10 Meter Amp

I recently acquired a 250 watt 10 meter amp that works well with 100 watt rigs when power into the amp is set to 10 watts.
The output is fine. It runs cool to the touch even when I get long winded into a nice wire antenna cut to the middle of the 10 meter SSB voice band allocation. I get peak power up to 320 watts. The problem I encounter is when I turn the amp on. It immediately attenuates the receive signal by 20db.
What should I look for first?
This is a generic, brick type amp made before the surface mount RM Italy stuff came out. Sadly, it did come from an 11 meter environment. Estate sale stuff.
Of note, I replaced a burnt fuse as soon as I bought. It was inexpensive. Sold as not working.
Thanks ahead of time.

Look at the T/R relay. And...is it a so-called "bilateral" unit? In other words, is there a receive preamp built in?

Dirty relay contacts in either the T/R changeover or the preamp in-out circuits will cause what you're seeing. Another possibility is the carrier-operated relay turning itself on at power-up with no drive applied. For that I'd see about getting a schematic then looking at any time-delay capacitors on the detector side of that circuit.

Ah, yes. There is a pre amp switch. It does nothing in the on position.

Quote:

---

Originally Posted by KG4CGC

Ah, yes. There is a pre amp switch. It does nothing in the on position.

---

I'd suspect its changeover relay (if so equipped).

What's the model of the amp?

Palomar 250HD Class AB.

Is this it?

http://www.cbtricks.com/Amp/palomar_..._fet/index.htm

No. It more resembles a Texas Star.
I will add some pics later this evening or tomorrow evening.
Thank you for your interest.

Whew.

I don't wanna take the rap if it was "Dave Made".

Suspect the pre-amp stuff. Maybe you can by pass it.

Attachment 16814

Quote:

---

Originally Posted by KG4CGC

Attachment 16814

---

I think yours is a predecessor to the amp in the link I posted (the hi/med/lo power switch controls bias and input attenuation). Open yours up, Charles, and compare the actual PCB with the schematic in the link. Similarities?

I can see one issue right off the bat but it looks like a slight bit of corrosion on the resistors.

Attachment 16816
Attachment 16817

Attachment 16819
Attachment 16820

Attachment 16822
Attachment 16823

When you power it on, does one of the relays pull in? The smaller relay is the receive preamp. The transistor that drives that relay is probably failed.

You could find the Normally Open poles of the main relay and, tack a wire across them and see if that helps you locate the problem.

Having operated a broadband Class AB2/C solid state 2M amp with receive preamp at a club special event station I can say the best thing is simply disable the receive preamp and forget it. All it did was drag in more noise and didn't improve the receive one bit. I'd say from that experience the only thing a receive preamp is useful for is when the rig has a crap receiver to begin with. Those cheap Radio Shack shortwave receivers sure need one, the 20dB gain 3 leg chip I used did the trick and I gave the souped up RX to a local kid who became a ham after hearing the world on a wire out the window. BTW the special event was a big disappointment being able to reach only one repeater being we were down in a deep valley, and HF was dead. The only Amateur Radio demonstration those poor kids got at the camp was me talking with a club member with an HT at the other end of the lake. <sniff>

Now we get to transmit that has problems of it's own. Throttling a 100W transmitter to drive a 200W CB amp is a total waste of time and effort to put another half an S unit on the receiving end. Remember signal increase with power is logarithmic, each doubling of power puts out another 3dB and one S unit equals 6dB, so all you get on the receive end is half an S unit with that amp. There's a good reason why CW/SSB rigs put out 100W, that's all that's needed on any band provided your antenna is efficient.

As an aside, I had fun testing a 500W CB amp after repairing it. It was the same sort of thing, I lowered the drive and audio gain on my scratchy Apache so I didn't push the grid driven 4CX250 into Class C. I doubt Dave could get away from his pill addiction long enough to even conceive of using those ubiquitous VHF/UHF transmitting tubes. The one S unit and a spit increase didn't do nearly as much QRM cutting as the audio when it's set up correctly, that clipper with an M derived pi section LP filter with a sharp cutoff at 3KHz cuts like a hot knife through butter. That 4CX250 put out a clean signal as opposed to the 12 sweep tubes in the D&A Phantom 500 aka The Green Dragon. I knew a guy who had one and couldn't use it, not only did it sound like shit, but it put the Tennessee Valley Indians on the warpath for blocks around, the neighbors nearly killed him.

Attachment 16824

Quote:

---

Originally Posted by koØm

When you power it on, does one of the relays pull in? The smaller relay is the receive preamp. The transistor that drives that relay is probably failed.

You could find the Normally Open poles of the main relay and, tack a wire across them and see if that helps you locate the problem.

---

That relay looks like an Omron DPDT of the type used in the FT-726R's various modules, the FT-980's RF board and other rigs of the period. They go bad due to their open-frame construction, which allows the contacts to oxidize due to atmospheric interaction.

