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Zenith 7H995 - Printable Version

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Zenith 7H995 - vecher - 10-21-2020

ALL, Have a zenith that the Rectifier is open SE-1.. I was going to replace it anyway But what do I use?

Would a 1N4007 work? or do I need something different..

[Image: https://live.staticflickr.com/65535/50512586558_0bc7724e62_c.jpg]

Here is schematic partial..

[Image: https://live.staticflickr.com/65535/50513298186_d6ac10ed56_c.jpg]


PS one other question There are a few resistors marked WW I assume that means wire wound Do they have to be wire wound?

Thanks

Skip

RE: Zenith 7H995 - RodB - 10-21-2020

Yes, you can use the 1N4007. You will have to increase R20 to around 33 to 47 ohms to get the B+ down to 90 volts.

RE: Zenith 7H995 - Paul Philco322 - 10-21-2020

Maybe leftovers from World War 1, resistors.....Icon_smile.

Paul

RE: Zenith 7H995 - vecher - 10-21-2020

Ok I heard or read somewhere that the selenium Rectifiers are marked backwards.. that the Plus marking is the Cathode and minus marking the Anode..Is this true? if so how does the 1n4007 go in..

Skip

RE: Zenith 7H995 - Radioroslyn - 10-21-2020

As far as I've seen no they are marked properly. The + is where the positive voltage exist the rectifier. The ring (cathode) on the 1n4007 goes to the + side of the filter cap.

RE: Zenith 7H995 - morzh - 10-22-2020

Skip

You are right, Plus is the Cathodes. Because Plus in teh Seleniums shows the Output Voltage polarity, not the Pos/Neg Electrode (Anode Cathode), as the Diode or teh Bridge has Positive Voltage coming out from Negative Electrodes (Cathodes).

So, in Seleniums, the B+ (and the positive of the electrolytic cap) will go to the pin marked "+" and the one marked "-" will go to the negative (and possibly Chassis in he later models).

RE: Zenith 7H995 - David - 10-22-2020

Guess I am the red headed stepchild!

RE: Zenith 7H995 - radiorich - 10-22-2020

hello morzh,
I am sure more then once people have hooked that diode up in backwards and really made a mess.

Sincerely Rich
P.S. look at some the old electrolytic capacitors the stripe is positive not negative !

RE: Zenith 7H995 - vecher - 10-23-2020

Ok Finished up the Capacitor replacement.. Replace some resistors also.. Brought up on variac .. at 120 Volts B+ is 90.5 volts..I used a 40 Ohm resistor.. at 120 Volts draws less then 300 ma.. So all is good there.. ON FM band stations come in load and clear all over dial.. BUT on AM only receive 1 station and only when I put my hand near the loop antenna..

Hopefully all it needs is a alignment on AM .. Any comments?

Thanks
Skip

RE: Zenith 7H995 - Paul Philco322 - 10-23-2020

Is antenna connected properly? I had a Zenith, played pretty good FM but a weak wire to antenna left AM very weak.

It was obvious but I overlooked it.

Paul

RE: Zenith 7H995 - vecher - 10-24-2020

Good thing to check..I label everything.. but as you say its easy to get something in the wrong place..

I have to bring it to garage.. that's were all my alignment equipment is..

Funny thing I expected the AM to be good.. and the FM have some problems... But i am glad its not the FM...

Skip

RE: Zenith 7H995 - vecher - 10-24-2020

Ok Aligned the AM.. The IF's were way off.. Also aligned the dial..

AM now working correctly.. I did find a bad tube.. The 12AT7.. I did check all tubes on tube tester and all tested Good..
Like they say you don't know if a tube will or will not work in radio until you try..

So now both AM and FM working great..

Skip

RE: Zenith 7H995 - David - 10-25-2020

When working on AM/FM sets it is good to have several 12AT7 an 12DT8 on hand never know when they will be needed.