Collins 831G schematic

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TPT
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Collins 831G schematic

Post by TPT »

Station was using a BE FX50 as an exciter, the Darlingtons went out and it went to a full 50 watts. While I had the BE out for repair, put in a 35 watt Bext exciter to keep the station on. Reflected power problems--as much coming back as being fed to the IPA.

Now the 831 has a tube IPA, the match to the IPA is a pair of open plate capacitors mounted on a "yoke" behind the tube sockets, one marked coupling which was fed from the jack for the exciter feed, the other marked "tune." Both caps had a Teflon shaft out of the cover of the IPA assembly.

We could adjust coupling for minimum reflected, but as soon as I removed the screwdriver from the shaft, it de-tuned. Thought was the over-drive had warped the plates of the little variable cap. Got a replacement, installed it, --now won't tune at all. Pulled the "tuning" cap--and found it wasn't connected to anything (the tab on the cap had never been soldered). So I am baffled. Transmitter is on the air, with about 8 watts drive, about the same reflected, Left exciter at around 7 watts to save the output stage. Which gives us about half-power out on the transmitter.

Have the book, but there is no schematic in the book for the IPA stage. Supposedly drawing in a "bedsheet" schematic in the "back pocket" of the manual, long gone from this 1975 transmitter. So--anyone have a copy of the IPA schematic? Any other thoughts?
kcbooboo
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Re: Collins 831G schematic

Post by kcbooboo »

While not identical, the Continental Electronics 816-R2A is similar.

Go to https://contelec.com/knowledgebase then scroll down to the "816R-2A,-3,-4 Technical Manual" and click the check mark in the Manual column. There's a full schematic very early in the file that has the two 4CX250 IPA and that should help considerably. There's also tuning instructions later in the manual. It's about 10MB.

Bob M.
TPT
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Re: Collins 831G schematic

Post by TPT »

Thanks...overall schematic is on page 161 of that manual. Looking at it, I can't figure out how this thing ever worked.

The "Couple" cap, C34, is mounted to a "U" shaped bracket which is connected to the grids of the two 4CX250B tubes. The variable side of this open frame cap has a tab that the incoming RF feed is soldered to, wire off the shield of the coax goes to ground. So far, makes sense.

The "Tune" cap is also mounted to this bracket, and, per the schematic, is connected to ground. Except the cap in this transmitter was never connected to anything--the mounting tab is clean--no trace of solder. So, if it was connected to ground it must of been with some kind of mechanical connection. Maybe knocked it off trying to get the coupling cap out.

To add to the confusion, there is a a mechanical "inductor" --L9 on the schematic that feeds the screen on the first tube (V1) through a fixed cap. 50 ohm resistor connected to the grid circuit across this "inductor" to provide a termination.

Of course, this whole mess is in a box that is only accessible by pulling the center panel in front of the high voltage section.
kcbooboo
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Re: Collins 831G schematic

Post by kcbooboo »

Sorry. I had clicked the "Schematics" tab on the left and it was the next page. I didn't notice it was page 161 and that was the only schematic in there.

L9 is on the control grid or input of the two tubes, which are run in parallel. The screen grid runs to another supply via the metering circuit. The 50 ohm resistor is probably there to provide a fixed 50 ohm load in case L9 goes open. The setting for L9 should be found earlier in the manual.

Most of the CE transmitters had the IPA replaced with a solid-state unit, probably due to the difficulty of finding 4CX250B tubes and other issues. The solid-state IPA is on its own slide-out chassis as I recall. I don't know if the tube IPA is also removable as a complete unit. A much more difficult area to work on is underneath the base of the PA or its socket.

Bob M.
kcbooboo
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Re: Collins 831G schematic

Post by kcbooboo »

By the way, I found an 831G manual (about 5 MB) without the schematic, but the rest seems to have been scanned in color and the photos are much better than the black&white 816R2 scan on CEC's site.

Bob M.
TPT
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Re: Collins 831G schematic

Post by TPT »

They had a 831 manual--in pieces. I've scanned that (luckily, I have a sheet-fed scanner), but, as I mentioned, the full scale schematic was in a pouch in the original manual cover--long gone.

