[NLRS] Amp Driving Me NUTS...

[Bill Ockert]

Hi Scott,

At the risk of asking a completely stupid question, have you checked what 
you are putting into it?
I ask because a couple of years back I picked up a used 5 in 150 out brick 
at a hamfest for $10.  '
The seller said it had been back to the manufacturer twice and that they 
said nothing was wrong
with it but he could only get out about 20W using his handheld to drive it. 
I bought it mostly out of
curiosity.  Got it home, tried it out, 20W.  Opened it up and spotted a 
variable cap on the input,
tweeked it a bit... 150W output.   That spiked my curiosity so I moved the 
cap back and put a
Bird with low wattage slug between my handheld and the brick.  SWR was 
through the roof and
the handheld was shutting down.  Tweeked it back for best match to the 
handheld and all was good.

It is a long shot that this is the problem but thought I would mention it.

73 de Bill ND0B

----- Original Message ----- 
From: "KBØNLY" <kb0nly at mchsi.com>
To: <nlrs at mailman.qth.net>
Sent: Wednesday, May 11, 2011 9:40 AM
Subject: [NLRS] Amp Driving Me NUTS...


>
>
> Anyone got some amp repair experience that can offer some suggestions 
> here???
>
> I have a RF Concepts 4-110 that I just bought, needs repair, had about 40w 
> of output when I got it.  This uses a MRF646, 45w driver, and a pair of 
> MRF648’s, 60w finals, for 100-120w of output.  They seem to vary a bit but 
> that’s not the issue here.
>
> My first thought was the driver was bad, upon opening it up I found this 
> unit didn’t have the optional attenuator propagated on the board, the last 
> two of this exact model amp that I had rebuilt I found the MRF646 driver 
> was blown from being overdriven.  So the first thing I did was grab a new 
> 646 from my spares and replace it, along with new caps for input and 
> output matching as long as I was doing it.  This didn’t increase the 
> output of the amp any, still about 40w.
>
> The next step I took was to replace the MRF648 finals with a fresh new set 
> I also had in my spares, the power output actually dropped to about 25w of 
> output.  What the heck??  I checked and rechecked and rechecked 
> everything.  Replacing all the caps on the input and output matching, 
> checked the bias and set it to .70 volts per device while keyed up without 
> drive applied, everything looks like it should, I’m stumped!!!
>
> Anyone offer some constructive ideas here??  The forums online have been 
> nothing but stupid responses like send it here or there for repair.  I 
> would have done that in the first place if I wanted to... HI HI
>
>
> 73,
>
> Scott KBØNLY
>
>
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