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DC503 Not Working


scm@...
 

It looks like you found the place where a needed signal is missing. The next step is to trace backwards to see why. Next to check is pin 6. If it is LOW, the multiplex clock will be disabled. If it isn't LOW, something is wrong with the circuitry around U330 (short, open connection, bad component). If it is LOW, we have to trace backwards to look for a problem where the signal for pin 6 comes from


Stephen
 

On Mon, Jul 27, 2020 at 10:34 AM, satbeginner wrote:


Hmm, should be ok-ish.
I measured it in circuit.

Is that cool accessible?
So you can measure the voltage across it when powered on?
It’s pretty accessible, yes. It’s on the top « Trigger » board.

Is it possible to trace the voltage from where it is coming from to the C-L-C
combination?
Hmmm... maybe. I’ll have to look into that tomorrow. It’s 12:00AM over here....

I mean, maybe there is a broken wire or dodgy connection before it gets to the
C-L-C filter.
Could be.. but if you don’t mind me asking, how is it relevant to the display issue?


Stephen
 

On Mon, Jul 27, 2020 at 10:57 AM, <scm@...> wrote:


It looks like you found the place where a needed signal is missing. The next
step is to trace backwards to see why. Next to check is pin 6. If it is LOW,
the multiplex clock will be disabled. If it isn't LOW, something is wrong with
the circuitry around U330 (short, open connection, bad component). If it is
LOW, we have to trace backwards to look for a problem where the signal for pin 6 comes from
Ok. I will check that tomorrow and report back. It’s past 12:00 AM over here. We are worlds apart.


Stephen
 

Thank you all.


Roger Evans
 

Stephen,

You have (at least) two distinct issues with comments coming in which are relevant to one or the other and are randomly interleaved.

1. The LCD display does not scan so typically only one LED is lit. This you have traced to U330. One thing that hasn't been mentioned here is that the output from U330 pin 10 also goes to U332 pin 6 so a fault in U332 (which I don't think you have changed) would stop the multivibrator from working. If U332 is socketed, just remove it and see if you can get the square wave on U330 pin 10.

2. The -22V supply is somewhere around 0V. This completely changes the bias of the analogue input stages and makes it less likely that any inputs will actually trigger the digital circuitry. Here, I at least, am confused as to whether there is too much current draw or a fault in the voltage regulator. If you can measure the voltage across R546 it should help to narrow down the problem. If the voltage across R546 is more than about 0.6 - 0.7V it will turn on Q540 and shut down the regulator as intended - then you have to track down what is drawing too much current. If the voltage is much less than this then there is likely to be a fault in the -22V regulator and you could measure the voltages on the base, emitter and collector of Q548. It is also worth checking that R546 is close to its nominal value, if it has drifted high it will shut down the regulator prematurely.

Roger


Stephen
 

On Mon, Jul 27, 2020 at 10:52 PM, Roger Evans wrote:

Interleaved answer.


Stephen,

You have (at least) two distinct issues with comments coming in which are
relevant to one or the other and are randomly interleaved.

1. The LCD display does not scan so typically only one LED is lit.
You mean that when the unit is on, only 1 “zero” should show?
I’ve never had one of these. I thought 4 should be show (from picts I’ve seen).

This you have traced to U330. One thing that hasn't been mentioned here is that the
output from U330 pin 10 also goes to U332 pin 6 so a fault in U332 (which I
don't think you have changed)
I have changed ALL the IC’s on that board but the 2 that I mentioned in an earlier post.

would stop the multivibrator from working. If
U332 is socketed, just remove it and see if you can get the square wave on
U330 pin 10.
Will do.

2. The -22V supply is somewhere around 0V. This completely changes the bias
of the analogue input stages and makes it less likely that any inputs will
actually trigger the digital circuitry.
When I measured 0V across that capacitor, the top “Trigger Board” was removed. Hence the cap was not
connected to the coil.

Here, I at least, am confused as to
whether there is too much current draw or a fault in the voltage regulator.
Both Voltage regulators were also changed. I really changed EVERY IC’s on that board but 2.

If you can measure the voltage across R546 it should help to narrow down the
problem.
Will do and report back.

If the voltage across R546 is more than about 0.6 - 0.7V it will
turn on Q540 and shut down the regulator as intended - then you have to track
down what is drawing too much current. If the voltage is much less than this
then there is likely to be a fault in the -22V regulator and you could measure
the voltages on the base, emitter and collector of Q548. It is also worth
checking that R546 is close to its nominal value, if it has drifted high it
will shut down the regulator prematurely.

Roger
Thank you Roger.


scm@...
 

I finally printed the manual out and figured out what scan clock disable (pin 6) is. It is an input from the backplane connector (pin 27A). This should not be low resistance to ground, either on the DC503 or on the backplane. It probably won't be, however. I suspect that since there seems to be some intermittancy involved, that something is marginally disrupting the multivibrator. Check the driven circuits as Roger suggested. Otherwise suspicion returns to the multivibrator, itself. Resolder all the connections, clean the socket and IC pins carefully and, possibly for good measure, replace the capacitors (I know they look OK DC wise).


Stephen
 

On Tue, Jul 28, 2020 at 12:39 AM, <scm@...> wrote:


I finally printed the manual out and figured out what scan clock disable (pin
6) is. It is an input from the backplane connector (pin 27A). This should not
be low resistance to ground, either on the DC503 or on the backplane. It
probably won't be, however. I suspect that since there seems to be some
intermittancy involved, that something is marginally disrupting the
multivibrator. Check the driven circuits as Roger suggested. Otherwise
suspicion returns to the multivibrator, itself. Resolder all the connections,
clean the socket and IC pins carefully and, possibly for good measure, replace
the capacitors (I know they look OK DC wise).
I will do and check all what you guys very kindly suggested, and report back as soon as it’s done.
I will replace those two 0.1uF capacitors for good measure. All I have readily available at the moment are rated at 600V instead of the 100V the schematic calls for. Overkill and way much bigger as you can imagine, but that’s all I have. Will try to make them fit as the top board needs to sit on top without shorting them.


Stephen
 

BTW, it may be worth mentioning that R331, which is connected to pin 6 of U330(B), reads 1K in circuit.


scm@...
 

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). 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.


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


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.


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


scm@...
 

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
 

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.


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 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?


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?



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


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.