I know the mica caps are rarity to go bad, but what if they have a crack in the body? would that be a sign? I have gone over this thing looking for bad tubes, loose conections etc and the AM will not work unless I wrap my hand around the ocsilator coil then will p/u half the band from about 950 on up, the other bands, and the electric tuning work great, the micas are my last hope, unless the osc coil is shot? is there anyway to test these?
any help is appreciated.

:I know the mica caps are rarity to go bad, but what if they have a crack in the body? would that be a sign? I have gone over this thing looking for bad tubes, loose conections etc and the AM will not work unless I wrap my hand around the ocsilator coil then will p/u half the band from about 950 on up, the other bands, and the electric tuning work great, the micas are my last hope, unless the osc coil is shot? is there anyway to test these?
:any help is appreciated.

Since the set works normally with the push-buttons on BC band, suggest you do the following: Refer to the 87K1 schematic @ NA, it appears similar according to the differences info @ NA for the 87T2.

- Be sure there is continuity for all band swith positions and circuits.
- Check the filament voltage at the 6A8, 6.3 vac or slightly higher, and that the tube tests O.K. as an oscillator.
- Be SURE you are using ALL metal tubes!
- Refer to the schematic AND the diagramatic to locate the windings on the oscillator coil, since most windings are capacitor coupled a "straight through continuity to check all windings test" will not work.
- Check all windings for continuity, most if not all the windings should be less than 25 ohms. Some may even be about 1 ohm, (the highest shortwave band).
- Since it is dead on all bands when in tuneable mode I would suspect circuits common to that mode are the place to look.
- L6 (osc coil) connection is to a grid on the 6A8 (blue wire, if insulation hasn't fallen off) that functions as the plate of the triode connected osc. It must have a reduced level of plate voltage via the Candohm resistor (or replacement resistors) that are riveted to the chassis.
- Check this circuit with your voltmeter and rotate the band selection switch. Voltage should be available at that grid for all band settings, if not, one or all of the plate windings or associated connections are open.
- The other windings can only be checked with the receiver off by continuity, again, refer to the diagramatic AND be sure that the bandswitch is not creating a false reading, remember it must be a low value of resistance.
- This same method can be applied to the antenna coil.
- Check the mica capacitors with your ohmeter, all should show open circuit. Without a capacitor bridge or some other means of measurement you will not know if the capacitor has shifted value or is indeed open. Only substitution of the value could be done.
- A cracked mica capacitor is a flag but until it is accuratly measured, consider it funtionable, to be replaced soon.
- Be sure to check the antenna choke, must also have continuity, but likely is O.K. because buttons work.
- This choke often opens at the coil connections to the imbedded bypass capacitor, simply melt off some of the beeswax with a heat gun and resolder the connections.
- Should you find an open in the oscillator coil all is not lost as often it is a break at a solder lug or on the first turn. Simply carefully repair. Do not permanently remove any turns as there is not enough or no adjustment that can be made to other parts of the tuned circuits to compensate and will result in calibration errors or oscillator dropouts.
- RCA is fussy about lead dress in the antenna and oscillator circuits. If you replace a wire do so with the same size, type and of the SAME length. See above paragraph. This would not stop the set from working but could cause calibration error or uncontolled feedback.
- The oscillator coil IS common to a variety of models if you can't repair it. Cross-referencing the models using the RCA RED BOOKS will give you the model types that would work.
- If the break is in a winding UNDER a universal winding (looks like a criss-cross winding), generally, this is not repairable without a great deal of time involved.
- The IF coils are wound with a Litz wire, if individual strands break the resistance goes up along with a change in "Q". This will effect IF bandpass and how the AVC functions read: How and to what extent the tuning eye performs as well. This is a tough repair as the coils are universal wind. There are also wax dipped metal/mica stack capacitors involved. No history of these is known? Resistors R5 & R6 are within the 2nd IF can, these, often, shift value dramatically, more than 20%, this will effect the tone and volume. The resistors are replaceable. The coils are interchangable among a number of models.

It is good to know what is wrong even if it cannot be easily repaired. At least your troubleshooting skills have hit the mark.

