TECH: Test Blanking Circuit on Bench

[phlegmer]

Greetings, Have a BK System7 that I'm working on. The BK itself is
actually in another location that isn't readily available. The last
time I had the boards in, it appeared that the blanking signal was low
(not the 4v-5v required) and therefore wasn't playable. However, I was
able to do the 5v jumper trick that is talked about in Clay's doc and
the game worked great. I now have the MPU and the Driver boards at my
house and I have replaced some of the blanking circuit components
described that could go out: C84, IC7 (7404), Q1, and IC23 (555).
According to the doc, when using Leon's test, the blanking signal will
alternate high and low every second. I put on my DMM on pin 37, it
seems to bounce around more like twice a second. This was the same
symptom I saw prior to replaced some of the blanking circuit components.
Question: Is there a good way to verify that the blanking circuit is
ok on a test bench? Thanks!


--
phlegmer
This USENET post sent from http://rgparchive.com

[David Gersic]

On Mon, 13 May 2013 11:48:37 -0400, phlegmer <phle...@gmail.com> wrote:
> Question: Is there a good way to verify that the blanking circuit is
> ok on a test bench? Thanks!

Get a 'scope, or at least a logic probe, instead of trying to use a
meter to show logic transitions. A meter can really only reliably show
"high" or "low" logic, not an active signal.


--
| David Gersic http://www.zaccaria-pinball.com |
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[barakandl]

Replace C31 and test Q5. I have used an electrolytic cap at C31
before with no troubles.

[wayout440]

Disregard the blanking signal when using the test ROMs. It cannot be
reliably tested with test ROMS: Per Mark's Guide:
Update 2/10/04 - If you are testing your board using the test chip, the
blanking signal may or may not be present. This is due to the fact that
the test chip pulses port PA2 on IC18 only about twice a second, which
may or may not be fast enough to reset the timer circuit. DISREGARD THE
BLANKING SIGNAL WITH THE TEST CHIP! Blanking can only be successfully
tested once your board is running properly with the game ROMs.
http://pinball.flippers.info/system6repairpart4.asp


--
wayout440

[Barry]

If you have the MPU using game ROMs and Driver board connected on your
test bench do you still have a blanking problem. If the blanking signal
is not reaching the driverboard "strong" enough because of interconnect
resistance you might consider replacing the driverboard 40 pin connector
and check for problems with the male pins on the MPU.


--
Barry

[phlegmer]

barakandl;2072954 Wrote:
> Replace C31 and test Q5. I have used an electrolytic cap at C31
> before with no troubles.

Thanks I'll might as well since I've replaced a plethora of components
already. The only problem is I can't quite make out what the specs are
for that cap. The copy of the schematic I have has some non-legible
text for the description. Maybe need a new copy of the schematic.
Would you or someone else know the specs? Thanks

[phlegmer]

Barry;2073029 Wrote:
> If you have the MPU using game ROMs and Driver board connected on your
> test bench do you still have a blanking problem. If the blanking signal
> is not reaching the driverboard "strong" enough because of interconnect
> resistance you might consider replacing the driverboard 40 pin connector
> and check for problems with the male pins on the MPU.

Thanks Wayout for the additional info.

Barry, I guess I didn't know one could safely hook the boards up on a
test bench sans Test Rom. I'll check that out. I have already replaced
the interconnects on the boards so I hope that's still not the problem.

Thanks all


--
phlegmer

[barakandl]

Check on firepowerpinball.com, they have good sys6 schematics. From
memory it is 1uf tantalum originally, electrolytic works as well.

[phlegmer]

Mine is actually a System 7. Would the caps be the same for C31?

[wayout440]

phlegmer;2073404 Wrote:
> Thanks Wayout for the additional info.
>
> Barry, I guess I didn't know one could safely hook the boards up on a
> test bench sans Test Rom. I'll check that out. I have already replaced
> the interconnects on the boards so I hope that's still not the problem.
>
>
> Thanks all

OK, let us know what you find.


--
wayout440

[phlegmer]

Finally had a chance to give it a try. I think the symptom is still the
same as before I swapped out components.

Here are some readings I took with game roms in on the test bench in
hopes to help you help me :)

Power Supply = 4.91v and 12.02v
TP1 = 12.02v
TP2 = 3.98v
TP3 = 72.5mv
TP4 = 3.4v
TP5 = 4.64v
TP6 = 2.37v
TP7 = 4.69v
TP8 = 4.73v
TP9 = 4.81v

Pin 37 = 3.4v
1J3 Pin4 = 3.4v

If I'm understanding it correctly, 3.4v just isn't enough on Pin 37
(TP4) get things rolling.

