The AM Forum

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

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.

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

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

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!

Thanks.  I will try to track down a copy of that ER issue, and a large ceramic replacement.

You are welcome, I imagine if you shoot an email or PM Bill via this board he probably can send you the article.

Rodger

Roger,
         I actually have a printed copy of Bill's article somewhere around the shack.
I also did a few other simple mods to perk up the audio in both directions. If you fatten up the audio a little bit, they also will make quite good receive audio. I will have to look for my skizmatic when I get home tonight.

                                                             The Slab Bacon

Slab:

I'd be very interested in seeing the mods, too.

Thanks,

Frank, W2FAL

Slab,

I would really appreciate a copy of your notes.  I used to use the G-76 quite a bit with a Drake AC-3 power supply (modified with a relay to switch HV like the "real" Gonset supply).  The AC-3 is now serving with an original R-4/T-4X station and I have to finish wiring a homebrew supply I built for the Gonset and this would be a good time to go back through the G-76 and make any changes.

Rodger WQ9E

I also would appreciate a copy of your notes Slab.

Tnx & 73,

Jack
--WA9NQW@nconnect.net

I have to go and dig for them, Its been a while since i did mine. Once I find them, I'll post them here for everyone.
                                                                          The Slab Bacon

As a follow-up to my original post and for those that are interested, I found the cause of the arcing in the G76 and it was me, with an assist from a can of DeoxIT.

I thought the arcing was near the modulator tubes, but once the case was off, I could see where it was occurring - the  "Transmitter" switch - so I was obviously seeing a reflection.  Fortunately, I was able to remove the wafer from the switch without removing the switch from the front panel, etc.  That saved a lot of time.  What I learned from this is that one should not spray DeoxIT on switch contacts just because.  I figure the Deoxit loosened up gunk and washed it down to the bottom of the function switch - where the HV comes in from the power supply.  I cleaned the carbon off using some cleanser and a brush (Thanks K4HBI, with an assist from NJ2R and K9JR.).  Before and after pix are attached.  It makes about 80W into a dummy load on 80 and 20 - without snaps, crackles or pops.  Haven't checked any other bands yet.  Grid drive on 80 is low and I need to find out why, but it does make 80W there, so maybe I shouldn't fix if it ain't broke - yet.  I still need to go through the transmitter alignment.

While troubleshooting, the spot function died.  After I found the problem, - an almost completely open 10K, 1W resistor in the spot circuit - I realized that the spot signal was getting weaker and weaker over the past month or so, probably as the resistor was going up in value.  That's the second bad resistor.  The other was the 47K, 2W in the clamp circuit.  Also, in terms of bad components, I had a bad npo trimmer in the receiver dial end-to-end adjustment.

Hopefully, after the transmitter alignment, all I need is an antenna and a mic....

Frank, W2FAL

One ought never spray cleaner directly onto a switch... sometimes the switch absorbs the cleaner and dust, etc., and becomes somewhat conductive.  Or, it can catch fire (ask anyone who's tried to clean the bandswitch on a SX-42 with the power on  ;D)

The way to do it is to moisten a Q-tip with some de-oxit and gently scrub the wafer rotor contacts, then crank 'er around a few times to get the de-oxit distributed thru the switch contacts.  Less is more here.