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DC503 Not Working


Stephen
 

This is a partial copy of a post a posted while trying the extender work.

Something very unexpected and quite puzzling just happened.
As many pointed out, I forgot to connect pin 6A. I just did.
Some of you may have seen my post regarding the DC503 that is not showing more than 1 digit (0) at any one time, if at all, when I power it on. Never ever.

Now that pin 6A is finally connected, not only the DC503 power on normally with the extender, but all digits are there!!! Not when connected inside the TM506 itself!! It’s not counting anything, though.
Everything is not lost, it gives me hope that at least something is showing now...

It’s also missing some segments. If anyone knows if the individual LED’s can be replaced, that would be nice to know of a modern replacement. I found some, but the panels look too thick, I think.


Harvey White
 

That's a start place.  Knowing how it works, you can start at either end and work towards the other.

Harvey

On 8/5/2020 5:07 AM, Stephen wrote:
On Tue, Aug 4, 2020 at 02:03 PM, Harvey White wrote:

Vocabulary thingie......

A pulse, likely sequential, is (at least at on time) called a strobe.

Take a display with a common anode for all the segments.  It needs to be
connected to VCC (power) to light.  However, there's a limit where the
display continuous power per segment is exceeded. This is not a good
design.  However, if you pulse the LED (segment) with more current than
the continuous rating, but for less time, then the display survives.
Due to a quirk with human vision, it appears brighter.  So what you do
is connect the source of each digit to a signal (strobe) which in
sequence selects each digit.

The segments drivers themselves are selected so that the current through
each segment (assuming common anode with digit strobe to +vcc, and
segment driver (through resistor) to ground) is the nominal peak current
that they want.

So if all displays were driven in parallel, there'd be one driver chip
per display, no pullup transistor to VCC, and  a lot of chips.  If you
multiplex the displays (please look this up), then you have a sequential
pullup to VCC (called a strobe), a pulldown to ground (called the
segment driver, which determines which segments on which digit are on
when....) and a current which is more than needed for a single digit
(perhaps # digits times current per segment, because you don't know if a
segment is on or off, so you do it per segment).

So:

a scanned display turns on each digit sequentially, with more current
per segment than permitted (steady state but less than pulsed) and with
less chips, but more complex than a few digits. However, as the digit
count goes up, the circuit is simpler. Microprocessors love this kind of
thing.

Harvey
Ok, so you suggest I sweep each digit for a signal at its input? At the ribbon cable connections then...



Stephen
 

On Tue, Aug 4, 2020 at 02:03 PM, Harvey White wrote:


Vocabulary thingie......

A pulse, likely sequential, is (at least at on time) called a strobe.

Take a display with a common anode for all the segments.  It needs to be
connected to VCC (power) to light.  However, there's a limit where the
display continuous power per segment is exceeded. This is not a good
design.  However, if you pulse the LED (segment) with more current than
the continuous rating, but for less time, then the display survives. 
Due to a quirk with human vision, it appears brighter.  So what you do
is connect the source of each digit to a signal (strobe) which in
sequence selects each digit.

The segments drivers themselves are selected so that the current through
each segment (assuming common anode with digit strobe to +vcc, and
segment driver (through resistor) to ground) is the nominal peak current
that they want.

So if all displays were driven in parallel, there'd be one driver chip
per display, no pullup transistor to VCC, and  a lot of chips.  If you
multiplex the displays (please look this up), then you have a sequential
pullup to VCC (called a strobe), a pulldown to ground (called the
segment driver, which determines which segments on which digit are on
when....) and a current which is more than needed for a single digit
(perhaps # digits times current per segment, because you don't know if a
segment is on or off, so you do it per segment).

So:

a scanned display turns on each digit sequentially, with more current
per segment than permitted (steady state but less than pulsed) and with
less chips, but more complex than a few digits. However, as the digit
count goes up, the circuit is simpler. Microprocessors love this kind of
thing.

