Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

donate to eham
   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: Replace one of a pair?  (Read 4855 times)
JS6TMW
Member

Posts: 1257




Ignore
« on: September 11, 2015, 09:03:54 PM »

(OK, please no "told'ya so's"  Huh).  I fired up a new RM Italy KL300 with a few watts input and after a few seconds of running some power out into a dummy load on 14 MHz I smelled a resistor heating up and then burning, and shortly afterwards it quit dead. The output MRF455s didn't even get warm. One of them is now base-emitter open circuit but the other measures OK. I suspect one was a dud or they were badly mismatched to begin with, so I am thinking about just replacing the one bad one. They are kinda pricey and two would come close to the cost of the new amp. I don't see anyone selling claimed matched pairs, so I assume there is no problem with just replacing the bad one. Opinions?

Steve AI6KX
Logged
REASTON
Member

Posts: 61




Ignore
« Reply #1 on: September 11, 2015, 09:19:09 PM »

Replacing one RF power transistor from a pair is about the worst repair decision you could make. You don't know for sure the condition of the other one, do you?

You asked for an opinion so here it is.......you have a piece of junk amp and you are going to put more good money into it to fix it? If I were you I would throw that piece of crap in a trash can, save your money, and buy a decent amp. You then would be money ahead.
« Last Edit: September 11, 2015, 09:23:10 PM by REASTON » Logged
KM3F
Member

Posts: 910




Ignore
« Reply #2 on: September 11, 2015, 10:09:58 PM »

I looked up your model and see it's not legal for sale in the US.
The PDF file of the schematic shows no filtering whatever.
No spec for frequency other than HF.
5 watts max on AM/FM and 10 pep on SSB. True CB levels.

You got what you deserved.
Sorry to say it like it is.
Good luck.
Logged
JS6TMW
Member

Posts: 1257




Ignore
« Reply #3 on: September 12, 2015, 12:44:32 AM »

10-4 good buddies, I saw those coming.

Divine retribution from the Made in America Gods. Oh, how it smarts.

It was nevertheless sold in the US.

I am not planning to use it there however since I live in Japan, won't be using it for anything but CW and mainly in other countries on mini-DXpeditions, and I have built an excellent filter for it.

The options for small portable amps when QRP is too little are mighty few.

Steve AI6KX
« Last Edit: September 12, 2015, 07:47:50 AM by EXW6BMZ » Logged
KM3F
Member

Posts: 910




Ignore
« Reply #4 on: September 12, 2015, 05:10:01 PM »

Steve, I'm not looking to assaile you  but you must take responsibility for your decisions.
My answer referring to US FCC approval was based on not knowing your in another country since you did not mention  it in your first post..
These amps were first marketed in 1987 according to what I saw on a search.
The 455 devices are old time today, expensive and limited availability.
I could take that amp, replace with different a devices, make some changes and get it to work again for about $75.
But it still needs filtering and driven at a reasonable level.
When your on the air with such, it does not make a difference to other ops whether your in Japan or the US as far as interference generated is concerned using such an amplifier.
It has the same effect no matter where you use it.
Along with being a bit firm, I gave you more information because I want you to learn from this situation..
If it has no value then please forgive my effort.
Good luck.
Logged
JS6TMW
Member

Posts: 1257




Ignore
« Reply #5 on: September 12, 2015, 05:34:16 PM »

OK, I appreciate your more helpful comments.

I was about to start on several modifications to the feedback and frequency compensation, and add a biasing source, but I decided to try out the amp as it came out of the box. As I said, I already have built a filter in a small shielded box that couples to the output SO-239.

If you could just mention what lower-cost devices you would use, that would be a big help since I could not find anything myself.

I will be driving it with a Weber CW Tribander, which puts out about 4 watts.

Steve AI6KX (I am waiting for my Japanese reciprocal license)
Logged
KM3F
Member

Posts: 910




Ignore
« Reply #6 on: September 12, 2015, 10:23:08 PM »

Download the specs for your MRF455 and compare to specs for 2SC2290.
They are very close to the same.
Pay attention to P in / P out and best IMD drive power levels.
Go to RF Parts and view the price of the 2SC2290 or from other sources.
The mounting should be drop in once you get the old device off the board.
How to get the old device loose from the board without overheating:
Use small flat blade screw driver sharpened to a knife edge.
Use to cut the device tabs connections to the board near the device ceramic.
Remove mounting screws and lift out.
Unsolder tabs from pads and clean up the pads.
Mount the new units in place with heat transfer compound and torque to about what you found removing them. Pay attention to device orientation for EBC.
Finish trimming and soldering the new tabs down and check for solder crosses.
Good luck.

Logged
JS6TMW
Member

Posts: 1257




Ignore
« Reply #7 on: September 13, 2015, 02:23:06 AM »

I can't see much price difference between the MRF455 and the 2SC2290, unfortunately. In fact RF Parts has the MRF for about 10 bucks less.

I do indeed need two. The one that tests OK on the junctions with a DMM doesn't put out any power as a single-ended amplifier, so I guess I fried it too.

Thanks for the unsoldering tips. I didn't have any trouble lifting the bases off the boards to test them.

Oh well, today was a nice day for antenna work....

Steve in Okinawa

Logged
JS6TMW
Member

Posts: 1257




Ignore
« Reply #8 on: September 14, 2015, 04:27:35 AM »

The MRF458 is an 80-watt RF transistor that is spec'ed as "low-beta" but still has minimum gain (30 MHz) of 12 db (compared to MRF455's 13 db. They were used in Ten-Tec rigs at one time. The reason I mention them is that RF Parts sells them for only $25, or $60 for a matched pair. At that price it's worth fixing my POS amp What would be the problem in sticking them in my amp? I get at least 4 watts out of my QRP rig for drive, and am not looking for more than 100 watts CW out.

Steve AI6KX
Logged
JS6TMW
Member

Posts: 1257




Ignore
« Reply #9 on: September 15, 2015, 03:18:58 AM »

I guess this post should be retitled, "Get a Pair"

There are some cheep 2SC2290's out there. Worth a try even if not genuine TOSH?
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!