Topics

DC503 Not Working


Stephen
 

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.


SCMenasian
 

U330B and U330C are the scan clock multivibrator. If these stop or stop intermittantly, the symptoms you describe will appear. U332B is part of the counter which, along with U333 (which you have already changed) controls the display multiplexer. There is a scan clock disable line shown in the block diagram. It may be stuck. Look for bad connections as well as bad ICs associated with these circuits. Try adding an external test point by soldering a 1 foot wire to U330, pin 10 as a start to bring the scan clock signal out so you can see it. Move it around, depending on what you see. I'm looking at the right manual now.


Roger Evans
 

Stephen,

If you have almost zero volts across C548 then the -22V regulated supply is at almost zero volts. The most likely culprits are C548 itself and C549 which sits after L549 to provide an additional stage of smoothing. You should lift one pin of each of C548 and C549 to check that they are OK. You might as well check L549 for continuity while the other parts are disconnected.

Roger


Stephen
 

On Mon, Jul 27, 2020 at 12:05 AM, <scm@...> wrote:


U330B and U330C are the scan clock multivibrator.
This is one single chip, right? U330. Believe it or not, I have Clean the socket, and replaced that one too.
I have actually replaced 98% of all the chips on that board. This is insane!

If these stop or stop
intermittantly, the symptoms you describe will appear. U332B
Also replaced and socket cleaned.

is part of the counter which,
along with U333 (which you have already changed) controls the
display multiplexer. There is a scan clock disable line shown in the block
diagram. It may be stuck. Look for bad connections as well as bad ICs
associated with these circuits. Try adding an external test point by soldering
a 1 foot wire to U330, pin 10 as a start to bring the scan clock signal out so
you can see it. Move it around, depending on what you see.
I'm looking at the right manual now.
Thank you

One funny thing though, whether a digit is there, or if nothing appears on the display at start up, when I turn the 506 off, digits appear randomly where none where before, before fading off for lack of power.


Stephen
 

On Mon, Jul 27, 2020 at 12:22 AM, Roger Evans wrote:


Stephen,

If you have almost zero volts across C548 then the -22V regulated supply is at
almost zero volts. The most likely culprits are C548 itself and C549 which
sits after L549 to provide an additional stage of smoothing. You should lift
one pin of each of C548 and C549 to check that they are OK. You might as well
check L549 for continuity while the other parts are disconnected.

Roger
L549 is fine. When I checked the voltage across C548, that top board was removed. Hence L549 wasn’t connected.

Thank you for pointing that out Roger


SCMenasian
 

Check to see if the multivibrator capacitors are bad (possibly leaky?). Also, check for signal on pin 10.


Roger Evans
 

Stephen,

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

Roger


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
I haven’t had the opportunity to check yet. I’m not home at the moment. Will report back when I do, Roger.

Thank you for your help.


Stephen
 

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.
Will certainly do when I get back home.
Will report back.

Thank you gain.


Stephen
 

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


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


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 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


Stephen
 

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


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
 

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


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


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