Harvey
Ok, so you suggest I sweep each digit for a signal at its input? At the ribbon cable connections then...


Harvey White
 

Vocabulary thingie......

A pulse, likely sequential, is (at least at on time) called a strobe.

Take a display with a common anode for all the segments.  It needs to be connected to VCC (power) to light.  However, there's a limit where the display continuous power per segment is exceeded. This is not a good design.  However, if you pulse the LED (segment) with more current than the continuous rating, but for less time, then the display survives.  Due to a quirk with human vision, it appears brighter.  So what you do is connect the source of each digit to a signal (strobe) which in sequence selects each digit.

The segments drivers themselves are selected so that the current through each segment (assuming common anode with digit strobe to +vcc, and segment driver (through resistor) to ground) is the nominal peak current that they want.

So if all displays were driven in parallel, there'd be one driver chip per display, no pullup transistor to VCC, and  a lot of chips.  If you multiplex the displays (please look this up), then you have a sequential pullup to VCC (called a strobe), a pulldown to ground (called the segment driver, which determines which segments on which digit are on when....) and a current which is more than needed for a single digit (perhaps # digits times current per segment, because you don't know if a segment is on or off, so you do it per segment).

So:

a scanned display turns on each digit sequentially, with more current per segment than permitted (steady state but less than pulsed) and with less chips, but more complex than a few digits. However, as the digit count goes up, the circuit is simpler. Microprocessors love this kind of thing.

Harvey

On 8/4/2020 6:46 PM, Stephen wrote:
On Tue, Aug 4, 2020 at 11:17 AM, Harvey White wrote:

Once you get the extender, check the strobes on each of the digits.
Strobes???



Stephen
 

On Tue, Aug 4, 2020 at 11:17 AM, Harvey White wrote:


Once you get the extender, check the strobes on each of the digits. 
Strobes???


Harvey White
 

Once you get the extender, check the strobes on each of the digits.  Then check the source of those strobes.  Then check the chip that's driving those (likely) transistors.  A typical way is to have a counter driving a decoder which then drives the digits individually.  You may find that this counter does all of that in one chip.  If it's a complicated chip, then look for a reason (if no strobes) that the chip is being inhibited.

Anything where digits freeze or are irregularly displayed indicates the scanning electronics.  Anything where there's a missing segment on ALL displays indicates the common 7447 decoder or the like.  Anything where you have one segment on one display missing tends to indicate a problem on that display.

Even if the display is wonky, you might see (and perhaps should see) digits indicating counting.  The counter is very sensitive to the input settings even on a good day.

You're going to need the JAMMA extender.  My research suggests that if you have all the lower pins connected on both sides, equivalent to 1 to 13 when you turn the extender upside down, all you need to do is mark both connectors promently, and reverse the connectors.  On the JAMMA connectors I saw, pin 7 was missing on both sides.  No other pins were missing, which suggests the reversal.

I wouldn't do much more until you can get that counter out on an extender.  Make the extension wires long enough to get the plugin at least one plugin length away from the front of the frame.

Harvey

On 8/4/2020 5:59 PM, Stephen wrote:
I got to working on bit on this unit today again. I rechecked all the voltages, and they’re all good.
For good measure, I replaced all the tantalums on the lower board. 2 with electrolytics, the 15uF/50V and the 6.8uF, which I couldn’t find. And of course nothing has changed, I yet have to find time to modify the JAMMA extender. After all that has been changed, there is close to no improvement...



Stephen
 

I got to working on bit on this unit today again. I rechecked all the voltages, and they’re all good.
For good measure, I replaced all the tantalums on the lower board. 2 with electrolytics, the 15uF/50V and the 6.8uF, which I couldn’t find. And of course nothing has changed, I yet have to find time to modify the JAMMA extender. After all that has been changed, there is close to no improvement...


