CBM 3032 (PET 2001N-32), no video?

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CBM 3032 (PET 2001N-32), no video?

Sherry Haibara
Hello everybody! Just acquired a beautiful CBM 3032 (PET 2001N-32) today, and
I'm already running into problems... Which was kind of expected, given its
age!
I managed to get a first successful boot of the machine with crisp video.
BASIC initialised correctly and I was able to execute the classic "PRINT
'Hello world'" command with no problem.
I then powered off the machine as I had to go away for a while. When I came
back, I tried to turn it on but to no avail. I hear a slight humming noise
from the power supply and sometimes, but not always, there seems to be a
sound right after flipping the power switch that reminds me of degaussing,
but I'm unsure if that's really what's going on.
I don't hear any other noise, including the usual pitched noise that comes
from CRT monitors when they're working. I also didn't hear any obvious
"failing noise", like crackling or pops, and I didn't see any smoke coming
from the machine. The fuse is fine. I can see the electron gun of the CRT
from the back of the case, but it's staying off - while on the first
successful power on I could see it glowing in orange.
There doesn't seem to be any obviously burnt component on the main board,
and I've also already reseated the ROMs. I checked the +5V voltage on the
user port and it's correct. I also checked the voltage on the main power
connector (J8?) and it shows -9 V, which as far as I understood should be
the correct value. The +12V rail also seems fine.
The symptoms seems a little suspicious to me, in that - if I'm not mistaken
- if there was a problem with the ROMs or the CPU, I should at least get a
garbage screen of some sort, which I'm absolutely not getting. There just
doesn't seem to be any sign of life from the CRT.
So my question is - where should I begin troubleshooting? I don't have an
oscilloscope, but I do have a multimeter and some basic soldering
experience. I'm not too comfortable with going around the high voltage CRT
video circuitry, but I know a person that may be able to perform
troubleshooting on that part.
Thanks!



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Re: CBM 3032 (PET 2001N-32), no video?

Bill Degnan


On Sat, Feb 24, 2018 at 3:02 PM, Sherry Haibara <[hidden email]> wrote:
Hello everybody! Just acquired a beautiful CBM 3032 (PET 2001N-32) today, and
I'm already running into problems... Which was kind of expected, given its
age!
I managed to get a first successful boot of the machine with crisp video.
BASIC initialised correctly and I was able to execute the classic "PRINT
'Hello world'" command with no problem.
I then powered off the machine as I had to go away for a while. When I came
back, I tried to turn it on but to no avail. I hear a slight humming noise
from the power supply and sometimes, but not always, there seems to be a
sound right after flipping the power switch that reminds me of degaussing,
but I'm unsure if that's really what's going on.
I don't hear any other noise, including the usual pitched noise that comes
from CRT monitors when they're working. I also didn't hear any obvious
"failing noise", like crackling or pops, and I didn't see any smoke coming
from the machine. The fuse is fine. I can see the electron gun of the CRT
from the back of the case, but it's staying off - while on the first
successful power on I could see it glowing in orange.
There doesn't seem to be any obviously burnt component on the main board,
and I've also already reseated the ROMs. I checked the +5V voltage on the
user port and it's correct. I also checked the voltage on the main power
connector (J8?) and it shows -9 V, which as far as I understood should be
the correct value. The +12V rail also seems fine.
The symptoms seems a little suspicious to me, in that - if I'm not mistaken
- if there was a problem with the ROMs or the CPU, I should at least get a
garbage screen of some sort, which I'm absolutely not getting. There just
doesn't seem to be any sign of life from the CRT.
So my question is - where should I begin troubleshooting? I don't have an
oscilloscope, but I do have a multimeter and some basic soldering
experience. I'm not too comfortable with going around the high voltage CRT
video circuitry, but I know a person that may be able to perform
troubleshooting on that part.
Thanks!


For now rule out the CRT and connections within.

I have the same system and had the same issue.  A bonk on the side of the display brought my video back....It may be a loose connection moreso than a CPU or logic issue..


Bill



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Re: CBM 3032 (PET 2001N-32), no video?

