42-PT-93 Please Help - Printable Version
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RE: 42-PT-93 Please Help - dseverson - 05-06-2015
Hi, Nick.
You're right. I realized my mistake when I was daydreaming about this radio today. I'm hoping to find an original or suitable replacement speaker setup so I put an ad in the want ads.
RE: 42-PT-93 Please Help - Radioroslyn - 05-06-2015
Actually what you have will work fine. Just need to add a resistor from the plus side of 26A to the plus side of 26B. If it hums alot increase the value of 26A and B.
Simple fix. Osc coil is probably OK. That was a red haring.
Terry
RE: 42-PT-93 Please Help - dseverson - 05-07-2015
Just need to order some more caps then I'll let you know how it turns out.
RE: 42-PT-93 Please Help - dseverson - 05-12-2015
Well, I put in the 47uf caps and a 1000ohm resistor between the positive leads. I hear a hum through the speaker kind of like a fluorescent light. When I touch the center tab of the volume control, I hear one station faintly but it stays the same up and down the dial. Should I put in higher rated caps or is something else wrong?
RE: 42-PT-93 Please Help - Radioroslyn - 05-12-2015
Guess what it's something else. So sounds like the audio, IF and detector stages are working. Are all of the tubes good and in the proper sockets?? Did you adj any of the trimmer caps?? Are the leads on the osc coil connected?
Terry
RE: 42-PT-93 Please Help - codefox1 - 05-12-2015
Schematic here: http://www.nostalgiaair.org/PagesByModel/176/M0014176.pdf
Agree, check above suggestion, problem is most likely "left" of the IF. Make sure Loop antenna and antenna coils are connected properly and shows continuity. Voltage checks should be done as well. See if connecting a length of wire to external antenna changes anything, and if you have a known good one, swap out the 7A8, as they can be difficult. Many now replace the carbon resistors whilst doing a recap job.
RE: 42-PT-93 Please Help - dseverson - 05-12-2015
Well, I put in the 47uf caps and a 1000ohm resistor between the positive leads. I hear a hum through the speaker kind of like a fluorescent light. When I touch the center tab of the volume control, I hear one station faintly but it stays the same up and down the dial. Should I put in higher rated caps or is something else wrong?
RE: 42-PT-93 Please Help - Radioroslyn - 05-12-2015
You guessed it something else! Check all tubes good and in the proper sockets? No solder bridges across connections or gnd?
Measure voltage with - probe connect to off/on sw. 7B7 pin 2 and pin3. 7A8 pin3, pin5, and pin 2.
Report back.
Terry
RE: 42-PT-93 Please Help - dseverson - 05-12-2015
OK, dumb question. Which tab on the on/off switch do I connect the negative probe to and does the radio need to be plugged in?
Tubes are in the correct sockets and I can't see any solder bridges. The antenna coil has no continuity so I will rewind it. I noticed someone fixed a couple breaks in the wire so I have to assume there are others. I haven't touched any of the trimmer caps. The leads to the osc coil are connected.
I'll get back later with the voltage readings once I'm sure how to do it.
RE: 42-PT-93 Please Help - Radioroslyn - 05-12-2015
OK, dumb question. Which tab on the on/off switch do I connect the negative probe to and does the radio need to be plugged in?
Well if you think about it if you are going to be measuring voltages the off/ on sw would have to be on. And it would need to be plugged in. If the sw is on it doesn't matter which side of the sw you are connected to, it's shorted.
Every once in a while I get an easy one.
Terry
RE: 42-PT-93 Please Help - dseverson - 05-12-2015
Sometimes it takes a while for the gears in my head to engage. I'll measure the voltages tonight.
RE: 42-PT-93 Please Help - dseverson - 05-13-2015
With the DMM set to 500v, 7B7 shows 4v on pin 1 and 163v on pin 2. 7A8 shows 162v on pin 3, 85v on pin 5, and 161v on pin 2.
RE: 42-PT-93 Please Help - Radioroslyn - 05-13-2015
Ok hows about pin 3 of the 7B7 and pin 4 of the 7A8. - attached to off/on sw. Other voltages look ok .
Pin 3 should have +150 or so pin 4 should have - 3 or -4 volts.
Terry
RE: 42-PT-93 Please Help - dseverson - 05-13-2015
Pin 3 on 7B7 shows only 90v. Pin 4 on 7A8 shows -3v.
RE: 42-PT-93 Please Help - Radioroslyn - 05-13-2015
Well the problem is your set works we just don't know it. What did you end up using for the antenna coil??
Terry