Thanks Guys
Jim WK5Y
----- Original Message -----
From: Tom Parker
To: Repeater-Builder@yahoogroups.com
Sent: Monday, October 01, 2007 9:14 PM
Subject: Re: [Repeater-Builder] Micor UHF PA
Jeff you just gave away all the secrets... Good, concise job!
Jeff DePolo wrote:
The coupling cap between stages that is at the end of the semi-rigid coax
can develop a bad connection as Tom said. Usually if it does, the pitting
that occurs due to the arcing renders the cap's end plate too damaged to
re-solder reliably, so replacing it is a good idea.
The 25.5 pF caps across the collector-emitter can go bad. Usually this will
manifest as somewhat-reduced output (depending on how many are bad), and/or
power that jumps around.
The little solder tabs that connect the RF ground on the substrates to DC
ground along the top edge of the board will often develop bad connections
and need to be re-flowed. When these open up you can have anything from low
power out to spurs.
The 0.01 uF DC decoupling caps that connect the output coax's shield to RF
ground can go bad too, or develop a cold solder joint; usually these fail
catastrophically so they're easy to spot.
If you suspect a bad cap, sometimes you can locate the bad one by applying a
little pressure with an insulated tuning tool while the transmitter is on.
You might get lucky and catch it arcing as it makes and breaks the
connection.
Last but not least, a cracked ceramic substrate will cause all kinds of
grief if the groundplane (underside of the substrate) breaks. A crack in
the substrate itself isn't a problem, but if the copper cracks along with
it, scrap it.
--- Jeff
> -----Original Message-----
> From: Repeater-Builder@yahoogroups.com
> [mailto:[EMAIL PROTECTED] On Behalf Of Jim Russell
> Sent: Sunday, September 30, 2007 9:08 PM
> To: Repeater-Builder@yahoogroups.com
> Subject: Re: [Repeater-Builder] Micor UHF PA
>
> Thanks
>
> Jim WK5Y
>
> ----- Original Message -----
> From: Tom Parker <mailto:[EMAIL PROTECTED]>
> To: Repeater-Builder@yahoogroups.com
> Sent: Sunday, September 30, 2007 2:25 PM
> Subject: Re: [Repeater-Builder] Micor UHF PA
>
>
> Also check the 52 pf coupling capacitors between each
> amp stage. We see these go bad and intermitten from time to
> time. Still too cheap not to replace, but sometimes all they
> need is a little heat and solder reflow.
>
> thp
>
> Milt wrote:
>
>
>
>
> Jim,
>
> You might be thinking about the 33pf(? value)
> on the collector/emitter junction on the flat pack RF
> transistors. Or the leaching problem that is mentioned in
> the service manual. I always replaced the chip caps on the
> base/emitter and collector/emitter junctions when we had to
> replace a defective transistor. There were PK-xxx packages
> for replacing the lower level transistors that included the
> transistor, chip caps and solder.
>
> Milt
> N3LTQ
>
>
> ----- Original Message -----
> From: Jim Russell
> <mailto:[EMAIL PROTECTED]>
> To: Repeater-Builder@yahoogroups.com
> <mailto:Repeater-Builder@yahoogroups.com>
> Sent: Friday, September 28, 2007 11:24 PM
> Subject: [Repeater-Builder] Micor UHF PA
>
> Ok guys help this old man out. I
> worked in a Motorola Shop back in the 80's. I seem to
> remember a problem with Micor UHF PA's where the solder
> connection on a perticular chip capacitor would go bad
> causing the power to drop to next to nothing. I think the
> solution was to resolder the cap with silver solder. Do any
> of you know of a better solution. I have one of the animals
> acting intermittant.
>
> Jim WK5Y
>
>
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