8G00TZ1

 


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Model 8G00TZ1

Filament Dropping Resistors.

This Zenith Transoceanic model 8G00TZ1 uses a miniature half wave rectifier for AC operation.  The operating voltage of this radio (for bench testing) is specified at 117vac.  Replacement of the filament voltage dropping resistors are critical to maintain good B+ and sensitive operation.

It should be noted the Sam's specified replacement resistor is 1000 ohms,   However, a note at the bottom of the table instructs the technician to set the resistance at the original 970 ohms.  This is critical to maintaining good B+ for different line voltages.

970ohm.jpg (166992 bytes)The power resistors are epoxies to the chassis for improved heat dissipation.  The original resistor (R19/20 Riders,  R78 Sam's)  is used as a terminal/mounting strip.

Model 8G005TZ1, chassis 8C40TZ2 with 117Z3 half wave rectifier miniature tube.

Important 970.jpg (15706 bytes) click this.

Most critical to proper B+ is R80 (or R18 on Rider's), 56 ohm (Green, Blue, Black).  A higher value put the B+ out of range.

 

 

For Model 8G005YT with the 8C40 chassis -  Calculations for R18 - 20 (Rider's) filament dropping resistors for the 8C40 chassis that use a 117Z6 octal tube. 

 TO heater resistors.jpg (125176 bytes)

The calculations for  the 970 ohm "candohm" resistor applies to other chassis as well (i.e., 8C40TZ2).

 

Replaced filament resistors in a 8C40 chassis using a 117Z6 full wave octal rectifier tube.  The first 1K ohm resistor measured 983 ohms.   Therefore, no parallel resistor was added. 

Once the filament resistors were replaced (R78 a & b and R80 8C40ZT2 chassis) the line voltage was varied while monitoring B+.  The radio functioned well on the SW bands over a wide range of Line voltages.  With R80 too high (a commonly available resistance) the oscillator failed on lower line voltages.  

chassis B4.JPG (191756 bytes)This is the Chassis just removed from the radio case.

After chassis.JPG (140900 bytes)This is the restored chassis with all bad capacitors and resistors replaced.

 

Electrolytic "Can" capacitors.

All Wax/paper capacitors are routinely replaced.  Electrolytic capacitors are cut open, gutted and refilled with modern equivalent capacitors.  The covers are secured back on to the electrolytic to maintain the original look. 

See the Capacitor Page for detailed examples of on the 8C40 chassis capacitor restuffing.

 

UL Safety Rated Capacitors.

It should be noted there is a 0.05 uf capacitor (C13 chassis 8C40, Riders) connecting chassis ground to B- (circuit ground) that if shorted would present one side of the power cord to the chassis.  Therefore, when performing a restoration, it is recommended this capacitor should be replaced with the UL Safety rated type.  C29 in the same chassis is directly across the power cord.  It is also recommended that this capacitor be replaced with the UL safety type.  See http://www.justradios.com/safetytips.html for information regarding UL safety capacitors.   Watch for this condition where a capacitor is connected across circuit ground to chassis in other radios.

UL rated safety Capacitors installed in the 8C40 chassis for C13 and C29.

safety caps.jpg (56960 bytes)

wpe4.gif (35261 bytes)

Click on this above image to enlarge and see fault path in red.   Schematic snippet from http://www.nostalgiaair.org/Resources/421/M0025421.htm

 

Parts placement.

It is important to maintain the relative position of all replacement components to the originals.  Alignment is affected by the components proximity to each other.

The yellow capacitor below the detector coil (right) was particularly sensitive to placement under the coil for proper alignment.  When the capacitor was against the chassis as was the much larger original, one of the SW bands was extremely weak.  Once moved the test signal came in loud and clear.

Cap position.JPG (155959 bytes)

To help maintain the components original position, the original part was clipped away as close to the body of the component as possible.  Two hooks were formed on the remaining lead wires.  The new component was formed with two opposing hooks and soldered into place.  Position insensitive parts (i.e., in the power supply section) were replaced by de-soldering the whole original component.

Alignment considerations.

During alignment of the Broadcast Band, the oscillator adjustment at 1600k Hz, oscillator trimmer had to be opened up almost 0.25 inches with no adjustment range remaining  (too far for my liking).  It should be closer.  About 2 to 3 mm.  

Moving the lead wires that connect the trimmer (C19, ch 8C40) to the oscillator coil help considerably.  The wires shown on the right were grouped together.  Experimentation with a plastic stick determined the green striped wire (connected from the trimmer to the oscillator coil) had the greatest effect.  Once separated,  the trimmer capacitor could be closed down thus yielding more adjustment range.  Attempting the same technique for the detector coil trimmer (C12) adjustment yielded no additional benefit.

All antenna trimmer capacitors were aligned once the chassis was reinstalled into the restored case.

BC_Oscilator_1600k_Adjust.JPG (27005 bytes)

 

Here are some additional pictures during a G500 restoration.

IM000138.JPG (279473 bytes) Capacitors in place of the old foil can cap.

IM000139.JPG (247150 bytes) Better look.

IM000140.JPG (255835 bytes) Hot melt the cardboard cap.

IM000141.JPG (276451 bytes) Push on until hard.

IM000142.JPG (268255 bytes) Cleaning the dirt off of the chassis.

IM000143.JPG (282548 bytes)Removing the worn speaker grommet

IM000144.JPG (276807 bytes)  Had to cut it off.

IM000145.JPG (296091 bytes) Replacement Grommets

IM000146.JPG (301332 bytes) Grinding rivets on old power resistor.

IM000147.JPG (287659 bytes)  Removal of the old power resistor.  It will go bad soon.  So better to replace it now while on the bench.

IM000149.JPG (332109 bytes)  Completed under chassis.  New power resistors are Epoxy(ed) to chassis.  this adds mechanical strength and good heat conduction.

 

 

 

 

 

Exterior

Below are three photos of the completed 8G005YT, chassis 8G40 Transoceanic.

TO exterior 0305.JPG (189800 bytes)The case was disassembled, surface mounted hardware removed and all parts cleaned.  

TO dial 0305.JPG (169146 bytes) All of the brass was shined up with Brasso and a coat of Minwax Finishing Past Wax.  A replacement clear plastic dial cover was installed in place of the missing one.  Non brass metals were treated with Tarn-X.

TO done chassis 0305.JPG (197773 bytes) The completed and aligned chassis displays all the new capacitors, resistors, UL safety capacitors and two additional line fuses.  The can capacitors present on the top of the chassis have been "re-stuffed".

IM000030.JPG (73780 bytes) Additionally; The black case was treated with Kiwi Black Leather dye, a coat of Kiwi Liquid wax and a buffing.    The plastic bezel was treated with Glazit.

Battery page with general TO links.

Thank you!

Please email any comments or questions.

 

Transoceanic History

Try this detailed story at http://www.radiomuseum.org/forum/zenith_trans_oceanics.html.  You may need to register.  For a time it was free registration for US participants.  I am not sure how long this is being offered.  On a side note, from what I have gathered the charge/fee is to keep the riff-raff off of their well maintained and informative forum. I frequent there.

 

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