[NLRS] Killed my amp? Probably

Thu Sep 3 17:29:29 EDT 2015

My old and very cheap 2m Mirage died that way from bad relay contacts. 
Replacing the relay was very difficult because all the high current 
wires and power transistors were soldered in after the circuit board was 
mounted to the heat sink. So I soldered the relay contacts in the 
transmit position and worked up an external 2 pole Doe-Key relay and PTT 
controls from the FT-857D because a root cause of such a relay failure 
is switching with full RF drive using RF sensing. Only had more troubles 
because I made up the RF cables from LMR-240 and one had a splinter of 
foil that shorted the amlifier output. I think the driver transistor 
survived but at least one of the PA transistors is melted into a short. 
Since then I have acquired a 2m Diawa with a little more power but it 
needs to be hard keyed for SSB or CW and I've not integrated it into the 
station yet. If my antenna was bigger and higher I'd be doing decently 
with the barefoot 50 watts of the 857, still did good enough to get a 
couple VHF contest certificates for Iowa section in the past year.

So 20 watts to the amp that takes 30 watts will naturally produce less 
than full output. If its truly linear to 30 watts in, 20 watts drive 
should produce 2/3 the rated output. The loss of power could be any RF 
part anywhere between the coax connectors that is changing value from 
the heat of RF current. Relay contacts are the prime suspects.

73, Jerry, K0CQ

On 9/3/2015 4:12 PM, David Palm wrote:
>
>
> Unfolding saga -- now I see I'm wrong.  I don't have a 4-110, I have a
> 4-310 which should take up to 30 watts of drive.  So I'm well under that
> limit, but last night I watched the output of this amp fall off as I was
> key down and now it's only putting out 30 watts for 17 watts in.
>
> I'm also seeing a bit of SWR -- not huge, but noticeable -- when I put the
> amplifier in line.
>
> Any other ideas on what to check on this dude?  I've got it open on my
> bench.
>
> 73,
>
> David  W9HQ
>
>
> On Thu, Sep 3, 2015 at 3:51 PM, Barry Malowanchuk<ve4ma at shaw.ca>  wrote:
>
>>
>>
>> Better RF Parts or else you may get a Chinese copy
>>
>> Barry VE4MA
>>
>> ----- Original Message -----
>> From: "Dr. Gerald N. Johnson"<geraldj at netins.net>
>> To: nlrs at mailman.qth.net, "Badger Contesters"<
>> badgercontesters at mailman.qth.net>
>> Sent: Thursday, September 3, 2015 2:00:06 PM
>> Subject: Re: [NLRS] Killed my amp? Probably
>>
>>
>>
>> RF Parts probably also has the part.
>>
>> For the future you need to attach a cable to the amp input with 1.25 dB
>> loss so the 20 watts from the 897 is only 15 at the amp. That's just a
>> few feet of RG-58. ARRL handbook chart shows 12 or 13 dB / 100 feet off
>> the top of the graph though. So ten feet should be pretty close. And the
>> driver probably is bad.
>>
>> 73, Jerry, K0CQ
>>
>> On 9/3/2015 11:58 AM, David Palm wrote:
>>>
>>>
>>> OK, time to tell on myself.  I was checking out my gear last night.  222
>>> MHz ampflier is working fine and playing well with the transverter there.
>>> So good to go.
>>>
>>> On 432 it's a different story.  Here's what you get when you don't use
>>> stuff for a long time and forget when you have.  I connected the rig to
>> the
>>> amplifier, an RF Concepts 4-110.  I had long forgotten that this amp can
>>> only take a max of 15 watts input.  My FT-897 puts out just over that.
>> So,
>>> I was keying the amp and getting about 80 watts out, which is actually
>>> rather low, especially with that much drive.  But then as I watched, the
>>> output just started dropping before my eyes, until I basically am getting
>>> little to no amplification at all.
>>>
>>> I presume that I burned out the driver stage.  The schematic says that
>> this
>>> is an MRF646.
>>>
>>> Those of you who know about such things, does my diagnosis seem correct?
>>> And do you know of any better source to get an MRF646 than this guy, who
>>> has them for $20 apiece:
>>>
>> http://www.ebay.com/itm/Motorola-MRF646-/281687443775?hash=item4195e1493f.
>>> Hopefully this will not be a big problem to swap out and be back in
>>> business.  If you've done such things before and have some good advice,
>> I'd
>>> love to have it.
>>>
>>> Doh!
>>>
>>> 73,
>>>
>>> David  W9HQ
>>> ______________________________________________________________
>>> NLRS mailing list
>>> Home: http://mailman.qth.net/mailman/listinfo/nlrs
>>> Help: http://mailman.qth.net/mmfaq.htm
>>> Post: mailto:NLRS at mailman.qth.net
>>>
>>> This list hosted by: http://www.qsl.net
>>> Please help support this email list: http://www.qsl.net/donate.html
>>>
>>>
>>
>> ______________________________________________________________
>> NLRS mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/nlrs
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:NLRS at mailman.qth.net
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>> ______________________________________________________________
>> NLRS mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/nlrs
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:NLRS at mailman.qth.net
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>>
> ______________________________________________________________
> NLRS mailing list
> Home: http://mailman.qth.net/mailman/listinfo/nlrs
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:NLRS at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
>