[Lawrence Austin]

https://philcoradio.com/library/download...8-1949.pdf

Hello, I have been working on my Philco 48-1266. I am NO expert but have brought my other radios back to life. Four consoles: 36 and 37 Philco, 38 Zenith and 49 Westinghouse. Three tabletops: 2- 48 Westinghouse and a 48 Philco. Even wound coils for the first time on the 36 Philco.

First the audio section is working great! The push pull 6V6 will blow you out of the room playing a record. I can get the 3 local stations at a very low volume. The volume seems to be even lower in the lower freq. like 570kHz. Tuning it on the dial or the pushbuttons sound quality is exactly the same. Trying to align the IF starting on the audio side first 2 trimmers work fine but the third and fourth don't do anything. The other AM trimmers all work.

All electrolytics and paper caps were replaced. Out of spec resistors replaced. All coils checked for resistance. Tubes changed and tested. Voltages all seem good. Should I start testing the small caps out of circuit? My main worry is having to open the IF can with the 2 non-working trimmers. Looks like a headache to get the 1/4" nuts under the band selector. I did check the fixed caps in the other 2 IF cans. Any help narrowing down what to look at would be greatly appreciated.

[RodB]

Hello Lawrence,

I look a quick look at the schematic for your radio and the first thing that popped into my mind is the AM/FM switch. Section 4, contacts 7, 8 and 9, of the wafer switch selects either AM or FM by shorting out the unused secondary. You're going to tell me that you have cleaned that wafer switch, but give it another go with the cleaner, preferably Deoxit D5, and a brush. We've had people do the cleaning 4 or 5 times before getting results.

[Lawrence Austin]

Thank you RodB for the suggestion,

I got out the toothbrush, cleaner and gave it another go without change.
Placed the selector in the AM position and used wire to short the blue to green and separately the brown to yellow of that IF like the switch does. Still no change.

[MrFixr55]

Hi Lawrence,

Does FM work?

Check cathode voltage on the 2nd IF / Detector / AVC tube.  It should be very low. If high, check the push buttons.  Alternately, ohm between the cathode of the 2nd IF and ground. It should be about 150 ohm.  If the switch is dirty, the resistance will be much higher.  The "PB1" (Radio / Phono switch?) does several things, including opening the cathode of the 2nd IF / Detector through a 150 Ohm resistor from ground to disable the radio when the phono is used.  Clean the heck out of PB1. 

One terminal of each of the secondaries of the various IF transformers connect to the grid of the next stage (except for the last one which connects to the plate of the detector diode.  The other terminal does not connect to ground; it connects to the AVC Line.  The purpose if the AVC Line is to vary the negative bias on the grid returns of each of these transformers.  The more negative the voltage, the less the gain. AVC generally varies between 0V and about -10V or more, depending on the strength of the signal.  Shorting AVC to ground may bring the set alive (or saturate it).  Alternately, a power supply can be used to force the variation of AVC voltage.  You may be able to measure the AVC voltage across capacitor C305.

One strange thing is that it appears that the 2nd IF stage is not under AVC, but there is a 2.2Megohm resistor between the grid return of the secondary of that transformer and ground.  that does nt make sense to me.  It could be an issue with the drawing.

Hope this helps.

[Lawrence Austin]

Thank you MrFixer55,

Yes, it was PB-1 was not connecting the radio. I see why they had to kill the radio section when phono is selected. The bleed through of the radio without it being connected by the switch is significant. I thought I checked it but maybe I didn't. Thank you for pushing me in the right direction!

Still need more cleaning of the selector switch. Also, I need to look at the phono cartridge match. The one I used is listed for use in a 78 RPM Rock-Ola jukebox. I thought the phono volume was good but the working radio has about 2-3X the volume. I think the fidelity when playing a record gets cut too soon since the volume control must be set much higher than the radio. Might try increasing the 1M resistor across the cartridge.

Video of set: FM then AM then phono
https://youtu.be/k5JrEgbnVio

[RodB]

Lawrence,

The 78 rpm record player my grandson restored on his Zenith produced better quality when the needle was extended further out of the holder. Maybe if yours is adjustable you can try extending it out a little.

