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 on: April 15, 2018, 09:14:23 AM 
Started by N1BCG - Last post by N1BCG
It turned out to be the bias supply, ammeter, grid leak resistor, and RF choke in series with bypass caps to chassis. Basically what Phil posted...


In my case, R2 is 7k, C1 is 0.05, C2 is 0.01, C3 is 0.02, and Lfg is 2.5mH because of the original design requirements of the transmitter.

 on: April 15, 2018, 09:14:06 AM 
Started by WBear2GCR - Last post by w8fax
Many years back I obtained a KWS-1 and of course the knobs were messed up. I have been a toolmaker for a hundred years, so it was a good challenge. I made a solid aluminum fixture that the knobs would sit in securely and the center located very accurately, and located radially by using the set screw hole so I could red-rill it later.. I had some end mill cutters ground to a few sizes that I thought would work to re-size the hole and repair the out of round. Then, on a Bridgeport vertical milling machine, I bored the hole out as far as I thought was safe. I then filled the back of the knob and hole with aluminum base JB Weld, avoiding the area where the gear is. By filling the void around the repair, it added some support to the wall of the hole. After letting this cure a couple of days, I re-bored the hole to the proper size, plus a thousandth or two. My fixture was made so I could also use it to re-drill the set screw. Success. I re-installed the knobs and they worked great. I still avoided over torquing them on installation tho'. Since the knobs are pretty much scrap and useless until repaired, I thought to offer my service and repair to others. Now understand, I offered this for free since I already had the tooling made up and the repair was simple. Just send me a buck or two for postage. I explained the repair that I did. I repaired a few for guys, warned them of over torquing the set screw, and they were happy to get otherwise useless parts repaired. Then....it started. I received some nasty e-mails from Collins Collector "experts" accusing me of trying to make money off of the repairs and also destroying or damaging valuable Collins parts, by rendering them "non-original". That was it. Screw it. I withdrew my offer to fix anymore at any price, and even tho' I still have the tooling somewhere I think, I stand by that decision. The repair must have worked, as I never heard of any  failures to repaired parts.  I am posting this so someone who has the skill can re-duplicate or improve what I found to be a solution and repair some otherwise useless parts to a pretty cool old transmitter.

 on: April 14, 2018, 09:21:33 PM 
Started by N1BCG - Last post by DMOD
Here is my suggestion using a 12AU7A triode, with a stage gain of 17, unbypassed.

Bypassing the 168 ohm cathode resistor with a 100 uF@16V electrolytic will get you more gain at the expense of increased distortion.

What you need is a decent amount of voltage drive into the grids of the 6146, so you want the load resistance of the speech amplifier to be about 5 to 10X the source resistance, which is what you see here.

+8dbm = 1.95V rms into 600 ohms.

1.95VX1.414X17 = 47V Peak per plate.

It would be helpful if you supplied more info such as a block diagram of the complete system.

And I trust this is for personal use.  Smiley

Phil - AC0OB

 on: April 14, 2018, 06:11:33 PM 
Started by W8ACR - Last post by W8ACR
I may have a chance to acquire an ARC-2 in half decent shape along with power supply. Does anyone have any experience with these transmitters? I guess it is both a transmitter and receiver in one box?


 on: April 14, 2018, 06:04:38 PM 
Started by W8ACR - Last post by DMOD
The grid voltage meter is damped and would never be fast enough to show 1/2*F pulsations.

I just noticed that there is a .001 uF RF shunt cap at the bottom of the grid RFC so that should attenuate some RF.

My view is the diode is just an additional noise source.


 on: April 14, 2018, 05:20:53 PM 
Started by W8ACR - Last post by W8ACR
OK Phil,

Thanks, your circuit makes good sense to me. I will say that the rheostat would never be at short unless by mistake.

Also, if the grid voltage went too positive, wouldn't that affect the voltmeter reading, even at RF frequencies? The voltmeter reading in operation is rock solid. It never flinches even a bit.

Lastly, can one calculate the positive peak grid voltage, or does it have to be measured?


 on: April 14, 2018, 04:52:09 PM 
Started by W8ACR - Last post by DMOD
Here are the problems I see with in this circuit:

1) When the Pot wiper goes to zero ohms it shorts out the Bias Supply.

3) Anytime the grid goes more positive than the bias supply, the diode conducts bringing back the voltage down to the bias supply voltage with it's accompanying switching transients (at half the RF drive frequency) and the grid sees a very low impedance situation, with no degree of freedom.

Personally I like to see the grid voltage with no modulation so I can actually see what it is.

Below is my suggestion for this situation.

Phil - AC0OB

 on: April 14, 2018, 04:39:49 PM 
Started by W8ACR - Last post by W8ACR
I just reacquired my T-368E that slipped away 15 years ago. I want to try to spiff up the audio a bit. Does anyone out there have suggestions for a home-brew speech amp circuit or commercial unit that makes the transmitter sound better? Also, is the stock modulation transformer one of the limiting factors? I do have a UTC CVM-4 that I could probably slip in there if that would be worth the effort.

Thanks, Ron

PS: After I posted this, I searched and found the WA1HLR audio mods as posted by K4HX. I would still like to hear from others who have tackled this problem in other ways. Thanks.

 on: April 14, 2018, 04:12:16 PM 
Started by W8ACR - Last post by W8ACR
I didn't want to hijack the other thread. I would like a critique of the grid bias circuit that I use in my big rig which is a Taylor 822 modulated by 811's. It has similarities to the circuits discussed in the other thread. The RF exciter is a Johnson Viking II. The grid coil is a B&W JVC. The bias transformer provides about 110VDC of bias voltage. The 822 plate B+ (2000VDC) is always on. With no RF drive, the tube is 100% fixed bias from the bias supply, and is biased well beyond cutoff (which is about -80VDC). When RF drive is applied, the bias becomes 100% grid leak - if I understand it correctly.

I can adjust the grid bias voltage to whatever I want by adjusting the RF drive and the rheostat. FWIW, the circuit seems to work just fine.

I guess my main questions would be:

1. is the 1N5408 diode good, bad, or indifferent?
2. what is the effect of the voltmeter, if any, on the rest of the circuit?
3 is there any reason that I should not use a rheostat as a grid leak?

Thanks, Ron

 on: April 14, 2018, 03:47:25 PM 
Started by N1BCG - Last post by DMOD
Here is my suggestion using a 6SN7 or other low Mu (20) triode, with a stage gain of 14.

It would be helpful if you supplied more info such as a block diagram of the complete system.

Phil - AC0OB

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