Zenith 7H920 Restoration - Printable Version

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+--- Thread: Zenith 7H920 Restoration (/showthread.php?tid=26141)

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RE: Zenith 7H920 Restoration - Vlad95 - 01-01-2026

Convinced.
But you've calculated too much resistance.
10 V at 150 mA. We need to add a 66 ohm resistor, not 374.
As a result, you will need 40uF, not 7.1, which is too much for a good solution.

Or maybe I misunderstood something.

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462ron - 462ron - 01-01-2026

Here is an article on dropping the voltage on a series strung filament (heater) using a capacitor. Scroll down to the section on using and calculating the needed values. It’s a bit involved.
https://www.vintage-radio.com/repair-restore-information/valve_dropper-calcs.html 

Ron

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RE: Zenith 7H920 Restoration - RodB - 01-01-2026

I think 150 ma is too conservative. The filaments are 150 ma but there's a pilot lamp and B+ in parallel with the filaments. That should add 100 ma or more. I've done calculations for dropping capacitors before and they always came out with a result of an AC capacitor so big I couldn't find one for sale let alone find space for it. I hadn't looked at the schematic till now and didn't realize this is a transformer-less set. Looks like instead of a hot chassis you will end up with a glowing chassis. Maybe T&E is your best bet. Start with 100 ohms, 25 watts and go from there.

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RE: Zenith 7H920 Restoration - RodB - 01-01-2026

On a separate note, I've done FM alignments like this with a 10.7 MHz carrier and I've never been able to get the L20 adjustment to zero. First thing to get out of the way is to make sure all the resistors in the Limiter and Discriminator circuits are spot on. Next, I've always found FM is sensitive to variations in B+ voltage, so solve your line voltage issue. I had a Sears set that was working then replaced the selenium rectifier with silicon diode and FM wouldn't work until I brought B+ back to correct value.

What you are monitoring with your DMM is the AGC, so it's really signal strength and that's why you don't need modulation. Step 5 has always been the last step in the alignment, don't understand why it's first. Maybe it's to get the transformer in range so the rest of the adjustments will be more effective.

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RE: Zenith 7H920 Restoration - morzh - 01-01-2026

Rod,

You are right, but then I am right also.

The pilot lamp is a separate load, in parallel to the filaments; so I do not need to protect the lamp, as I don't care about it, plus it is todays lamp for 130V, and I bought 10pcs from Amazon at $1.50 ea. This is a lifetime supply.



Vlad,

You would be right. But, as said before, I calculated a capacitor. Not a resistor.
Hint: there is a phase shift of 90 degrees.

So the formula that applies is not that for series ohmic resistances (or reactances of the same type) which is

R1 + R2 = R3,

but for reactances (in this case a capacitive one, let's call it Zc) in series with Ohmic resistance ®, which is: (we will call the total impedance Z)

R-square + Zc-square = Z-square (Formula #1)

This is, as you might remember, what we learnt in...9th year of school and, of course, in the course of circuit theory.

So. Here's the calcs:

The sum of the filaments' voltages (nominal) is 115V.
We will call existing resistance "R-115" and the impedance needed to obtain the same current at 127V "Z-127".
The capacitive reactance is Zc.
The capacitance is C.

The resistance of the existing filament string is
R-115 = 115V/0.15A = 767 Ohm.

The impedance needed for 127V is
Z-127 = 127/0.15A = 846.7 Ohm.

Now we solve for Formula #1:

R-square + Zc-square = Z-square
767-square + Zc-square = 846-square.
Or
588289 + Zc-square = 716844

Now

Zc-square = 716844-588289 = 128555

And the Zc is
Zc = SQRT(128555)
Zc = 359 Ohm.

So. Your capacitive reactance that would increase the impedance by some 77 Ohm is not, well, 77 Ohm but 359 Ohm.
Phase shift. Have to deal with it.

The communist party teaches us the water bopils at 90 degrees C!!!

Now, calculating the cap is easy, it is (F = 60Hz, C is capacitance in Farads)
Zc = 1/(6.28*F*C).
So,
C = 1/(Zc * 6.28 * F) = 1/ (359 * 6.28 * 60) = 1 / 135271 = 0.0000074 F = 7.4 uF.


Done.

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RE: Zenith 7H920 Restoration - Vlad95 - 01-01-2026

Dаmn it, you're right. 
I'm getting old. 