Stephen
 

On Sat, Aug 1, 2020 at 03:34 AM, Roger Evans wrote:


My DC503 also has nothing connected at the lower of the two points in your
picture. On my board there are four of these holes in a row with no wires or
components attached, partky hidden by the shaft of the function switch. They
are 'vias' used to transfer a PCB trace to the opposite side of the board for
routing purposes. There is another row of 7 similar vias towards the
connector end of the same board.

Roger

Interesting. The schematic shows pin 12 connected to a switch....


Roger Evans
 

My DC503 also has nothing connected at the lower of the two points in your picture. On my board there are four of these holes in a row with no wires or components attached, partky hidden by the shaft of the function switch. They are 'vias' used to transfer a PCB trace to the opposite side of the board for routing purposes. There is another row of 7 similar vias towards the connector end of the same board.

Roger


Stephen
 

I always discover new stuff...
It seems the pin, I think, 12? of U354 Is connected to nothing.... But I pretty sure it should be...

See pict: https://groups.io/g/TekScopes/photo/250733/0?p=Created,,,20,2,0,0

There is continuity between these to points, but nothing is connected...
I’m not sure if this is related to my display issue, but I’m sure it’s not right.
Does any have one of these and tell me what should be there, if anything?

Thanks.


Stephen
 

On Mon, Jul 27, 2020 at 07:06 AM, Colin Herbert wrote:


I think you are getting terms a bit confused. Zero resistance _is_ a short.
Perhaps you intended to write "infinite resistance"? Easily done.
Colin.
Yes, you’re absolutely right. What I awkwardly meant to say is that I “read” nothing on the meter, and that the continuity tester didn’t beep.

Thanks for correcting me.


Richard R. Pope
 

Hello all,
I had someone do this with a Mazda RX-2. The Mazdas of that day and age had an amp meter in the dash. There was a 50 amp main fuse under the hood. The wiring for the amp meter ran through the main harness under the hood and through the main harness under the dash. These fuses were $5 each and that was expensive in the early 1970s.
Well this car started blowing the main fuse so the owner replaced the fuse with a piece of coat hanger instead of finding the short. Well the results were predictable. He heavily damaged both the harness' under the hood and under the dash. The alternator had some shorted diodes and that is what was blowing the fuse. It wasn't pretty but the car didn't catch fire. Fortunately!
GOD Bless and Thanks,
rich!

On 7/31/2020 6:55 PM, Harvey White wrote:
It tells me that something is drawing too much current.

Generally, it's a tantalum capacitor. The tantalum may NOT read a dead short, only some do. It could read low, draw an amp on a circuit where the circuit was supposed to draw 100 ma. I'd want to remove the fuse, check the power current vs. the fuse rating. Use the 20 amp range on a meter.

If it's not a tantalum, then something ought to be getting rather hot in operation.

The previous owner "fixed" the problem the same way that an "expert" electrician fixes a power problem by putting a penny behind the fuse.

Harvey


Harvey White
 

It tells me that something is drawing too much current.

Generally, it's a tantalum capacitor.  The tantalum may NOT read a dead short, only some do.  It could read low, draw an amp on a circuit where the circuit was supposed to draw 100 ma.  I'd want to remove the fuse, check the power current vs. the fuse rating. Use the 20 amp range on a meter.

If it's not a tantalum, then something ought to be getting rather hot in operation.

The previous owner "fixed" the problem the same way that an "expert" electrician fixes a power problem by putting a penny behind the fuse.

Harvey

On 7/31/2020 6:22 PM, Stephen wrote:
I changed the 2 caps, as someone suggested. I’ve rechecked the voltages, and they are all good.
I’ve also re-cleaned the sockets. And nothing has changed. I’m waiting to modify the JAMMA connector to checks all that you guys suggested.
However, I did notice something maybe worth mentioning, and that might give you experts a clue as to what’s not right:
1- The plugin fuse (Fast 2A), blows every once in a while when I power the TM on and off 5-10 seconds apart Or so, to check if the “0” comes on in the same position, or at all. The previous owner had soldered a 4A fuse. Maybe because he had the same problem and thought 2A wasn’t enough... I don’t know why he’d do that. Grrr...
Does that tell you something?



