If the single digit is much brighter than normal, the display scanning is likely not working.
If the digits are otherwise fine, then it's either drivers to the LEDS, or the decoding is bad.? IIRC, the sign may be driven separately and not multiplexed.
If the display is still scanning (look for positive going pulses on U1701, they may not go to ground, but should go to 5 volts, signals D1 inverted to D5 inverted)? then the problem is in the decoding, all of which is in the digital section U1603).? Look for a clock on pin 8, and activity on the other pins.
Best I can do, since I haven't looked at any of mine for this problem.
Harvey
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On 4/6/2024 12:59 PM, Bob W8RMV wrote:
Hi Members,
I've had this TM504 for 10 years maybe, and it has right to left: DM501A, DC503A, AM502 & PS503A.
All has been good until recently. The DM501A has a start up issue.
When it is started from cold, all that is displayed is a - (minus sign). It will not respond to any buttons pushes except the minus sign will disappear if I activate AC from DC.
Then after about 5 or 10 minutes it will suddenly start working on all functions.
I took the 501 out & checked all the electrolytic around the regulators. I was surprised to find they were all 85°C caps but they all tested good & do not show any distress. I looked for cracked solder connections & did not see any. I have not remelted the regulator pins, yet. I started to reseat the IC's but halted that.
The DM501 only seems to "fit" the left slot so I cannot power it on the other 3 slots. Maybe the left slot has an issue?
So I thought I'd ask for guidance here. I do not have a single port "TM504" for testing, but I wonder if there is anything I should look for in the DM504?
Thanx - Bob