Suggestions?

Thanks!


--
phlegmer

[wayout440]

phlegmer;2074724 Wrote:
> Finally had a chance to give it a try. I think the symptom is still the
> same as before I swapped out components.
>
> Here are some readings I took with game roms in on the test bench in
> hopes to help you help me :)
>
> Power Supply = 4.91v and 12.02v
> TP1 = 12.02v
> TP2 = 3.98v
> TP3 = 72.5mv
> TP4 = 3.4v
> TP5 = 4.64v
> TP6 = 2.37v
> TP7 = 4.69v
> TP8 = 4.73v
> TP9 = 4.81v
>
> Pin 37 = 3.4v
> 1J3 Pin4 = 3.4v
>
> If I'm understanding it correctly, 3.4v just isn't enough on Pin 37
> (TP4) get things rolling.
>
> Suggestions?
>
> Thanks!

Yes I would agree - it does sound low.
"check the Blanking signal at test point 4 or pin 37 of the
interconnect. It should be +5 volts. The most likely cause for a
blanking signal failure is a timer chip failure, a bad capacitor at C31
or a a failure of transistor Q5 (a 2N4403)"
If I had a similar game I would measure mine to confirm, perhaps a Sys 7
owner could check. You need a minimum logic high and depending on the
types of integrated circuits that might be interpreted too low
http://www.allaboutcircuits.com/vol_4/chpt_3/10.html


--
wayout440

[phlegmer]

Well, I guess the next step is to replace Q5 and C31. There's not much
going to be left as far as original components after resolving this
issue. :)

I'll post an update after I get the parts and install.

Thanks


--
phlegmer

[phlegmer]

I just replaced:

C31
Q5
and C27 (just because I had it)

Still the same 3.4v.

Interesting on my schematic (dated 10/10/80) it says that Q5 is 2N4401
and not 2N4403 as posted. I installed what my schematic listed

Any other suggestions?

[Barry]

On test bench board powered:

What is the logic state of pin 8 of IC7? Does pin 8 of IC23 measure
4.9VDC? This is the power input to the 555 timer.

With meter attached to ground and TP 4 of CPU ground pin 8 of IC7, is
there any change in the voltage reading of TP4?

Then ground pin 6 of IC23 the 555 timer along with pin 8 of IC7. Any
voltage change on TP4?

If the voltage doesn't go up to close to 5V it would seem to be isolated
to IC23 and IC23 connections.


--
Barry

[Barry]

If the driver board is still attached to the CPU on the test bench,
remove it and repeat the grounding tests associated with the input logic
to the 555 timer.

If the Blanking voltage goes up when testing the timer inputs without
the driver board, then suspect a bad AND gate IC on the driver board
that is pulling the Blanking level to a marginal logic high (3.7V.)

[phlegmer]

Hope I did this right....

With MPU and Driver boards connected:

*"What is the logic state of pin 8 of IC7?"*
-Not sure what kind of answer you are looking for. It meters out to
1.9v and the osziFOX (which I hope I'm using correctly), does show a
shallow square wave.-

*"Does pin 8 of IC23 measure 4.9VDC?"
*-4.84v...close enough?-

*"With meter attached to ground and TP 4 of CPU ground pin 8 of IC7, is
there any change in the voltage reading of TP4?"*
-I clipped the meter to TP4 and TP10 (ground). When doing this test, I
clipped another wire to TP10 and then used it to ground pin 8 of IC7.
It went from 3.41v to 64.5mv. So yes, it did change.-

*"Then ground pin 6 of IC23 the 555 timer along with pin 8 of IC7. Any
voltage change on TP4?"
*-Pin 6 of IC23 by itself meters out to 1.46v. I clipped another wire
to TP10 (a total of 3 now). While grounding pin 8 IC7, I then grounded
pin 6 of IC23. No voltage change.

*"*-*If the driver board is still attached to the CPU on the test bench,

remove it and repeat the grounding tests associated with the input
logic to the 555 timer."

*-I removed the driver board and repeated the above test. Voltage then
goes down to 0v. Shouldn't I have Leon's test installed when there is
no driver board?-

Conclusion IC23 issue?

Thanks


--
phlegmer

[phlegmer]

Just to refresh...IC23 has already been replaced. :)

Suggestions?

