Warning: Creating default object from empty value in /homepages/11/d132647312/htdocs/Amfone/mkportal/include/SMF/smf_out.php on line 47
Gates BC-1G problem




 
The AM Forum
October 22, 2018, 07:43:36 PM *
Welcome, Guest. Please login or register.

Login with username, password and session length
 
   Home   Help Calendar Links Staff List Gallery Login Register  
Pages: [1]   Go Down
  Print  
Author Topic: Gates BC-1G problem  (Read 1348 times)
0 Members and 1 Guest are viewing this topic.
WV9R
Member

Offline Offline

Posts: 73


« on: October 02, 2018, 11:55:36 AM »

I converted my 1G to 75 meters and all was good for a time. I was on the air for about an hour one night with no problems. The next morning I fired it up and when I keyed it instead of 3.880 on the frequency counter it showed 3.620 or there abouts. I can see the oscillator at 3.880 but the carrier and output is at 3.620 or so. I can vary the actual frequency of the carrier with the RF driver tune control. I've checked several things but I'm stumped. K3FEF has been helping me troubleshoot the problem and has been a great help, but we are stuck. We've isolated it to the L4 coil is oscillating with plate voltage on. With the oscillator unit unplugged it will still have a 250 watt carrier on 3.620  depending on where the RF driver tune control is set, the frequency will go up or down if the control is varied. I've removed all the tubes except for 1 833 final. Still have a 125 watt carrier. If I disconnect the wire that comes from the center tap of the 10 volt supply the coil doesn't oscillate and no power output. The 10 volt line goes through 3 3 ohm resistors in parallel then through 2 5000 ohm resistors in parallel. I measure approx. .05 vac on this wire, and also approx. 2.5 vdc on this wire. Something obviously changed when I shut the transmitter down but what?  I was talking with Rob K5UJ and all was good then after I signed Don K4KYV came in and said I sounded good. I don't know what happened and hope someone can point me in the right direction. I replaced all the caps on the RF Driver Board.  HELP!  Huh  I've attached the schematic. Thanks for looking. Rodger WQ9E any thoughts.

* Gates BC-1G.pdf (4750.24 KB - downloaded 52 times.)
Logged

Ray
WV9R
KA3EKH
Member

Offline Offline

Posts: 444



WWW
« Reply #1 on: October 02, 2018, 01:36:23 PM »

C5,C6,C7 and C8. would replace C7 first. did you adjust C40?
Logged
WV9R
Member

Offline Offline

Posts: 73


« Reply #2 on: October 02, 2018, 01:57:08 PM »

All of the caps you noted have been changed, and yes C40 was adjusted
Logged

Ray
WV9R
KA3EKH
Member

Offline Offline

Posts: 444



WWW
« Reply #3 on: October 02, 2018, 04:49:25 PM »

Did you check the condition of the two taps on L4 for oxidation? If the C7 side tap is week or flakey then you would have problems.
Going to assume that you already swapped the two 833 from the modulator to the PA and the problem remained. I once saw a 833 installed backwards, plate and grid swapped and that had colorful results but oddly did not damage the tube or transmitter.
Logged
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1180


« Reply #4 on: October 02, 2018, 07:28:17 PM »

I would examine the area and connections around the Neautralization Cap C40 and then mark the present dial position of the Neautralization Cap C40, and then go above and below that mark slightly and see what happens.


Phil - AC0OB

Logged

"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
WV9R
Member

Offline Offline

Posts: 73


« Reply #5 on: October 02, 2018, 10:15:24 PM »

I've done all of the suggestions and still have problems. I really doubt that I have all bad tubes, although possible I guess, but what else could/would oscillate except the tubes? I'm rally starting to wonder . Huh
Logged

Ray
WV9R
N1BCG
Member

Offline Offline

Posts: 598


« Reply #6 on: October 02, 2018, 11:30:21 PM »

What modifications were made to the neutralization circuit for 75M? The amount of feedback on 3880 will be dramatically different than on the bícast band. Does it seem to neutralize properly?
Logged
KA3EKH
Member

Offline Offline

Posts: 444



WWW
« Reply #7 on: October 03, 2018, 08:40:33 AM »

