Zenith 70/71 - Printable Version
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Zenith 70/71 - dconant - 11-18-2025
I have been asked to restore this radio which is in great shape. Has anyone worked on one of these and if so anything I should be aware of. I don't know if it has been recapped or not yet but the owner has been playing with it. He says he can't turn the volume down.
That schematic scares me. Thinking I might have bit off more than I want to chew. Guy is already asking how I am doing with it. He dropped it off 3 days ago.
Dan
RE: Zenith 70/71 - morzh - 11-18-2025
Typical Zenith drafting style; otherwise no more complex than any radio for the period.
RE: Zenith 70/71 - Paul Philco322 - 11-18-2025
Real pretty set. Good luck. Paul
RE: Zenith 70/71 - MrFixr55 - 11-18-2025
Hi Dan,
Neat radio. Shame that only 1 of the 45s is a globe tube, the other is the "ST" style. This is a relatively high end TRF (Tuned Radio Frequency) set.
The volume control on this set is quite interesting. It is at the very beginning of the circuit and consists of 2 functions.
The first function is to "shunt" the antenna to ground at minimum setting. The resistance element is connected to the antenna and the wiper is connected to ground. This is a common method of regulating volume in sets before AVC (Automatic Volume Control). The second function is to bias the RF amps. The lower the volume, the more the bias. However, Zenith did not vary the cathode bias to regulate volume. It seems that they vary the resistance to one leg of a capacitor whose other leg connects to an RF coil in the plate circuit. I assume that the closer that one lead of the capacitor is to ground, the more filtering of RF from the plate circuit of the first RF (and conversely the lower the signal to the grid circuit of the succeeding RF amp and detector).
Servicing the set should be rather easy. Verify that the coils are not shorted or open. Verify both sections of the volume control. Replace the caps, and you should be good to go.
Because this is a Non-AVC set, there is some strangeness to volume. First off, if the volume control is set rather high for a distant station, and you tune to a strong local station, the RF amp may become saturated, and you may stop hearing stations. Just turn the volume to minimum for a few seconds and slowly raise volume. Some stations may be so strong that they will be picked up by the wiring between the tuning cap and the grids of the RF amp tubes. Make sure that all shields are in place.
Hiope this helps.
RE: Zenith 70/71 - dconant - 11-19-2025
Thank you MrFixr55. Since the owner has played it I think the coils must be good. I'm going to replace the ecaps in the power section and then bring it up to see (hear) for myself what is what.
Dan
RE: Zenith 70/71 - dconant - 11-20-2025
Ok I have replaced the ecaps and powered it on. The volume control does not really do too much. Strong stations are too loud and can't turn down so I will take it apart and see what going on. I'm not sure what I am looking at on the schematic for the volume control. Are there two windings in there?
The speaker is shot. The cone is very stiff and it does not move back and forth. Someone has put tape all around the perimeter of the cone and painted it with something maybe hard glue. Probably would sound ok at low volume.
RE: Zenith 70/71 - morzh - 11-20-2025
Yes I think it is a dual potentiometer.
RE: Zenith 70/71 - MrFixr55 - 11-21-2025
Yes, it is. You may have to unsolder the connections and "ohm" that section of the volume control that is across the antenna and ground connections. You will likely find it open. Check the antenna coil primary also.
As stated in my other post, the common method of controlling volume on early sets was to have a 2 section volume control where one section shunts the antenna to ground at 0 volume and provides a negative grid bias or positive cathode bias to the RF (and IF) amp(s). The volume control in the Zenith shunts the antenna coil but does not provide bias to the RF or Amps. Apparently, it controls the resistance between ground and one leg of a cap whose other leg goes to the RF Coil.
Hope this helps.
RE: Zenith 70/71 - dconant - 11-22-2025
Checked the volume control today and got very erratic readings. Took it apart and found the carbon track just about gone on both paths. Can't get much of any kind of readings off it with a VTVM. Anyone have any idea if this can be repaired or replaced? The part number 68-226.