The IPA assembly is in a box mounted below the final tube socket, the tubes are accessible from the left cabinet, as is the output tuning (a vacuum variable). However, the two coupling and tune capacitors for the grid match have to be accessed from the center cabinet.
kcbooboo
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Re: Collins 831G schematic

Post by kcbooboo »

It sure seems like the capacitor has to have a ground connection somewhere. Hard to believe the unit would even work without one, unless the IPA grid tuning inductor was doing all the work and was set improperly.

If the entire IPA assembly can be removed it might be easier to work on.

There's a PA grid tuning adjustment that has to be made while the center cover is removed and the unit is transmitting, and you have to be real careful what you grab onto. I don't know why they didn't add some sort of long or remote tuning shaft to that capacitor. Granted you don't have to change it often but it still requires bypassing interlocks and running the TX with lethal voltages exposed. The CE at the station that I help with, usually has me standing by next to the PLATE OFF button just in case.

Bob M.
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jthorusen
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Re: Collins 831G schematic

Post by jthorusen »

The schematic for the 831G is a large format drawing that is about (if memory serves) three feet by five feet. It's huge. That's why it is in a pocket in the back of the book.

If the station has a blueprint file cabinet, look there. Also check any other files where someone might have folded up a huge drawing.

Good Luck,
James K. (Jim) Thorusen
KB6GM
Central Coast Electronics
www.centcoast.com
NW Oregon Consulting Bdcst Eng.
TPT
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Re: Collins 831G schematic

Post by TPT »

The 816 manual has a copy of the schematic, which can be blown up enough to see what is going on. Next step is to crawl in to see if I can figure out what is actually in there compared to the schematic. May just take a clip lead off the "tune" capacitor to ground to see if that helps. L-9 is a slider adjustment--can't see how that would change.
kcbooboo
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Re: Collins 831G schematic

Post by kcbooboo »

Are you sure you aren't fiddling with the neutralizing cap, because that apparently has only one end connected.

The Teflon shaft extensions should not be RF conductive, unless they're full of carbon or the caps are retuning themselves after you release tension on the shafts.

Any clip lead is likely to introduce inductance you probably don't want or need. The cap really should go to a solid ground.

Have you tried talking to anyone at CEC? I realize that could be a futile attempt but there may be someone they know who you could talk to. Of course they'll strongly try to get you to convert the tube IPA to a solid-state version that the CEC transmitters all use.

Bob M.
TPT
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Re: Collins 831G schematic

Post by TPT »

Which we (Fireman No, 1 and me, Fireman No. 2) have been urging them to do--(convert to 50 ohm input, that is). Then get a solid state kilowatt transmitter to drive the final (believe about 600 watts drive is needed). Which could then be used as a back-up when something else breaks.

The two small open-frame capacitors have the Teflon shafts coming out of the back of the cage that surrounds the back of the IPA assembly. "L9" is apparently a coarse tune shunt --I think I see it, haven't messed with it, don't know why it would change. The missing ground on the "tune" cap must be in there somewhere--either it is "mechanical" and I can't see it, or there was a clip to ground off the solder tab on the cap. Nothing was soldered to that tab.
kcbooboo
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Re: Collins 831G schematic

Post by kcbooboo »

I agree, there has to be a mounting bracket for that cap somewhere, on or around L9. Those transmitters used a lot of sliding metal shunts to set rough tuning. There's a chart in the manual that describes three different lengths for slide tuning. It took me a while to realize the single solid line, double solid line, and double dotted line were also used at the bottom of the page to let you know which inductor used which line on the graph. They should have labeled the lines on the graph itself.

I think the solid-state IPA is rated for up to 700w. If you end up replacing your IPA with a 1kw TX, remember you'll need a way to turn its output on and off as a function of the plate on/off circuitry, because you probably don't want to feed all that power into the PA when it's not expecting it, or possibly worse, not feeding drive to the PA when it's expecting it. And if the exciter unlocks that might want to shut down the IPA.

Bob M.
edwinbrand
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Re: Collins 831G schematic

Post by edwinbrand »

What transmitter did you end up feeding the 831 IPA with? And how many watts did it take?
TPT
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Re: Collins 831G schematic

Post by TPT »

Little Bext 30 watt exciter, about 20 watts in, a whole bunch back.

Looks like transmitter needs new finals, will probably play some more when the finals are changed.
alleo
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Re: Collins 831G schematic

Post by alleo »

Chris Hall reelaudio@gmail.com he knows how to drive the 831 with solid state drivers,
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