[Premium-Rx] URR-74/WJ8718 help

[L D Ritta]

Hello Ahmet
I know there is a transistor which is used to control the brightness.
Well I removed all the leds but one.I noticed the leads on the leds
are dirty they have a black substatance I cleaned up one and I
noticed the b segment now lights up. However the f segment still
does not light no matter which led I plug in. BTW the DS8857J
still gets vey hot even with one led pluged in. I recon there is a short
or something in the driver for the f segment. At the very least I will
have to remove the old chip and install a socket. The leds have sockets,
but they are wider that a standard 14 pin dil type. I wonder if you can 
still
get those anymore.

73 Lee

----- Original Message ----- 
From: "Ahmet G." <ahmet-m at usa.com>
To: "L D Ritta" <ufp at optusnet.com.au>; "Ahmet G." <ahmet-m at usa.com>; 
<premium-rx at mailman.qth.net>
Sent: Monday, October 13, 2008 7:01 AM
Subject: Re: [Premium-Rx] URR-74/WJ8718 help


>
> 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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>
> =
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