DX350 oscillation issue

[MKwrench]

I grabbed a used DX350 and Black Widow DX250 to fiddle around with They both look nice and clean inside and appear to be all original, except for the DX250 having 2879s in it. I rigged up my test equipment as a station monitor switchable between the antenna and test load.
On a dummy load they both look great and show impressive (to me) numbers on my meters.
I adjust the deadkey to 50w for testing the 250 on high where it swings close to 300w PEP.
Then the 350 with a 65w carrier will swing to over 350w PEP.
My antenna is an attic dipole with a 1:1 feedpoint choke and great match across the 11m band. When I switch to the antenna vs the dummy load I get a strange oscillation that I found gets better when the carrier is raised on the 350 or the channel frequency is raised on the 250.
The 1st video is the 350 acting up on the "AM in" setting on the antenna. The 2nd is High on a dummy load. A few more pictures show AMP voltage, Radio voltage and the extra CM choke I rigged up directly behind the amp, and my attic dipole measurments.
I read somewhere that the metalized mica caps can drift causing issues, coud this be whats up here? What do you think?

 

[nomadradio]

Don't remember seeing the metal-clad mica caps go bad. Does make me suspect either a loose circuit-board screw or missing/ damaged negative-feedback resistor/capacitor issue. I have seen the capacitor on the secondary of the input transformer go bad and create issues. Worth checking.

73

 

[MKwrench]

I'm glad you responded, I'm a little more confident about getting in there and checking things out. I'll start with the feedback sets as recommended.
I'm really not familiar with the insides of these yet.

 

[brandon7861]

Why does that balun look like it is at the radio end instead of at the dipole? The balun needs to be at the antenna's feed point, not at the radio.

Add a common mode choke at the feedpoint, maybe more than the toroid too. Wind some coax coils if you can.

A 1:1 SWR does not mean the parasitic oscillation (different frequency) sees a 1:1 SWR. If improving the common-mode rejection does not improve things, change coax length to hopefully make the antenna system not support the self-oscillation frequency.

 

[MKwrench]

The antenna has one at the feed point already, I just added that other one to see if it changed anything. I agree that it's going to be the antenna causing the issue, I'm finding out that the further I go into the 10m band from 11m the better that carrier behaves.
The feed point balun is a mix 43 240 core, dual bifilar wound with 14ga speaker wire 6 wraps per side, the new one I just put together (is behind the radio now) uses Teflon wire instead, I was going to swap it out tomorrow. The radials on the dipole itself are 12 gauge solid core insulated wire from some leftover Romex I had laying around.

 

[MKwrench]

I just tried it on another antenna, which is just a magnet mount mobile stuck to my AC unit outside...lol
And it works perfectly fine.

 

[MKwrench]

Okay I think We got it, My feed line is two sections of LSR240 (supposedly equivlent to LMR240), One 50' run from my attic feedpoint balun down to the basement, and anorher 20' or so from that point to the transceivers. Its where I had them connected that caused my problems.

I didn't have a "barrel" connector or a SO end to fit the 240 line, but I did have an old TVI trap to use in place of a "barrel" with two PLs. I thought it was a convenient way to kill 2 birds with one stone and mounted it to the basement rafters (or floor joist... whatever).

Just now, I removed the balun/choke from behind the amp, the little antenna that was fine gets the fuuzzieness now too. Then, I removed the TVI trap in the dipole line and the issue dissapeared.....
Now I see the issue closer to the top of band 5 on the Stryker. I'll deal with that crap another day, It still works nice up there on SSB, so I can still yap in the 28s.

Cool, I'm glad I dont need to repair anything in the amp, I'll work on the aesthetics instead.
But, stay tuned I havn't tried the 250 again yet,....

 

[MKwrench]

On another note: Something I just read says that my parallel wound choke cuts the common mode resistance in half, I thought it was the other way around, while increasing the power handling.
Then asking AI, suggests "twisting" a singular bifilar pair for maximum common mode choking and a better impedance match. I had always thought that twisting was a big mistake and that flat windings are best.
Maybe this is another thread....

 

[MKwrench]

Now after swapping jumpers and meters around a few times chasing the issue at the other bands, I discover that the jumper between the radio and the amp make a huge change an RG58 3' gives me a wide range without the issue, a 8X 1' gives a little less and a LMR240 3' is terrible with a real thin range. I believe this has to do with the amount of loss in each cable crossing a "padding" threshold that changes reflection.

When I put a meter between the amp and radio.... SWR is insane at the high power level, but completely fine everywhere else. Even on the dummy load, but only on high.
I'm searching for a thread where I thought someone mentions a variable cap to chill the input in the amp. Is anyone familiar with that?

It's an issue I'd like to fix even if it's not the exact issue I'm chasing, but I'm sure it's relative... In the meantime I'll enjoy what I have without running it on high.

 

[MKwrench]

Popped the amp back open to inspect, AI is telling me that a dirty switch or cracked solder joint near the input transformer can be an issue. I found the other thread I was looking for, It was by Shadetree where C8 was replaced by a variable cap to tune the input.
The schematic I have from CBTricks.org shows a 470pf should be used, mine has an 82pf. HandyAndy who commented in that other thread mentions a 333pf.
The others seem to be close to what they need to be C9 is 1500 vs 1200pf, C14 is 1200 vs 1000 and C16 is 82pf vs 91pf.
I have a few 470s, but they're ceramic disks, I'd rather use a silver mica like it came with.
Here's some pictures. Is there an easy/safe way to clean up all of that nasty flux?

 

[brandon7861]

The first thing I would try is ditching the bifilar parallel wire winding and run 6+6 turns (maybe even 5+5) of some RG316 or RG400 teflon coax in a W1JR crossover method on a type 61 core (or stacking a couple 43's if you have to).

That gets rid of the impedance mismatch at the "transmission line" used to make the balun. It also gets rid of the very lossy PVC dielectric on the speaker wire.

I would also make sure the core isn't heating up. I think 43 gets pretty resistive at the top end of HF (not 100% on that). I wouldn't really expect it to be getting hot on account of the thousands of people doing the same, but maybe the core was mislabeled or from an unreliable source?

Even if the core is from a reputable source and rated for this use-case, excessive common-mode current (gonna happen in an attic) will still turn to heat in the core.

edit to add: I'm sure this point goes without saying, but make sure the coax feeding the dipole is perpendicular to the dipole for as far as you can run it. Any coax parallel to it will pick up excessive RF and add to the common-mode load the balun must eat.

As for the input tune, I would leave that alone. If the tune is good at low power, it should be good at high power too. This sounds like a common-mode or feedback issue.

 

[MKwrench]

Sounds like a plan, I rewrapped my dual balun with single teflon insulated wire and the measurments weren't that great. I'll grab some RG400 and reuse two of my 43s. I dont have any 61 cores handy.
I did get it to act up into the test load too and I can achieve the same output carrier on low or high but it will only ever act up on high, thats what made me want to revisit the amp.