Philco Model 81B - Printable Version
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Philco Model 81B - texan george - 09-25-2022
Starting on a new project, model 81 that has numerous problems so this will take some time. The on/off vol switch was replaced with a high resistance vol control and the cap block item 15 was removed. I put a notice in the wanted section for these parts and sent a email to oldradioparts for help. Meanwhile the output transformer was replaced years ago and will probably work but I would like a newer version, maybe a P-T125CSE if that would handle the load. On a good note, the cabinet is in good shape. I am going thru the coils and starting to replace the caps. I am sure I will have some questions for yall as I get into this.
Thanks,
George
RE: Philco Model 81B - Radioroslyn - 09-25-2022
For the price of a replacement opt I'd dress up what you have providing it's working. The volume/off-on control is an odd value at 20,000 ohms.
https://philcoradio.com/library/download/service%20info/service%20bulletins/Philco%20Service%20Bulletin%20154.pdf
RE: Philco Model 81B - David - 09-25-2022
When looking at old sets amazing what you find! The Silvertone tube (Sears) all that I have found have a date on the sticker some a phone number for servicing the set.
RE: Philco Model 81B - texan george - 09-25-2022
Thanks Terry, I will see how it does when I get to that point. It seems to check out ohm wise.
David, sticker doesn't have any legible writing on it.
Take care,
George
RE: Philco Model 81B - texan george - 09-30-2022
Don't know what I did but things are not right. I replaced all the caps and resistors. A lot of the old ones were hanging tied together and I had a hard time trying to get back according to the schematic. I have a box I made with an AC voltmeter and 1 amp meter I hook to my variac for power checks. When I applied power with no tubes installed, after less that 10 volts I hit the 1-amp mark on the ammeter, so I shut it down. Need to find where the short is without burning something up. I think I am at the point where I can't see the forest from the trees. Any ideas to get me back on track?
Thanks,
George
RE: Philco Model 81B - Radioroslyn - 09-30-2022
With the chassis unplugged measure the resistance across the ac plug. Should see about 10 1/2 ohms with the switch on. If much lower cord could be shorted or the pt. Does the pt look like it's been over heated?
RE: Philco Model 81B - texan george - 10-01-2022
Terry,
I replaced the power cord before I applied power. With the switch on I measure 9.1 ohms on the power cord, 242 ohms between pins 8 and 10 for the 80 plate, 144 ohms between one of those and the center wire no. 9 and 349 ohms between the other one to pin 9. The filament wires 6 and 7 measure .8 ohms. I did notice that some of the pt wires were brittle, and I put heat shrink sleeving on them. Would pulling the pt and replacing wires I can be worth the effort or is the pt shorting out?
I appreciate your response and the help you have given me.
George
RE: Philco Model 81B - MrFixr55 - 10-01-2022
If there are RF suppression caps across the transformer or to ground, take them out of the circuit. A really leaky or shorting cap can do this. Alternately, connect directly to the power transformer primary with everything else out of the circuit (power cord, switch, rectifier and all other tubes removed.
This was a somewhat slightly more "deluxe" version of the 4 tube Model 80 loss leader, with 77 pentodes for 1st and 2nd detectors instead of the 36 tetrodes used by the original 80. This radio also received "police" band.
20K Ohm is likely correct, or maybe it should actually be lower! This is not the usual 500K "pot" (potentiometer) between the 1st and 2nd audio stages. The pot is essentially between Antenna and Ground and varies the strength of the signal fed to the input transformer. This is common on many early sets without AVC and that employed grid leak detectors or biased "power" detectors, including the Radiola 16, 17 and 18. Other sets like the Radiola 60 used the pot to either bias the RF and IF stage. Other sets of this vintage varied the antenna signal and varied the RF and IF bias.
This radio employs 6V tubes so any transformer from a 5 tube set that you can pull out of the junkbox with 6V and 5V windings along with the HV windings could work. I say 5 tube (if not an early Philco) because a 42 or 6F6 draws about twice the heater current that a 41, 6K6 or 6V6 draws. Of course, mounting it could be fun.
RE: Philco Model 81B - Radioroslyn - 10-02-2022
The resistance measurements are confusing as your end to end total is lower than the midpoint to one end.[Image: https://philcoradio.com/phorum/images/smilies/icon_eek.gif] But I'll put a pin in it for now. The rest seem ok. Is your variac fused? If so I'd give it a try and ignore the meter. But I live dangerously. It could be that your meter is missing it's shunt. That is a resistor in parallel with it to control the amount of current that flows through it. Without it the meter will read high.
RE: Philco Model 81B - texan george - 10-02-2022
Thanks Terry,
The measurements are confusing to me as well. I will take them again. Yes, my variac is fused with a 5 amp, so I can just plug the radio into it and see what happens.
I will let you know what happens.
RE: Philco Model 81B - falcon123 - 10-02-2022
dos your multi-meter have an ac amp function of 5 or 10 amps? my fluke does. if so, use the meter in series with your variac
RE: Philco Model 81B - texan george - 10-02-2022
Measured the plate pins again and the resistance went down to 100 ohms across and 135 and 160 to the center. I went ahead and powered it up on the variac and about 35 volts the tuning lamp came on. Took it up gradually to 110 volts. The filiment voltage for the tubes got up to 2.9 volts. Then it smelled like something burning and the pt got pretty hot and was humming. Shut it down. I believe that the pt is bad.
RE: Philco Model 81B - Radioroslyn - 10-02-2022
>I believe that the pt is bad.
I'm not there yet.
Do you see a wire that come out of the pt and connects to the chassis? This should be the ct for the 6.3v heater winding. Unground it. It could be that you have a short that 1/2 of the heater winding is shorted to the chassis. Perhaps the dial lamp socket or one of the 6v tube sockets. This would give you the 2.9v or so at the lamp as you are seeing just 1/2 the heater voltage. Check for a low resistance to chassis gnd after you disconnect the gnd wire from the pt.
Using an ohm meter to detect these small amounts of resistance differences maybe difficult (a 1/2 to a 1/4 of an ohm). So use a small percentage of the ac volt applied to the primary may work just as good. Say 20% of 120 is 24vac to the primary should yield about 1.2v across the 6v winding.
RE: Philco Model 81B - texan george - 10-02-2022
I disconnected the center tap for the 6.3 heater voltage, set variac for 24volts and measured .8 vac across all the heaters. Not sure what that tells me.
George
RE: Philco Model 81B - Radioroslyn - 10-02-2022
Forgot to move the decimal point it's 1.2v. It's kinda low. Ok you won me over I think the pt has a short.
Pm sent