[Barry]

I was originally thinking of forcing the Blanking circuit but it appears
all of your inputs to the circuit are good and of proper value, such as
the Reset signal and the pin 8 output of IC7.

The only thing I can think of is to measure the resistance from each of
the pins on your IC23 to the next connecting component and make sure you
are very close to zero resistance. I would check for possible shorts
caused by a solder splash. 555 Timers are inexpensive, try another one
from a different source.

Double check all polarized capacitor orientations with the schematic.

If the board had an existing socket that you used when you replaced the
timer, IC23 you might want to take a close look at the socket.

I guess you have replaced all of the capacitors associated with the
circuit and measured the resistors for proper values. Given good
inputs, good components, it would appear there is a circuit short or bad
connection somewhere associated with the IC23 and connections causing a
poor output.


--
Barry

[phlegmer]

Barry;2077056 Wrote:
>
> I guess you have replaced all of the capacitors associated with the
> circuit and measured the resistors for proper values. Given good
> inputs, good components, it would appear there is a circuit short or bad
> connection somewhere associated with the IC23 and connections causing a
> poor output.

I don't think I ever did check all the resistors. Can that be checked
in circuit?


--
phlegmer

[Hans]

Twice per second blinking on the test ROM is actually pretty normal to be honest, only if it's very slow would I be concerned. I've only encountered a sketchy blanking circuit a couple times, and never to the point it caused the board to stop working. The worst I ever saw was caused by a bad capacitor causing very short output pulses on the blanking, and this manifested in about half the lamp matrix going out. But it still ran.

-Hans

[phlegmer]

Barry;2077056 Wrote:
>
> The only thing I can think of is to measure the resistance from each of
> the pins on your IC23 to the next connecting component and make sure you
> are very close to zero resistance.

Sorry, don't know if I understand what you are meaning by this. Are you
talking about checking the resistance between pins on the 555 (IC23)?

Barry;2077056 Wrote:
>
> Double check all polarized capacitor orientations with the schematic.
>

From what I can tell, this looks correct.

Barry;2077056 Wrote:
>
> If the board had an existing socket that you used when you replaced the
> timer, IC23 you might want to take a close look at the socket.
>

Nope it did not have a socket. I installed the socket and the new IC23.
Same symptom as before. Because it's the same symptom, I'm leaning
towards that it isn't the 555 (IC23).

Barry;2077056 Wrote:
>
> I guess you have replaced all of the capacitors associated with the
> circuit and measured the resistors for proper values. Given good
> inputs, good components, it would appear there is a circuit short or bad
> connection somewhere associated with the IC23 and connections causing a
> poor output.

Correct me if I'm wrong but I'm pretty sure in order to get a good
resistor reading, one of the legs needs to be pulled to be out of
circuit. I did test the resistors in blanking circuit and all but one
appeared to be within specs. R22 says it should be 6.9kohms but it
measured 2.38kohms. For fun I check for other resistors not in the
blanking circuit and several measured different than what they
should...again, probably a result of them still soldered to the board.

Any specific suggestions on what / how to check where the voltage is
getting dropped below the required 5v?

Thanks


--
phlegmer

[Barry]

You check continuity between pins of IC23 and their associated component
or test point.

Pin 1 of IC23 one probe the other probe to the ground test point.
Should be close to zero resistance.

Pin 2 of IC23 one probe, the other probe to the junction of R21 and
R22.
Also check to Q21 base.

Pin 3 of IC23 one probe, the other probe to TP 4.

Just follow the schematic checking each of the pins of IC23 to where the
pin should be connecting.

I can't think of anything else you can do beyond interconnect testing
since you have replaced all of the components.


--
Barry

[phlegmer]

Thanks, hopefully I'll get to it tonight and check.

One thing I do recall noticing is that there were a couple pins on IC23
that metered the same 3.4v that is on header pin 37 / TP4. I know one
was pin 7 but don't recall what the other pin was.

Coincidence?

Thanks


--
phlegmer

[phlegmer]

Barry;2077596 Wrote:
> You check continuity between pins of IC23 and their associated component
> or test point.
>
> Pin 1 of IC23 one probe the other probe to the ground test point.
> Should be close to zero resistance.
>
> Pin 2 of IC23 one probe, the other probe to the junction of R21 and
> R22.
> Also check to Q21 base.
>
> Pin 3 of IC23 one probe, the other probe to TP 4.
>

All appear near 0 including the following...

