[KCMike]

  An excellent restoration.

[davemc]

Well, I finished the alignment of all ten bands. There were clear peaks on all bands except for the 19 meter band spread band. I was supposed to start with the trimmer at maximum tightness and then back off to the SECOND peak.  I only found one peak, although there was reception on the band.
 
I was quite disappointed with the performance of the radio.  One would think that a radio with a tuned RF amplifier stage and two IF amplifier stages would be quite sensitive. But it did not appear to be that sensitive. My antenna is only a 40 foot length of insulated wire strung across the basement ceiling (and I am in a rural area). Adding a ground connection did not help. I rechecked all the tubes, and all of them were good. I checked the voltages, and all seemed reasonable, but the available documentation does not have the voltages marked. The AVC voltage varied from -2.7 volts (with no antenna) to a maximum of -6.5 volts.  That seems low to me, but there are no high power stations nearby. I plan on checking this voltage again with a strong signal from the signal generator. Some of the voltages I measured were:
 
B+ 233, B- 89
RF amplifier Plate +213, Screen +144, Suppressor -6.65
First Detector Plate +233, Screen +87, Grid -3.37
Oscillator Plate +131, Oscillator Grid -15
First IF amplifier Plate +234, Screen +87, Grid -2.8, Cathode +4.41 (with R301 maximum CW)
Second IF amplifier Plate +233, Screen +87, Control Grid and Suppressor -3.8
First Audio Amplifier Plate 68, Grid -3.45
Audio Outputs (6V6) Plates 223/216, Grids -11, Screens +233
 
All capacitors were changed/restuffed except for the mica capacitors. All resistors are in tolerance except for the 47K filter resistor inside the third IF transformer R302A (it measured 26% high) and R403 68 ohms (measured 23% high). I did not want to risk tearing into the IF transformer to change R302A. R403 was hidden under the band switch and would have been very dangerous to try and change – lots of hair-thin wires around.
 
Adjustment of the variable resistor on the back of the chassis (R301) does not seem to change the performance. This resistor varies the cathode bias of the first IF amplifier over a narrow range (4.36 to 8 volts). There is no mention of this resistor or how to adjust it in the available documentation.
 
If anyone has any troubleshooting suggestions, I would really like to hear them. I plan on checking the AVC voltage with a strong signal from my signal generator.  Since the background noise is high, I also plan on shutting down all computers and monitors and listening at night vs. in the middle of the day. There are some mica capacitors in the RF signal chain that I did not check.  I suppose I should check or change them.

[davemc]

I checked the four mica capacitors that are in the RF signal path (C400, C407, C409, and C415).  All were good - values were close to spec and no leakage indicated (up to 40 megs - the limit of my DVM). I then connected up the signal generator and monitored the AVC voltage.  On the broadcast band with no signal, AVC was -2.7 volts.  Set for a loud output, I measured -8.7 volts.  The AVC then seemed to gradually increase to -10.2 volts at the maximum output of my generator.

There seems to be a quite high level of "hiss" which generally goes away on stronger local stations, but it is still there on weaker stations. I am not quite sure what to do next. Although all tubes were tested, should I try substituting tubes in the RF and IF section at this point? Another concern is the screen voltage for the converter and IF amplifiers.  I measure about 87 volts.  The tube manual for a type 78 (first IF amp) says the screen voltage should be 100 volts with 250 volts on the plate. All of the resistors that are part of the circuit that supplies the screen voltage are in tolerance. But my power transformer is not original.  I may play around with the screen voltage and see if that makes any difference.

I pulled out my restored Philco 37-670 and compared it side by side with the 46-888, using the SAME antenna.  The performance, including the hiss level, was comparable.  However, the 37-670 only has ONE IF amplifier stage.  I would have thought that the 46-888 would blow it away, but they appear comparable (subjective).  

[davemc]

In order to eliminate the issue of the RF/IF screen voltage being lower than called for in the tube manual, I used a resistance substitution box in parallel with one of the resistors supplying screen voltage to the RF amp, converter, and IF amps. I raised the voltage to 100 volts (from 87). There was a noticeable increase in volume. The circuit in my radio, while original (based on solder joints and components used), does not match the published schematic. But there must have been a reason Philco changed the circuit. I left my radio as it was (only replacing out of tolerance resistors). There are several other places where my radio does not match the published schematic.


My other concern was the low AVC voltage, which varied from -2.7 to -6.5 volts depending on signal strength. Somewhere I recently read that AVC voltage usually max out at -20 volts in radios using variable MU tubes in the RF and IF sections. In radios using sharp cutoff tubes, the AVC is usually limited to -5 volts, else the controlled tubes may be cut off. The Philco 46-888 uses 7C7 tubes for the RF and one IF amplifier stage. The second IF amplifier stage is a 78, which is variable MU. I'm not sure of the characteristics of the 7J7 heptode section. And one end of the AVC line is connected to a negative voltage supply, which would tend to limit the range of the AVC. So bottom line is that the AVC may be working correctly.

So I am wrapping up this restoration.  The radio is back in its cabinet and works great! I am just disappointed with reception performance, especially on short wave. But I have only listened during the day, and with a crappy antenna.

[462ron]

Davemc, I too have the same problem with a 37-116. I have gone all through it, replacing caps and out of tolerance resistors, changed tubes, aligned it a few times and double checked all coils in the front end. It plays good but I am disappointed in its reception. I have a 41-250 that will run rings around the  37. The 41-250 has 2 IF stages but no RF but yet out performs the 37-116 that has an RF and 2 IF. One difference is the 37 is a long wire antenna and the 41 uses a built in loop. I plan on restoring my 620 next  to compare it's reception using a long wire antenna.

Ron

[Ron Ramirez]

It seems strange to me that your radio is performing less than optimally, Dave.

I restored my 42-788 which is very similar, and it seems to play very well indeed. As I recall, its circuitry matched the schematic. I had no problems with it once I overcame the miles and miles of rubber-covered wiring and replaced all paper and electrolytic capacitors and many of its resistors as well. I understand that you did the same with your set's caps...so I have nothing more to offer at this point...