Amp works , radio works but they wont work together

[321 treeclimber]

Thanks to a couple of great guys here on the forum,my 300 skipper is workin and my comstat 25a works well. Unfortunately they wont work together. How do i link them up? Is there a tuning procedure for this?
321

[tubefan]

Can you give a little more info? Comstat wont key the amp? Low power? 

[Sandbagger]

May 28, 2019 13:45:19 GMT -5 321 treeclimber said:

Thanks to a couple of great guys here on the forum,my 300 skipper is workin and my comstat 25a works well. Unfortunately they wont work together. How do i link them up? Is there a tuning procedure for this?
321

The Comstat has a PI tuning network in the back, similar to the load and plate controls on an amplifier. Its purpose is the same, to match the impedance of the final tube to the load impedance and provide maximum power output into that load. The problem is that your antenna's impedance is not going to be the same as the input stage of the amplifier, so you'll have to retune the load and plate trimmers on the Comstat to put out the proper amount of power into the amp. Which also means that you'll have to retune again when going back to barefoot power.

The other thing it could be is a problem in the RF keying circuit in the amp. Most amps are sensitive enough that 1 watt of power should key it. If it takes a lot more than that, I'd look for burned or damaged parts on the keying circuit, especially the 10 pF input cap or the 22uH choke after it. Another potential issue could be the 1000uF cap that filters the 12V supply. If the supply voltage sags, that could cause poor RF sensitivity. Hopefully the previous owner didn't try to drive that amp with a high output export radio. That would be BAD..........

[321 treeclimber]

I see. When i used a cobra 29 it was no problem. Then i tried the comstat 25a and that puts out barely 2w dead key. I didnt try to retune.then i tried my pearse simpson cheetah and that worked well. I'll try again tomorrow but it makes sense.

[2600]

If you have a spare SWR meter and coax jumper, hook the radio to the 'radio' side of the spare SWR meter, and the spare jumper from the meter's "antenna" jack to the radio jack of the Skipper.

Key the radio, tune the Skipper for max peak output, and then check the SWR meter that's between the radio and the amplifier. Set the switch to "cal", turn the cal knob to the mark on the meter, and switch to SWR for the reading, just as if you were checking an antenna.

I'll guess that the reading is around 3 to 1. Usually more.

This is probably holding down the carrier power coming from the Comstat radio into the Skipper.

Palomar "skipped" as it were, adding a circuit that matches the radio's 50-ohm output to the cathode of the driver tube.

As a result, the input impedance of the Skipper tends to run around 200 ohms.

Not 50.

Quickest thing to try first is to wind 5 or 6 turns of insulated (solid) wire around a half-inch diameter drill bit or dowel rod. Solid wire will hold its shape. Stranded wire won't. The layout in the Skipper's driver section is not the same in all of them, but the idea is to insert this coil between the wire that feeds the drive power into pin 2 of the driver tube. Don't have an example picture of this, but that's the idea.

The coil alone sometimes does the job well enough to get the input-side SWR below 2 to 1. Sometimes adding a trimmer capacitor to the input side of the coil is necessary, and will allow getting a proper impedance match and a reading below 2 to 1. Stretching the coil slightly to "tune" it is usually necessary to obtain the lowest SWR on the amplifier's input.

But that's the root of the mystery. The Skipper's input impedance is not 50 ohms. Not all radios respond to a high SWR the same way.

73

[321 treeclimber]

Wow...ok, this sounds like an interesting endeavor. I'll start putting effort into the initial test then I'll let you know where its at. Probably tonight. Thank you 321

[321 treeclimber]

Hey tubefan,sorry i didnt see your post.the comstat seemed to initially key the amp but it would then kick back out. Im sure you've since read the 2600 post,so now i have a solid direction to move forward. thanks for your inquiry. You gentlemen have given me great advice,insight and confidence to make my own repairs. This site is the best thing to happen to a novice and or /idiot,like myself. 321

[321 treeclimber]

Ok, sorry about the delay but unforseen..blahblahblah but yes, swr with modulation swings up as high as 3.5+. Dead key is 2.9-3.0 i looked at a schematic for the unit and it shows a 17uH inductor at number 2 pin. Speaking as an electronic illiterate,whats next? Do i add a trimmer cap? Or build a whole new lc circuit?

[2600]

The 17uH inductor on pin 2 is a choke coil. The radio's RF drive is supposed to ignore it, as if it weren't there. It carries the driver tube's DC-only current from pin 2 to ground.

The suggestion above is to unhook the connection that feeds the radio's drive power from the relay to pin 2 on the driver tube. The coil is inserted in the break.

If the coil alone can be tweaked by stretching/squeezing the turns to a SWR below 2 to 1, a trimmer capacitor may not be necessary.

And if the coil alone isn't sufficient, a compression-type trimmer cap would go from the relay end of the new coil to ground. The capacitor and coil together would then get adjusted for lowest input-side SWR with the amplifier keyed and tuned for max swing.

73

[321 treeclimber]

I got it. Will build the coil tonight. By the way 2600, i tried to order the set of two drop ins that you gave me the link to,but im worse at ordering online than i am at electronics....let me know if theres anything else i could possibly need for this skipper. I'll get it.

[2600]

Never thought of Fleabay as being tricky to place an order.

But I've been fooling with computers for almost 50 years, so my opinion probably isn't worth much on that.

The four electrolytic capacitors are the big deal, along with relays.

And tubes, of course.

73

[321 treeclimber]

Hahaha yeah well im easily stumbled by computers. It seemed simple, follow the prompts,check out as a guest,i dont have an ebay account, get to the end credit card info in... and then it says address is wrong. Check address,its right. Do it again. Same thing. That's why i only buy on sights where theres a phone number to call a live person. Gigaparts,mouser,dxengineering and the like. Im sure its me because i truly despise the automated systems. I like to talk to a sales person BUT i realize the world is to big to have thousands of telephone receptionists answering my calls. So,with all of that said, i will try again. ENDEAVOUR TO PERSEVERE! This amp deserves it.

[oldgeezer]

If your amp does not have a tuned input, I am selling a tuned input kit for $19. You cannot buy the parts for $19.

[321 treeclimber]

10-4 geezer, I'll keep that in mind. By the way what mic did you use on that courier 23+ ?

[oldgeezer]

Aug 3, 2019 4:58:37 GMT -5 321 treeclimber said:

10-4 geezer, I'll keep that in mind. By the way what mic did you use on that courier 23+ ?Non amplified Turner SSB crystal.