Forums :: NEW! Web Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives
Support This Site: Contributors :: Advertise


It is currently Jun Tue 28, 2022 11:39 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 25 posts ]  Go to page Previous  1, 2
Author Message
 Post subject: Re: solved-Collins 30s-1 lower ouput with kwm-2a vs kwm-2
PostPosted: May Wed 25, 2022 2:45 am 
Member
User avatar

Joined: Nov Mon 02, 2009 7:01 am
Posts: 5846
Location: Lincoln City, OR 97367
Greetings to Larry and the Forum:

At the time the 30S-1 was made, Collins was in love with the idea of a grounded screen for its amplifier stages. The screen of the 4CX15000 in the Collins 831G FM broadcast transmitter is arranged that way. The 30S-1 is the same. A quick look at the schematic will show that the 4CX1000 screen is indeed grounded. "How do they do that?" I hear you cry.

The screen has to be positive with respect to the cathode. The absolute potential doesn't matter; only the potential difference. The way you ground the screen is to make the cathode negative by the amount of the required screen voltage. The 30S-1 has a separate screen power supply to create this negative voltage. As it says in the manual, the screen supply voltage must now be added to the plate supply voltage to determine the actual plate voltage on the tube. Note that the bias supply now stacks on top of the screen supply, since the control grid has to be negative with respect to the cathode.

The advantage of this technique is that the screen of the PA is grounded for RF as well as DC; no screen bypass capacitors are required. The disadvantages are several, the principal one being that since the entire cathode current of the PA (sum of plate and screen currents) now passes through the screen supply, it must be capable of delivering considerably more current (and therefore power) than a conventional screen supply.

To measure the screen voltage in the 30S-1, you need access to TB201, terminal 2. The screen voltage is measured from this point to ground. It should be in the neighborhood of 200 volts. Note that this voltage is NEGATIVE with respect to ground. I strongly suggest that you arrange some method of bringing the test leads for your meter out under the door and operate the 30S-1 with the door closed and all interlocks in place. Safety first!

As far as the tuning issue is concerned, if the screen voltage is correct and the tube is good, you will need a vector network analyzer to look at the 30S-1 tank circuit. This can be one of the inexpensive Nano VNA units or something more sophisticated. Proceed as follows:

With all power removed from the 30S-1 (primary circuit disconnected) connect a 6.8K non-inductive resistor from the 4CX1000 plate to ground. Ideally, this should go to the cathode, but that would be hard to get to and I think ground will do for this test. Using a suitable tool, wedge the antenna change-over relay into the transmit mode. Connect the VNA to the RF output connector on the 30S-1. Examine the return loss on the VNA. You should be able to get a large value of return loss (little power reflected back to the VNA) at 50 ohms if the 30S-1 tank circuit is correct. Of course, you will have to adjust the 30S-1 tune and load controls to achieve this. If you cannot get a good null on the VNA, then there is some problem with the tank circuit in the 30S-1. If you can, note the positions of the tuning and loading controls and compare to what you have when trying to operate the amplifier into a load.

Good Luck,

_________________
Jim T.
KB6GM


Top
 Profile  
 
 Post subject: Re: solved-Collins 30s-1 lower ouput with kwm-2a vs kwm-2
PostPosted: May Wed 25, 2022 5:59 pm 
Member

Joined: Jan Fri 29, 2021 10:08 pm
Posts: 148
Location: High Sierra > 10,000' elevation
Hi Jim

Thanks very much for the pertinent information; I obviously have some work to do.
I did connect my MFJ-269 antenna analyzer before the antenna change-over relay and could get good swr on 40m & 20m (that is all I checked), but will now try your suggestions.

73,
Larry, kq6xy


Top
 Profile  
 
 Post subject: Re: solved-Collins 30s-1 lower ouput with kwm-2a vs kwm-2
PostPosted: May Sun 29, 2022 1:33 am 
Member

Joined: Jan Fri 29, 2021 10:08 pm
Posts: 148
Location: High Sierra > 10,000' elevation
The problem turned out to be the bias setting. Once I set it to 200ma I got ~1kw out with tuning as expected.

Thanks to all that replied.

73,
Larry, kq6xy


Top
 Profile  
 
 Post subject: Re: solved-Collins 30s-1 lower ouput with kwm-2a vs kwm-2
PostPosted: May Sun 29, 2022 4:18 am 
Member
User avatar

Joined: Nov Mon 02, 2009 7:01 am
Posts: 5846
Location: Lincoln City, OR 97367
Greetings to Larry and the Forum:

Too many zero-bias triodes out there. For any tetrode amplifier (or any tube that is not designed as a zero-bias tube), the first thing one should always check is idling current.

Glad you found it.

Regards,

_________________
Jim T.
KB6GM


Top
 Profile  
 
 Post subject: Re: solved-Collins 30s-1 lower ouput with kwm-2a vs kwm-2
PostPosted: May Sun 29, 2022 3:57 pm 
Member

Joined: Jan Fri 29, 2021 10:08 pm
Posts: 148
Location: High Sierra > 10,000' elevation
Thanks Jim

I should have checked it earlier. All the other voltages etc. were fine, so I just assumed it was something more serious since I've had the amp for almost 20 years and only recently started to work on it.

73,
Larry, kq6xy


Top
 Profile  
 
Post New Topic Post Reply  [ 25 posts ]  Moderator: Sandy Go to page Previous  1, 2

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 6 guests



Search for:
Jump to:  




































Privacy Policy :: Powered by phpBB