RM KL400 problem

This is the place to discuss any general CB radio related topics. Getting started, installations, operation etc.
User avatar
Phoenix
Radio Addict
Radio Addict
Posts: 601
Joined: 27 Apr 2008, 09:08

RM KL400 problem

Post by Phoenix » 12 Sep 2017, 18:37

Purchased this a while back and just got round to given it a go....Smells awful so took the bottom off to reveal a awful looking Capacitor (see photo with the component in front of of the ferrite chokes) What does this CAP do ? What should i replace it with ? Whats caused it and will anything else of gone down with it ?

Thanks for now guys

Image
26TM114

Tim
Radio Addict
Radio Addict
Posts: 879
Joined: 10 Jun 2016, 12:35
Call Sign: 26AT239

Re: RM KL400 problem

Post by Tim » 12 Sep 2017, 19:20

Looks like someone has been in there with a welder by the look of the awful solder mess. I would send it back whence it came pronto.

User avatar
Phoenix
Radio Addict
Radio Addict
Posts: 601
Joined: 27 Apr 2008, 09:08

Re: RM KL400 problem

Post by Phoenix » 12 Sep 2017, 19:41

Yes it looks a little rough around the output transistors but the welding is sound ha ha just need to know a bit about the CAP to sort it
26TM114

User avatar
ch25
Super Member
Super Member
Posts: 223
Joined: 03 Dec 2016, 11:07
Call Sign: n0call

Re: RM KL400 problem

Post by ch25 » 12 Sep 2017, 21:15

Did you try to take a look into schematic? Maybe that cap is right there.

User avatar
Buick Mackane
Legend
Legend
Posts: 6787
Joined: 21 Aug 2012, 17:30
Location: A Country Called Scotland

Re: RM KL400 problem

Post by Buick Mackane » 12 Sep 2017, 21:26

Radio Someone Still Loves You

User avatar
Phoenix
Radio Addict
Radio Addict
Posts: 601
Joined: 27 Apr 2008, 09:08

Re: RM KL400 problem

Post by Phoenix » 13 Sep 2017, 05:59

Not found the circuit diagram for this revision the .PDF is for a very early 400 and the board and components are different
26TM114

wa10
Top Poster
Top Poster
Posts: 1150
Joined: 19 Feb 2008, 00:37

Re: RM KL400 problem

Post by wa10 » 14 Sep 2017, 23:34

The cap is a compensation cap to cancel some of the inductive reactance of the primary winding on the transformer at higher frequencies, helps keep output up & flatten impedance excursions as you move up the bands,

the correct value changes with different transistors, its not critical but it does effect output power on 11mtrs,
usually dm19 1300pf 500v or thereabouts dipped silver mica, older versions used 2 or 3 ceramic caps in parallel,

the two large feedback resistors either side of the transistors don't look right in that pic, could be lighting issue,
standard they should be 68ohm (blue grey black),

whoever put the 455's in should have their iron confiscated.

good luck.

User avatar
Phoenix
Radio Addict
Radio Addict
Posts: 601
Joined: 27 Apr 2008, 09:08

Re: RM KL400 problem

Post by Phoenix » 18 Sep 2017, 11:29

Wa10 thanks for your comments...Yes the soldering is not great on the transistors and it looks like the resistors are the the wrong value..can u just confirm the wattage and type of the 68 ohm you refer to also will it be 68ohm for mrf 455 transistors thankyou
26TM114

wa10
Top Poster
Top Poster
Posts: 1150
Joined: 19 Feb 2008, 00:37

Re: RM KL400 problem

Post by wa10 » 18 Sep 2017, 11:55

in the the mrf455 & sd1446 versions they used 68ohm 2w, they get very hot on high power sometimes falling out of the board,
i have tried different parallel combos over the years, now i use 100ohm 5w, they don't overheat & never had stability issues using a little less feedback.

i had a look in the two kl405's i have for repair, both have 68ohm 2w but one set have been cooked more than the other & look like
black brown black.