Francesco Messineo
In reply to this post by Sherry Haibara
On Sat, Feb 24, 2018 at 9:02 PM, Sherry Haibara <[hidden email]> wrote:

> Hello everybody! Just acquired a beautiful CBM 3032 (PET 2001N-32) today, and
> I'm already running into problems... Which was kind of expected, given its
> age!
> I managed to get a first successful boot of the machine with crisp video.
> BASIC initialised correctly and I was able to execute the classic "PRINT
> 'Hello world'" command with no problem.
> I then powered off the machine as I had to go away for a while. When I came
> back, I tried to turn it on but to no avail. I hear a slight humming noise
> from the power supply and sometimes, but not always, there seems to be a
> sound right after flipping the power switch that reminds me of degaussing,
> but I'm unsure if that's really what's going on.

the monochromatic CRTs don't need (and don't have) any degaussing.
What you're hearing is the power transformer building it's magnetic
field and the windings move a bit.

> I don't hear any other noise, including the usual pitched noise that comes
> from CRT monitors when they're working. I also didn't hear any obvious
> "failing noise", like crackling or pops, and I didn't see any smoke coming
> from the machine. The fuse is fine. I can see the electron gun of the CRT
> from the back of the case, but it's staying off - while on the first
> successful power on I could see it glowing in orange.
> There doesn't seem to be any obviously burnt component on the main board,
> and I've also already reseated the ROMs. I checked the +5V voltage on the
> user port and it's correct. I also checked the voltage on the main power
> connector (J8?) and it shows -9 V, which as far as I understood should be
> the correct value. The +12V rail also seems fine.

if the CRT heater doesn't come on (that's the orange glow on the CRT
neck), check the AC secondary
output on the transformer. There's one winding for the monitor supply,
it should be about 18Vac.
If that's ok, then it's something on the power rectifier and
regulators on the monitor PCB that are failing.
The PET monitor heater is fed by regulated DC.

> The symptoms seems a little suspicious to me, in that - if I'm not mistaken
> - if there was a problem with the ROMs or the CPU, I should at least get a
> garbage screen of some sort, which I'm absolutely not getting. There just
> doesn't seem to be any sign of life from the CRT.

yes, on 2001 and 2001N (30xx) variants of the PETs you get at least a
garbage screen if the CPU is not starting.
However, the lack of heater glow on the CRT's neck indicates a failure
in the monitor power supply, as I already said.

> So my question is - where should I begin troubleshooting? I don't have an
> oscilloscope, but I do have a multimeter and some basic soldering
> experience. I'm not too comfortable with going around the high voltage CRT
> video circuitry, but I know a person that may be able to perform
> troubleshooting on that part.

18Vac on the transformer, then it goes inside the monitor to a 4 diode
bridge and a big 4700 uF/25V capacitor, you should get about 18-20 Vdc
on the capacitor when the supply is working well. Then that goes to an
LM7812 that regulates the DC voltage to 12V, it's mounted on a quite
big heatsink, you have to check that its output is indeed about 12V.
Those monitor tend to develop cracked solder joints after all these
years, so you might find that the diodes and maybe the capacitor need
their solder reflowed and renewed.

HTH
Frank IZ8DWF

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Re: CBM 3032 (PET 2001N-32), no video?

Sherry Haibara
This post was updated on .
Just checked the voltage on the winding for the monitor supply, it shows
16.5 Vac, which is a little lower than the 18Vac you mentioned. Is that
within tolerance? Otherwise I’ll have to check the monitor PCB, it may
indeed be a cracked solder joint… It’s my first experience with a CBM, but
other computers of that era tend to have this kind of issues as well, I
guess it’s rather common.
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Re: CBM 3032 (PET 2001N-32), no video?

MiaM
Den Sat, 24 Feb 2018 14:19:23 -0800 skrev Valerio Castelli
<[hidden email]>:
> Just checked the voltage on the winding for the monitor supply, it
> shows 16.5 Vac, which is a little lower than the 18Vac you mentioned.
> Is that within tolerance? Otherwise I’ll have to check the monitor
> PCB, it may indeed be a cracked solder joint… It’s my first
> experience with a CBM, but other computers of that era tend to have
> this kind of issues as well, I guess it’s rather common.