I've repaired them by removing the plastic cover, soaking the edge of a piece of card stock in DeOxIt D5-100 and gently dragging the stock through both N/C and N/O contact sets. Use minimal pressure on the wiper arm when doing this.

Those relays are long since unobtanium. I think I bought the last available OEM ones from Yaesu a couple of years ago.

An ETA:

Quote:

---

Throttling a 100W transmitter to drive a 200W CB amp is a total waste of time and effort to put another half an S unit on the receiving end.

---

I have a Communications Power HF-150 here in the shack. It'll put out 120w PEP or so when driven with 10w, and has switchable low-pass filters along with a receive preamp and AM-FM/SSB-CW time delay selection for its RF COR circuit.

That amp works very well with a 10W-class HF transceiver. Think TS-660 (on 15 through 10), the FT-726's HF module (ditto) or my TS-130V (on any band the rig covers). It's also remote-controllable so I can put the amp next to the power supplies and 6/2/4x0-MHz amps and control all of them from my operating console. Since each Yaesu module offers a keying output for accessory activation I'm thinking about modifying the amp for direct keying.

I'd borrow a bit from this thing and put a bandpass filter between the driver and amp, then a lowpass filter on its output. And match your exciter to the amp...no more than 10-15W PEP drive, else you should construct an input padding circuit.

Thanks for the input. I will be doing some testing and cleaning over the next week.
I'll get back here with what I did, observed and the results. I'll keep my eyes and ears open to listen to and watch the relays when I power it up. If it's as simple as a little transistor I might need some help sourcing it. If it's the relay I'll probably need to bypass the whole preamp circuit whether that's just jumping the relay or not.
Last resort, take it to the shop. I've know the man since I was 8 years old. He runs a brick and mortar place in Greenville. He has done repairs since forever and sells whatever makes money. High end car stereos has made him more money than CB ever did.

As an aside, I was in a local 10m ragchew net that originates in Greer and extends to Spartanburg (east). The difference between running the Corsair wide open at 100 watts (barefoot) and running it at 6.5 watts into the amp which produced peaks of over 300 watts was the difference between straining to hear me and then being told, "Armchair copy OM." That's only a 30 to 50 mile jump but on 10m, that's just how it is with local comms. Of course, on 80m we could hear each other, comfortably, down to the 25 watt range. Same thing with a couple of friends almost due south of me in Greenwood. We wait for a local opening to hit 10m but on 80m, armchair copy. When that local opening occurs, a couple of guys in the north across the NC state line join in, of course they have the advantage of another 2500' plus in altitude so when they hear us they know what's up with the weather.

Another thing I would do is to replace those 10-ohm bias resistors...especially the one which appears overheated, damaged and open to the world. Use 1/2w or 1w metal-film of the same resistance and tolerance values.

Quote:

---

Originally Posted by N8YX

Another thing I would do is to replace those 10-ohm bias resistors...especially the one which appears overheated, damaged and open to the world. Use 1/2w or 1w metal-film of the same resistance and tolerance values.

---

Got a hundred ordered. 100 was the same price as 1.
If anyone needs a few just let me know. No charge.

Quote:

---

Originally Posted by KG4CGC

Got a hundred ordered. 100 was the same price as 1.
If anyone needs a few just let me know. No charge.

---

I think we have a sticky "Parts Exchange" thread. Put 'em in there. :icon_wink:

"I have a Communications Power HF-150 here in the shack. ... That amp works very well with a 10W-class HF transceiver."
That's a whole different story than throttling back a ham rig to drive a CB amp. That's like the 100W P-P 80-10M broadband solid state amp I had back in the day, only it lacked a RX preamp. Fine with me, like I said if you have a good RX to begin with all it does is drag in more noise.

While on the subject of amps, Motorola made a 5W in 50W out VHF Class C amp for use with police HTs in a mobile Convertacom, a shoe with contacts for antenna and power. When I was living in West Creek working the world at K2PG Superstation I found that amp at a hamfester, bought it for a song, re-tuned it for 2M and hooked it up to Phil's antique packet station using an HT. That sure sped things up, ack and frack finally got together and sent resend pack(et)ing.

The resistors have arrived and as soon as I get a couple of other things done this weekend I'll get back on this project.

The delay: weather.
If it's not raining too much tomorrow (Monday) I should get the new 10m antenna hoisted back up in the air. Today I was able to get the supports up in the trees before it got too dark to see.
The antenna will consist of either 8ga wire or 4 strands of 12ga twisted together. Haven't decided yet. Maybe I'll use both.
The new 10m ant will occupy the previous space that the OCF was in. It will be higher than the old one, stronger and more wide banded. I hope. Once that's back in the air I will further test the relays on the amp. I know what you're all saying: Dummy Load. I have an old Heathkit Cantenna that is a little over half full. Not sure what to put in it. I was warned however that, the current liquid is the dangerous (toxic) stuff and I really don't like keeping it around. I think they have superfund sites full of transformers that have the same stuff in them. If I knew the proper way to get rid of it without the hazmat guys coming out it would have been done, hihi.
I'll take pictures of the other side of the board and I think the best thing to do with regard to the preamp is to just bypass it. Any advice towards that end would be very much appreciated.