Dan
RE: Zenith 70/71 - KCMike - 11-22-2025
You might try Mark Oppat.
https://www.oldradioparts.net/controls.html
He was able to help me out last winter while working on an Victor/RCA model R-14
It had an odd ball dual volume control also.
RE: Zenith 70/71 - dconant - 11-23-2025
Trying to find out what the output transformer and field coil values might be for this radio I discovered one side of the primary of the output transformer is open. Would the radio still play this way? I didn't think so but then I don't know.
Anyone have any idea what the values for the field coil and output transformer might be? I'm measuring about 2.3K on the field coil and the good side of the output transformer reads about 1.3K.
Thanks,
Dan
RE: Zenith 70/71 - Arran - 11-23-2025
Dan;
I seem to recall that the radio would play with half of the output transformer primary being open, though not as well, since one of the #45 tubes is still getting it's plate voltage. 1.3K Ohms sounds high for half of an output transformer primary, if that was across both outer legs I could see it. Is it mounted on the chassis or on the speaker? Are you measuring directly from the leads on the output transformer, or through the speaker cable?
Regards
Arran
RE: Zenith 70/71 - MrFixr55 - 11-23-2025
Hi Dan,
Yes, the radio would play with only 1 of the two Push Pull output tubes working or only one of the 2 "legs" of the output transformer good.
Attached is a page of the RCA Receiving Tube Manual version RC13 from 1937, describing the "45" power triode:
Based on the info included in the above document, my rough guestimate is that a transformer with a primary impedance of 4KOhm - 5KOhm (plate to plate) will work. Impedance is not the same as resistance. Primary impedance depends on proper matching of the secondary's impedance to the speaker voice coil.
Unfortunately, the diagram does not have any specifications for output transformer, voice coil or field coil resistance or impedance. It is also quite difficult to determine how it is connected. By the early 1930s, the field coil was between the first and second filter caps and usually had a resistance of about 1KOhm.
Hope this helps.
RE: Zenith 70/71 - MrFixr55 - 11-24-2025
Hi Dan,
For now, regarding the volume control, you can use a 10K pot across the antenna terminals, either externally, or in place of the existing volume control. I'm a little surprised that it is a carbon type potentiometer, most of the volume controls of this type and vintage were wire wound.
Regarding the speaker, there were several iterations of the original dynamic loudspeakers, developed by GE engineers Rice and Kellogg. The first iteration had very little movement. The voice coil was centered in the field electromagnet by a dish-like metal device, cemented to the voice coil and screwed to the electromagnet pole. This certainly did not allow for much movement. The second iteration was known as the "spider", a device that looked like the insert used to play 45RPM records on a turntable. This device allowed much more movement of the cone but held it centered so it would not rub against the magnet pole, causing distortion. However, this device was rather flimsy. The third iteration was an external version of the spider, kind of a "leaf spring-like iteration. Finally, a corrugated external ring was used to center the voice coil and continues to this day.
I have repaired 2 Philco 20s and have two of my own. The ones I repaired had the first iteration with the hard centering device. One of mine had the 45RPM type spider; the other has a modern replacement speaker. My two sound quite similar. The two that I repaired having the "hard" device are loud but quite tinny sounding.
There are several companies / individuals who re-cone speakers. Ig you want to keep the radio original but improv ed, have the speaker repaired or re-coned using the more modern spiders.
Hope this helps.
RE: Zenith 70/71 - dconant - 11-24-2025
Arran, I too thought 1.3K sounded high for one side but I am not 100% sure I am reading the full primary or one half. If I look at the transformer I can see I am reading one half based on the center wire going into the transformer is really the center tap. I am reading right on the transformer terminals. See picture of output transformer. I get about the same reading if I go through the speaker plug. Readings are while speaker is disconnected from the radio.
MrFixr55, I have included pictures of the pot. Although they are not the best pictures you can see the carbon tracks are pretty beat up. Please go into more detail about how I can use a 10K pot across the antenna. Like how would I wire that up and what would I do with the other winding?
As for the speaker you can see from the pictures it is using the 45 record type spider. What are those staples doing in there if that is what they are? If I get the speaker reconed I still need to deal with the output transformer. If I get the recone job it would be silly to leave the transformer as is.