I'd check the signals from the main logic board up to the monitor. An
oscilloscope is ideal but a simple volt meter could be usable. If you
don't have anything to measure with you could use a series resistor and
the input of some crap audio amplifier/speaker setup. You should hear a
rattling sound on VSYNC and with really good ears might hear something
on HSYNC but probably not. Video out should give a rattling sound if
there is something on screen. If you hear that (or see something on an
oscilloscope) you could try pushing SHIFT + CLR/HOME and see if the
signal almost goes away. If so you probably had a correct startup
screen and just cleared the screen.

If you want to see if the CPU starts, you could connect a datasette and
see if the motor turns on and off as you press play/req/fwd and stop.
(If the datasette seems to work but you are unsure if the motor really
stops in stop mode, you can open the lid and feel if the capstan axle
turns even in stop mode. That is an indicator of that the CPU doesn't
run the correct code from ROM).


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Re: CBM 3032 (PET 2001N-32), no video?

Sherry Haibara
Thanks for the suggestion! Unfortunately I don't have a datasette to check
the CPU at the moment, I'll see if I can get my hands on one.

I did take some measurements on the video connector with the multimeter
though, and this is the result:
J1 (video signal): 4.47V
J3 (vertical drive): 4.26V
J5 (horizontal drive): 1.6V
I have seen that someone was mentioning on an old thread on vcfed that the
correct values for J3 and J5 should be around 4.6V and 1.8V, which don't
seem to be too far from what I've measured. The thread also mentioned that a
voltage greater than 4V on J1 is associated with a blank screen; the monitor
is indeed blank, though I can't say if that's because the monitor itself has
a problem or if it's not being driven correctly by the video circuitry.

Incidentally, I removed the three screws from the back of the monitor and
I've noticed the cables of the yoke are directly soldered to the board; I
assume any reflow operation would have to be done with the CRT still
attached to the board? I know I'm a newbie, but I was also wondering how to
safely detach the monitor from the base and where should I look to remove
the top cover of the monitor assembly.



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Re: CBM 3032 (PET 2001N-32), no video?

André Fachat
In reply to this post by MiaM


Am 25. Februar 2018 00:25:24 schrieb Mia Magnusson <[hidden email]>:


> I'd check the signals from the main logic board up to the monitor. An
> oscilloscope is ideal but a simple volt meter could be usable. If you
> don't have anything to measure with you could use a series resistor and
> the input of some crap audio amplifier/speaker setup. You should hear a
> rattling sound on VSYNC and with really good ears might hear something
> on HSYNC but probably not. Video out should give a rattling sound if
> there is something on screen.

Attaching a speaker is a cool idea!

I would have suggested using a small logic board on the user port (that has
the video signals) to create a composite signal and attach a standard monitor.

But the drawback is that you don't know if your composite logic and monitor
setup work if you haven't tried it before on a working pet

André



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Re: CBM 3032 (PET 2001N-32), no video?

Francesco Messineo
In reply to this post by Sherry Haibara
On Sat, Feb 24, 2018 at 11:19 PM, Valerio Castelli
<[hidden email]> wrote:
> Just checked the voltage on the winding for the monitor supply, it shows
> 16.5 Vac, which is a little lower than the 18Vac you mentioned. Is that
> within tolerance? Otherwise I’ll have to check the monitor PCB, it may
> indeed be a cracked solder joint… It’s my first experience with a CBM, but
> other computers of that era tend to have this kind of issues as well, I
> guess it’s rather common.


yes, 16.5Vac is ok. If the heater doesn't glow, it's definitely
something in the rectifier/filter/regulator part in the monitor PCB.

Frank

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Re: CBM 3032 (PET 2001N-32), no video?

Francesco Messineo
In reply to this post by Sherry Haibara
On Sun, Feb 25, 2018 at 1:45 AM, Sherry Haibara <[hidden email]> wrote:

> Thanks for the suggestion! Unfortunately I don't have a datasette to check
> the CPU at the moment, I'll see if I can get my hands on one.
>
> I did take some measurements on the video connector with the multimeter
> though, and this is the result:
> J1 (video signal): 4.47V
> J3 (vertical drive): 4.26V
> J5 (horizontal drive): 1.6V
> I have seen that someone was mentioning on an old thread on vcfed that the
> correct values for J3 and J5 should be around 4.6V and 1.8V, which don't
> seem to be too far from what I've measured. The thread also mentioned that a
> voltage greater than 4V on J1 is associated with a blank screen; the monitor
> is indeed blank, though I can't say if that's because the monitor itself has
> a problem or if it's not being driven correctly by the video circuitry.
>
> Incidentally, I removed the three screws from the back of the monitor and
> I've noticed the cables of the yoke are directly soldered to the board; I
> assume any reflow operation would have to be done with the CRT still
> attached to the board? I know I'm a newbie, but I was also wondering how to
> safely detach the monitor from the base and where should I look to remove
> the top cover of the monitor assembly.


