Hi: Steve, Colin, Todd,
Thanks so much for your replies, they are very much appreciated.
Steve, I've done little work with solid state rf pa design but long for
understanding. I will try to describe the circuit:
It consists of two 2SC2694's in parallel DC wise. The bases are fed in
phase, each by a separate LC network that originate from a common point
which provides RF and DC bias. The emitters are grounded.
The collectors tie to a common wide trace on the board where DC is fed by a
choke. The tank coil comes off the center of that trace with capacitors to
ground on either side of the coil, forming what looks like a pi network. The
load side of this network is then coupled to the filter board by a series
capacitor.
The pi network capacitors need replacement (2 in the "tune" position and one
in the "load" position.) These are 200pf, 68pf, 47pf monolithic chip
capacitors which measure about 0.15" x 0.20". The series output coupling
capacitor also needs replacement. It is a ceramic 27pf device with leads.
My concern and lack of understanding is in the voltage rating needed for
these capacitors and possibly the current rating of the coupling capacitor.
As Colin mentioned, @ 50 ohms, 100 watts requires 71 volts RMS. That's
nearly 100 volts peak. Tech support at ICOM said 100 volt capacitors would
be fine.
One of the steps in the tech manual calls for the radio to be keyed at full
power in the FM mode with no load, and then the APC adjustment made so that
the DC supply current is 10 amps. Am I wrong to be uncomfortable with doing
that?...with new $100+ finals? And it would seem to me that tank circuit
voltages might exceed 100 volts while making that adjustment.
Anyway, the chip caps that I've found so far seem to come in 100 volt max
ratings, and some minimum order quantities are 4000. Possibly I'm looking in
the wrong places?
Some history: I returned the radio under warranty some years back, for an
intermittent rx issue. I got it back with scratch on the front panel and a
report that the pa tank coil connection was at fault. (I made them give me a
new front panel which I have yet to install) After that the rig never seemed
to put out as much power and it ran hotter and down powered on high temp
fairly soon in the tx mode. I shelved it for several years and recently
decided to go through the entire adjustment procedure to verify that all is
up to spec before offering it for sale.
I started with the pa board. It drew proper idle current but when I applied
drive, the current went way up (~35A) and max power out was about 40W. It
was obvious that someone had replaced all of the major components on the
board. There were several cold solder joints in the output circuit and a
couple of not-soldered joints too. The screw that grounded the common
emitter connection to the heatsink was backed out several turns and not
making contact. I guess someone had a bad day.
In troubleshooting, I accidentally fried the finals. The bad chip capacitors
were not apparent until I pocked them and they fell apart. The series
ceramic capacitor in the output has a crack that I found through close
inspection with a magnifying glass.
I now have new pa transistors, shining solder connections, and a beautifully
clean board. I just need 4 capacitors and I'll bet she'll run better than
new. In fact, I probably wont want to sell it!
Any help, comments, criticisms, etc, would be very much appreciated.
tnx es 73
Jeff
----- Original Message -----
From: "Steve Thompson" <g8gsq@qsl.net>
> -----Original Message-----
> From: Jeffrey Madore <K1LE@arrl.net>
>
>
> >Gentlemen,
> >
> >I'm repairing the pa board (100w) in an Icom 275H and need to replace all
3
> >of the monolithic chip capacitors in the output circuit, as they are
burned
> >open. I also need to replace the ceramic DC blocking capacitor between
the
> >tank coil and the antenna.
> >
> >My questions are:
> >
> >Where is a good place to purchase these caps? Icom has all but one of
them,
> >but they want $6 to $8 each for a chip cap. I haven't bought these types
of
> >devices but that seems like a lot. In addition, Icom tech support says
that
> >they can't supply a replacement component for the one that is no longer
> >available...I would have paid their price if they could have just given
me
> >what I needed.
> >
> >Secondly, the Icom service manual shows only capacitance values. What
would
> >be a conservative voltage rating for caps in a 12 volt 100w pa tank
circuit?
> >
> >Chip capacitors are really new to me. Is there any one type that would be
> >preferable in an RF power application? What % tolerance is acceptable?
>
> Without seeing the schematic I'm guessing, but they could be 'Hi-Q'
ceramic
> chips, often referred to as 'ATC', although ATC is but one manufacturer
> (Dielectric Labs, Johanson and Murata are some of the others). Mostly they
> come in 250 or 500V DC rating but in a low voltage PA, it's probably the
> high current handling that's needed. Usually they are 0.1" cube. I'd be
> surprised if you needed better than 10% tolerance, whether they are in the
> amp section or the filters. Trade price for most brands in the UK is
around
> $1.50 each, 3-5x that for ATC brand (and I've yet to find them better
> despite the ads). What are the values?
>
> Steve
>
>
From: "COLIN LAMB" <k7fm@teleport.com>
> Hello Jeffrey:
>
> Under full load, 100 watts will be 71 volts RMS with a 50 ohm load.
> However, when you do not have an antenna on, the voltage will be much
> higher. Yours could have burned out when there was no load or as a result
> of lightning or other external event. You should at least have the same
> voltage as original.
>
> RF Parts sells monolithic capacitors for installation. They will be able
> to provide what you need. You need one with short or no leads, for good
> performance at vhf. It makes no difference who sells them as your radio
> does not know if it is an Icom part. Call RF Parts and tell them what you
> need and they will have replacement parts. It should be an easy fix.
>
> 73, Colin K7FM
From: <ToddRoberts2001@aol.com>
> <PRE>Hi Jeff, I saw your posting in the AMPS group asking about chip
capacitors. I
> might be able to give you some suggestions but you gave no specifics as to
> what value capacitors you need? What capacitance do you need? Do you know
> what voltage rating you need? RF current rating? 73 Yours Truly, Todd
Roberts
> WD4NGG.
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