About ready to throw in the towel while I'm Ahead - Valiant

[ka4koe]

Started tracing the audio from the microphone pins onward.

1. First problem, intermittent in the microphone jack.  Fixed that.

2. Good audio swings noted on modulation scale. Audio sounds distorted with hum on second receiver.

3. Waveform looks normal at output of 1st audio, good amplification.

4. Went to output of 2nd audio, pin 6. Waveform looks okay.

5. Loaded up transmitter initially with 7.5 mils grid, around 300 mils plate. Idle modulator current around 60 mils.

6. At this point large SNAP, blowing 6 ampere line fuse I had installed the other day.

Frankly, blowing the line fuse tells me something else is not right and I don't want to inadvertently do more harm at this point.

Pointers? Advice?

tnx

Philip

[ka4koe]

I've solid stated the 866s and replaced all electrolytics, fused the line cord and gone to a 3 conductor with grounding pin.

[w8fax]

Check out my thread http://amfone.net/Amforum/index.php?topic=33384.0 that goes thru my audio and arcing/sparking problems. I had a LOT of help from the forum too. Some of my sparking was not readily findable. So, I started bringing it up on a variac so I could possibly see what was happening. I found a couple of term strips that were arcing. One of my 866 sockets had been previously changed also, so apparently that had been a prob in the past. I also had audio problems and found one of the resistors in the plate lead to one of the 6146's had gone bad. Lots of stuff and often frustrating. But the info on the forum is priceless and I have been on the air quite  a bit both CW and AM....but as has been said....for now....good luck.

[KA2DZT]

Philip,

Is the 6amp fuse the correct value fuse for the Valiant??  Is it a slo-blow fuse??  I would try to load less, maybe only 200ma, and see if something still breaks down.

Fred 

[ka4koe]

Think I may have it figured out with the help of a friend. Here is what I think initially happened:

1. The audio jack was loose, with no audio applied initially when I first got transmitter.
2. Advanced audio gain trying to get deflection on modulator current.
3. Modulator/audio stage went in oscillation.
4. Modulator resting current noted to be low (about 45 mA). Advanced R61 to get around 60 mA.

After the FLASH/BANG, suggested I check the 6146's in the modulator. Sure enough, at least one of the 22 ohm resistors was burned clean in half. The excitement happened on a voice peak while I was tracing the audio from the input onward.

So, the initial oscillation may have damaged one of the 6146's, or both. This would account for

1. Low resting current.
2. Hum on transmitted signal.
3. Distorted audio.

Note that we were getting swings with modulation with audio, but had to advance gain control up to around 5-6.

So, I have a pair of 22 ohm resistors and a pair of 6146's that will need to be ordered.

I think it was originally spec'd at 8 amps, but not 100%. I recall a posting here where 6A was recommended for fusing.

Opinions?

Philip

[ka4koe]

New modulator tubes ordered and plate resistors bought from Rat Shack. Tubes arrived today. Installed new modulator resistors and heat shrinked the connections.

Also noted that none of the HV wires coming out of transformer are insulated with either a grommet or heat shrink where they pass thru the hole in the chassis. Gonna fix this as well.

Philip