Bad Audio Driver Xfmr or...?

[Steve - WB3HUZ]

Nice troll.

[Todd, KA1KAQ]

Blasphemy!  You should have seen the hue and cry when I showed how to fix the endpoint adjust on a 51J PTO by removing a turn...

Ha! That's what you get for dealing with self-described 'collectors'.   But you'll be pleased to know John, that removing a turn is now considered 'acceptable' and even necessary in cases of stubborn PTOs. You were ahead of your time, you trend setter!

I just had to let Fred know about my conversation with Skip about the replacement driver transformer, via email. Skip said:

"The iron I sent may create one small issue--the lug pattern is round. I am not sure whether the cutout in the audio deck is large enough-There are 2 options--use a small 1" diameter sanding drum and add a slight radius to the square cutout. If you do this I won't call the Collins police. The other method would be to raise the critter on short standoffs.   None of this may be necessary, but I am thinking ahead."

Skip's main concern is helping me get my transmitter back on the air. That's the true radio spirit!

I'm with Derb and others as far as drilling and blasting goes, but like John, I received similar tongue-lashings years ago when I mentioned the mods I did to the audio chain of the Oh-my-gawd-RARE KW-1. Nothing that couldn't be reversed, mind you, but the fact that I had the gall to even disturb an original Collins solder joint was enough to send a few over the edge. While I appreciate the historical significance, I also understand this: it's significant to few, and it's only a radio if you use it as such. Otherwise it's a paperweight (or doorstop, in this case).

It never made sense to me to try to make a 1950s radio into a 1990s radio by adding features, changing the basic character and so on. Easier and more fun to get something else. But I've never had a problem with making simple changes to improve performance of what it does do, so it can do it better. Add in the possibility of losing a hard-to-replace piece of iron or such due to a crapped out "original" cap, and it suddenly becomes crystal clear.

In the end, it all depends on whether or not you described yourself as a user or collector. Collectors quite often don't use their gear, preferring instead to display trophies. In the old radio world, particularly the amateur radio side, it's a pretty small audience.

[WA1GFZ]

Todd,
The transformer may still be good. Consider mounting a new one on a plate then put stand offs on the plate to match the hole pattern of the rig. You never know when you may want to sell it to a sucker with deep pockets who must have it unmolested.
If the transformer is bad I would rewind it then reinstall it later for the same reason as above.
I looked at the schematic for the first time. I think it is a pretty good deisgn. How many rigs have double tuned stages.

[Todd, KA1KAQ]

Todd,
The transformer may still be good. Consider mounting a new one on a plate then put stand offs on the plate to match the hole pattern of the rig. You never know when you may want to sell it to a sucker with deep pockets who must have it unmolested.

Nah, I've had several offers over the years, as recently as last year from a newcomer to this list who has since bought 2 for his 'collection'. Before these repairs I had about $50 into it or less. I plan to use it until it can't be used anymore, then be buried in it.   I'm keeping my fingers crossed that the transformer is still good, but will have the spare from Skip ready to go just in case.

If the transformer is bad I would rewind it then reinstall it later for the same reason as above.
I looked at the schematic for the first time. I think it is a pretty good deisgn. How many rigs have double tuned stages.

They get a really bad rap from the fools who choose to worship the name, but it's the best built bandswitching kw-class rig of the era and then some. Stable, full coverage including 160, built in filtering and so on. Top quality components throughout. A few simple mods to the audio and it sounds pretty decent. Today you could certainly build something as good or better for less money. Not the case in 1953.

[w3jn]

Ya gots the right attitude fer shore, Todd 

That xmitter's got it all - buzzardly looks, plenty a wrinkle paint, superb design (splatter choke issue aside), good power, easy to work on.  Plus, the price was right and you have the bonus of dick envy... kinda like showing up to a car show in a super rare '68 Dart Hemi.  What more could a guy want?

I'm starting to bond the same way with my '750 in the same way.  Although the newness hasn't worn off I don't see myself ever getting rid of it.

[WA1GFZ]

When you own a transmitter like either one of those there is no real need to have anything else....unless you want a class e rig for the hot WX.
Tube wise rigs you own the best so no need search to for something better.
I saw my first GPT when I lived in LA. A guy had the whole station set up in his back yard. a beautiful machine.

[k4kyv]

But I still wouldn't trade my homebrew HF-300 rig for a KW-1.