I've always been able to extract the boards from those monitors (and
work on them) without desoldering the joke wires, one time I did
remove also the CRT itself from the metal case, but usually the wires
are long enough to allow the board sit just outside the case for
probing and working on it.
If you want to detach the joke wires, make a picture to be sure you
reattach each wire where it belongs.

Frank

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Re: CBM 3032 (PET 2001N-32), no video?

Sherry Haibara
In reply to this post by Francesco Messineo
Took a couple of measurements on the monitor PCB. The AC input is ok
(16.5Vac), but the voltage across the capacitor and on the LM7812 voltage
regulator seems to be very off.
The voltage on the capacitor oscillates between 0 and 8.7 volts in a
rhythmic fashion (it goes like: 0-1-8.7-1-0). On the LM7812 (I assumed it's
the one mounted on the big heatsink right next to the capacitor) I measure
22V DC on two pairs of pins and 0V on the other.
The 4 diodes on the bridge test fine with the diode test of the multimeter,
showing 0L on one side and a 0.55V drop on the other.



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Re: CBM 3032 (PET 2001N-32), no video?

Francesco Messineo
On Sun, Feb 25, 2018 at 1:40 PM, Sherry Haibara <[hidden email]> wrote:
> Took a couple of measurements on the monitor PCB. The AC input is ok
> (16.5Vac), but the voltage across the capacitor and on the LM7812 voltage
> regulator seems to be very off.
> The voltage on the capacitor oscillates between 0 and 8.7 volts in a
> rhythmic fashion (it goes like: 0-1-8.7-1-0). On the LM7812 (I assumed it's
> the one mounted on the big heatsink right next to the capacitor) I measure
> 22V DC on two pairs of pins and 0V on the other.

this doesn't sound right... The voltage from the input capacitor
should show exactly on the input (left) pin of the LM7812 (measured
with the center pin as ground) and if so, the right pin of the LM7812
should show 12V, unless it's broken or there's a short somewhere on
the 12V supply.

> The 4 diodes on the bridge test fine with the diode test of the multimeter,
> showing 0L on one side and a 0.55V drop on the other.

also carefully check the solder joints.

Frank

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Re: CBM 3032 (PET 2001N-32), no video?

Sherry Haibara
Quick update. I built a rudimental PET video mixer according to D'asaro
schematics to test if the main board was working correctly. The discrete
components I used don't match exactly the ones on the schematics (resistors
and capacitors are a little off), so the result is that the external
monochrome monitor I'm using as test is not syncing correctly on the video
signal.
Still, I'm able to see some output. When I turn on the CBM, I see a mostly
green screen, followed by a black screen, followed by what seems to be a
single line of text that is repeated multiple times on the screen and that
keeps oscillating in a diagonal fashion. I'm not entirely sure of what I'm
seeing, but this seems to suggest that the video generation circuitry on the
main board is doing something, and that there is some processing going on
(could it be the BASIC prompt?). I may try to build a better one in the next
days if I can find the right components.
I will now try to refresh the solder joints on the diode bridge, on the
capacitor and on the LM7812 rectifier and see if I can get the heater of the
internal monitor working (and/or if this leads to some difference in the
measurements).



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Re: CBM 3032 (PET 2001N-32), no video?

