zenith 7h820

[doodie]

I am new at this restoring old radios. when I got this radio it was
not working I replace all wax caps with new ones filter capacitors too
and replace all bad tubes and got it working on AM real good but
nothing on FM, I am lost I have try everything IF transformers check
out good, I check all tubes can someone help me .
Thanks

[philo]

First off...how old is the radio?

What's the model number?

if it's pre WW-II the FM band was 42-50 mhz
and that differs from todays broadcast band

[Paul P]

Do you have the schematic and alignment diagrams?
http://www.nostalgiaair.org/Resources/458/T0000458.htm
Page 19-8 shows the standard FM band (2nd position) and 19-9 shows the old
unused band third position.

You will not hear stations on the old band. You should hear your FM
generator if it is working on that old band.

Try and see how far you get through the FM alignment procedure. That may
tell you where or what stage a failure lies.

Check all your resistors for tolerance. You may have a bunch of bad ones.
For examples of how many resistors can be bad in a radio chassis see:
http://www.ppinyot.com/A/Airline.htm and
http://www.ppinyot.com/links/resistors.htm

Good Luck and keep us posted!

Paul P.


"doodie" <scadhom...@gmail.com> wrote in message
news:1a7dbed2-d5e0-442e...@f1g2000yqi.googlegroups.com...

[doodie]

Zenith 7h820 1948 and I know one band does not working

[doodie]

On Jan 10, 9:33 pm, "Paul P" <use...@ppinyot.com> wrote:
> Do you have the schematic and alignment diagrams?http://www.nostalgiaair.org/Resources/458/T0000458.htm

> Page 19-8 shows the standard FM band (2nd position) and 19-9 shows the old
> unused band third position.
>
> You will not hear stations on the old band.  You should hear your FM
> generator if it is working on that old band.
>
> Try and see how far you get through the FM alignment procedure.  That may
> tell you where or what stage a failure lies.
>
> Check all your resistors for tolerance.  You may have a bunch of bad ones.

> For examples of how many resistors can be bad in a radio chassis see:http://www.ppinyot.com/A/Airline.htm andhttp://www.ppinyot.com/links/resistors.htm

>
> Good Luck  and keep us posted!
>
> Paul P.
>

> "doodie" <scadhomedes...@gmail.com> wrote in message

>
> news:1a7dbed2-d5e0-442e...@f1g2000yqi.googlegroups.com...
>
>
>
>
>
>
>
> >I am new at this restoring old radios. when I got this radio it was
> > not working I replace all wax caps with new ones filter capacitors too
> > and replace all bad tubes and got it working on AM real good but
> > nothing on FM, I am lost I have try everything IF transformers check
> > out good, I check all tubes can someone help me .
> > Thanks

I don't know much on alignment procedures but I did check out your
links on resistors, I will check them
and can you tell the procedure on checking them

Thanks

[philo]

Stupid me... sorry you put the model in the subject line

sheesh

[Paul P]

I don't know much on alignment procedures but I did check out your
links on resistors, I will check them
and can you tell the procedure on checking them

Thanks

----------------------------------------------------------------------------------------------------------------------------------------


I check resistors with an ohm meter. I look to see if they are in
tolerance. However, there are a few things to consider:

In circuit resistor testing can have other resistances in parallel. If the
resistor is low out of tolerance that does not automatically indicate a bad
resistor. Although it can be bad.

Any resistor that measures high out of tolerance is bad.

If you are really curious about the exact resistor's value you must
disconnect one end. After proper reconnection time and effort you just as
well could have shot-gun replace it anyways. You will see a few on that
Airline that were good in tolerance. But I replaced them anyhow.

If I find more than two European "plastic coated" resistors out of tolerance
I replace them all. I find that Grundig and other European manufacturers
(post war) used resistors with a coating of plastic like substance. Those
plastic coated resistors are rather good and I keep them. In later years
50s 60s European manufacturerers started to use the uncoated carbon comps
and PC boards. Those all get replaced.

I reopened my first AA5 restoration several times, spent time finding a bad
resistor and replacing them one at a time. Then I got smart and shot-gunned
them all and have been using that practice ever since.