[MrFixr55]

Hi Lawrence,

Is the cartridge original? Do you have a pic of the cartridge. If this is a 78 RPM Only changer, there may be a limit to what you want to do. Generally, the cartridge has to put out about 1V for this amplifier (a rather good amp BTW, with 1 stage of amplification, a cathodyne phase inverter, 2 6V6s (Beam Power baby brother of the 6L6). Generally, these phonos used a crystal cartridge which is not a HiFi device to say the least. If you replaced the cartridge with a ceramic cartridge, the fidelity would be somewhat better but the output voltage is a lot lower (about 0.1 -0.5V). It may be nice to add a preamp and a GE RPX or similar monaural magnetic cartridge. It is somewhat period correct (I believe that the RPX came out in the late 1940s or early 1950s and the RPX II came out around 1956 or so.) I said add a preamp. There needs to be at least 2 tube stages for the RPX. Alternatively, a solid state preamp, the type sold often in the 1970s would work. This would definitely boost the fidelity. There are some modern magnetic cartridges that may work here, that were made for "Scratch DJs" and "Turntablists". However, you would need a needle meant for 78RPM records and the higher tracking force encountered with turntables of this vintage. The advantage here is that with a stereo cartridge, paralleling the outputs in phase allows playing lateral groove (Victor and Columbia and most other brands) records while paralleling out of phase allows you to play vertical grove (Edison) records.

You would be surprised how good a 78 RPM record in good condition sounds with a good cartridge and amplifier.

[Lawrence Austin]

Hello,

Thought I would report back. The cartridge I am using is mono only Astatic P-51-3. It has a 3mil stylus and has a very rigid pickup to handle the weight of the tonearm. Tried both a solid state and tube phono preamp. Both were over driven by the cartridge itself. Disconnected the 1M resistor across the cartridge while listening to the phono and the bass response, volume increased. The original cartridge had an even lower volume output but as with everything else I remove is stored inside the cabinet for any future owner to have. Repositioning the cartridge made a great improvement also. The back of the cartridge had to be lowered closer to the record and placed 1/4" spacers between the cartridge and tonearm. The extreme downward slope of the tonearm had the needle digging backwards in the groove. On the first video it creates a swish-swish sound that is gone now. Intended to use the same record but UT keeps blocking it so had to use a lesser-known record for demonstration.  Even though the phono is still quieter than the radio fidelity is good now!

Phono

[MrFixr55]

Hi Lawrence,
I'm glad that you are making progress. Unfortunately, I can't find any documentation on this cartridge other than it's used in various Seeburg, AMI Rowe and Rock-ola jukeboxes. Make sure that you are using a 3mm stylus (for 78RPM). I may be wrong but I believe that the stylus should either be vertical or pointing slightly toward the front of the tonearm. This is called the vertical tracking angle, and does change with the amount of records on the turntable. Therefore, you can experiment by stacking records to determine if the sound improves or gets worse. Another measurement is azimuth. This is the left-right tilt of the cartridge (and therefore the stylus) staring at the front of the cartridge This angle should be perpendicular to the record and doesn't change with the amount of records on the turntable. The last measurement is overhang. This is how forward or behind the spindle the cartridge is. Since the tonearm moves in an arc, this measurement is a compromise. Draw an imaginary straight line radiating from the spindle, extending to the end of the record with the stylus laying on that line. The cartridge should be "square" with that line, meaning that a line passing through the stylus and perpendicular to a theoretical fore and aft line passing through the center of the cartridge should lie on that imaginary line. Think of your car driving down a straight road. Your car should be straight with the rear wheels perfectly in line with the front wheels, if this makes sense. Bottom line is that cartridge alignment is very important and can vary from model to model depending on relation of the mounting holes to the stylus, height from the mounting face of the cartridge to the stylus, etc.

Finally, there is tracking force. It should be under 10 grams.

To determine the output voltage of this cartridge, I looked at 2 schematics for the jukeboxes that this cartridge is speced for.

One model that uses the cartridge is the Rock-ola Model 1422. The amplifier uses one stage of audio voltage amplification using a 6J5 triode, feeding 2 6L6s through an interstage transformer. This setup would need a cartridge with high output.

However, another model, the Rowe Models A, B, C or D which use this same cartridge. The amplifier for Model A or B uses a 6SJ7 First AF amp stage and 1/2 of a 6SL7 as a second stage amp. The other half of the 6SL7 is used as a phase inverter, getting its signal from a voltage divider in the plate circuit of 2nd AF amp. This circuit drives a pair of 6L6s. Since both the 6SJ7 and 6SL7 are rather high mu tubes, this circuit should be rather sensitive and should have a much higher gain than the Rock-ola 1422.

If you were using a preamp designed for a magnetic cartridge, the Astatic P-51-3 would certainly overload it. If you have a preamp with both magnetic and ceramic inputs, try that. Otherwise, if you have a turntable with a magnetic cartridge, and a preamp for a magnetic cartridge, pipe that into the phono input using a Y connector to join the 2 channels (assuming that this is a stereo turntable) and compare the sound you get with the sound from the Philco turntable.

Hope this helps.