Pin 4 of IC23 to the following all near 0
IC36 Pin 34
R29
R30
Q2
TP8

Pin 5 of IC23 to the following all near 0
C17

Pin 6 of IC23 to the following all near 0
C31
R23

Pin 7 of IC23 to the following all near 0
Pin 3 of IC23
TP4
Header pin 37

Pin 8 of IC23 to the following all near 0
C63

This is turning into quite the mystery....


--
phlegmer

[phlegmer]

ADDITIONAL INFO:
On bench with game ROMS, 7 segment flashes "0" and the 2 LED's also
flashes once at the same time as the "0". I believe it means it passed
the initial test.

With Leon Test Rom, all diagnostics pass.

Went over the repair guide again
(http://pinball.flippers.info/system6repairpart4.asp).

From the guide: "-To use the test LED, connect the negative end to
ground and then touch the positive end to each output on the PIA
(pins 2 through 17)...-- If you don't get a flash on one of the
pins, or one of the pins gives you a continuous signal, then you
will need to replace the PIA-.\"

On IC36, pin 9 appears to be dead. I'll try and get a replacement soon.
If I'm understanding this correclty, the other PIA (IC18) would affect
the blanking and not IC36.

Also noticed that on both PIA's, pins 2-9 were more dim than 10-17.

From the guide: \"-The crystal is connected between pins 38 and 39 on
IC1. You should see a waveform if using your scope probe on these
pins or approx. +3 volts if using your DMM.-\\\"

I'm getting around 1.4v for each pin. Not sure if this is an issue or
related to blanking.

Again from the guide: \"-If your game seems to be booting properly with
the Game ROMs however you're not getting a plus on pin 4 of IC18
(the PIA that drives the blanking signal), check the outputs of
IC25 (a 4020). If the interrupt signal is not being generated
at the correct speed, then IC18 will not generate the required
\\\"pulse\\\".-\\\"

Not sure what is meant by \\\"a plus on pin 4 of IC18\\\" Does that
mean any voltage or perhaps around 5v? When I test mine, it shows 2.4v.

[barakandl]

Occasionally a PIA output is tied to ground (always low 0v). Double
check the schematic, continuity beep it to ground to make sure. To
test the switch matrix PIA you have to ground an entire connector.
Using a logic probe to test the PIAs with the test rom is easiest.

Pulse is a signal that is going high(5v) and low(0v) back forth. Your
DMM rounds this out and you see an average voltage. 2.5vdc on a DMM
you can assume it is pulsing.

The crystal is effected by your DMM probes. Check the clock output
for a pulsing signal. I think it is pin 37 on the CPU.

You should get identical highs and lows on all cycling PIA outputs
with test rom. Any variation is suspect.

The blanking signal is usually a symptom of a not booting board set,
not usually the cause of a non booting boardset.

**** i havent read most of the previous posts

[barakandl]

> **** i havent read most of the previous posts- Hide quoted text -
>
> - Show quoted text -

Every driver board i work on now i add a LED to the blanking signal.
Nice mod to do. Ground - 150 ohm resistion - negative side of diode -
positive side of diode - blanking signal. There is a spot near the
interconnect where there is space to do this.

[Andy_B]

I think he meant to type "pulse"


--
Andy_B

[wayout440]

phlegmer;2079046 Wrote:
> ADDITIONAL INFO:

>
> Not sure what is meant by "a plus on pin 4 of IC18" Does that mean any
> voltage or perhaps around 5v? When I test mine, it shows 2.4v.
>

According to the datasheet for the 6820, the output logic high value for
the data bus is "Vss +2.4V". Since Vss is tied to ground on the game,
the 2.4VDC you are measuring appears to be correct for a logical "high",
however you need to check it with a logic probe or oscope to be certain
it is a pulse, and not locked in a continuous high state.


--
wayout440

[phlegmer]

Got distracted with other squirrels but managed to carve out a little
time tonight.

Changed out the crystal (boy are the new ones tiny compared to the old
ones), no change.

I was able to borrow another driver board from a friend. Mated the MPU
and the loaned driver board, BINGO! I now have 4.72v compared to the
3.3v. The issue must be with my driver board. As mentioned before, the
interboard connectors have been replaced. I re-inspected the soldering
and they all appear nice, shiny and solid.

Now that we know that something is pulling my driver board down ~1.5v,
suggestions what to interrogate first?

Thanks


--
phlegmer

[wayout440]

Here is where I would start:
Compare the known good board to the bad board using logic probe (or
better yet an oscope) for the following signal:
Pulse on pin 4 of IC18 (the PIA that drives the blanking signal), check

the outputs of IC25 (a 4020). If the interrupt signal is not being
generated at the correct speed, then IC18 will not generate the required

pulse.


