12-28-2012, 11:12 AM
For as long as I can remember I’ve heard Philco techies whine about the difficulty in servicing the RF stages of the 1937-1938 Philco’s- the 37-620 through 37-670 and the ‘38’s. In truth these assemblies are designed for EASY service! The most difficult part of the operation is marking and unsoldering about 8 wires that go the the main chassis. This particular description was done on a 37-670 but the procedure applies to the whole series.
Set the band switch to band one (BC). To remove the RF assembly, the dial must be removed. Remove the one screw in the center-rear of the assembly. Remove the screws holding the IF chassis (you can leave the rear side screws just loosened); pull the IF chassis out about ¾ inch and lift out the RF stage.
On the RF stage each side cover can be removed with three screws. Next remove the two screws holding in the band switch detent/shaft and pull out the shaft- STOP WORRYING! Look closely at the back end of the shaft- there’s a little raised bump on one side. That little raised “bump” will line-up to the notches in each of the switch rotors. The rotors are not going to fall out and don’t worry about realignment!
There are two wires connected to the bottom of the tuning cap on each stage, one on each side; simply unsolder these wires.
The front switch stage is held in by two screws; the middle and rear stages of the switch are retained by one screw at the base of each stage “plate”; remove the screw to the front stage, and lift the stage up slightly until you see the two solder joints to the tube socket; make a note as to where they are attached and unsolder these two wires. The front switch stage will now lift out completely.
The middle stage will usually have only one wire to disconnect; then you can lift the middle stage out.
The rear stage has two joints and then it will lift out.
You can now replace any resistor or capacitor. Total time to disassemble the RF stage down to switch stages is about four minutes on a slow day.
In each switch stage you can insert the band switch shaft and look at each individual wiper to ensure that it lifts and lowers in each position of rotation. If any wiper isn’t moving as it engages the rotor, gently bend it so that it does visibly move when engaged. If you don’t do this it’s likely that you will have intermittents or inoperative bands. There are an awful lot of wipers do don’t rush through the operation. (Note: the author uses a 1/8” diam. fiberglass rod, shaped on one end like a wedge; this forms a very coarse abrasive that is used to clean crud off of the rotors, just enough to expose the silver color; it does not remove the plating unless you go nuts in the process)
When reassembling, be sure that the wires going to the tuning cap will go through their proper holes (don’t forget to solder them either!). The only precaution is that you have set the rotors all to one position, preferably the BC position from where disassembly began. (if you didn’t preset the rotors, it’s no big thing- remember that the shaft has that “bump” and will only go in one way unless you are reckless and force it!). The shaft should slide back into position with a little jiggling of the individual stages.
These RF stages were designed for EASY SERVICING! Take advantage of it! A total disassembly, cap and resistor change will take less than an hour; most of that time must be spent on examining and repairing switch wipers.
Set the band switch to band one (BC). To remove the RF assembly, the dial must be removed. Remove the one screw in the center-rear of the assembly. Remove the screws holding the IF chassis (you can leave the rear side screws just loosened); pull the IF chassis out about ¾ inch and lift out the RF stage.
On the RF stage each side cover can be removed with three screws. Next remove the two screws holding in the band switch detent/shaft and pull out the shaft- STOP WORRYING! Look closely at the back end of the shaft- there’s a little raised bump on one side. That little raised “bump” will line-up to the notches in each of the switch rotors. The rotors are not going to fall out and don’t worry about realignment!
There are two wires connected to the bottom of the tuning cap on each stage, one on each side; simply unsolder these wires.
The front switch stage is held in by two screws; the middle and rear stages of the switch are retained by one screw at the base of each stage “plate”; remove the screw to the front stage, and lift the stage up slightly until you see the two solder joints to the tube socket; make a note as to where they are attached and unsolder these two wires. The front switch stage will now lift out completely.
The middle stage will usually have only one wire to disconnect; then you can lift the middle stage out.
The rear stage has two joints and then it will lift out.
You can now replace any resistor or capacitor. Total time to disassemble the RF stage down to switch stages is about four minutes on a slow day.
In each switch stage you can insert the band switch shaft and look at each individual wiper to ensure that it lifts and lowers in each position of rotation. If any wiper isn’t moving as it engages the rotor, gently bend it so that it does visibly move when engaged. If you don’t do this it’s likely that you will have intermittents or inoperative bands. There are an awful lot of wipers do don’t rush through the operation. (Note: the author uses a 1/8” diam. fiberglass rod, shaped on one end like a wedge; this forms a very coarse abrasive that is used to clean crud off of the rotors, just enough to expose the silver color; it does not remove the plating unless you go nuts in the process)
When reassembling, be sure that the wires going to the tuning cap will go through their proper holes (don’t forget to solder them either!). The only precaution is that you have set the rotors all to one position, preferably the BC position from where disassembly began. (if you didn’t preset the rotors, it’s no big thing- remember that the shaft has that “bump” and will only go in one way unless you are reckless and force it!). The shaft should slide back into position with a little jiggling of the individual stages.
These RF stages were designed for EASY SERVICING! Take advantage of it! A total disassembly, cap and resistor change will take less than an hour; most of that time must be spent on examining and repairing switch wipers.
Pete AI2V