Calculation of the quenching capacitor

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RE: Zenith 7H920 Restoration - RadioSvit - 01-01-2026

Влад , Майкл... Шото ви хлопці запрацювались ... Забули що Новий Рік чи що ? Ану гайда до столу . З Новим Роком вас всіх .

Vlad, Michael... You guys have worked hard... Did you forget that it's New Year or something? Come on, let's go to the table. Happy New Year to you all.

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RE: Zenith 7H920 Restoration - RodB - 01-01-2026

You going to boil that cap in oil?

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RE: Zenith 7H920 Restoration - Vlad95 - 01-01-2026

РадиоСвит

Та отож
So, then

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RE: Zenith 7H920 Restoration - morzh - 01-01-2026

https://www.youtube.com/watch?v=TtTl2ldB86k

Youtube link to the reception in the basement video.

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RE: Zenith 7H920 Restoration - morzh - 01-01-2026

Peter,

I am applying an honest effort to finish my wife's Vinegrette and Olivier.
And there is still 1/2 bottle of Veuve Cliquot. I am the only one drinking French Brut in this family.

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RE: Zenith 7H920 Restoration - morzh - 01-01-2026

Rod


Not sure why the cap would deserve such a fate. I am not a monster.

As for the alignment, I also added a 1M resistor between the lugs of the disc xfmr and the lead of the DMM, in order to not de-tune a circuity with added extra lead's parameters.

I did what I could

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RE: Zenith 7H920 Restoration - KCMike - 01-02-2026

Just watched your video on the 7H920.
Good job Mike. 
  It was picking up lots of stations on both bands.

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RE: Zenith 7H920 Restoration - morzh - 01-02-2026

Thanks Mike!


As for the alignment, I was able to complete it.
There were several factors.

1. My SG output was unreliable: this protection device I made years ago from two BNC connetors was making intermittent contact. I think that 0603 50 Ohms resistor either cracked or disconnected.

2. I used a 1:1 scope probe (it measures 280 Ohm end-to-end, so I thoughgt it would protect at least some) to feed the signal to the Limiter grid.

3. I used 2M resistor in series with the DMM Red lead, and I twisted the leads to minimize the pickup. I intend to manufactire a shielded probe for it.

So, the voltage immediately became negative (as it should) and I was able to peak the Lug 6 and minimize (no it is not zero but a few mV, and I found the minimum) the Lug 3.


Seems to work fine.




Now to the rectifier. I was worrying about the temp rize, but I was assessing it with my finger and thermocouple. It showed 150F.
I looked at the datasheet of a similar resistor, whose thermal resistance is about 56 K/W.
I have exactly 6.25V across that 82 Ohm resistor.
This is 0.48W

This should give me 26.7K temp rise, and the ambient there is about 19C, so it is 46C or 115F. 115F is still very uncomfortable to touch and will burn over time.

It is possible my resistor Thermal resistance is a bit higher (the datasheet is YAGEO) and mine is some noname bought through eBay.

But. The good news is, even 150F is safe temp for wires around; they should be rated at least for 100C (212F).

So I might just leave it be.

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RE: Zenith 7H920 Restoration - morzh - 01-02-2026

PS. I went and measured the charge voltage across the 1st cap when using an 82 Ohm resistor in series with a 1N4007 diode, when plugged directly into my wall outlet (127 VAC).

The original WV rating for the caps is 150V.
I used 160V Hi-rel Nichicon caps.

So, while on 117VAC, the voltage was within 155V or so.
When plugged into 127VAC, the DC voltage peaked at 165VDC and gradually, over 10-15 seconds, drop to 135VDC (the required voltage is 130VDC).
First this bothered me, then I remember my caps are 160V, not 150V.

The surge voltage rating for the Japanese-made aluminum caps is 115% of rated WV for those under 350V and 110% for those over 350V. The voltage can be applied for 30 seconds (the test) or, according to other sources, for 1 minute.

So, I want to shave off 5VDC. THis will require me to use a 150 Ohm resistor instead of 82 Ohm I am using now. At 75mA it will take off extra 5.2V.

This will bring the surge to 160V (the rated voltage) and the stabilized voltage to 130V exactly, when working off of 127VAC.

I will be using 10W cemented resistor, as my wattage will now come to 0.82W and I want the resistor warm, not hot. I just ordered those on Amazon and they will come tomorrow.

And, I have that last one 6.8uF cap for the filament string.

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