope you find it helpful. David

RE: Zenith 7H920 AM/FM set 7 tube - KCMike - 12-02-2018

Try this one, see if it's what you need. Good luck with your radio.
https://www.tubesandmore.com/schematics/zenith-radio-corp/7h920

RE: Zenith 7H920 AM/FM set 7 tube - jcassity - 12-02-2018

ok getting to the last caps now.

i will check the in/out voltages of the sel rect.

i asked terry what the typical expected resistance of one is ,,, but from what you said, i am glad i asked because i was going to upsize the resistor to offset the new diode

RE: Zenith 7H920 AM/FM set 7 tube - jcassity - 12-02-2018

here is a video of the "gargle" i hear.

https://www.dropbox.com/s/renn0klzjyjr7b6/VID_20181202_130752.mp4?dl=0