Zenith model 7S657R
Zenith model 7S657R
by Rogm1017 Sun Jan 24, 2021 4:58 pm
Hello, new to radio restoration and attempting my first. Purchased this radio for 50.00 , cabinet is in great condition and thought it would make a nice piece when finished. Looking for a little help on the restore. This radio has been sitting for many years unused, so the first thing I was going to do was a recap and check all resistors. After pulling chassis I noticed a lot a black sticky goo on the underside of the chassis right below where the transformer sits. Is this the indication of a bad transformer? If so how do I know what kind of replacement to get and where. Any help would be appreciated. Thanks...
Last edited by Rogm1017 on Sun Jan 24, 2021 4:59 pm; edited 2 times in total (Reason for editing : Add pic)
Rogm1017- New Member
- Number of posts : 1
Registration date : 2021-01-24
Re: Zenith model 7S657R
by Chas Sun Jan 24, 2021 11:49 pm
Perform a simple test of the power transformer.
Remove the rectifier tube and the two pilot lamps in event they are shorting.
Power the set, if a crackling, sizzling starts anytime in the next 30 minutes the transformer is bad. If the tubes stay lit and the transformer relatively cool to the touch the transformer is good...
This radio was designed with the transformer at its limit and to use a failure prone rectifier the earlier version of the 6X5. What happens is the the cathode sleeve shorts to the filament or the plates. This causes failure to the transformer form overload.
The are multiple remedies, choose what fits you knowledge and pocketbook...
Replace the transformer with a used one.
Have the transformer re-wound and add a 5 volt secondary for a 5Y3. or not and use power solid-state diodes for a rectifier.
In any transformer replacement use the updated 6X5 and place type 47 pilot lamps in series with each plate. If there is a short in the tube the pilot lamps would burn out, that, is the Zenith recommendation.
Obvious all the other capacitors in the radio will have to be replaced.
Other triage must include testing by resistance using a VOM, all the other RF/IF coils, the output transformer speaker field and voice coil. These are the tougher components that can fail.
The loctal sockets are at times also troublesome.
http://www.nostalgiaair.org/pagesbymodel/383/M0025383.pdf
Remove the rectifier tube and the two pilot lamps in event they are shorting.
Power the set, if a crackling, sizzling starts anytime in the next 30 minutes the transformer is bad. If the tubes stay lit and the transformer relatively cool to the touch the transformer is good...
This radio was designed with the transformer at its limit and to use a failure prone rectifier the earlier version of the 6X5. What happens is the the cathode sleeve shorts to the filament or the plates. This causes failure to the transformer form overload.
The are multiple remedies, choose what fits you knowledge and pocketbook...
Replace the transformer with a used one.
Have the transformer re-wound and add a 5 volt secondary for a 5Y3. or not and use power solid-state diodes for a rectifier.
In any transformer replacement use the updated 6X5 and place type 47 pilot lamps in series with each plate. If there is a short in the tube the pilot lamps would burn out, that, is the Zenith recommendation.
Obvious all the other capacitors in the radio will have to be replaced.
Other triage must include testing by resistance using a VOM, all the other RF/IF coils, the output transformer speaker field and voice coil. These are the tougher components that can fail.
The loctal sockets are at times also troublesome.
http://www.nostalgiaair.org/pagesbymodel/383/M0025383.pdf
Chas- Member
- Number of posts : 174
Age : 74
Registration date : 2008-07-24
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