I did a RCA thatís almost identical to that design but I stayed with just operating on 160 because of the large amount of capacitance in the two 833 tubes. They have no issue and behave very well on 160 but got squirrely on higher frequencies.
What changes did you do in your output tank to get it to work on 80? And what changes did you do in the interstage plate to grid from the 807 driver tubes to the PA?
L4 may have too much inductance the way you have it taped currently and that inductance is enough to allow that stage to osculate. Have you tried a new set of tuning solutions for the two taps? What did you do with C4 when you converted it to 80 meters? Would think the neutralizing path C40 would be good being that was large enough for broadcast band and going way up in frequency its ok just may be an issue of L4 being too large for the return path to work.
The weird thing is that you did not have any issues before, at least that you may have been aware of. You got a spectrum analyzer?
I am going to assume that you had to big time reduce C42 and C43, going down to just one capacitor thatís small and C44 and C45 got reduced too.
Logged
KB5MD
Contributing
Member
*
Offline Offline

Posts: 548


« Reply #8 on: October 03, 2018, 04:30:41 PM »

Never had this problem with the Gates BC-1T but I also have an RCA BTA 1R1 that I use on 40.  The RCA will generate an off frequency carrier if it does not have enough excitation on the desired frequency. The exciter is an old Gonset GSB100 and if it is turned off, the RCA will still transmit an out of the ham bands carrier.  Turn the exciter on and the frequency will change to the exciter frequency.
Logged
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1180


« Reply #9 on: October 03, 2018, 05:07:17 PM »

 I've attached the schematic. Thanks for looking. Rodger WQ9E any thoughts.

Please show us YOUR schematic with the values of the changed components.  

BTW, 3620 is the second harmonic of 1810.  Was this rig on 160m before?

I am thinking it may have been on 160m and maybe one or more of the 160m frequency determining components were not changed out.


Phil - AC0OB
Logged

"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
KL7OF
Member

Offline Offline

Posts: 2031



« Reply #10 on: October 03, 2018, 06:18:44 PM »

Please post the solution when you get it solved Ray...I know you will figure it out...I have one of these transmitters on 160 M  (easy conversion) Good Luck ..Steve KL7OF




   
Logged
wa1mtz
Member

Offline Offline

Posts: 21


« Reply #11 on: October 03, 2018, 07:30:06 PM »

I did a restoration on a BC-1many years ago, and I found lots of resistors that had changed value in both the exciter and AF driver, some went way up in value some went way down and discoloured, sounds like a definite oscillation going on, can't wonder if you're using a xtal or driving with something? look for LF  paths, any discoloration in any section of chokes? I had a problem here with one of my HB rigs and used a small toroid core wound with 8 T of # 16, put that between rf choke & tube  toroid is the kind used in switching supplies in a computer
Logged
WV9R
Member

Offline Offline

Posts: 73


« Reply #12 on: October 03, 2018, 09:34:07 PM »

The rig was on 1.050 when I got it. I followed TimTron's mods to convert the transmitter. The mods are the ones Tim did to the BC1T at the ARRL headquarters. The neutralizing cap appears to work as it should. I cleaned all the connections and had to re-solder a couple. I am using crystals in the oscillator.
Logged

Ray
WV9R
KA3EKH
Member

Offline Offline

Posts: 444



WWW
« Reply #13 on: October 04, 2018, 08:42:06 AM »

I have done lots of work on broadcast transmitters before, but that was in the broadcast band. I have converted a RCA MX and a old Gates BC1but not the newer series like the G or T and never attempted trying 80 meters with 833 tubes.
All that being said I decided to try to look at just whatís involved with the TrimTron method and so I watched the video up on YouTube:

https://www.youtube.com/watch?v=cqM-nbiMBOE&feature=youtu.be

He appears to be concerned with the quality of the filament bypass capacitors C5 and C6 and appears to manufacture a new plate and much better ground attachment for the PA
Would speculate that may help to tame any oscillation in the output tank.
Did you do that or is it still the original filament bypass set up?

A footnote, was real interested to see what he did on the video but have to say that although his work on the transmitter was interesting whoever shot and edited the video did a poor job. Would have been better if they tried to make it informative and not entertaining. But thatís just me.


Logged
KA3EKH
Member

Offline Offline

Posts: 444



WWW
« Reply #14 on: October 05, 2018, 09:02:45 AM »

On the RCA MX series broadcast transmitters there is a huge steel plate between the front side that has the tubes mounted there and the backside where the tuning components go. Wonder if this isolation between the tubes and the tanks help to prevent oscillation in the PA? The Gates is all open and perhaps with the huge surface aria of the 833 tubes thatís where the oscillation can occur? Although I still think that filament bypassing may have something to do with it.