User avatar
Phoenix
Radio Addict
Radio Addict
Posts: 601
Joined: 27 Apr 2008, 09:08

Re: RM KL400 problem

Post by Phoenix » 18 Sep 2017, 13:13

Thats great with the test's you've done would you advise i move over to the 100ohm 5w resistors also any tips for getting the welding off these 455's without generating too much heat ( worried i may damage the devices) thanks so far your info is appreciated.
26TM114

User avatar
Phoenix
Radio Addict
Radio Addict
Posts: 601
Joined: 27 Apr 2008, 09:08

Re: RM KL400 problem

Post by Phoenix » 18 Sep 2017, 13:55

This is an image taken off the net....NOTE the MS1051's Outputs also see the balance resistors are the same value as mine 12 ohm so im thinking the balance resistors in my amp are correct.. BUT the OutPut transistors have been changed and are the wrong ones for this board....Arrrrhhhhhh plot thickens HELP :wtf:

Image
26TM114

wa10
Top Poster
Top Poster
Posts: 1150
Joined: 19 Feb 2008, 00:37

Re: RM KL400 problem

Post by wa10 » 18 Sep 2017, 19:34

I have tested a few rm's with ms1051's starting with the millennium version ,

in theory they are a good transistor, higher rated than 455's & 1446's something like pd=290w vs about 170-180w for the small flange 455/1446 transistors and better thermal resistance but they don't seem to work so well in the Rm circuit as it comes from the shop,
low output at the same drive level as 1446's and you can't drive them harder without smoking the power selector & input attenuator resistors,

its possible that yours had 1051's in & RM changed a few parts to work better with the 1051's,
if I was repairing yours I would use the components that have worked OK with 455's & 1446's with that type ferrite in the past,

increasing feedback by using less than 68ohm lowers gain and increases the current through the resistors that @2w are not up to the job of full power FM even using 68ohm,
i lost count of the RM & zetagi amps i have fixed with burnt feedbacks or feedbacks dropped out of the board they get so hot,
never had an issue with 5w 100ohm, gain increases a tad and they never burn or fall out of the board,

RM used 5w 68ohm in the HLA amps ( 18 years late ) which are the same basic amp as the 2 & 4 pill cb amps with filters & protection on a larger heatsink,

for a cb amp i don't need that much feedback to tame the transistors increasing gain as you move down in frequency or to minimise frequency dependent impedance excursions so i use 5w 100ohm that run cooler,

i use a large ceramic trimmer cap across the primary, peak the output then back it down a little,
remove the trimmer & measure it then replace it with fixed caps close to that value but you can use what the 455 amps use without the faffing finding a suitable trimmer cap and capacitance meter,

if it did have 1051's its worth checking the other components to make sure they have not changed anything else around the outputs such as the 2 ceramic caps in series with the feedback resistors or changed a resistor in the bias circuit to compensate for the different hfe of the transistors,

bear in mind the resistors in one of these kl405's look just like yours but measure 68ohm, they don't look at all like 68ohm because they have been baked, try measuring yours to see what they are.


there is a knack to soldering that style transistor without making a mess or lifting the foil traces,
its all about using a suitable iron and timing, too small or short time and it won't all flow it goes snotty & dull like your amp, too hot or too long on the job and you lift traces & risk damaging the transistors,

since you don't need to remove them i would use an iron with a large tip & thermal mass, if melting all the solder on a pad takes more a few seconds the iron is too small wattage,
or lacking thermal mass in the tip,
or you did not tin the tip with a decent volume of solder to get the solder on the board melted fast,

i have owned many fancy & expensive soldering stations over the years that were crap for most jobs compared to cheap Antex irons including my weller & pace stations,
for cleaning something like that up i use a 25w 12v antex xs on 14v with the largest tip & a cheap solder pump and solder wick then resolder them with eutectic solder or 60/40 if im out of eutectic.

if your at all in doubt leave it alone, scruffy & working beats neat & broken.

User avatar
Phoenix
Radio Addict
Radio Addict
Posts: 601
Joined: 27 Apr 2008, 09:08

Re: RM KL400 problem

Post by Phoenix » 19 Sep 2017, 05:13

Thanks for the reply again really useful info and soldering tips ...well i leave the 455's as they are they do work ...if i need to change the resistors then i maybe should just clip the legs but will check the value first....Apart from Maplin where's the best place to get the big resistors and the SM cap ? I also need a bag of loose electrolitics for a project im starting all of different values but single...thanks so far pal appreciated
26TM114

wa10
Top Poster
Top Poster
Posts: 1150
Joined: 19 Feb 2008, 00:37

Re: RM KL400 problem

Post by wa10 » 19 Sep 2017, 12:14

i used to get the 5w 68 & 100ohm & 3w 10ohm metal oxide resistors & caps from farnell, the minimum order is a pita when you just want a few bits n bobs to fix your mates cb amp,

transistors & caps from RF parts in the USA, the only place i know of that sell matched transistors for cb amps,

mouser are pretty good but also have a minimum order.

User avatar
Phoenix
Radio Addict
Radio Addict
Posts: 601
Joined: 27 Apr 2008, 09:08

Re: RM KL400 problem

Post by Phoenix » 19 Sep 2017, 17:58

Thankyou
26TM114

Post Reply

Who is online

Users browsing this forum: Yahoo [Bot] and 7 guests