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Help for DC503A universal counter repair


Attilio
 

Hello everyone,
I am trying to repair a DC503A universal counter. I found a blown fuse on the line from which the 5Vdc is taken, I replaced it and the display turned on (only the first digit on the right), I replaced the LS7301 chip and finally all the digits lit when I press the button reset (all 8).
Now it works and indicates correctly, but has a problem, initially the digits with the decimal point on
remain off, then it has improved and the digits with the decimal point lit are on, all this without doing any action on the circuits. The problem remains that some digits go out slowly especially with slow gate times.
I need help to find the problem and I have a question: is it normal that without input signal all digits of the display are off (only the decimal point is lit, if present)?

Thanks for the attention
--Cheers
Attilio


Stephen
 

I’m sorry I can’t really help. But one thing is for sure, if you’re on any of the A Gate Time positions, you should definitely have a dot and some 0000.


Nenad Filipovic
 

Hi Atilio,

I need help to find the problem and I have a question: is it normal that
without input signal all digits of the display are off (only the decimal point
is lit, if present)?
Not on my unit, at least one zero should be lit. Without signal, the number of (zero) digits lit and decimal point position depends on the averaging setting (AVGS). Changing AVGS also affects the Hz/kHz/MHz... LED indicator.
I'd surely try cleaning all the switches, especially the two main selectors.

Nenad Filipovic


Attilio
 

Hi Nenad, hi Stephen
I have all the digits of the display off with no input signal, with a selector on "Frequency" and the "Timing" selector on "100 us" I have on the decimal point of the third digit from the right and the "MHz" led is on, the "GATE" led flashes.
While passing from one selection to another, the digits light up showing 8.
What could I watch?

--Cheers
Attilio


Attilio
 

Hi everyone,
I have isolated the "Blanking" output from U1520 (pin 11), obviously the non-significant zeroes remain on (visible on the display), but in this way the frequency meter works well and the display remains on. But there is a second problem, FREQUENCY, all AVG functions and TIME MANUAL work OK while instead the PERIOD B, WIDTH B and TIME A - B functions do not work.

Do you have any advice for the repair.

Thanks for the attention.
-- Cheers
Attilio


Attilio
 

Hi everyone,
sorry but i probably didn't understand how PERIOD B and PERIOD B AVG work. From more in-depth tests I have understood that it is possible to use PERIOD B to measure medium-long periods (example 100 ms) while the PERIOD B AVG (average) can be used to measure short periods (example 100 ns). Am I wrong or does it work like this?

The problem of blanking remains, solved for now by disconnecting pin 11 of the U1520 (LS7031), but in this way the leading zeroes remain on.

-- Cheers
Attilio


Attilio
 

Hi everyone,
I solved the problem by inserting a pull-up resistor on the blanking line (pin 11 of the LS7031).
One problem remains: the zero before the decimal point does not stay lit, I think it is the DPI input, pin 10 of the LS7031, but I leave it like that, just know it.

I doubt that the original Tektronix LSI LS7031 is slightly different from the standard LS7031, or it may be that the original is from 1983, while the one I found is from 1980, or finally that the new LS7031 is faulty, I don't know...

--Cheers
Attilio