Video of my RCA MX:

https://www.youtube.com/watch?v=LETZZRauMbQ


Logged
k7mdo
Contributing
Member
*
Offline Offline

Posts: 303


« Reply #15 on: October 05, 2018, 10:26:25 AM »

On my BC1g I added a large broad aluminum filament bypass structure to replace the "long" ground leads on those caps....  The original ground side lead was very long and wandered very close to the oscillator box and cage....  I will photo the thing today and add a picture.

Tom

Ray, I don't know if it helped because I never ran the transmitter without the modification. 
Logged
WV9R
Member

Offline Offline

Posts: 73


« Reply #16 on: October 05, 2018, 08:43:13 PM »

That's (the plate for the bypass caps) one thing I still have left to do. Possibly this weekend. Tom how is yours coming along? Where are the connections made on your coil? A pic of the coil would be great. What kind of resistors did you use in the modulator? And lastly did you have low grid drive to the 807's?
Logged

Ray
WV9R
WV9R
Member

Offline Offline

Posts: 73


« Reply #17 on: October 05, 2018, 08:52:30 PM »

Ray KA3EKH Great job on the RCA!  I can only hope!
Logged

Ray
WV9R
WV9R
Member

Offline Offline

Posts: 73


« Reply #18 on: October 06, 2018, 12:56:10 PM »

Well I found the problem, Thank you to everyone for the suggestions Now the hunt is on for parts...possibly a vacuum variable...  the attachment is a pic of the failed cap.


* IMG_2831.JPG (341.83 KB, 1080x1920 - viewed 93 times.)
Logged

Ray
WV9R
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1180


« Reply #19 on: October 07, 2018, 10:19:12 PM »

Looks like a fixed cap to me with one in series with another.

I have a few of those from a BC-1T so let me know the value.



Phil - AC0OB
Logged

"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
KA3EKH
Member

Offline Offline

Posts: 444



WWW
« Reply #20 on: October 08, 2018, 08:40:35 AM »

What cap is that? almost looks like what's in the PA output tank.
Logged
WV9R
Member

Offline Offline

Posts: 73


« Reply #21 on: October 08, 2018, 12:24:29 PM »

Yes in the PA tank output.  2-250pf @10kv Phil.
Logged

Ray
WV9R
KA3EKH
Member

Offline Offline

Posts: 444



WWW
« Reply #22 on: October 08, 2018, 03:52:14 PM »

that's like C42/C43 and not C44 or C45. two 250@10Kv come out to 125pf @ 20Kv. Think back on my RCA with just one 833 ended up with a 50pf 20Kv vacuum bottle for the plate tank tuning. The loading capacitors were huge like 0.001 but the tuning capacitor was small. I always attributed that to the huge plates and all the additional capacitance in the design and that's what scares me about trying to use that tank above eighty.
Logged
DMOD
AC0OB - A Place where Thermionic Emitters Rule!
Contributing
Member
*
Offline Offline

Posts: 1180


« Reply #23 on: October 08, 2018, 10:44:13 PM »

Yes in the PA tank output.  2-250pf @10kv Phil.

They are mica capacitors, thick aluminum bases with ceramic cylinders.

Both measure 252 pF on my B&K LCR meter.

They are 10kV type 1960, 7 Amps@3MHz, made by Acushnet.

What capacitance does each of your's measure?


Phil - AC0OB
Logged

"What kind of Koolaid do they make you drink in the Physics Department?" Charlie Epps to Dr. Larry Fleinhardt, NUMB3RS   Smiley
WV9R
Member

Offline Offline

Posts: 73


« Reply #24 on: October 09, 2018, 10:52:06 AM »

Phil,
  That is what I need. My originals were 400pf each. To get on 75 I used 2 250pf @ 6kv and that's what cracked. so what I'm looking for is exactly what you have there.
  Ray
Logged

Ray
WV9R
Pages: [1]   Go Up
  Print  
 
Jump to:  

AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
 AMfone © 2001-2015
Powered by SMF 1.1.21 | SMF © 2015, Simple Machines
Page created in 0.108 seconds with 19 queries.