Now I must admit a few of my own radios I may not shot-gun them all right a
way and take the gamble. Any "client" restoration get the full treatment!
I'd hate to have a radio shipped (70 bucks round trip) to me for a simple 7
or 10 cent resistor. That would piss-off a client real fast! Plenty of
eBay radios have one two or several parts replaced so it is "Operational".
But that is like putting air in a 1968 original Firebird auto tire, shining
up the tire walls with Armor-All and saying the tires are good.

I use carbon fiber resistors (<=18MHz - my personal rule) as replacements
for two primary reasons:
1) They are less expensive than carbon composition.
1a) Easy to find.
2) As I later found out, too much soldering gun heat damages carbon
composition resistors. So be quick about soldering them in place when you
are close to the resistor's body. Carbon film can be damaged too but they
are more tolerant of my Weller soldering gun.


I would be surprised if your Zenith used the plastic coated resistors.

Sorry if I go on too much......

Let me know what you decide to do.

Paul P.

[doodie]

The more you go on the more I learn.

I do think I will replace my resistors and see if that helps
But Let me see if I am right, my Zenith has what looks like them
reddish-brown resistors carbon composition
like the one on
http://www.ppinyot.com/links/resistors.htm
but you would use carbon film resistors as replacements,
and should I use the same values what's shown in my schematic.

thanks D.Snow

[Paul P]

The more you go on the more I learn.

I do think I will replace my resistors and see if that helps
But Let me see if I am right, my Zenith has what looks like them
reddish-brown resistors carbon composition
like the one on
http://www.ppinyot.com/links/resistors.htm
but you would use carbon film resistors as replacements,
and should I use the same values what's shown in my schematic.

thanks D.Snow

-----------------------------------------------------------------------------------------------------------------------------------

I always use the schematic value. Some times (I suspect) a repair shop may
make tweaks to get a radio to work if the part needed is not on hand (or for
some other reasons). The other reasons also includes a factory modification
or change. You need to look at some of the later year documentation for
hints of manufacture changes. But Zenith is pretty good at documentation.

Make sure your documents match the chassis number along with the model
number. 7h820 is the model number. I have been using "7h820 chassis 7E01"
documents for this explanation. This may NOT be your chassis number.

Often the color bands of the original resistors are discolored. The
schematic (parts list) is your best bet.

You can get all you need from Radiodaze and Just Radios in one order. Just
make sure you order the higher wattage resistors when needed (i.e., 1 and 2
watt resistors). I put in 1/2 watts in for 1/4 or 1/2 watts (I do not have
1/4 watts in the bins).

Do you have the parts list? Keep the wire wound resistors unless they test
bad. Replace all the 1/2 watt resistors. I never had problems with carbon
fiber in FM radios. I have had problems with carbon fiber resistor in
communications receivers and transmitters above that 18 meg range. So don't
worry about it for your Zenith would be my recommendation.

C21 on the volume control's wiper is a shielded cap. I use aluminum sticky
duct tap to shield a cap with an added ground wire. Most of the time people
do not bother adding the shield. If you get away with it good for you.
See: http://www.ppinyot.com/saba/saba.htm for the shielded cap modification
(middle of page). Skip adding the little brown silver mica cap for your
Zenith. Some of these European radios can be finicky so I added the cap to
ground.

You will want to replace the selenium rectifier with a silicon rectifier
(1N4007) and a series resistor to bring the B+ down to specifications. The
resistor usually is 5 watts wire wound at 75 to 150 ohms. You have a half
wave rectifier circuit so the lower value will be needed. You may have to
experimentally determine the value of that resistor. You should be able to
adjust (replace) R21 (22 ohms) with a higher resistor to maintain 135 volts
(this is highly dependant on the line voltage). I would try about 100 ohm 5
watt wire wound (22 ohms plus 75 for the new diode).

Other news group participants may have a better resistance to use in place
of R21 and get you closer.

You have a transformerless HOT CHASSIS radio. Use an isolation transformer
before connecting any test gear (including your finger) to the radio. And I
believe this chassis is actually connected to one side of the line cord!!!!.
Make sure all insulating features are in good shape and keeping all knobs
and chassis screws isolated from the hot chassis.