After I get that done I will revise the OCF in the hopes that I will be able to load it up on 160m. If not, no worries. Last time I had a 160m antenna that was agreeable with the tuner, it was NVIS and just a dandy worm toaster. The tuner is an MFJ-948. If the antenna for 160m isn't close to size, it will arc the caps.
With regard to running the Corsair at 5 watts: it was designed to run at whatever level you set to from 3 watts to 100 watts without any degradation in overall performance. Ask me how I know.

Cantenna, then and now

Quote:

---

Originally Posted by WØTKX

Cantenna, then and now

---

Thank you.
Mine is red. Was told it was bad stuff. Can mineral oil be mixed with the other stuff?

Red? That sounds like automatic transmission fluid.

Quote:

---

Originally Posted by WZ7U

Red? That sounds like automatic transmission fluid.

---

Or transformer oil from the 1970's.

Or Marvel Mystery Oil.

Drain it all out, with appropriate mystery recycling.

Wipe it clean, maybe rubbing alcohol on the rags.

If there isn't much of the old stuff left mixing it with plain mineral oil won't hurt anything.

Mineral oil in bulk is cheaper at feed or farm stores. Like for horses.

Used my MFJ clone the past week. My amp is back up and running. Yay!
.

I would think Marvel would flash over at some point, but point taken. I suppose the mystery fluid could've started out as off road diesel and gets thicker with each application.

Thank you, Dave.

The antenna is back up at 70' in a almost perfect inverted-V configuration. I decided on twisting 4 strands of solid 12ga. I started with a total of almost 10' and twisted it down to 8' 3". Used an aluminum mobile mirror mount to attach the legs and coax in a neat and strengthy package. Choke balun and a heavy ferrite collar near the PL-259. It was heavy. Probably the heaviest antenna I've built to date. 100' of coax adding to it swinging like chimp in the breeze. Stress relief to the feedpoint with heavy dacron cord through the balun and mount.
It's broadbanded.

And now to break the amp down further ...

As I was about to take the board off of the heatsink I discovered something that was hiding from me.

Corrosion.

It would seem that in this amp's past there was some water damage that fried the preamp and froze the smaller relay. After replacing the the old resistors with new half watt versions I began shooting Deoxit and doing some gentle scraping. Guess what?

It's working and it sounds really good!

Yes. I did remove the cover of the small relay and shoot it full of Deoxit. That's an exaggeration but it is clean now however, it does not do anything. No noise is heard and no action is resulting when I flip the switch to the preamp. That's OK though. We didn't need that added noisy buggar anyway.
Removing the cover to the large relay proved to be a bit more difficult. I bro0ke one of the tabs that holds it in place. That's OK. The entire unit will stay covered with its case so I think the sloppy fit of the relay cover won't be that big a deal.

Let me say that I appreciate most everyone's input in this thread regarding this topic. You are a group of people that I hold in high regard and may the deity(s) or whatever, of your choice bestow upon each of you special dispensations. OK, just looked that up. Forget what it says on the internet. I mean some type useful or otherwise desirable blessings from or through the æther. Health and Wealth together are usually very much appreciated so, Health and Wealth to each of you!

You did the work man!

And yes, pre-amps are dumb for most 10 meter setups.

Would shake your hand and maybe a hug, for congrats.

I suggest a new mutual greeting for these times:

https://media.giphy.com/media/N0PFV6nGA72XC/giphy.gif

Uh, maybe share hand sanitizer or fistbump with latex?

Quote:

---

Originally Posted by WØTKX

And yes, pre-amps are dumb for most 10 meter setups.

---

Three of the receivers in use here incorporate pre-amps: The R-7, R-70 and R-71A. Most times they're turned off. The two Icoms are often run with the Attentuators turned on as well. That said...late at night, when man-made QRM/QRN is at a minimum, those pre-amps can come in handy when working a station way, waaayyy out there. Especially if the other guy's setup isn't as strong on the TX end of things.

Back in my 11M daze (when there were people to talk to on the band) one of my buddies lived a considerable distance from me, but still within the state. I had an Ameco RX pre-amp inline with my antenna feedline and he had similar on his end. 10-15dB of amplification may not sound like much but it made the difference between copy and no copy. (I had a 2-el Yagi fixed in my friend's direction, while he had an omnidirectional setup. His antenna came in handy for his wrecker business, long before the advent of affordable cell phones.)

My CPI CP-2000/BC-2000 combo has a receive pre-amp in the Base Console accessory. When I have the rig inline, I almost never run the pre-amp. I suspect those devices were marketed at a time long before the arrival of noisy switching power supplies and PWM controllers on the consumer electronics scene.