[Heathkit] SB-634 questions

[William Houghton]

Hi Sandy,

The Mostek MK5017 chip provides the clock function. There is a disagreement 
between the schematic in the Heathkit manual I have and the short MK5017 
datasheet I found on the web. The clock chip has a 24H/12H mode. It also had a 
50Hz mode.

According to the datasheet 24H mode is selected by connecting a diode between 
pins 5 & 21. 50Hz mode is selected by connecting a diode between pins 5 & 22.

The Heathkit schematic shows these as being in the opposite positions. My guess 
is that a previous owner removed the diode that selects 24H operation.

The timer runs from a counter chain formed by three 7490 decade counters. If it 
is incrementing by 2, this likely means that the D0 bit of the lowest counter 
output is stuck low. This is pin 12 of IC204. So instead of seeing 0000, 0001, 
0010, 0011,0100, 0101, etc the DM8880 BCD-to-7 segment decoder/driver is seeing 
0000, 0000, 0010, 0010, 0100, 0100, etc.

I'd try pulling the chip out of its socket and cleaning the leads. Check for 
solder bridges, etc. You could try swapping  IC204 with IC203, or IC202. Then 
the problem should then move to either the tens or hundreds if it is a problem 
with the 7490 itself.

73,

Bill




________________________________
From: Alexander N. Gerli <ac1wye at gmail.com>
To: heathkit at mailman.qth.net; heath at puck.nether.net
Sent: Sun, November 25, 2012 7:45:36 PM
Subject: [Heathkit] SB-634 questions

Hi, guys,

Time to put my SB-634 back in service. Naturally, I can't find my manual. I
got a spare unit and the old '634 I had is now a parts-car.

The 'new' one needs some tweaking. The digits are perfect (that's why I got
it) however, it reads on 12-hour time, not 24-hour. Gotta be a switch or
wire jumper somewhere on a board to change that?

Then, the timer digits are weird. Yeah they're perfectly clear and they
reset just like they're supposed to. BUT, they're incrementing
2,4,6...instead of 1,2,3...Anyone got an idea about that? My guess is that
Joe-Ham-Before-Me wired an IC wrong on the timer board or something?

Thanks for your help!

73,

Sandy AC1Y

Alexander N. (Sandy) Gerli
'Corgi Cottage'
548 Allred Mill Road
Mount Airy, NC 27030-2202
919-625-7181

A Scotsman walks about 900 miles each year
Scotsmen drink about 40 gallons of whisky each year
Scotsmen get about 25 miles per gallon


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