Regards,

Charles

Thanks,Charlie I will try what you suggest, one thing I did not mention is the AM did work, but once in a great while would cut out( and the tuning eye would widen) then would cut back in , then would cut out etc, at first it would only do this one in a while, but with in a matter of a few weeks this would increase to the point that the AM stoped working all together, when it did cut out would sound like a little electrical pop, (thus my thinking a cap was breaking down)and seeing it only effected the am and nothing else I figured one of the micas.the shortwave bands still play great.
::I know the mica caps are rarity to go bad, but what if they have a crack in the body? would that be a sign? I have gone over this thing looking for bad tubes, loose conections etc and the AM will not work unless I wrap my hand around the ocsilator coil then will p/u half the band from about 950 on up, the other bands, and the electric tuning work great, the micas are my last hope, unless the osc coil is shot? is there anyway to test these?
::any help is appreciated.
:
:Since the set works normally with the push-buttons on BC band, suggest you do the following: Refer to the 87K1 schematic @ NA, it appears similar according to the differences info @ NA for the 87T2.
:
: - Be sure there is continuity for all band swith positions and circuits.
: - Check the filament voltage at the 6A8, 6.3 vac or slightly higher, and that the tube tests O.K. as an oscillator.
: - Be SURE you are using ALL metal tubes!
: - Refer to the schematic AND the diagramatic to locate the windings on the oscillator coil, since most windings are capacitor coupled a "straight through continuity to check all windings test" will not work.
: - Check all windings for continuity, most if not all the windings should be less than 25 ohms. Some may even be about 1 ohm, (the highest shortwave band).
: - Since it is dead on all bands when in tuneable mode I would suspect circuits common to that mode are the place to look.
: - L6 (osc coil) connection is to a grid on the 6A8 (blue wire, if insulation hasn't fallen off) that functions as the plate of the triode connected osc. It must have a reduced level of plate voltage via the Candohm resistor (or replacement resistors) that are riveted to the chassis.
:- Check this circuit with your voltmeter and rotate the band selection switch. Voltage should be available at that grid for all band settings, if not, one or all of the plate windings or associated connections are open.
: - The other windings can only be checked with the receiver off by continuity, again, refer to the diagramatic AND be sure that the bandswitch is not creating a false reading, remember it must be a low value of resistance.
: - This same method can be applied to the antenna coil.
: - Check the mica capacitors with your ohmeter, all should show open circuit. Without a capacitor bridge or some other means of measurement you will not know if the capacitor has shifted value or is indeed open. Only substitution of the value could be done.
: - A cracked mica capacitor is a flag but until it is accuratly measured, consider it funtionable, to be replaced soon.
: - Be sure to check the antenna choke, must also have continuity, but likely is O.K. because buttons work.
: - This choke often opens at the coil connections to the imbedded bypass capacitor, simply melt off some of the beeswax with a heat gun and resolder the connections.
: - Should you find an open in the oscillator coil all is not lost as often it is a break at a solder lug or on the first turn. Simply carefully repair. Do not permanently remove any turns as there is not enough or no adjustment that can be made to other parts of the tuned circuits to compensate and will result in calibration errors or oscillator dropouts.
: - RCA is fussy about lead dress in the antenna and oscillator circuits. If you replace a wire do so with the same size, type and of the SAME length. See above paragraph. This would not stop the set from working but could cause calibration error or uncontolled feedback.
: - The oscillator coil IS common to a variety of models if you can't repair it. Cross-referencing the models using the RCA RED BOOKS will give you the model types that would work.
: - If the break is in a winding UNDER a universal winding (looks like a criss-cross winding), generally, this is not repairable without a great deal of time involved.
: - The IF coils are wound with a Litz wire, if individual strands break the resistance goes up along with a change in "Q". This will effect IF bandpass and how the AVC functions read: How and to what extent the tuning eye performs as well. This is a tough repair as the coils are universal wind. There are also wax dipped metal/mica stack capacitors involved. No history of these is known? Resistors R5 & R6 are within the 2nd IF can, these, often, shift value dramatically, more than 20%, this will effect the tone and volume. The resistors are replaceable. The coils are interchangable among a number of models.
:
:It is good to know what is wrong even if it cannot be easily repaired. At least your troubleshooting skills have hit the mark.
:
:Regards,
:
:Charles