[Premium-Rx] URR-74/WJ8718 help

[Ahmet G.]

Hi. 

I believe there is a small copper finger that touches DS8857J
to heat sink. DS8857J being ceramýc package is able to handle
the current.   On the 8718A display board there is power transistor
as far as i remember.

display segments go blank from time to time due to connection
problems between the board and display legs. this happened on my
8718. when you move the board or press on the displays, they light up
again.
 
Also, have you checked the display segment LED diodes for continuity
to make sure they are working fine / not burned out ?

Also usually if an IC is NOT workýng properly, it is much colder than
it should be. Especially for a ceramic IC pkg.  So perhaps it is good
to check to see if the ICs are warm enough ( most plastic packages 
operate at about 45C ).  Of course this may not be true.

have a great day.
Ahmet

> 
> Howdy Ahmet
> In the WJ 8718/URR-74  on the display board there is a bcd to 
> decimal decode and
> a bcd to 7 segment decoder. there are no transistors between the 
> DS8857 and the displays.
> so that is not the problem. unless you can prove otherwise.
> The only other chip is a  MOS to LED 8 digit driver which goes 
> between the cathode of the displays
> and 4028 bcd to decimal decoder.
> BTW is the running time meter supposted to be noisy, it goes tic 
> tic tic it is very annoying.
> and I noticed the DS8857J is very hot I mean you could burn your self.
> 
> 
> 
> 
> Lee
> ----- Original Message ----- From: "Ahmet G." <ahmet-m at usa.com>
> To: "L D Ritta" <ufp at optusnet.com.au>; <premium-rx at mailman.qth.net>
> Sent: Saturday, October 11, 2008 8:33 PM
> Subject: Re: [Premium-Rx] URR-74/WJ8718 help
> 
> 
> >
> > Hello
> >
> > DS8857 is an IC made by National Semiconductor
> > It is a BCD to 7 Segment display driver, Decoder.
> > Current: 60mA
> > Type: CA Common Anode
> >
> > I believe you can replace this with a similar driver IC from the
> > 74xxx series drivers that you can find easier in the market.
> > Such as: 7446, 7447, 74247 which are CA types
> >
> > They have same connection diagrams and functions, each one with an
> > additional function such as Ripple Blanking on the DS8857.
> >
> > Usually there are power transistors that regulate the higher currents
> > on the display boards ( they are usually attached to heat sinks) and
> > these transistors tend to burn out before the driver ICs. One of the main
> > reasons is the fact the heat sink connection is lose or accumulated
> > dust thereby blocking heat flow.
> > So you may want to check to see if the transistors are ok before replacing
> > the driver IC.
> >
> > Hope this helps.
> >
> > Ahmet Gundes
> >
> >>
> >> Hello Everyone
> >> Well I got my my URR-74 working after I changed my back panel for
> >> one from a WJ-8718.
> >> However the display does not work, the f and b segments are out on
> >> all displays. This looks like
> >> the U8 on the display board which is a DS8857 however I can not
> >> find any data on it on google.
> >>
> >> Am I correct in my logic and someone know what I can use to replace
> >> it. your ideas please.
> >>
> >> 73 Lee _______________________________________________



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