Francesco Messineo
On Sun, Feb 25, 2018 at 6:58 PM, Sherry Haibara <[hidden email]> wrote:

> Quick update. I built a rudimental PET video mixer according to D'asaro
> schematics to test if the main board was working correctly. The discrete
> components I used don't match exactly the ones on the schematics (resistors
> and capacitors are a little off), so the result is that the external
> monochrome monitor I'm using as test is not syncing correctly on the video
> signal.
> Still, I'm able to see some output. When I turn on the CBM, I see a mostly
> green screen, followed by a black screen, followed by what seems to be a
> single line of text that is repeated multiple times on the screen and that
> keeps oscillating in a diagonal fashion. I'm not entirely sure of what I'm
> seeing, but this seems to suggest that the video generation circuitry on the
> main board is doing something, and that there is some processing going on
> (could it be the BASIC prompt?). I may try to build a better one in the next
> days if I can find the right components.

it looks you're seeing the right output but the monitor can't sync to
both the sync signals you're trying to feed it.

> I will now try to refresh the solder joints on the diode bridge, on the
> capacitor and on the LM7812 rectifier and see if I can get the heater of the
> internal monitor working (and/or if this leads to some difference in the
> measurements).

the heater of the monitor is really independent from the main PET
board. It should light as soon as you apply the AC power to the
monitor PCB, even if no PET is connected. It's fed with the regulated
12V DC through a dropping resistor only.
If it doesn't come on, there is no 12V dc on the monitor PCB or some
connection is broken (wires going to the CRT neck, the dropping
resistor, etc.)

Frank
>
>
>
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Re: CBM 3032 (PET 2001N-32), no video?

Sherry Haibara
This post was updated on .
> the heater of the monitor is really independent from the main PET
> board. It should light as soon as you apply the AC power to the
> monitor PCB, even if no PET is connected. It's fed with the regulated
> 12V DC through a dropping resistor only.
> If it doesn't come on, there is no 12V dc on the monitor PCB or some
> connection is broken (wires going to the CRT neck, the dropping
> resistor, etc.)

The pins of the regulator seem indeed to be in a very sorry state. I'm
attaching a picture (let me know if it's visible). It looks like someone has
attempted to replace it but didn't do a very good job, those traces might be
broken at this point. How should the pins and the traces look like? Are they
supposed to be connected, and in what order? My guess so far is that the
left pin (top in the image) should be connected to the trace with the 'U'
shape, the right pin (bottom in the image) should be connected to the bottom
trace and the central pin should be connected to the trace on the left.

After having applied some fresh solder to the pins, I now see 8V DC across
the two side pins and between the left pin and the central pin. There are 0V
between the right pin and the central pin. It sounds like the central pin
and the right pin are shorted together.

<http://cbm-hackers.2304266.n4.nabble.com/file/t374814/IMG_20180225_192003-02.jpeg




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Re: CBM 3032 (PET 2001N-32), no video?

MiaM
In reply to this post by Sherry Haibara
Den Sun, 25 Feb 2018 05:40:10 -0700 (MST) skrev Sherry Haibara
<[hidden email]>:
> Took a couple of measurements on the monitor PCB. The AC input is ok
> (16.5Vac), but the voltage across the capacitor and on the LM7812
> voltage regulator seems to be very off.
> The voltage on the capacitor oscillates between 0 and 8.7 volts in a
> rhythmic fashion (it goes like: 0-1-8.7-1-0). On the LM7812 (I
> assumed it's the one mounted on the big heatsink right next to the
> capacitor) I measure 22V DC on two pairs of pins and 0V on the other.
> The 4 diodes on the bridge test fine with the diode test of the
> multimeter, showing 0L on one side and a 0.55V drop on the other.

Unless there is some mistake while taking the measurements, I can only
see theese explanations:

* The main rectifier capacitor is more or less totally worn out causing
  a too big ripple for anything to work. You can try connecting another
  capacitor with similar specs in parallell with the existing one.

* Bad diodes in the rectifier, measuring correctly with the low voltage
  your measuring instrument uses but conducts in the wrong direction
  whit enough voltage across them. Not that likely but also not totally
  unlikely.

* Severe overload on the 12V regulator. That would also cause a big
  ripple across the main capacitor but then some stuff would also run
  very hot.

* Extremely unlikely: Short circuit between the secondary windings on
  the transformer. Easy to measure, just disconnect the transformer
  from the monitor and ohm measure between this winding and the other
  windings.

You could try disconnecting the output from the 12V regulator (and
disconnect the transformer) and feed the monitor from some other 12V
supply like a PC PSU or similar.


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Re: CBM 3032 (PET 2001N-32), no video?

