Gonset G76 Questions

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W2FAL:
I recently picked up a G76 and when I finally got around to trying out the transmitter on high power, I found that someone had let the smoke out of a 47K resistor in the screen clamp circuit. I was tuning up into a Bird watt meter and dummy load and everything was going just fine on low power.  When I went to high power, there was arcing in the area of the clamp and screen control tubes.  That got my attention, so I opened it up and that's when I found the cracked resistor.

I'd like to replace the resistor, but I have conflicting documentation as to what I should replace it with.  The one I removed, which is not the original, looks like it was maybe a 5W resistor.  It is about 1 3/4 inches long and about a 1/4 inch in diameter.  One version of the schematic doesn't even have the OB2 screen control or 12AQ5 clamper tube on it.  Another does have the tubes, but with a single 47k, 2W resistor connected to the plate of the 12AQ5.  Then I have yet another version that has two 47K, 2W resistors in parallel.  (Also, I have one block diagram showing no clamper/screen tubes and one that does. The one that does has the tubes labeled as V18/V19, but the schematics have them labeled as V17/V18.)

Does anyone know the history behind these design changes?  Has anyone run into this arcing problem?  Do I use a single 47k or two in parallel?  Does it need to be a higher wattage?  Anyone have any ideas as to what the real problem is, as I don't believe that this bad resistor is the sole cause of the arcing, but I don't really know for sure.  BTW, the resistor that I removed measured around 50K.

Thanks for any help.

Frank, W2FAL

N3DRB The Derb:
gonset was famous for doing stuff like this. Frank AHE has a G76, tap him when you get a chance. I've never worked on one.

The Slab Bacon:
that resistor looks like 470K (yellow, purple, yellow)?? Check your skizmatic to be sure of its value. Replace it with any 2 watt resistor and you'll be good to go. The smaller "metalized film" ones are nice because of their smaller size.

tont froget to find out why that resistor smoked out. This is usually caused by something else breaking down.

                                                         The Slab Bacon

WQ9E:
Frank,

I was looking through old ER's last night for some info on crystal grinding and the issue I pulled also has the article "The Gonset G-76-Revisited" by Bill KD0HG.  According to Bill the correct value is to use is a 50K 10 to 12 watt resistor, the original 47K 2 watt has too low a power rating and the parallel 47K units used in some was the wrong resistance rating.  Bill will probably see this thread later and add additional comments.

My G-76 also came with the crispy critter 47K and I replaced it with a large ceramic resistor (same resistance rating) several years ago.

You might want to take a look at Bill's article, it is in Electric Radio March 1994, starts on P. 20.

73, Rodger WQ9E

W2FAL:
Quote from: The Slab Bacon on February 15, 2008, 07:08:48 AM

that resistor looks like 470K (yellow, purple, yellow)?? Check your skizmatic to be sure of its value. Replace it with any 2 watt resistor and you'll be good to go. The smaller "metalized film" ones are nice because of their smaller size.

tont froget to find out why that resistor smoked out. This is usually caused by something else breaking down.

                                                         The Slab Bacon


The lighting wasn't the best, so the colors didn't reproduce accurately.  It is a 47K.  I guess I'll pick up a couple of them and troubleshoot by explosion.

Thanks!

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