Use a Dim Bulb Tester or Variac to bring the power up after you replace a
bunch of parts. This will save you if you make a wiring mistake. I make
them once in a while too. See:
http://www.ppinyot.com/dimbulb/dim_bulb_tester.htm

NOTES
M Ohm equals times 1,000 ohms; Megohm is times 1,000,000 ohms.
Mmfd or mmfd is pico farad (x 10 ^-12). Keep those unless they are marked
Micamold.
Use 630 volt caps for all cap replacements (except the electrolytic filter
caps). It keeps ordering simple.
Dry electrolytic can be replace with wet electrolytic. Observe + and -
polarity. Get a voltage equal to or bigger than the marked value.
Capacitance go to the next highest standard value available.

Paul .

[doodie]

> See:  http://www.ppinyot.com/saba/saba.htmfor the shielded cap modification

Well first of all,I wont thank you for your time and knowledge for
helping me on this.
I tried to get help on this from Antique Radio forum
with no success.Like I said I am new at this restoring old radios.

I got the photofact (It's a big help for me) chassis 7E01 schematic
and the one from nostalgiaair


When I got this radio it was

not working I replace all wax caps with new ones filter capacitors too

and replace all bad tubes,cleaned everything and got it working on AM
real good but
nothing on FM,and this is my first study on resistors.

I don't know what you mean by selenium rectifier and series resistor
I don't understand all of that one paragraph.
I looked on Radiodaze and all I could find is a power resistor with
5 watts.
Guess I need to study more on resistor.

D.Snow

[Brenda Ann]

"doodie" wrote in message
news:10c03c3b-17f0-4ae9...@k28g2000yqn.googlegroups.com...

I don't know what you mean by selenium rectifier and series resistor
I don't understand all of that one paragraph.
I looked on Radiodaze and all I could find is a power resistor with
5 watts.
Guess I need to study more on resistor.

D.Snow

--------------------------------------------------------------------------------
--------------------------------------------------------------------------------

The selenium rectifier is a squarish device with several plates. It's
generally either blue or orange, and is mounted to the chassis, generally on
the front or the back. This part is almost always defective and needs to be
replaced, as Phil said, with a solid state diode and a 47 ohm resistor in
series. The resistor can be on either end of the diode, and only needs to be
a 2 watt resistor. Make sure to pay close attention to the polarity of the
old part. The side with the + sign goes to the side of the new diode with
the white line on it.

[Paul P]

"Brenda Ann" <newsg...@fullspectrumradio.org> wrote in message
news:pfydnS1k_ou8j4zS...@giganews.com...

I am of the opinion and get the feeling you should get an Isolation
Transformer as soon as possible to reduce your risk of electrocution.
Research "Hot Chassis Radios" on Google and see:
http://en.wikipedia.org/wiki/All_American_Five for starters.


The blue stack of plates in the lower right side of this chassis is a
selenium rectifier. They are different colors but are made of plates.
http://www.ppinyot.com/transoceanic/T600/IM000007.JPG

Replace it with a new silicon diode and a 2 watt resistor (according to
trusted Brenda Ann). See the upper left (the chassis has been spun 180
degrees) http://www.ppinyot.com/transoceanic/T600/IM000009.JPG. See the
little black tubular diode soldered onto the first two lugs of the new
terminal strip. The square white resistor is the new increased resistance
"series resistor". That one pictured is 5 watts.

There is an existing 22 ohm 2 watt resistor in series with the existing
Selenium rectifier diode. If you must replace the 22 ohm resistor you may
combine the need for an additional series resistor of 75 ohms ( mentioned by
Brenda Ann) and the old 22 ohm value. That can be covered by a single 97
ohm 2 watt resistor. But 97 ohms is not a standard resistance. Use a 100
Ohm resistor. Read about how resistors can be combined in series or
parallel.

This is a new 1N4007 diode you will need to buy:
http://image.made-in-china.com/2f0j00rCKTSyqGbPcf/Rectifier-Diodes-1N4001-1N4007-.jpg
The solid line (around one side of the diode) is the positive side of the
diode. Look for the + sign on the old selenium diode and match them up. Or
wire it according to the schematic.

http://www.radiodaze.com/category/216.aspx lists all of their resistor
categories. Carbon film and Metal film resistors are >1 watt, Metal Oxide
are offered in 1 and 2 watts. Power category have 5 and 10 watt resistors.
You should be covered.

