DC503 Not Working


Following someone’s request not to talk about this on my initial thread about the TM506, I’m posting this new thread regarding the DC503 I purchased with it.

As some of you may recall from my previous thread/posts, this unit turns on, but only one digit is lit at any one time when I power in on. And it’s never the same one. Although There are some digits that I almost never see lit.
I can tell “something“ is being counted when I plug in a function generator. Some lonely digits go from 1 to 9 when I play around with the controls. So before I even try to debug the counter section, if it’s even needed, I need first the display to work properly.
Since it’s harder than I thought to probe around without an extender, I decided to go ahead and change many components related to the display:
- U340
- U350
- U333
- U335
- U337
- U352
- U354
- Q360
- Q364
- Q368
- Q372
- Q376
- Q380
- Q384
Some more were changed as well at the time of this new post. The 2 voltage regulators for example.
Not much of an improvement... Except that now it sometimes scans the entire display area on startup, but stops at one digit, or nothing at all.
Obviously something else is not right. Voltages at TP’s are all good.

Because my supply store doesn’t have any tantalum 15mfd/50V, I decided to check the voltage across C548 to see if I get away with a 15mfd/35V, just in case it’s bad. It’s not shorted, but there is close to 0V across that capacitor. Also, someone had stuck a 4A fuse instead of a 2A in there.
Something else could have gotten damaged if the fuse didn’t do it’s job. So I’m trying to check and/or replace what I can.

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