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DC503 Not Working


SCMenasian
 

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.


Stephen
 

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


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.


SCMenasian
 

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 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.


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
 

Thank you all.


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
 

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?


SCMenasian
 

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


satbeginner
 

Hmm, should be ok-ish.

Is that cool accessible?
So you can measure the voltage across it when powered on?

That way you would know if there is a real current flowing, or the voltage is already low when it arrives.

Is it possible to trace the voltage from where it is coming from to the C-L-C combination?

I mean, maybe there is a broken wire or dodgy connection before it gets to the C-L-C filter?

Leo


Stephen
 

On Mon, Jul 27, 2020 at 10:12 AM, satbeginner wrote:

And, the coil, was it really low resistance, or just 'conducting'?
The coil measures 2.2 ohms


Stephen
 

On Mon, Jul 27, 2020 at 10:12 AM, satbeginner wrote:

Interleaved

My 2 cents:

Just temporarily remove the two chips you couldn't replace, to see what the
voltage does at power on.
In order to do that, I’m gonna have to take in apart because those chips are on the main board underneath a pretty substantial “triggering” top board.
And, the coil, was it really low resistance, or just 'conducting'?
I didn’t check its resistance, just the conductivity. But I will right away.

Leo


satbeginner
 

My 2 cents:

Just temporarily remove the two chips you couldn't replace, to see what the voltage does at power on.

And, the coil, was it really low resistance, or just 'conducting'?

Leo


Stephen
 

Pin 10 of U330 checked. All I have is about 180mV DC riding on top on 25 to 30mV of AC gibberish.


Stephen
 

On Mon, Jul 27, 2020 at 09:23 AM, Harvey White wrote:

Interleaved

If the two capacitors are ok (when one leg lifted), yet are not shorted,
and there's still a short measured across them, then what else can be
shorted?
I didn’t measure a short across them; neither in circuit nor with one leg lifted.
They read the correct value (within specs), and there is no resistance reading with one leg lifted.

TTL Chips will measure a low resistance from vcc to gnd  when
properly biased, so many times, you may be tempted to blame the
capacitor.  Assuming, then, there's a short somewhere.
I have replaced ALL the chips on that main lower board, except for U270 & U272 which I couldn’t find.
This is frustrating as hell.
I have yet to check for a signal on pin 10 of U330.

Harvey


Harvey White
 

If the two capacitors are ok (when one leg lifted), yet are not shorted, and there's still a short measured across them, then what else can be shorted?  TTL Chips will measure a low resistance from vcc to gnd  when properly biased, so many times, you may be tempted to blame the capacitor.  Assuming, then, there's a short somewhere.

Harvey

On 7/27/2020 1:53 PM, Stephen wrote:
On Mon, Jul 27, 2020 at 01:18 AM, <scm@...> wrote:

Check to see if the multivibrator capacitors are bad (possibly leaky?). Also,
check for signal on pin 10.
C330 and C335 read the right value within specs. they are not shorted And apparently not leaky. I read zero resistance across them.

Will check signal on pin 10 of U330 and report back soon.



Stephen
 

On Mon, Jul 27, 2020 at 02:16 AM, Roger Evans wrote:


Stephen,

So is either C548 or C549 showing high leakage (low resistance) with one leg
unsoldered?

Roger
C549 is not shorted nor leaky, and it reads within specs as well.


Colin Herbert
 

I think you are getting terms a bit confused. Zero resistance _is_ a short. Perhaps you intended to write "infinite resistance"? Easily done.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Stephen
Sent: 27 July 2020 18:56
To: TekScopes@groups.io
Subject: Re: [TekScopes] DC503 Not Working

On Mon, Jul 27, 2020 at 02:16 AM, Roger Evans wrote:


Stephen,

So is either C548 or C549 showing high leakage (low resistance) with one leg
unsoldered?

Roger
C548 reads the right value within specs and zero resistance across. Not shorted either.
Haven’t checked C549 yet. Will do.


Stephen
 

On Mon, Jul 27, 2020 at 02:16 AM, Roger Evans wrote:


Stephen,

So is either C548 or C549 showing high leakage (low resistance) with one leg
unsoldered?

Roger
C548 reads the right value within specs and zero resistance across. Not shorted either.
Haven’t checked C549 yet. Will do.