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48-1262 low voltage - Printable Version

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48-1262 low voltage - ke5yum - 04-04-2013

Good morning, all

Well, the bads micas don't appear to be an issue, but I do have low voltage. The values appear to be about half of what they should be.

All paper/wax and electrolytics have been replaced with correct values. voltage is 120 ac with a variac. Tubes all tested good when I tested them with a tube tester.

Using the section 1 trouble shooting guide from the service manual, I get the following readings:

Test point A 82.7 (167)
Test point C 110.0 (214)
Test point D 97.0 (181)

Oscillator:

Pin 3 51.8 (110)
Pin 6 52.7 (115)
Pin 7 2.9 (4.2)

R102 reads 166K (120K); it's a leakage resistor.

After the recap last week. I powered it up and did hear a faint signal from the local station.

I am thinking I now have a bad 50X6 tube. Any thoughts?

Terry
KE5YUM

RE: 48-1262 low voltage - BrendaAnnD - 04-04-2013

Looks like you're only getting half of your voltage doubler. Recheck your connections in the power supply... voltage doublers can be tricky if you're not used to them. Pay special attention to C101 and 102, which, along with the 50X6 form the doubler circuit. Check the polarity of the caps, then check it again. Note the negative end of C101 connects to the positive end of C102.

RE: 48-1262 low voltage - ke5yum - 04-04-2013

Thanks, I will check those tonight. I did consider that I wired something backward, but since I didn't smell any smoke or hear popping, I assumed that my replacement was ok.

RE: 48-1262 low voltage - morzh - 04-04-2013

Icon_smile I assembled my first Greinacher (Villard) doubler when I was...13 I think. At the time I did not know anything about Greinacher, Villard or Cocroft and Walton, for that matter. But the doubler worked.

RE: 48-1262 low voltage - ke5yum - 04-05-2013

I checked my attachments as BrendaAnnD suggested. C101 and C102 appear correctly orientated to me, and all the connections have continuity. I am having a electrical engineer friend look it over next week to make sure since I'll be out of town. If the wiring is correct, what do I check next?

RE: 48-1262 low voltage - ke5yum - 04-05-2013

A thought occured to me (since I am new at this) that my readings may be off due to my voltmeter. I am using a Fluke multimeter; do I need to be using a VTVM?

RE: 48-1262 low voltage - BrendaAnnD - 04-05-2013

If anything, your Fluke should read a bit high compared to the listed voltages on your schematic. It could still be a weak or bad rectifier. You can check this by installing a pair of solid state diodes across the two sections of the tube (white band to cathode).

RE: 48-1262 low voltage - ke5yum - 04-05-2013

I'll need some help with this one. If I understand, you want me to bypass the cathodes to rule out a bad tube.

From the schematic, it looks like pin 7 goes to R100 and pin 2 goes to ?????.

Also, I am not sure what "white band" means.

I am looking forward to learning on this one.

Terry

RE: 48-1262 low voltage - ke5yum - 04-05-2013

disregard previous post. I have the info that I need and have figured out what to do. Terry