B+ referrers to the high voltage used on the tube plate. And any circuit
needed to make or supply that voltage. It originally meant a high voltage
battery of 22, 45, 90 volts. Do not lick the terminals of those batteries!

On the schematic there is a voltage marked that the Selenium rectifier puts
out of 135 volts. There is also a voltage of 117 volts AC marked on the
power plug symbol. If all is working properly you will read 135 volts DC on
that point when there is 117 volts AC on the power plug. (130 to 140 volts
at 117 input is good. (Read about volt meters loading circuits under
test)).

When you change from the old technology of the selenium rectifier to the
newer silicon diode (as a rectifier) the voltage will increase. The new
diode is much more efficient at rectification than the old one. That
efficiency and resulting increase in voltage must be compensated. So adding
in a new 75 ohm 2 watt resistor will mimic the losses of the old selenium
rectifier (diode). Read about silicon and selenium diode voltage drops.

The 75 ohms is from experience. The actual and exact value may have to be
separately calculated and determined by experimental methods.

Paul.

[doodie]

On Jan 14, 1:06 am, "Paul P" <use...@ppinyot.com> wrote:
> "Brenda Ann" <newsgro...@fullspectrumradio.org> wrote in message

>
> news:pfydnS1k_ou8j4zS...@giganews.com...
>
>
>
>
>
>
>
>
>
>
>
> > "doodie"  wrote in message
> >news:10c03c3b-17f0-4ae9...@k28g2000yqn.googlegroups.com...
>
> > I don't know what you mean by selenium rectifier and series resistor
> > I don't understand all of that one paragraph.
> > I looked on Radiodaze and all I could find is a power  resistor with
> > 5 watts.
> > Guess I need to study more on resistor.
>
> > D.Snow
>
> > --------------------------------------------------------------------------------
> > --------------------------------------------------------------------------------
>
> > The selenium rectifier is a squarish device with several plates. It's
> > generally either blue or orange, and is mounted to the chassis, generally
> > on the front or the back. This part is almost always defective and needs
> > to be replaced, as Phil said, with a solid state diode and a 47 ohm
> > resistor in series. The resistor can be on either end of the diode, and
> > only needs to be a 2 watt resistor. Make sure to pay close attention to
> > the polarity of the old part. The side with the + sign goes to the side of
> > the new diode with the white line on it.
>
> I am of the opinion and get the feeling you should get an Isolation
> Transformer as soon as possible to reduce your risk of electrocution.

> Research "Hot Chassis Radios" on Google and see:http://en.wikipedia.org/wiki/All_American_Fivefor starters.

>
> The blue stack of plates in the lower right side of this chassis is a

> selenium rectifier.  They are different colors but are made of plates.http://www.ppinyot.com/transoceanic/T600/IM000007.JPG

>
> Replace it with a new silicon diode and a 2 watt resistor (according to
> trusted Brenda Ann).  See the upper left (the chassis has been spun 180

> degrees)http://www.ppinyot.com/transoceanic/T600/IM000009.JPG.  See the

> little black tubular diode soldered onto the first two lugs of the new
> terminal strip.  The square white resistor is the new increased resistance
> "series resistor". That one pictured is 5 watts.
>
> There is an existing 22 ohm 2 watt resistor in series with the existing
> Selenium rectifier diode.  If you must replace the 22 ohm resistor you may
> combine the need for an additional series resistor of 75 ohms ( mentioned by
> Brenda Ann) and the old 22 ohm value.  That can be covered by a single 97
> ohm  2 watt resistor.  But 97 ohms is not a standard resistance.  Use a 100
> Ohm resistor.  Read about how resistors can be combined in series or
> parallel.
>

> This is a new 1N4007 diode you will need to buy:http://image.made-in-china.com/2f0j00rCKTSyqGbPcf/Rectifier-Diodes-1N...

I understand now, I thank am going to place a order for some
resistors
and gave it a try.
thanks for your time
D. Snow

[doodie]

Well I replace the selenium rectifier with a new silicon rectifier
(1N4007) and a series resistor and It WORKS, AM and FM Works GREAT
I have not replace the 1/2 watt resistors or should I.
thanks Paul P. for your help