MiaM
In reply to this post by Sherry Haibara
Den Sun, 25 Feb 2018 10:58:42 -0700 (MST) skrev Sherry Haibara
<[hidden email]>:
> Still, I'm able to see some output. When I turn on the CBM, I see a
> mostly green screen, followed by a black screen, followed by what
> seems to be a single line of text that is repeated multiple times on
> the screen and that keeps oscillating in a diagonal fashion.

The easiest way to check if you see actual video and not something from
the sync signals would be to use the keyboard to clear the screen
(shift + clr/home) which should leave you with a single cursor, and
then just type stuff on the keyboard and see if it fills the screen.

Then you can be rather sure that most of the main board works fine.

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Re: CBM 3032 (PET 2001N-32), no video?

Francesco Messineo
In reply to this post by Sherry Haibara
On Sun, Feb 25, 2018 at 7:43 PM, Sherry Haibara <[hidden email]> wrote:

>> the heater of the monitor is really independent from the main PET
>> board. It should light as soon as you apply the AC power to the
>> monitor PCB, even if no PET is connected. It's fed with the regulated
>> 12V DC through a dropping resistor only.
>> If it doesn't come on, there is no 12V dc on the monitor PCB or some
>> connection is broken (wires going to the CRT neck, the dropping
>> resistor, etc.)
>
> The pins of the regulator seems indeed to be in a very sorry state. I'm
> attaching a picture (let me know if it's visible). It looks like someone has
> attempted to replace it but didn't do a very good job, those traces might be
> broken at this point. How should the pins and the traces look like? Are they
> supposed to be connected, and in what order? My guess so far is that the
> left pin (top in the image) should be connected to the trace with the 'U'
> shape, the right pin (bottom in the image) should be connected to the bottom
> trace and the central pin should be connected to the trace on the left.

that is correct.
You should measure only between the center pin (negative) and the
other pins: one is the input coming from the rectifiers and the
smoothing capacitor, and the other pin is the output.
On the input you have to measure at least 17V, more likely about 20V,
on the output pin, you should have about 12V.

> After having applied some fresh solder to the pins, I now see 8V DC across
> the two side pins and between the left pin and the central pin. There are 0V
> between the right pin and the central pin. It sounds like the central pin
> and the right pin are shorted together.

your measurement is strange and only compatible with a shorted 12V
rail (or broken rectifiers or maybe dead capacitor).
Power down everything and measure the resistance between the output of
the 12V regulator and ground (center) pin.

Frank

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Re: CBM 3032 (PET 2001N-32), no video?

Sherry Haibara
I removed the regulator from the board. The situation is now the following:
- the resistance between ground and output of the regulator (on the board)
is 25 Ohm
- the resistance between ground and output of the regulator (on the
regulator itself) is 10 KOhm
- the voltage between ground and the output pin of the capacitor / input of
the regulator is now oscillating between 0 and 2V approximately.
I have also disconnected the L701 inductor, or at least what I think is the
L701 inductor, but that didn't make any difference. Incidentally, I think I
might have damaged it while pulling it out... Is it a difficult part to find
a replacement for?
I'll probably be able to make some more measurements later this week.



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Re: CBM 3032 (PET 2001N-32), no video?

Francesco Messineo
On Sun, Feb 25, 2018 at 10:16 PM, Sherry Haibara <[hidden email]> wrote:
> I removed the regulator from the board. The situation is now the following:
> - the resistance between ground and output of the regulator (on the board)
> is 25 Ohm
> - the resistance between ground and output of the regulator (on the
> regulator itself) is 10 KOhm
> - the voltage between ground and the output pin of the capacitor / input of
> the regulator is now oscillating between 0 and 2V approximately.

that's not good at all. You have to understand if the diodes and the
smoothing capacitor are good and the tracks/solder joint are ok.
Really there isn't much else that can go wrong now.
F

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Re: CBM 3032 (PET 2001N-32), no video?

Sherry Haibara
I'll take the capacitor and the diodes out to test them separately.
At this point I'm planning to replace the smoothing capacitor, the
regulator, the four diodes and the L701 inductor just to be safe. Do you
know a modern equivalent of the 1N5403 that is easier to find? I'm also
unsure about the correct replacement for L701, the schematics show a "10F"
value; F probably stands for ferrite, but is 10 expressed in mH?



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