The later KW-1 serial #'s sound pretty good when the speech amp stage is converted back to the original design.  I helped Jay, W5JAY re-convert his.  Sometime after Collins released the KW-1, they decided to change over to "communications quality" audio, and altered some of the coupling circuitry to restrict the frequency response to the much touted 300-3000~ range of that era, undoubtedly influenced by the increasing popularity of the hyelloey sounding audio of SSB.

To protect the mod transformer I would remove the splatter filter however.

The audio driver stage for the 810's is inadequate.  It really should be a quad of 6B4G's in pushpull parallel.  That may be one reason for the splatter filter.  The anaemic audio driver stage could have generated just enough waveform distortion to make audible splatter.  So I would avoid overdriving the audio.

Check out the engineering design data on class-B audio drivers.  A single pair of 6B4's or 2A3's simply does not deliver enough audio power with enough reserve to modulate a pair of 810's at the power level run in the KW-1 without excessive distortion.  Even the Raytheon broadcast transmitter that runs 250 watts with 810's/810's, with only 1600 volts on the plates, uses a quad of 6B4's as the audio driver.

[wa1knx]

ha, nice troubleshoot by everyone. I fixed w7md's damons globe king 500b last year. intermittent in the audio. we pulled the decks and re soldered every audio conx. put em back and problem was still there. So I pulled the decks again , tacked in a half dozen test point wires out the back so that I could put the scope on to find it. Just in doing that, it was FIXED! damon left in the test point wires   Like quantum physics, a watched problem will cease to fail and avail its secrets.

[N3DRB The Derb]

I found my 2A3's were working pretty hard to drive my 805's in my old HB rig. The only silicon in that rig was a 5 watt zener diode to fix the neg bias voltage to the 2A3's. It stayed at -62 no matter what. I f I was a purist  would have rigged up vr tubes I guess.

[Todd, KA1KAQ]

I'm starting to bond the same way with my '750 in the same way.  Although the newness hasn't worn off I don't see myself ever getting rid of it.

Ah, yes....radio bonding. It's a beautiful thing.  Of course John, your rig had the benefit of a few more years of technological advances. Translation: more sh#t to fix! Of the gov't equipment I've seen from that era, TMC takes the trophy hands down for quality and....max stuff inside. I've got it easy in comparison.

As of about 1905 last evening ol' #89 was back on the air making RF and audio. Made a quick aircheck with Al VE3AJM and Bob VE3BGH and got good reports. The parts arrived Friday and Saturday, so I started on it Saturday afternoon. Finished up and got the deck back into the cabinet just before 1900 Sunday (with ample breaks). Replaced all electrolytics and wax/paper caps. Also found a crappy looking 1500 ohm resistor in the AF amp with a chunk missing. A lot of folks must've had their fingers crossed since the driver tranny still works and didn't need to be replaced, despite the tar leakage. Considering its age and already-abused nature, having the spare from Skip is no doubt a good thing. So there ya go, Frank - it wasn't bad after all. You were right.

I did notice one anomaly though, and it relates to the push/pull 6B4Gs. I had noticed that one got hotter than the other regardless of which socket it was in. After the rig warmed up for a few minutes, the 300V supply would drop down to around 200V from over 300. It would steadily creep down to around 280 or so, then drop suddenly to 200. No idea why. Grid bias on the 6B4s and 807 would also drop.

The hotter of the two 6B4s (a KenRad) reads about 110 in the 'good' scale on my tube tester, the other (RCA) around 85. So just for the hell of it, I pulled one out of another transmitter and it also measured around 85(CBS labeled RCA). Swapped it with the hot one, and voila! Everything stabilized, including the grid bias. No shorts showed on any of the tubes, but it sure looks like something was going on with the KenRad. Almost as if it was eventually shorting in the circuit and drawing excessive current. Assuming the tube isn't shorting in the circuit, do matched sets really matter that much?  I've never run into this before, so I'm guessing the tube is bad.

Thanks again for all the help, suggestions, or just moral support. Hope to hear everyone on the air soon. It beats the hell out of all this typing.

[WA1GFZ]

You might have a grid to cathode short or leakage allowing the resting current to take off. Maybe you could leave it in the tube tester for a while and see what happens or build a fixture and test it at full voltage. This could be the reason your 5V4 LV rectifier was bad. Too bad you couldn't add some fused to protect the iron.
Oh well remember a FET is just a triode without a glow.