Stephen
 

I changed the 2 caps, as someone suggested. I’ve rechecked the voltages, and they are all good.
I’ve also re-cleaned the sockets. And nothing has changed. I’m waiting to modify the JAMMA connector to checks all that you guys suggested.
However, I did notice something maybe worth mentioning, and that might give you experts a clue as to what’s not right:
1- The plugin fuse (Fast 2A), blows every once in a while when I power the TM on and off 5-10 seconds apart Or so, to check if the “0” comes on in the same position, or at all. The previous owner had soldered a 4A fuse. Maybe because he had the same problem and thought 2A wasn’t enough... I don’t know why he’d do that. Grrr...
Does that tell you something?


Stephen
 

I finally received the 2 JAMMA extenders that I bought from eBay.
I’m gonna finally be able to resume more adequately and comfortably the troubleshooting of this plugin.
Provided I don’t mess up the extender modification.


Stephen
 

I’ve been quite busy today and I haven’t had the chance to check what you guys suggested yet.
However, I did notice something that may or may not be worth noting: When I input a signal to input A,
let’s say 1Khz, and “if” And only if the only digit lit is the one in the middle (the 3rd from the left I think), when I’m in the Totalize A position, and press the start button, it counts from 0 to 9 repetitively. And this is all this unit does.


SCMenasian
 

If the voltage at pin 6 turns out to be LOW, measure the resistance from pin 6 to ground two ways - with the plugin installed in the mainframe or not. This will indicate whether the problem is in the plugin or the mainframe.


Stephen
 

On Tue, Jul 28, 2020 at 02:19 AM, Roger Evans wrote:

Interleaved.


When you measure R331 in-circuit there is a parallel current path through U330
and R330 (1k) so you might well see a low value of resistance and it would
probably be different if you reversed the polarity of the probes.
I measured it in circuit, but with the unit off. If it’s of any significance.

R331 being
low wouldn't be a problem since it is only required to establish a logic 1
level at pin 6. The problem would arise if for instance there was some damage
to the PCB giving a low resistance from pin 6 to ground or one of the
'optional' connectors on the TM506 backplane that someone had previously set
Apparently not. No optional jumpers or anything on the connectors themselves, but « Option 2 » is installed. However, nothing is connected to it.

for a different plugin. As long as the voltage on pin 6 is more than about
2.5V that is read as logic '1'. It will probably measure very close to 5V.
I will check all that.

The regulator for the -22V supply is not an IC, it is the discrete circuitry
around Q540, Q545, Q548 and the PNP pass transistor in the mainframe.
Maybe I didn’t express myself correctly. What I meant is that I also replaced U500 and U251.

Roger


Stephen
 

On Tue, Jul 28, 2020 at 02:12 AM, <scm@...> wrote:

Interleaved

According to my diagram, R331 should be 5.6k. Actually, if it's 1k, there
should be no problem (unless someone changed it in the past to try to cure the
real problem).
It appears to be stock. And I measured it in circuit. I’ll lift a pin and see what it says...

Move the test lead to pin 6 and measure the voltage there. If
it is logic HIGH, the multivibrator should run; logic LOW inhibits the
multivibrator.
Will do.


Roger Evans
 

When you measure R331 in-circuit there is a parallel current path through U330 and R330 (1k) so you might well see a low value of resistance and it would probably be different if you reversed the polarity of the probes. R331 being low wouldn't be a problem since it is only required to establish a logic 1 level at pin 6. The problem would arise if for instance there was some damage to the PCB giving a low resistance from pin 6 to ground or one of the 'optional' connectors on the TM506 backplane that someone had previously set for a different plugin. As long as the voltage on pin 6 is more than about 2.5V that is read as logic '1'. It will probably measure very close to 5V.

The regulator for the -22V supply is not an IC, it is the discrete circuitry around Q540, Q545, Q548 and the PNP pass transistor in the mainframe.

Roger