19gtl power output all over the place

[Uncle Ronnie 336]

so there is an intermittent problem that I thought I solved with a recap. but apparently not. im thinking it might be a bad driver but i want to make sure first.
power drops, audio flutters, eventually comes back but about half of it's power output. odd stuff. before it started acting up again, I did a mosfet conversion to it, and its workin nice when it does work. (master mod thread coming) looking forward to some opinions here before i spend time changing out parts that don't need it

 

[Uncle Ronnie 336]

previous fixed issues:
audio was dead bc of the 470uf c41

holy cigarette tar

 

[Uncle Ronnie 336]

smh changed driver. wasn't the problem. still getting the intermittent machinegun power drop on tx

 

[nomadradio]

It's old enough, I'd be wondering if the PLL is dropping out of lock briefly during transmit. Could be bad electrolytic capacitors, since they serve to isolate separate circuits, so transmit audio doesn't leak into the PLL, for example.

Not a common symptom.

73

 

[Uncle Ronnie 336]

all electrolytics have been swapped. and the rx is workin good

 

[Uncle Ronnie 336]

smh changed driver. wasn't the problem. still getting the intermittent machinegun power drop on tx

the more i think about it, i should check the 8v feed on things to see if the voltage is dipping on tx tomorrow. i does kinda seem like SOMETHING is coming outta lock

 

[9C1Driver]

I think the issue will end up being something related to the the mosfet conversion that was done. If the final was smoked I would have replaced it with the correct original. If it was a working radio I would have left the final section alone. If it's just a fun project carry on and maybe look over the mosfet mod work that was done.

 

[Uncle Ronnie 336]

so it IS a fun project with no angry customer on the other end. came in a lot with some 140's.

the thing is, it did it with the stock final too once i restored modulation, did the rest of the electrolytic recap bc why not, and i thought it came back into line until about 10 minutes AFTER i did the FET. when it's not sounding like a machine gun i'm getting 7 watts and around a peak of 28-30ish. if i get this one problem figured out than we will have a contender here and i'll post the FET procedure for scientific and research purposes. im seeing zener diodes being what's involved in the 8v on the schematic, but i wont know until tomorrow. im gonna head out for the evening

 

[Handy Andy]

That "machine gun" effect, if it's not affecting the RX/TX light - can be from many of the Electrolytic caps - some fail "shorting" on a voltage that is much lower than their rated working values.

That may be also from a cap not quite correct in value on the Audio Bootstraps or much of the Electrolytics that are used around the Audio amp - they work great when there is no RF - but put them in a radio with RF - that can be a lot of work to find the correct values - it's why radios like the 19 are so "ducky" they have fewer parts and lots more places for RF to come in and make it unstable.

So they use smaller values than what would work otherwise.

There may also be some Mylar (Green Chiclet) or Polystyrenes that are now showing their age.

 

[Uncle Ronnie 336]

yea thats what I was thinkin of swappin next lol. so electrolytics being underpowered. never heard of that. so if i replace a 16v with a 25v which happened in a few slots, that has wonk potential?
so not that it changed anything, when i recapped, the main rail was swapped with a 2000uf 25v and i did the limiter release cap mod too.

 

[Uncle Ronnie 336]

I just ordered a big tray of mylar assortment so lets see where this goes

 

[Handy Andy]

so electrolytics being underpowered. never heard of that.

Not "underpowered" breaking down before they reach their working ratings - a quality issue of performance.

There are several "categories" or sets of capacitors that have tolerance ratings that when they operate - can change their rated value in percentage - others can keep their values for a wider working range of power from microwatts up to several watts while another can fail reaching only 1 watt of power dissipation.

This rating is what some call "Prime". Their ability to work and stay in value without having to be swapped out because they drifted off or just didn't work like they are stated to.

When you work with Audio like the '19 the capacitors "react" to the power they have flowing thru them, it's a factor of their performance - when one or two go out of tolerance or not stay true to their values - they interact and make the Audio Amp unstable.

Your reference to "Machine gun" - made me think of this and I figured you may need to recheck the caps - compare those to a known good Cobra 19 if you have one to test or compare with.

 

[Uncle Ronnie 336]

ah ok cool. the caps are all brand new bc sometimes with old radios the desire is to "just be done with it" never to crack the case for another 30 years

 

[Uncle Ronnie 336]

no mylars had to be swapped. a slug near the PLL was just on the edge of not locking so the problem has been solved! mosfet mod thread coming soon.....

 

[Uncle Ronnie 336]

ok i lied, its a bit unstable so i guess some mylars will get swapped around the vco