Just received a 51-932 yesterday and it has a code 122 chassis. The schematic I have calls out for a 35Z5 rectifier and 35L6 output. Apparently the code 122 has a 35Y4 rectifier and 50A5 output and is wired as such. There is still a 100 ohm resistor (R5) in the heater string wired into the chassis which I assume was to drop some of the line voltage with the 35L6 version. Being that the output is now a 50A5 is that resistor really still necessary? Thanks in advance!
Hi Tony
Lucky for you
I just happen to have a 1951 Philco Home Radio Yearbook handy.
According to it, the only change made in Code 122 receivers is changing the 35Z5 rectifier to a 35Y4. No mention of a 50A5, so I must assume that change was made by someone later on? What does the tube layout sticker show?
Since R5 is still present in your radio, that convinces me even more that the 50A5 was added by someone later on. Of course the socket would have been rewired for the 50A5, since its basing is different from the 35L6.
Hi Ron:
The tube layout sticker shows both the 35Z5 and 35L6 - perhaps the cabinets and labeling were done in advance of the chassis and not changed accordingly. If you don't show the 50A5 in your book, like you I will assume it was done later on. That being said, should I remove R5? The set appears to work with it in place although I haven't checked any of the heater voltages to see how they are impacted.
You have two choices:
1. Change the 50A5 back to the original 35L6, rewiring the socket as needed; or
2. Leave it as is.
If you leave things as they are, it might not be a bad idea to leave R5 in place, given today's higher line voltages. 100 ohms is probably a bit overkill in this case, but some calculations using Ohm's law would yield a more accurate resistance to use given, say, 127 volts to represent today's line voltage as opposed to the 110-115 of yesteryear.
Carefully take some voltage readings and see what kind of filament voltages you have with R5 in place. That will help determine where you stand. But be careful - bear in mind that is an AC/DC set, and you should make your measurements with the radio plugged into an isolation transformer.
I think I'll leave the output tube as is and not bother with the rewiring required and put a note on the bottom of the radio about the change. If the filament voltages aren't too far off I may leave R5 in place as well. One thing I noticed with this set also is that the pilot doesn't have the initial bright illumination, dimming and gradual brightening as some other typical transformerless AA5 sets I have. This one appears to be off for the most part until the tubes warm up and start conducting - going to take a closer look at the schematic of each later and see if I can figure out what's different with the Philco.
One other question I had and forgot to post earlier. On the print there is a connection from the voice coil to pin 2 on the antenna connector jack and doesn't seem to be carried through at the plug. Any idea what that is for?
A convenient test point for the service tech to tie his AC meter to the output for aligning. Seen on many post-war Philcos.
Chuck
Thanks Ron and Chuck - hopefully this is the last issue I need to ask about with this set. Just recapped it and it sounds very good. However I still want to peak up the alignment and am a bit unsure how to do so with these coils as I haven't run into this type yet. The alignment indicates two adjustment points on each IF transformer - the top view diagram indicates a solid line for TC3 and TC5 which I will assume is the adjustment point on the top of the transformers. TC2 and TC4 are shown as dashed lines. I thought that there may be adjustments on the bottom of the transformer as well but in looking at them I'm not so sure. Any help is appreciated - thanks!
Also wanted to pass on something I found that maybe some of the newer forum members might want to keep in mind in troubleshooting these sets and others as well. After the recap, the audio sounded a bit on the distorted side. Turned out to be one of the rubber grommets that isolates the tuning cap from the chassis. It was split and the mounting 'peg' was coming in contact with the bracket connecting it to the chassis. For now I spun the grommet 180 degrees so it's resting on what is left of the grommet.
Tony
TC2 and TC4 are accessible from underneath the chassis.