--
wayout440

[phlegmer]

OK, I think I may have FINALLY found something of interesting. One of
the first things I did a long time ago when upgrading the components of
my driver board was to install a blanking signal LED. This was advised
on Clay's guide by taking an LED along with a 1/4w 150ohm resistor and
placing it between interconnect pin 37 and ground. (Even has a nice pic
as to where he soldered his.) The thought is when there is a blanking
signal one can easily, at a glance, see if it is up. One evening I had
a thought and decided to see what would happen if I de-solder the
blanking LED. Metered out the signal, wow! It's actually super close
to 5v now!

I took the same LED and soldered it onto my buddy's driver board. Same
exact symptom. The blanking signal was pulled down to a hair above 3v.
I metered out the resistor and it is almost spot on 150ohm. I then
snagged a different LED, even a different color, the same exact symptom.
Down to around 3v.

I guess I could live without the blanking LED but now I'm more curious
as to why this simple mod advised by Clay's guide would do this. The
LED does light up and as far as I know, resistors are not polarized.

Thoughts?

Thanks!


--
phlegmer

[c...@provide.net]

that's weird i do that LED mod all the time and have never
had an issue. when i was reading your post i thought
maybe the LED was bad, but you changed that. or maybe
attached "backwards"?

[barakandl]

On Saturday, October 26, 2013 12:37:02 AM UTC-4, phlegmer wrote:

You are installing the wrong resistor or doing this mod wrong. I put the blanking LED in often with never any trouble.

[phlegmer]

I just tried a 3rd totally different LED (larger orange) along with a
different 150ohm resistor (brown-green-black-black-brown), same issue.
LED's do have polarity and wouldn't light up if installed backwards.

Those of you who have this mod installed, can you do a quick test on TP4
and see what voltage is showing up? If the voltage still holds strong
near 5v, perhaps there is something a bit weak on the board's power
supply circuit?

Thanks

[phlegmer]

Most likely the power circuit is ok since I do get an equal voltage on
TP9 to that of what my bench power supply puts out (~4.82v). I did a
little poking around the blanking circuit and not sure if what I found
helps in troubleshooting. IC23 (LM555 Timer) does keep consistent
voltage on pin 8, which is the +VCC (~4.82v), when I apply the LED.
However pins 7 and 3, which both appear to go to TP4, do fluctuate
between 4.78v down to the 3.3v when LED is applied.

I have already replaced the IC23 with a new part as well as installing
SIP socket.

Any other thoughts or places to look?

[YFZ450]

150 ohm resistor is brown/green/brown not brown/green/black.
brown/green/black is a 15 ohm risistor.


--
YFZ450

[KenH]

It was a good idea, but for a 5-band resistor,

brown(1)-green(5)-black(0)-black(x10^0 = 1) + brown(tolerance)

wouldn't it be 150x1, or 150 ohms?

[YFZ450]

Not familiar with 5 band resistors. Maybe it's a precision resistor?

[YFZ450]

Yup, I stand corrected. 150 ohm 1%. Sorry

[phlegmer]

I agree it is, in a 4 or 3 color band scenario. I too thought I was on
to something when I double checked the resistor colors. However, my
multimeter was telling me that it was 149ohm. A 5 color band resistor
is actually brown-green-black-black-brown where the last brown is 1%
tolerance. Check out this resistor color band calculator -->
http://www.hobby-hour.com/electronics/resistorcalculator.php

Thanks


--
phlegmer

[phlegmer]

Here is a pic of the blanking LED.

Filename:-----------> Blank LED.jpg
Link to attached PIC: http://rgparchive.com/rgpforum/attachment.php?attachmentid=2261

[phlegmer]

I checked the blanking signal with a scope and it appears to be just a
solid line. The the only time I see it change is when I remove the LED,
then the line jumps up a tad.

Thanks

Filename:-----------> photo(1).jpg
Link to attached PIC: http://rgparchive.com/rgpforum/attachment.php?attachmentid=2262

[phlegmer]

Not sure why the pic was upside down....sorry.

Filename:-----------> photo(1).jpg
Link to attached PIC: http://rgparchive.com/rgpforum/attachment.php?attachmentid=2263

[phlegmer]

Here is a pic of the CPU board just in case there is something visually
obvious that I'm missing.

Filename:-----------> Board.jpg
Link to attached PIC: http://rgparchive.com/rgpforum/attachment.php?attachmentid=2265